Nervous System

Contents

Neurons and Nerves

The human nervous system has two main divisions (Figure 01a): the central nervous system (CNS), and the peripheral nervous system (PNS), which includes the somatic motor nervous system, and the sensory nervous system. The CNS consists of the brain and spinal cord. It acts as the central control region of the human nervous system, processing information and issuing commands. The autonomic nervous system (ANS) is the command network the CNS uses to maintain the body's homeostasis. It automatically regulates heartbeat and controls muscle contractions in the walls of blood vessels, digestive, urinary, and reproductive tracts. It also carries messages that help stimulate glands to secrete tears, mucus, and digestive enzymes. The nerves (Figure 01b) that are easily visible to the unaided eye are not single cells. Rather, they are bundles of nerve fibers (neurons) each of which is itself a portion of a cell. The fibers are all traveling in the same direction and are bound together for the sake of convenience, though the individual fibers of the bundle may have widely differing

Figure 01a Nervous System [view large image]

functions. There are no cell bodies in nerves; cell bodies are found only in the CNS or in the ganglia. Ganglia are collections of cell bodies within the PNS.

The main portion of the neuron, the cell body, is not too different from other cells. It contains a nucleus and cytoplasm. Where it is most distinct from cells of other types is that out of the cell body, long threadlike projections emerge. Over most of the cell there are numerous projections that branch out into still finer extensions. These branching threads are called dendrites ("tree" in Greek). At one point of the cell, however, there is a particularly long extension that usually does not branch throughout most of its sometimes enormous length. This is the axon (the axis). Figure 01b shows the three parts of the neurons: dentrite(s), cell body, and axon. A dendrites conducts nerve impulses toward the cell body, the part of a neuron that contains the nucleus and other organelles. An axon conducts nerve implses away from the cell body. There are three types of

Figure 01b Neuron and Nerve [view large image]

neurons: sensory neuron, motor neuron, and interneuron. A sensory neuron takes a message from the recptors in the sense organ to the CNS. A motor neuron sends a message away from

the CNS to an effector, a muscle fiber or a gland. An interneuron is always found completely within the CNS and conveys messages between parts of the system (Figure 3a). In addition to neurons, nervous tissue contains glial cells such as the Schwann cells covering the neurons with sheath. These cells maintain the tissue by supporting and protecing the neurons. They also provide nutrients to neurons and help to keep the tissue free of debris. The neurons require a great deal of energy for the maintenance of the ionic imbalance between themselves and their surrounding fluids, which is constantly in flux as a result of the opening and closing of channels through the neuronal membranes. Thus while the brain is only 2% of our body weight,

Figure 01c Neurons [view large image]

it consumes 20% of our energy and moreover 80% of this energy consumption is devoted to maintain the imbalance.

Neurons are dynamically polarized, so that information flows from the fine dendrites into the main dendrites and then to the cell body, where it is converted into all-or-none signals, the action potentials, which are relayed to other neurons by the axon, a long wirelike structure. The neuron is actually a very poor conductor; the signal drops to 37% of its original strength in only about 0.15 mm. Thus it needs amplification all along its length in the form of sodium-potassium pumps and gates (see Figure 01d). The amplification is initiated by detection of small changes in voltage across the membrane with the opening of voltage-sensitive sodium channels in the membrane of the neuron. Sodium ions rush into the neurons from the extracellular fluid, resulting in a transient change in the voltage difference between the neuron and the surrounding environment. The action potential travels like a wave from the cell body down the neuron via the repeating amplifications. Thus, the action potential enables the neuron to communicate rapidly with other neurons over sizable distances, sometime more than a meter away with a speed from 20 -200 m/sec. When the action potential reaches an axon terminal (the synapse), it causes the terminals to secrete a chemical messenger (neurotransmitter), generally an amino acid or its derivative, which binds to receptors in the post-synaptic neurons on the far side of the synaptic cleft. When the postsynaptic potential has reached a specific value an action potential is triggered and the signal is passed to the next neuron.

Figure 01d Action Potential [view large image]

Neurotransmitters

		firing less likely (see Figure 29a). It all depends on the type of neurotransmitter. An individual nerve cell can possess both kinds of synaptic connections (with a total of about 50000 synapses on the surface) to other nerve cells. Only if the excitatory charges (positive charge) exceed a threshold does the target neuron starting a nerve impulse of its own and is known as transduction. Figure 02b shows the various components in the synapse. The vesicle contains the neuro-transmitters in the axon. The receptor is located on the surface of the dendrite to pick up the neuro-transmitters. The
Figure 02a Synapse [view large image]	Figure 02b Neurotransmitter [view large image]	transporter is for recycling un-used neutrotransmitters back into the axon; while the glial cell provides nutrition and support for the neurons.

Figure 02c shows the process of signal transmission across the synapse:

Release - As the action potential comes down the axon, the calcium influx triggers an exocytosis of vesicles that contain the neurotransmitters, which are release into the synaptic cleft. Bind - The neurotransmitters then drifts across , binds to the postsynaptic receptors. Transduction - Depending on the integration of the excitatory and inhibitory inputs, the receiving dendrite may fire a signal for further transmission. Reuptake - The neurotransmitter transporters remove the un-used neutrotransmitters in the synaptic gap back to the axon for re-use. This step is to prevent continuous stimulation of the postsynaptic neuron.

Figure 02c Signal Transmission [view large image]

There are other ways to turn the signal off. One is simple diffusion into the extracellular space. Another way is to break down the neuro-transmitters with enzymes. Then there are the presynaptic autoreceptors

(not shown), which terminate the release once a neutrotransmitter drifts back upstream and hits one of these receptors.

manipulations and abuses. Natural neuromodulators can aid the release or inhibit the reabsorption of neurotransmitters; still others delay the breakdown after reabsorption, leaving them in the tip to be reused by the next nerve impulse. Mood, pleasure, pain, and other mental states are determined by particular groups of neurons in the brain that use special sets of neurotransmitters and neuromodulators. For example, mood is strongly influenced by the neurotransmitter serotonin. It is believed that depression results from a shortage of serotonin. It is difficult to treat depression directly with serotonin because the chemical has too many other side effects. However, depression can be successfully treated with drugs that act as serotornin neuromodulators (Figure 02d). Prozac, the

Figure 02d Neuromodulator [view large image]

world's top-selling antidepressant, inhibits the reabsorption of serotonin, increasing the amount in the synapse by slowing down its removal.

When a neuron cell is exposed to a neurotransmitter for a prolonged period, it tends to lose its ability to respond to the stimulus with its original intensity. This is known as habituation, which is the result of the cell producing fewer receptors for that particular neurotransmitter. If someone takes a drug that acts as a neuromodulator (such as cocaine), which causes abnormally large amounts of neurotransmitter (dopamine in this case, Figure 02e) to remain in the synapses for long periods of time, it would generate more pleasure messages. Such action reduces the number of receptors in the neuron. Next time a higher dosage is required to maintain the pleasurable sensation. The result is addiction. Cocaine is a stimulant discovered in the mid-1800s. Many physicians at first considered it a miracle drug, prescribing it for all sorts of physical and mental ailments; it was even added to soft drinks. Today United States law forbids the importation, manufacture, and use of cocaine for nonmedical purposes, and even the medical use is extremely limited.

Figure 02e Drug Addiction [view large image]

Neurotransmitters can be broadly classified into two groups - the "classical", small molecule neurotransmitters and the relatively larger neuropeptide neurotransmitters. The small molecule types are mainly amino acids and amines (a nitrogen atom bonds to a maximum of three hydrocarbon groups). The larger neurotransmitters are combination of two or more amino acids joined by peptide bonds. Some fifty different neurotransmitters have been identified. The form of receptors for the neurotransmitters varies depending on the location in the body and produces different physiologic symptom. Understanding the numerous neurotransmitters, their receptors, locations and interactions with one another has been central to the design of medicines for mental illness. Figure 02f shows the effects of three major neurotransmitters and the mental states induced by their interactions. Table 01 summarizes the properties of some important neurotransmitters.

Figure 02f Types of Neuro-transmitter [view large image]

Table 01 Neurotransmitters

The Brain

Some of the most basic features of brains can be found in bacteria because even the simplest motile organisms must solve the problem of locating resources and avoiding toxins. They sense their environment through a large number of receptors, which are protein molecules embedded in the cell wall. The action taken in response to the inputs usually depends on the gradient of the chemicals (see Figure 03a). Thus memory is required to compare the inputs from different locations. The strength of the signal is modulated by immediate past experience. This in turn regulates the strength of the signal sent by chemical messengers from

Figure 03a E. coli's Response to Chemical Gradient [view large image]

the receptor to the flagellar motors. Thus even at the unicellular level, the bacteria have already possessed the ability to integrate numerous analog inputs and generate a binary (digital) output of stop or go.

miniature versions of adults, but baby mammals and birds are dependent because of their poor capacity to thermo-regulate, the consequence of their need to devote most their energy to growth. Most mammals solve the problem with maternal care (Figure 03b), shelter, warmth, and milk. In most birds, both parents cooperate to provide food and shelter to their young. The expanded forebrain and parental care provide mechanisms for the extra-genetic transmission of information from one generation to the next. This transmission results from the close contact with parents during infancy, which provides the young with opportunity to observe and learn from their behavior; the expanded forebrain provides an enhanced capacity to store these memories. The expanded forebrain and the observation of parents

Figure 03b Maternal Care [view large image]

are probably necessary for the establishment of successful care giving behavior itself, as the young mature into adults that will in their turn have to serve dependent young. During the period of infant dependency, baby mammals and birds play,

		The human brain can be divided into three parts: the hindbrain, which has been inherited from the reptiles; the limbic system, which was first emerged in mammals; and the forebrain, which has its full development in human. Different views of the human brain are shown in Figure 03c, d, and e. Tables 01 lists the functions of the different parts of the human brain. The brain is separated into two hemispheres. Apart from a single little organ -- the pineal gland in the centre base of the brain -- every brain module is duplicated in each hemisphere. The left brain is calculating, communicative and capable of conceiving and executing complicated plans --
Figure 03c Human Brain 1	Figure 03d Human Brain 2 [view large image]	the reductionistic brain; while the right one is considered as gentle, emotional and more at one with the natural world -- the holistic brain. The cerebral cortex is covered in a thin skin

of deeply wrinkled grey tissue called the grey matter (densely packed neurons for information processing). Each infold on the surface is known as a sulcus, and each bulge is know as a gyrus. While the white tissue inside are axons -- tentacles which reach out to other cells (to relay information). The cortex can be broken down into many functional regions, each containing thousands of cortical columns (oriented perpendicular to the cortical surface). Columns are typically about half a millimeter in diameter and contain about one hundred thousand neurons. They are the units of cognition (the mental process

Figure 03e Human Brain 3 [view large image]

of acquiring knowledge by the use of reasoning, intuition or perception). Table 02 below lists the location and functions of the major components in the human brain.

Table 02 Human Brain

Table 03 The Five Stages of Human Brain

Figure 03f The Four Stages of Human Brain [view large image]

It is well known that the brain is an electrochemical organ; a fully functioning brain can generate as much as 20 watts of electrical power. Even though this electrical power is very limited, it does occur in very specific ways that are characteristic of the human brain. Electrical activity emanating from the brain can be displayed in the form of brainwaves. There are four categories of these brainwaves, ranging from the most active to the least active. Figure 03g is produced by an EEG (ElectroEncephaloGraph) chart recorder to show the different kind of brainwave according to the different state of the brain. These are all oscillating electrical voltages in the brain, but they are very tiny voltages, just a few millionths of a volt. Electrodes are placed on the outer surface of the head to detect electrical changes in the extracellular fluid of the brain in response to changes in potential among large groups of neurons. The resulting

Figure 03g Brain Waves [view large image]

signals from the electrodes are amplified and recorded.

be associated with creative, insightful thought. When an artist or scientist has the "aha" experience, there's a good chance he or she is in Alpha or Theta. These two kinds of brain waves are also associated with relaxation and, stronger immune systems. Therefore, many people try to train themselves to enter such states through various biofeedback7 techniques (with varying degree of success). Delta Waves occur during sleep. They originate from the cerebral cortex when it is not being activated by the reticular formation. In slow-wave sleep, the entire brain oscillates in a gentle rhythm quite unlike the fragmented oscillations of normal consciousness. The neocortical activity is often modulated by a rhythm of 40-80 Hz, called the Gamma wave (not shown in Figure 03g). When there are strong gamma oscillations in certain parts of the neocortex, human subjects do better on learning and memory tasks. A 2010 study indicates that brainwaves are for integrating various sensations to ensure all the relevant signals

Figure 03h Integration of Brain Waves

for one event arrive at the binding site at exactly the same time. This allows the receiving neurons to process the signals together, recombining them into a single sensation. For example, we see an apple as red and round, not one red thing and another round thing although red and

With all these delicate components inside the brain, it will not work properly without a critical modification of the capillaries. This is the blood brain barrier (BBB), which prevents harmful substances getting into the brain from the blood. Usually, the capillaries provide a small gap between the endothelial cells (the inner-most layer of cells) to let nutrients and oxygen going into the organs. The gap is large enough for bacteria, and large hydrophilic molecules to escape from the blood vessel causing diseases and other kind of damages to the brain.

Figure 03i Blood Brain Barrier

The special functions provided by the blood brain barrier is summarized below (Figure 03i):

1. The feet of the astrocytes (a type of glial cells) form a supportive layer around the capillaries.
2. It is suggested that the astrocytes may promote the formation of the tight junctions between the endothelial cells. The tight junction is the key component to seal the pathway for water-soluble substances from getting through.
3. Fat-soluble molecules such as CO₂, O₂, hormones, and alcohol (that's why we get drunk), etc. can pass through the membrane freely.
4. Nutrients such as sugar , amino acid etc. manage to come out by carriers and ion channels.
5. Additional enzymatic barrier bound to the wall is used to remove harmful molecules from the blood.
6. There is also the efflux pump to extrude un-wanted fat-soluble molecules back into the blood.

Spinal Cord

		The spinal cord (Figure 04a) lies along the middorsal line of the body. It has two main functions: (1) it is the center for many reflex actions, and (2) it provides a means of commu-nication between the brain and the spinal nerves that leave the cord (Figure 04b). The white matter of the cord is white because it contains myelinated long fibers of interneurons that run to-gether in bundles call tracts. These tracts connect the cord to the brain. The dorsal ones are primarily ascending to the brain, while the ventral tracts bring information down from the brain. The inner portion of the
Figure 04a Spinal Cord [view large image]	Figure 04b CNS and PNS [view large image]	cord is filled with a mass of nerve cell bodies called gray matter. Each spinal nerve emerges from the spinal cord as two short branches, the dorsal and the ventral roots. These roots join just before the nerve leaves the vertebral column.

Peripheral Nervous System

		The peripheral nervous system is outside the CNS. It consists of the various nerves that connect particular parts of the CNS with particular organs. Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial nerves (Figure 05) are either sensory nerves, motor nerves, or mixed nerves. All of them, except the vagus nerve, control the head, the face, the neck, and the shoulders. The vagus nerve controls the internal organs. Table 04 lists the functions of the various cranial nerves. All spinal nerves (Figure 06) are mixed nerves that take impulses to and from the spinal cord. Table 05
Figure 05 Cranial Nerves [view large image]	Figure 06 Spinal Nerves [view large image]	describes the symptom of spinal cord injury (SCI) with the particular spinal nerve(s).

Table 04 Functions of Cranial Nerves

Table 05 Symptom(s) of Spinal Cord Injury

Autonomic Nervous System

One division of the autonomic nervous system, called the sympathetic nervous system, dominates in times of stress. It controls the "fight or flight" reaction, increasing blood pressure, heart rate, breathing rate, and blood flow to the muscles. Another division, called the parasympathetic nervous system, has the opposite effect. It conserves energy by slowing the heartbeat and breathing rate, and by promoting digestion and elimination (of waste). Most glands, smooth muscles, and cardiac muscles constantly get inputs from both the sympathetic and parasympathetic systems. The CNS controls the activity by varying the ratio of the signals. Depending on which motor neurons are selected by the CNS, the net effect of the arriving signals

Figure 07 ANS Side View [view large image]

will either stimulate or inhibit the organ. Figure 07 shows the various organs and actions, which are related to the two different divisions.

the CNS (the preganglionic fibers). At the ganglia the fibers form synaptic junctions with the dendrites of as many as twenty different cell bodies. The axons of these cell bodies form a second set of fibers, the postganglionic fibers. It is these postganglionic fibers that lead to the organs. The chief ganglia involved in the autonomic nervous system form two lines running down either side of the spinal column. They are outside the bony vertebrae. These two lines of ganglia outside the column resemble a pair of long beaded cords. At the lower end, the two cords join and finish in a single central stretch. These lines of ganglia are sometimes called the sympathetic trunks (used by the sympathetic nervous system). Not all ganglia are located in the sympathetic trunks. Some are not; and it is possible for a preganglionic fiber to go right through, making no synaptic junction there at all, joining instead with ganglia located in front of the vertebrae. For the parasympathetic nervous system, some of the ganglia separating the preganglionic fibers from the postganglionic fibers are actually located within the organ the nerve is servicing. In that case, the preganglionic fiber runs almost the full length of the total track, whereas the postganglionic fiber is at most just a few millimeters long. The splanchnic nerves, which originate from some of the thoracic nerves, have their preganglionic fibers ending in a mass of ganglia lying just behind the stomach. It represents the largest mass of nerve cells that is not within the CNS and is sometimes called the "abdominal brain". It is a vital spot to be protected during boxing.

Figure 08 ANS Front View [view large image]

Figure 08 is the front view of a more detailed ANS anatomy.

Senses

		Senses organs receive external and internal stimuli; therefore, they are called receptors. Each type of receptor is sensitive to only one type of stimulus as listed Table 06, while Figure 09 shows many types of receptors. When a receptor is stimulated, it generated nerve impulses that are transmitted to the spinal cord and/or the brain, but we are conscious of a sensation only if the impulses reach the cerebrum.
Figure 09 Senses [view large image]	Figure 10 Cerebrum Mapping [view large image]

Table 06 Receptors

Sight (see location of the various components in Figure 09):



Sclera - This is the white of the eye. It is a tough whitish sheath covering most of the eye (see Figure 11). The inter-cellular space contains a kind of loose proteins composed of collagen and elastic fibers, which make the wall of the eyeball more pliant. Cornea - It is almost perfectly transparent at the front of the sclera for the protection of the eye. Light rays is refracted through this dome-shaped structure. Choroid - The soft layer inside the sclera. It has many blood vessels that nourish the eye. It also absorbs stray light.

Figure 11 Human Eye [view large image]

Ciliary body - The choroid becomes suspensory ligaments near the opening. It holds the lens in place, and controls the shape of the lens for near and far vision.

• Iris - Further toward the opening, the choroid becomes a thin, circular, muscular diaphragm known as the iris, which regulates the size of a center hole (the pupil) and thus controls the amount of light entering into the eye. The pupil gets larger in dim light and smaller in bright light.
• Aqueous humor - This is the anterior cavity between the cornea and the lens. It is filled with an alkaline, watery solution secreted by the ciliary body. Normally, the solution leaves the anterior cavity by way of tiny ducts that are located where the iris meets the cornea. When a person has glaucoma, these drainage ducts are blocked, and aqueous humor builds up. The resulting pressure compresses arteries that serve the nerve fibers of the retina. The nerve fibers begin to die due to lack of nutrients, and the person becomes partially blind.
• Lens - It is a fat disk that completes the focusing of light rays into a clear, sharp image on the retina. Actually, the cornea provides about 80% of the focusing power. The lens is stretched by the ciliary muscles to fine-tune the focus by changing the shape, so it becomes fat for near objects, thin for far ones.
• Vitreous humor - The posterior cavity behind the lens contains a viscous, gelatinous material. It forms the bulk inside the eyeball and, with the outer sclera, gives it firmness. It also helps to refract light rays toward the retina.

		Retina - Inside each light-sensitive cell (rod or cone) in the retina are up to 100 million molecules of photopigment, each of which contains a smaller molecule known as retinal (Figure 12a). When retinal receives light energy, it changes shape by twisting around its backbone. The altered retinal sets off a chain of chemical reactions inside the cell, which triggers an electro-chemical change in the cell membrane creating a nerve impulse. The retinal returns to its original configuration when the signal jumps across a synapse to a bipolar cell. Curiously, in order to
Figure 12a Retinal [view large image]	Figure 12b Retina [view large image]	reach the photoreceptors, incoming light must first pass through all the other layers of cells in the retina. There are five layers altogether (see Figure 12b). Starting from the outermost layer:

1. Pigment epithelium - Epithelial cells are the guards and protectors of the organ. They cover the surface and determine which substances are allowed to enter.
2. Rods and cones - These are the photoreceptors. The rods are responsible for night vision, and the cones for color vision. The retina has as many as 150 million rods but only 1 million ganglionic cells and optic nerve fibers. This means that there is considerable mixing of messages and a certain amount of integration before nerve impulses are sent to its final destination. The photoreceptors achieve the remarkable sensitivity (of a millionfold difference in luminance) with adjustment relative to the average background. But if the overall background level of illumination were to change drastically, as it does when we enter a dark room, we are effectively blind for a few minutes until the rod photoreceptors have adapted to this level of reduced intensity. Vision is most acute in the fovea centralis (see Figure 09), where there are only cone cells. Clear vision is possible only when the fovea inspects a scene. This provides a very restricted window of clarity. Thus in order to obtain a clear picture, the eyes have to dart about frenetically and automatically under the direction of the brain.
3. Bipolar cells - Nerve signals from the rods and cones pass inward to this layer. The bipolar cells together with the horizontal and amacrine cells form the network of pre-processing nerve cells. The network helps to simplify, and code information before it reaches the optic nerve.
4. Ganglionic cell layer - It receives input from the pre-processing network, and send output to the brain via the axons of the ganglion cells.
5. Optic nerve - It is made up of nerve fibers, which come from across the whole retina. They pass in front of the retina, forming the optic nerve, which turns to pierce the layers of the eye. The signals eventually end up in the occipital lobe to form an image.

Optic Pathway - As shown in Figure 13, information about the left visual world is transmitted to the right side of the brain and vice versa. As the visual fields of the eyes overlap in the front, this division is achieved by sorting retinal ganglion cell axons according to whether they look at the left (dotted line) or the right (solid line) visual field. So some axons from the right eye go to the right side of the brain and others to the left. The sorting occurs in the optic chiasm. The retinal axons proceed to the lateral geniculate nuclei where the first synapses are fromed with the visual neurons in the brains.

Figure 13 Optic Pathway [view large image]

Hearing (see location of the various components in Figure 09):


• Outer Ear -
  • Pinna - This is the external flap for collecting sound wave.
  • Auditory canal - The opening of the auditory canal is lined with fine hairs and sweat glands. The modified sweat glands secrete earwax to guard the ear against the entrance of foreign materials, such as air pollutants.
• Middle Ear -
  • Tympanic membrane (ear drum) - It pushes a set of three small bone (the ossicles) against an inner membrane.
  • Ossicles - The three parts in this structure are: hammer, anvil, and stirrup. These components multiply the sight vibration of the sound by about 20 times. When the stirrup strikes the oval window, the pressure is passed to the fluid within the inner ear.
  • Eustachian tube - It extends from each middle ear to the nasopharynx and permit equalization of air pressure. Chewing gum, yawning, and swallowing in elevators and airplanes help to move air through the eustachian tubes upon ascent and descent.
• Inner Ear - Whereas the outer ear and the middle contain air, the inner ear is filled with fluid. Anatomically speaking, it has three areas: the first two, the semicircular canals and a vestibule, are concerned with balance; only the third, the cochlea, is concerned with hearing. It is a very delicate and sophisticated organ about 1 cm³ in size.
  • Scala vestibuli - The incoming vibration (red arrows in Figure 14) spirals along this structure toward the apex of the cochlea. At the apex, a small gap allows the wave to pass into the scala tympani.
  • Scala tympani - The wave travel down (blue arrows) in this spiraling tube toward the round window.
  • Round window - It acts as a pressure relief membrane dissipating the vibrational energy.
  • Organ of Corti - This structure is set onto the basilar membrane which forms one of the arms of the Y inside the cochlea (see Figure 14). Pressure changes in the fluid outside the scala media create vibrations in the basilar membrane and in the Reissner's membrane, which forms the other arm of the Y. The vibrations in the scala media and the tectorial membrane shake the hair cells, which convert the vibrational energy into electrical nerve impulses. The signals are channeled by nerve fibers along the organ of Corti to the main cochlear nerve, and then to the temporal lob of the brain for processing into the sounds that we perceive.

    		According to the frequency of the sound wave, different parts of the basilar membrane along the organ of Corti are set into motion. In general, low-pitch sounds make the apex of the cochlea vibrate while high-pitched ones cause most vibrations near the base of the cochlea. Figure 15 shows such frequency distribution along the length of the cochlea for both the incoming and outgoing waves. The strength of nerve signals also depends on the volume of the sound. This is interpreted by the brain as loudness. It is believed that tone is an interpretation of the brain based on the distribution of hair cells stimulated.
    Figure 14 Cochlea [view large image]	Figure 15 Sound Wave [view large image]

  
  
  
  Smell (see location of the various components in Figure 09):
  
  

  Nasal cavity - This a large air space above and behind the nostril. Three shelf-like ridges of bone (nasal conchae) are there to deflect air. In normal breathing, air flows through the lower part of the cavity, past the rear of the soft palate and into the throat (Figure 16). A good sniff sends the odor eddying up into the roof of the nasal cavity, where it comes into contact with the olfactory apparatus. Mucus layer - Only those odor molecules that can be dissolved in the mucus, become the stimulants to the receptor sites on the cilia.

  Figure 16 Sense of Smell [view large image]

  Cilia - These are the hairlike projections from the olfactory sensory cells, and trigger the sensory cells to generate nerve signals if the receptors recognize the shape of the odor molecules. It is estimated that the human nose contains about 1000 different types of olfactory neurons, each type able to detect a particular set of chemicals.

  Bowman's gland - This gland makes the mucus. Olfactory sensory cell - It is embedded in the olfactory epithelium. Nerve signals pass upward along the cell body, which narrows into a wire-shaped nerve fiber, or axon. The axons from thousands of sensory cells group into bundles and convey their nerve signals to the olfactory bulb. Olfactory bulb - In this structure, the axons form complicated ball-shaped sets of connections with the mitral cells. These connection area are olfactory glomeruli, and there are hundreds in each olfactory bulb (see Figure 17). Each glomerulus receives signals from more than 25000 sensory cells and has tens of thousands of connections from the mitral cells in the bulb itself. Much sorting and processing of the signals takes place in the glomeruli. The resulting nerve messages are sent along the olfactory tract to the olfactory area in the brain.

  Figure 17 Olfactory Bulb [view large image]

  Olfactory area - Nerve signals representing smells are routed to two regions of the brain: the medial (inner) olfactory area and the lateral (side) olfactory area in the amygdala. Figure 18 shows the pathway of the odorants in the air, which initiates impulses moving along the nerve pathway to the brain. Since the nerve pathway is in part of the brain's limbic system, which also deals with memories and emotions, smell can evoke strong emotion from past experience about a certain odor. Smell is the sense in which habituation occurs most quickly. Habituation is the process in which a sense becomes accustomed to what it detects so that it is no longer perceived. Most odors can hardly be perceived just 30 seconds after they are first detected.

  Figure 18 Pathways [view large image]

  The sense of taste and the sense of smell supplement each other, creating a combined effect when interpreted by the cerebral cortex. For example, some of the molecules may move from the nose down into the mouth region and stimulate the taste buds there. Therefore, part of what we refer to as smell actually may be taste.

  
  
  Taste (see location of the various components in Figure 09):
  
  
• Tongue - Embedded within the surface of the tongue are four types of taste receptors localized in specific regions on the

		tongue (see Figure 19). Each detects a different class of chemical: sweet (sugars), sour (acids), bitter (complex organics), and salty (salts). The "hot" sensation of foods such as chili peppers is detected by pain receptors, not chemical receptors. But a report in 2006 reveals that contrary to popular belief, there is no tongue map. Responsiveness to the five basic modalities - bitter, sour, sweet salty and umami (a Japanese word
Figure 19 Tongue [view large image]	Figure 20 Papillae [view large image]	meaning the savory or meaty taste of amino acids) is present in all areas of the tongue.

Taste buds - The tasting, or gustatory, cells in the buds have hairy tips which detect chemicals in solution (secreted by the gland at the bottom of papilla). When stimulated by flavor molecules, these cells generate nerve signals, which they send to the taste center on the brain's cortex, and also to the hypothalamus, which is concerned with appetite and the salivating reflex. Taste nerve pathway - The nerve signals are carried by three nerves in each side of the tongue (cranial nerves) to a small part of the medulla (brain stem). The signals then travel to parts of the brain, such as the hypothalamus, the thalamus, and the gustatory part of the sensory cortex - the "taste center", where the signals are interpreted (Figure 21). The thalamus acts like a relay station, shunting the data onto appropriate cortical areas for processing. The sense of taste tells us what is good to eat. It evolved to pick out sweet, ripe fruits and energy-packed sugars

Figure 21 Sense of Taste [view large image]

and starches. Likewise, taste is is extremely sensitive to bitter flavors, because many poisonous berries, fruits and fungi are bitter-tasting.

Sensations (see location of the various components in Figure 09):


• Skin - Skin has a thin epidermis, which is mainly for protection, and a thicker dermis below. In addition to small blood vessels and sweat glands, it has tiny nerve endings in the various type of touch receptors (see Figure 09).
• Receptors -
  • Bulb of Krause - These are multi-layered capsules with many branched nerve endings. They are quick-change mechanoreceptors, triggered by rapid alterations in shap caused by pressure or vibrations, and may also help us to feel extreme cold.
  • Free nerve endings - They have a treelike branching system of naked nerve fibers. They are the most common sensory endings in the skin and detect just about anything - light touch, heavy pressure, heat, cold, and importantly, pain. Slight stimulation of these nerve endings may elicit the sensation that is known as itching.
  • Meissner's endings - They are found in the uppermost part of the dermis, especially on the hands, feet, lips, and inner surfaces of the eyelids. They are shaped like eggs and are both quick- and slow-change mechanoreceptors, detecting light touch and vibrations.
  • Merkel's endings - They are like tiny disks stuck in the underside of the epidermis, where they feel slight changes in its shape, thereby detecting light touch. They are both quick- and slow-change mechanoreceptors.
  • Pacinian endings - They have layers like an onion and are sited deep in the dermis. They pick up heavy pressure and also fast vibrations, such as those from a tuning fork.
  • Ruffini endings - They respond to sustained stress or gradually altering shape. This means that they are slow-change mechanoreceptors. They are found mainly in hairy skin and are sausage- or spindle-shaped. It is thought that they may also detect extreme heat.
• Proprioceptors - The sense of position and movement of limbs is dependent upon receptors termed proprioceptors (Figure 22a). They are located in the joints and associated ligaments and tendons that respond to stretching, pressure, and pain. Nerve endings from these receptors are integrated with those received from other types of receptors so that we know the position of body parts.
• Sensory nerves - Nerve impulses may reach the somatosensory cortex for analysis before a response is decided. These result in voluntary actions - a deliberate response. Sometimes the stimulus require immediate action (such as from the burning sensation), a reflex action is taken without the conscious control of the brain. These are the involuntary actions directed by the spinal cord. We only become aware of them when other impulses are sent to the brain to "inform" what has happened. The path which impulses travel along during a reflex action is called a reflex arc. Not all the body parts

		receive the same attention of the brain. The relative importance is often represented by mapping over the length of the sensory or motor cortex. These cortical maps (Figure 22b) are not drawn to scale; instead they are variously distorted to reflect the amount the neural processing power devoted to different regions. This accounts for the grotesque appearance of the human body in the homun-culus, which is a translation of the body's sensory map into the human form.
Figure 22a Propriocep-tors [view large image]	Figure 22b Homunculus [view large image]

Balance is an ongoing process that keeps our two-legged posture stable. Four main sets of sensory input are involved:


1. Information from the skin is important, especially from the touch and pressure sensors on different parts of the feet, which tell the brain if you are leaning. This sense is not available in a free falling environment such as in a spacecraft.
2. Eyesight is used to judge verticals and horizontals to which your body should be parallel and at right angle respectively.
3. The body's proprioceptive sense of stretch in muscles, tendons, and joints tell the brain about the positions and angles of the arms, legs, torso, and neck.
4. The sensory parts dedicated to balance is located deep inside each inner ear, next to the cochlea (see Figure 09). These parts are known collectively as the vestibular apparatus and are part of the same network of fluid-filled chambers as the cochlea. They consist of the utricle, the saccule, and the semicircular canals (Figure 23a). In certain parts of their linings are tinny hairs, whose roots are embedded in lumpy crystals or gels. The crystals or gels are attracted downward by gravity, and they are also pushed to and fro by the fluid in the chambers, which swirls as the head changes its position.

The functions of the these organs are shown in Figure 23a: (a) The ampullae of the semicircular canals contain hair cells with cilia embedded in a gelationous material. (b) When the head rotates, the material is displaced and the bending of the cilia initiates nerve impulses in sensory nerve fibers for maintaining dynamic equilibrium. (c) The utricle and saccule are sacs that contain hair cell with cilia embedded in the gelationous material. (d) When the head bends, otoliths are displaced, causing the gelationous material to sag and the cilia to bend. This initiates nerve impulses in sensory nerve fibers for maintaining static equilibrium.

Figure 23a Balance [view large image]

The vestibular nerve feeds its information chiefly to the cerebellum and to four structures in the medulla known as vestibular bodies. Using these data, as well as input from the other three sensory sources, the brain works out what to do, usually subconsciously.

It turns out that such structure of hair within gel to detect disturbance has been around hundred of million years in the shark and fish (Figure 23b). This is the neuromasts embedded in the skin of fish. They give the fish information about the flow of water. Amphibians and reptiles have a simple uncoiled inner ear. Jawless fish has only one semicircular canal instead of three in mammals (for detecting three dimensional movement). Ultimately, it is the Pax 2 gene that give rise to these structures. It is also known that the Pax 6 gene is responsible for the development of eye. The connection to ancient creatures goes even deeper when it is

Figure 23b Neuromast [view large image]

found that the box jellyfish carries a gene which is the combination of Pax 2 and Pax 6. The box jellyfish is an amazing animal with more than 20 eye pits and many eyes very similar to ours. They seem to double for ears as well.

Memory

As shown in Figure 03a, the ability to modify our behaviour in response to life's experiences is shared by all animals including the bacteria E. coli. Such feat requires the brain's willingness to learn. Learning results in the formation of memories and in humans this process reaches its most sophisticated form, allowing us creatively to associate different reflections on the past, to generate new ideas, and most importantly to acquire language as a medium of expression and communication. Memory requires the brain to be physically altered by experience and it is this remarkable property that makes thought, consciousness, and language possible. The basic mechanism of memory formation is highly conservative over

Figure 24a Memory Classification [view large image]

billion years of biological evolution. The difference in humans is that we have a lot more of the stuffs. There are about 100 trillion synaptic connections in our brain.

There are many ways to classify the memory. The concept of explicit and implicit memory refers to whether or not the recollection is produced consciously and intentionally. While the scheme of declarative and nondeclarative memory depend on the retrieval that can be declared verbally or not. Associative memory is triggered by clues; nonassociative memory can be habitual or sensitive. There are also short term and long term memory. One of the classification schemes is shown in Figure 24a. Table 07 is an attempt to put them all together. In the table, the declarative, and the procedural memory are explicit with the rest of nondeclarative memories being implicit. Only the working memory belongs to the category of short term memory fading away in hours, while the others are long term, and available for retrieval in years. Figure 24b shows the components,

Figure 24b Types of Memory [view large image]

locations, and pathways for many types of memory.

Table 07 Types of Memory

• Working Memory -Most of our memories are fleeting because few of the many experiences we have in the course of an average day are remembered for very long, nor do they need to be. Transient memories are absolutely essential to the process of understanding the meaning of events as they occur in the present. This type of very short-term memory for things being experienced now is known as working memory; it allows you to comprehend what you are reading or to figure out the meaning of what has just been said to your in a conversation. Working memory can be thought of as a low capacity information reservoir that is always full, sensations flowing into it continuously at about the same rate that they are forgotten. Some of the information held in short-term storage may be important enough to be remembered for a long time and must therefore be transferred to a more stable form of storage, which is represented by far more robust

alterations in the brain's chemical and physical make-up in the form of synaptic connections. It is not necessarily for an important experience to trigger the formation of long-term memories, other factors such emotion, practice, and rehearsal also facilitate the transformation. Experiments show that in all cases the most important underlying distinction between the short- and long-term memory formation is that the latter requires a dialogue between synapses and genes and the former does not.

Figure 25 Working Memory [view large image]

The working memory itself is located in the prefrontal cortex. As experimental techniques became refined, it has become clear that there is no rigid dividing line between a memory and a thought. A model of working memory has been developed to combine perceptions, memories and concepts together, and consists of three parts:


• Phonological loop - Memory in this area (see Figure 25) enables us to remember sequences of approximately seven digits, letter, or words. The language areas of the brain are mainly in the left hemisphere, around and above the ear. The language loop start with hearing words in the auditory cortex and/or reading words in the visual cortex. Perception of language results from the convergence of auditory and visual information in Wernicke's area. Expression of language is controlled by Broca's area; while the angular gyrus is concerned with meaning.
• Visual-spatial scratch pad - It is like a sort of inner eye, which receives and codes data into visual or spatial images. For example, it comes into play when we need to remember where we were on a page when we start reading a book again. Functional imaging suggests that this complex structure represents the "what" and "where" in short-term memory (see Figure 25).
• Central executive - This most important yet least well understood component of the working memory model, is postulated to be responsible for the selection, initiation, and termination of processing routines (e.g., encoding, storing, retrieving). It is believed that this component coordinates information from a number of sources, directs the ability to focus and switch attention, organizes incoming material and the retrieval of old memories and combines information arriving via the other two temporary storage systems. It performs various tasks such as reasoning or doing mental arithmetic - rather like the RAM (Radom Access Memory) of a computer.




Nondeclarative memory - Nondeclarative memory includes skill learning, implicit learning, priming, simple classical conditioning, and habituation. These forms of learning are similar in that it is experience which changes the neural makeup, and the conscious access to past episodes is not essential for the formation of these memories. Implicit memory is not flexible and does not allow for the recombination of learned information. Nondeclarative memory does not require the hippocampus or related structures. Instead, the implicit learning of skills and habits depends on the neostriatum (basal ganglia and its connections to the frontal lobes). The conditioning simple skeletal muscle reactions depends on the cerebellum. The amygdala is

Figure 26 Nondeclarative Memory [view large image]

essential for emotional conditioning. Nondeclarative memory can be classified to five main groups:

• Procedural memory - It is the repository of such skills as handwriting or driving. These skills are essential part of our memory store, but it is difficult to describe the "know-how" in words. In this sense the memory is said to be implicit or non-declarative (Figure 26); you just cannot explain how to ride a bicycle. The skills may be difficult to acquire, but once learnt they are never forgotten, even without occasional practice. Thus it seems that the know-ledge or information required for the execution of very complex motor routines or procedures is somehow laid down in a robust permanent memory store. The parts of the brain involved in the acquisition of complex motor skills are the cerebellum and putamen (see Figure 24). Deeply ingrained habits are stored in the caudate nucleus.
• Classical conditioning - Along with motor skills, conditioning is part of non-declarative memory. The desire for food at a particular time of day - regardless of whether hungry or not - is one example of such conditioning. A classical example is to associate the ring of a bell to food when feeding a dog. After repeating the training many times, the dog shows salivation at the ring of the bell even without food (see Figure 26).
• Fear memory - Recent study in delivering shocks to mice suggests that fear memory does not occur immediately after a painful event; rather, it takes time for the memory to become part of our consciousness. The initial event activates NMDA receptors - molecules on cells that receive messages and then produce specific physiological effect in the cell - which are normally quiet but triggered when the brain receives a shock. Over time, the receptors leave their imprint on brain cells. A phobia is an excessive or unreasonable fear of an object, place or situation. Examples include fears of specific things such as insect, snake, mouse, and flying. It seems that people can learn to suppress a fright reaction by repeatedly confronting, in a safe manner, the fear-triggering memory or stimulus. It is found that for specific phobias, up to 90% of people can be cured through such exposure therapy.
• Nonassociative memory - Nonassociative memory includes two forms of learning called habituation and sensitization. Habituation is defined as a decreased in response to a repeated stimulus such as a certain odor. On the other hand, sensitization is an increased responsiveness such as more sensitive in touching a cut in the skin. Nonassociative learning involves reflex pathways in the spinal cord and elsewhere.
• Remote memory - The memory of events that occurred in the distant past is referred to as remote memory. The underlying anatomy of remote memory is poorly understood, in part because testing this type of memory must be personalized to a patient’s autobiographical past. What is known is that, like semantic memory, remote memory eventually becomes independent of the hippocampus. One memory model shows a linear representation of how experience is processed as memory: Stimulus Sensory Registration Attention Short Term Memory Consolidation - Retrieval Long Term Memory Remote Memory. At the stage of sensory registration, there is a matching/assigning of the pattern to a meaning. Short-term memory is temporary and is limited in space. If short-term memory is not repeated, the information is lost fairly quickly. Long term memory is consolidated and stored throughout the nervous system. Remote memories represent the foundation memories upon which more recent memories are built. Since early acquired information is the foundation for new memories and may be linked to many more new memories, such memory is less subject to change and/or loss. Similar to the short-term memory, the remote memories are not usually affected by aging.




Declarative memory - Declarative memory covers the memory of facts such as events and names, which do not need to be repeated for them to sink in. Those experiences destined to be laid down as long-term memories are shunted down to the hippocampus where they are held in storage for 2 - 3 years. During this time the hippocampus replays the experiences back up to the cortex, and each rehearsal etches it deeper into the cortex. Eventually the memories are so firmly established in the cortex that the hippocampus is no longer needed for their retrieval. Much of the hippocampal replay is thought to happen during sleep. Dreams consist partly of a rerun of things that have happened during the day, fired up to the cortex by the hippocampus. The visual areas generate rerun of daily sightings (Figure27).

Figure 27 Long-term Memory [view large image]

Semantic, and episodic memory are the subclasses of declarative memory:

Episodic memory - It is about an event in one's life and everything about it, including emotional reactions. Remembering an episode, e.g., the attack on Pearl Harbour (Figure 28), is to create a memory for a unique event that only happened once and there is no opportunity for learning the event by rehearsal. Episodic memories are not very reliable, they are highly personal, selective, idiosyncratic and varying over time, but they may also be richly complex and movie-like in character. They constitute the stories we tell ourselves about our past, they are the things we would write about in our autobiography. Episodic memories can be recalled deliberately or are triggered by evocative sensory stimuli - particularly by the sense of

Figure 28 Declarative Memory [view large image]

smell. Episodic memory involves the use of the hippocampus for forming memories and the cortex for storage (see diagram D, Figure 24).

• Semantic memory - Semantic memory is the knowledge of facts - numbers, addresses, language and concepts - which the brain files in categories and which seems to involve the left temporal lobe. Retrieval is then carried out by the frontal lobes (see diagram E, Figure 24). We assume all of the facts that constitute our knowledge of things must be stored in an organized fashion to be useful. Though this has not been demonstrated, it seems likely that the brain stores our semantic memories as modules that have some logical links to one another; they are grouped by category for instance. On retrieval, the brain knows where to find the memory according to the address of that particular category. Semantic memory is essential to the understanding of how things work and thus to an under-standing of the world we live in. It is a body of knowledge that helps us to regulate our behaviour according to and dependent on reliable factual memories. Navigational skills, for example, depend on our ability to deploy a complex store of semantic memory, including detailed spatial memories and representations of the world.

mechanisms and principles involved in its formation of short- and long-term memories are conserved throughout the animal kingdom, including in humans. Aplysia exhibits a behaviour of protective reflex in which the sea slug withdraws its gill into the safety of the mantel cavity in response to a mild touch stimulus to another part of the body called the siphon (Figure 29a). If the stimulus is repeated a number of times, the gill withdrawal reflex becomes weaker until finally the animal ignores the touch stimulus. The waning of sensitivity to repeated stimulation is known as habituation and is a very simple form of learning found in all animals, including humans. Another type of learning is sensitization, when we are exposed to an unexpected or strongly unpleasant stimulus. Generally the sensitizing effect of

Figure 29a Memory in Aplysia [view large image]

a single alarming stimulus is short-lived, lasting perhaps for just a few minutes. But if the alarming stimulus is repeated a number of times our senses may be heightened for days and now such sensitization becomes a form of long-term memory.

As mentioned earlier, repeated activation yields a lasting increase in the efficiency of synaptic transmission -- a process called long-term potentiation (LTP) -- which is thought to underlie memory formation. LTP depends both on enhanced insertion of receptors for the neurotransmitter glutamate at spines (the sites of synapses) and on spine growth. Figure 29b shows the molecular basis of LTP as described in a 2008 neuroscience article. Essentially, it reports that the process is driven by the myosin V proteins, which shuttling receptors and membranes to make synaptic junctions better detectors of incoming signals.

Figure 29b LTP, Molecular Basis of [view large image]

A brief description for each step of the process is outlined in the following (also see Figure 29b):

1. Neurons receive synaptic transmission from other neurons (not shown) at dendritic spines.
2. Influx of Ca²⁺ through NMDA receptors activates the folded (inactive) form of the myosin V motor protein.
3. The active myosin V protein (in extended form) then moves to the dendrite shaft and binds to Rab11/Rab11-FIP2 on recycling endosome (reused membraneous sac) containing AMAP receptors.
4. Finally, it transports the cargo into and along spines via actin filaments (microtubules) to mediate insertion of AMPA receptors at the cell surface, as well as spine growth through membrane insertion.

		It is reported in 2007 that the seat of memory has been pinpointed in mouse. By monitoring 260 neurons in the hippocampus (Figure 29c), researchers have discovered that different experience is recorded in different area called "clique", which can be categorized from very general to very specific. Furthermore, such brain activities can be translated into binary codes (Figure 29d). Supposedly, we can read the mind from such codes and tell what it is
Figure 29c Seat of Memory [view large image]	Figure 29d Memory Code [view large image]	thinking by the process of backward translation. The followings are steps to uncover the memory code:

1. Recorded Experiences - The mice are exposed to three startling experiences - a puff of air on the back (to mimic an owl attack from the sky), a fall in a container (the "elevator" drop), and shaking in a cage (the "earthquake") - while a recorded plotted firing from a large set of CA1 neurons. Each row in the plot captures firing of a single cell over time.
2. Patterns of Mental Activities - The points in the 520 dimensional phase space (corresponding to the activities of 260 neurons before and after an event) are projected into a 3 dimensional phase space. Different mental activity falls into different area in such plot starting from "rest". Temporal analysis revealed that the activity patterns associated with those startling experiences recurred spontaneously at intervals ranging from seconds to minutes after the actual event, but with smaller amplitudes than the original response. Such patterns provide evidence that the information traveling through the hippocampal system was inscribed into the brain's memory circuits. The replay corresponds to a recollection of the experience after the fact.
3. Coding Cliques - It is discovered that neuron ensembles active during an event contain subsets -termed neural cliques. The cells in a clique all show very similar firing patterns and are not part of the other cliques.
4. Organization of Memories - Further analyses showed that each clique encodes a different aspect of an experience, ranging from the general to the specific. It can be visualized as a hierarchical organization with the most general clique at bottom, and the very specific on top. Any given pyramid can be a component of a more general polyhedron representing all events of a given category, such as "all startling events".
5. Translated into Binary Code - The clique activity is represented as a string of binary code with 1 as being active and 0 signifies inactivity. Thus the earthquake binary code is 11001 corresponding to: "starting event", "disturbing motion", "air puff", "drop", and "shake". While the elevator drop binary code is 11010 for the same sequence.

Higher Functions

The frontal lobes are where ideas are created; plans constructed; thoughts joined with their associations to form new memories; and fleeting perceptions held in mind until they are dispatched to long-term memory or to oblivion. This brain region is the home of consciousness, where the products of the brain's subterranean assembly lines emerge for scrutiny. Self-awareness arises here, and emotions are transformed in this place from physical survival systems to subjective feelings. The area of the frontal lobe most closely associated with the generation of consciousness is in the prefrontal cortex. Figure 30 shows four areas, which endow human with fucntions not available in other animal:

Figure 30 Higher Functions [view large image]

1. Orbito-frontal cortex - This area inhibits inappropriate action, freeing us from the tyranny of our urges and allowing us to defer immediate reward in favour of long-term advantage.
2. Dorsolateral prefrontal cortex - Things are held "in mind" here, and manipulated to form plans and concepts. This area also seems to choose to do one thing rather than another.
3. Ventromedial cortex - This is where emotions are experienced and meaning bestowed on our perceptions.
4. Anterior cingulate cortex - It helps focus attention and "tune in" to own thoughts.

The answer was still elusive at the end of Francis Crick's life, when he was struggling in vain trying to understand the role of claustrum in consciousness. Subjectivity poses the more formidable scientific challenge. Each of us experiences a world of private and unique sensations that another person can only appreciate indirectly. If the senses ultimately produce experiences that are completely and personally subjective, then we cannot arrive at a general definition of consciousness because there would be an infinite number of them. This is the "hard problem" of consciousness. According to some researchers, science cannot take on consciousness without a significant change in methodology, a change that would enable scientists to identify and analyze the elements of subjective experience. Others argue that we only need an underlying theory.

Figure 31a Consciousness [view large image]

Just like the Newtonian mechanics, one theory is sufficient to describe the multitude of orbits and trajectories.

The nature of free will is another issue that can be tackled by the new biology of mind. Free will is the ability to act or make choices as a free and autonomous being and not solely as a result of compulsion or predestination. According to Freud's discovery of psychic determinism - the fact that much of our cognitive and affective life is unconscious - there is not much left for freedom of action. Experiment on the correlation between electrical activity of the brain and movement (lifting a finger for example), reveals that the electrical activity precedes the movement by 200 milliseconds. It is proposed that the process of initiating a voluntary action occurs in an unconscious part of the brain, but that just before the action is taken, consciousness is recruited to approve or veto the action. In the 200 milliseconds before a finger is lifted, consciousness determines whether it moves or not. Thus, our conscious mind may not have free will, but it can freely modify inappropriate behavior (Figure 31b). This is the reason for the laws in our society to hold all of us accountable for our own action. It is suggested that we should update our idea of free will to mean self-control over our behaviour.

Figure 31b Free Will [view large image]

Default State (Network)

internal processing with a lot of activities in the prefrontal cortex and hippocampus (Figure 32). One theory likens it to daydream with a very serious purpose - for incorporating lessons learned in the past (by activating the hippocampus) into our plans for the future (using the higher function in the prefrontal cortex), such process is called default network. The association of default network with very slow brain wave (0.1 Hz or less) even in sedated humans and heavily anaesthetized monkeys lead to suggestion linking it with sorting and preserving memories. Since it devours huge amounts of glucose (used up 30% more energy than other area of the brain) in the process and its impairment is related to many brain diseases, it must be very important whatever function it serves. BTW, the brain seems to go into a truly resting state via

Figure 32 Default State [view large image]

meditation, which deliberately turns off the default network. Such feat actually requires a lot of training and is not easily attainable.

Altered States

is thought to be common to all mammals). Others are attained through learned techniques such as meditation. Some are induced by drugs. Other still - the vision and trance states - are highly controversial, and many people doubt their existence. To understand altered states one must assess subjective accounts of what it is like to "be in" these states, along with objective research that tries to identify their physiological basis and effects. Figure 33 shows the brain scan for some of the altered states listed below. The areas in light blue color represent over-activity in the upper row, while the same color indicates under-activity in the lower pictures.

Figure 33 Altered States [view large image]

Dreaming - Vivid visual dreams light up the visual cortex; nightmares trigger activity in the amygdala and the hippocampus flares up from time to time to replay recent events. The areas, which seem to be most commonly active are the pathways carrying alerting signals from the brainstem and the auditory cortex; supplementary motor area and visual association areas - all of which produce the "virtual reality" effect of dreaming. Activity is decreased in the dorsolateral prefrontal cortex, the area of waking thought and reality testing (Figure 33). Studies have shown that dreaming sleep occurs in a wide range of animal species. Figure 34 shows a dreaming cat. When its pons is surgically removed to permit movement during REM sleep, the very nice cat

Figure 34 Dreaming Cat [view large image]

becomes a vicious tiger when it is dreaming and throws itself at imaginary prey.

Daydream - Many surveys suggest that ordinary men and women, who are neither disturbed nor neurotic, spend a large part of each day in some sort of fantasy, reverie or daydream. This kind of quick fantasy rarely has a structured narrative. It is the moment when we stop paying attention to what we are seeing and hearing and switch into an inner theatre of the imagination where we can play at wish fulfillment (Figure 35). But there are other fantasies qualitatively different from these "wouldn't it be nice if ..." stories. These are sustained fantasies, which often seem to have been crafted, worked and reworked to meet some more profound psychological need. When one daydreams, normal inhibitions are bypassed. The evidence of the rather macabre biographies of

Figure 35 Daydream [view large image]

serial killers shows that they had frequently recurring violent fantasies before they turned to murder.

Meditation - One function of consciousness is to knit together our sense of self-identity. But many religious traditions believe that enlightenment can be achieved only by breaking the shackles of self and attaining "purer" states of consciousness through meditation (Figure 36). As well as its psychological benefits, the meditative state has marked physiological effects - these phenomena are measurable and reliably repeatable, and thus are a suitable object of scientific study. Such studies have revealed some remarkable effects: meditation can lower a subject's metabolic rate, decreasing blood pressure, pulse rate and muscle tension. One study shows that the subject could reduce his oxygen intake to one-third of the normal resting state. Scans of people in a self-induced state of "passive attention" have been shown to "turn off" areas of the

Figure 36 Meditation [view large image]

brain normally associated with seeking stimuli, including the parietal, anterior and premotor cortexes (Figure 33).

Hypnosis - Modern studies show that the brain waves of hypnotized subjects are much like those of the waking state. When subjects are hypnotized, they can speak, walk and carry out instructions. Yet there are some noticeable changes from normal consciousness: attention becomes very selective, with the subject ignoring everything but the hypnotist's voice; the subject rarely initiates thought or activity, but waits for suggestions from the hypnotist; and fantastic ideas or situations are more readily accepted as reality. It is almost as if the willing, relaxed subject relinquishes control over part of his or her consciousness to the hypnotist. The classic method of hypnotism is to put a subject into a relaxed frame of mind and ask him or her to concentrate on an object, such as a swinging pocket watch (Figure 37). Brain scans (Figure 33) show increased activity during hypnosis in the motor and sensory areas

Figure 37 Hypnosis [view large image]

suggesting heightened mental imagery. Increased blood flow in the right anterior cingulate cortex indicates that attention is focused on internal events. The brain activity seen in this state is quite different from that seen in normal waking or sleeping.

Sexual fantasy - This is the ability to use our imaginations erotically. It is found that people spend a surprising amount of time thinking about sex. There are vast cultural differences in what different societies consider acceptable material for sexual fantasy and fetishes. The Victorians considered fetish to be shocking and dangerous, the true dark side of sexuality; while the Freudian view treats fetishism as the result of linking unresolved childhood drives to object that seems "safe" such as the high-heeled shoe. Many therapists now consider that it is perfectly normal to have sexual fantasies, and some even believe that they can be used to achieve a more fulfilling sex life. Research into sexual fantasies is complicated and must rely on what patients report to their therapists, but some studies have found links with childhood events - either sexual violence or a strict, repressed upbringing. There is an obvious distinction between fantasy and action - a fantasy does not harm others. However, some people who have fantasies that involves inflicting pain to

Figure 38 Fantasy [view large image]

themselves or others (such as to the cat in Figure 38) claim that they feel compelled to act them out. Sometimes people with less extreme fantasies also choose to turn them into realities such as in the form of cross dressing.

Addiction - Drug addiction is caused by a similar train of events to hunger. However, unlike most types of food, addictive drugs cause changes in the receptors to which they bind, making them less sensitive. This creates tolerance and addiction. Most addictive drugs work by altering levels of neurotransmitters in the brain's reward circuitry centered on the limbic areas. Other brain areas are also involved and each type of drug works in a slightly different way to produce its characteristic effects. Opiates are drugs derived from the dried resin of the opium poppy (Figure 39), or synthetic versions of these chemicals, such as heroin, codeine and morphine. All have been used medicinally

Figure 39 Drug Addiction [view large image]

at some time for their pain-killing properties. They are used illegally for similar reasons: heroin gives the user a "high", reducing anxiety and producing a sense of temporary well-being.

poverty and discrimination also appear to be involved in increasing the risk of schizophrenia or schizophrenia relapse, perhaps due to the high levels of stress they engender. The disease is frequently accompanied by paranoia and delusions. Some may experience extremely bizarre hallucinations. Ironically, while some areas of the schizophrenic brain may be dead, in other ways the sufferer's brain is overactive. Most schizophrenics appear to have an excess of dopamine in the brain, the neurons become overloaded and relay inappropriate messages (see Figure 40 for a modern view). Lack of activity in the frontal lobes is a feature of states of mind in which consciousness is disturbed. This might account for the state's common reduction in planned or spontaneous behavior and social withdrawal. The anterior cingulate cortex - thought to distinguish between external and internal stimuli - is also underactive (Figure 33), which may be one reason schizophrenics confuse their own thoughts with outside

Figure 40 Schizophrenia [view large image]

voices. Recently in 2006, it is found that those with mutations in the PCM1 gene had a significantly lower volume of grey matter in their orbitofrontal cortex resulting in poor judgement, inappropriate social behaviour and not keeping themselves clean. PCM1 plays a role in cell division, which in the brain occurs more actively at adolescence - an age at which schizophrenia is commonly diagnosed.

when the cells in the hippocampus degenerate (see Figure 41). The ability to perform routine tasks also declines. As Alzheimer's disease spreads through the cerebral cortex, judgment declines, emotional outbursts may occur and language is impaired. Progression of the disease leads to the death of more nerve cells and subsequent behavior changes, such as wandering and agitation. The ability to recognize faces and to communicate is completely lost in the final stages. Patients lose bowel and bladder control, and eventually need constant care. This stage of complete dependency may last for years before the patient dies. The average length of time from diagnosis to death is 4 to 8 years, although it can take 20 years or more for the disease to run its course.

Figure 41 Alzheimer's Disease [view large image]

Transcendence - In this state, the subject experiences a feeling of merging the self with the rest of the universe. It used to be considered as the attainment of moving up to a higher level of existence. For example, in the ancient text of "Tao Te Ching"

("Book of Tao"), Lao Tsu (the author) mentioned that "Those who know do not talk. Those who talk do not know". A straight forward translation would mean someone is hiding knowledge from the prying eyes. A more subtle interpretation would attribute the silence as the difficulty to describe the experience by words. It has been known for some time that an area in the posterior partietal cortex (PPC) is responsible for the separation of self from the external environment. Impairment of this part of the brain would induce a feeling of union with the whole universe. A 2010 report in the Journal of Neuron confirms this observation by checking on people with brain cancer. Those who have some neurons in the PPC removed, tend to believe in miracles, extrasensory perception and other non-material phenomena; while the others with intact PPC do not

Figure 42 Transcendence [view large image]

have such sentiments. It could be that Lao Tsu's claim about transcendental experience is real, but such occurrence can be explained in term of modern medical knowledge without invoking another level of existence. He must be able to inactivate the PPC somehow, perhaps via meditation.

Vision - It is virtually impossible to carry out research into visions in the laboratory, because they do not happen on demand; as a result, the only evidence that visions do exist is the accounts of those people who have experienced them. Vision may occur in response to stress. They are often central to religious experience. Out-of-body experiences are not restricted to religious practices: they seem to occur in response to some kind of emergency situation. This is the case with near-death experiences.

There have been thousands of reports of near-death experiences, many noting the same types of sensations. Subjects feel as though they have left their bodies watching the scene unfold -- as if from above. Others saw deceased loved ones in a very peaceful and beautiful scene. Some people report traveling down a tunnel toward a bright light (Figure 43), where benevolent presences wait. Scientists have been unable to explain them conclusively. Some physiologists have suggested that hypoxia, or low oxygen levels in the brain, might cause a consistent pattern of hallucination in all sufferers. Other scientists argue that the experience stems

Figure 43 Vision [view large image]

from an acute bout of "REM intrusion" into the partially awakening state (in time of extreme stress) similar to narcolepsy - a neurological disorder characterized by uncontrollable bouts of sleep that can cause elaborate hallucinations and, sometimes, out-of-body experiences. The re-appearance of loved ones are explained as

the last of the dieing memory lingering on after the official declaration of death. Some neurologists suggest that tunnel vision is caused by lack of blood flow to the eye. The eye, the retina of the eye, is one of the most exquisitely sensitive tissues to a loss of blood flow. So when blood flow does not reach the eye, vision fails, and darkness ensues from the periphery to the center.

	An aplisa is like a squishy snail. In rain, in snow, in sleet, in hail. When it is angry, it shoots out ink. The ink is purple, it's not pink. An aplisa cannot live on land. It doesn't have feet so it can't stand. It has a very funny mouth. And in winter it goes to the south.
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