Neural systems 1 exam 3

Ramachandran soon found a complete map of Tom's phantom hand, but the map was activated only when Tom's face was stimulated! He realized that what he was seeing was a

direct perceptual correlate of the remapping that Tim Pons has seen in his monkeys. The explanation lies in the peculiar mapping of body parts in the brain, with the face lying right beside the hand

Since the homunculus represents the density of the sensory innervation of the body surface, it follows that this replication of the body surface is

distorted: those parts of the body having the greatest sensitivity, and thus the largest number of sensory receptors, contribute the greatest number of fibers to the pathway

While the fact that the homunculus is due to the specific connections that the sensory fibers on the body surface make in the cortex, the wiring is not

immutable (unchangeable) and the ability to change the map has some surprising, really astounding consequences! These remarkable changes are most clearly and dramatically seen in people who have lost a limb. These people experience "phantom limbs".

Notice that axons of the same stretch receptors that synapse on

motoneurons and generate stretch reflexes, have collaterals that innervate second order neurons in the spinal cord, whose axons form the spinocerebellar tract. In other words, the same sensory axon contributes to multiple channels

Impulses initially generated by the sensory receptors are carried over the

peripheral nerves, which project into the central nervous system (CNS). Once the peripheral nerve enters the CNS it divides into several branches, each making at least one synaptic contact with the cell body of a neuron lying within the CNS. These cell bodies are not scattered randomly throughout the CNS but rather are concentrated into a small aggregate

The sensory nerves coming into the brain and spinal cord together with motor neurons that innervate muscles comprise the

peripheral nervous system.

The first major component in motor neurons is the motoneurons that send their axons to the

periphery to directly innervate muscle fibers. The cell bodies of motoneurons lie in the midbrain and hindbrain for the cranial nerves that innervate the muscles in your head, neck and face, and in the spinal cord, which innervates the rest of your body

the vestibular portion of the inner ear monitors the

position of your head in space, and sends axons that synapse on the vestibular nuclei in the medulla. The cells in the vestibular nuclei then send their axons to the spinal cord as a distinct fiber tract, the vestibulospinal tract. This is the tract that influences your muscles in response to changes in the position of your body, such as when you fall or for any change your posture.

(The cerebellar channel) Both the vestibulocerebellar and spinocerebellar tracts convey

proprioceptive information to the cerebellum. Proprioceptors are the sensory receptors that sense the internal forces acting on the body, such as muscle stretch receptors, the receptors in joints and the receptors in the vestibular portion of the inner ear that respond to the position of the head in space. Thus the vetibulocerebellar tract informs the cerebellum about the position of the head and the spinocerebellar tract informs the cerebellum about the position of each joint and the state of each muscle in the body.

The cerebellum has a unique structure, in that it is composed of shallow

ridges called folia, that run from side to side

Since the axons of the cells comprising this nucleus are the

second group of fibers to relay the information within the CNS, the nucleus is called a secondary sensory nucleus, and the axons or fibers that originate from these cell bodies are called second-order sensory fibers, as opposed to the peripheral sensory neurons that are the primary sensory fibers.

Consequently, touching points on the upper arm also evoked

sensations in the phantom hand. One cluster of points on the face evoked sensations in the phantom hand, and a second cluster on the upper arm also evoked sensations in Tom's phantom hand. Both regions that evoked sensations in the phantom hand correspond to the body parts that are represented on either side of the hand representation in the brain

That is somatotopically organized

the fibers innervating the feet, leg, arms, hands, etc. each travel in the pathway with the same relationship to each other as they have on the body surface.

The basic function of the cerebellum is analogous to

the head coach of a football team. I use this analogy because neither the football coach nor the cerebellum does anything directly. The coach, for example, does not score touchdowns, does not throw passes, does not block, does not catch passes and does not kick extra points. He doesn't even coach. He has coordinators and position coaches to do that job. So what does he do? He coordinates everything and makes sure every aspect of the team functions in a highly cooperative manner. The coach has to know everything about the team, the players and coaches. A team with a good coach works well but functions poorly on teams with bad coaches.

Recall that the somatosensory fibers that project from the somatosensory nucleus in the medulla cross

the midline to ascend on the opposite side of the brain.

The explanation of this result was that the amputation of digit 3 removed

the neural innervation that had been provided by digit 3, thereby leaving neurons in the cortex that were no longer innervated (Fig.1B). When those inputs are removed, the nerve fibers that normally stimulate neighboring cortical regions, the regions that represented digits 2 and 4 in this case, sense the vacancy and respond by sprouting new axon collaterals that extend to innervate the vacant denervated region of the cortex (in this case the region that previously had been innervated by digit 3).

what is the somatosensory lemniscus

the pathway that conveys information about touch, pressure and temperaturE

The first-order sensory fibers (peripheral nerves) from the body surface enter

the spinal cord all along its length. The sensory fibers, however, do not synapse in the cord (except for a collateral that forms the local reflex channel), but rather project up the spinal cord to the medulla, where the fibers make their first synapse in a nucleus.

What is the second major component of the vertebrate nervous system

An effector or motor component that sends or projects axons to the muscles of the body and thereby enables the organism to act upon its environment.

(The lemniscal pathways) The term lemniscus derives from the Latin word meaning

a ribbon. These tracts are composed of large numbers of heavily myelinated axons that appear as great "ribbons" of fibers running up and down the brain; hence the term, lemniscus. These ribbons of fibers are the neuronal systems that convey sensory information that will enter into conscious experience. The sensory modalities of vision, audition, and somatosensation are all conveyed up the brain in their own lemniscal pathway.

(Reflex channel) The motoneuron transmits

action potentials along its axon to a muscle, resulting in a muscular contraction.

a nucleus in the central nervous system is an aggregation or collection of

cell bodies that have a common input and send their fibers to a common target (they have a common output).

The pathway that we will be concerned with is the corticospinal tract. This pathway or tract originates from

cells in the cortex that send their axons, without synaptic interruption, as a very large and distinct bundle of fibers, directly to the spinal cord Most of these axons terminate on "interneurons" in the spinal cord, but in primates, many end directly on motoneurons. The corticospinal tract is huge, and is the most prominent and important motor pathway in the human brain.

Together, the brain and spinal cord constitute the

central nervous system (CNS).

the thalamus is the gateway to th

cerebral cortex, in that all sensory information (except olfactory) must first synapse in the thalamus before that information can be relayed to the cerebral cortex.

By the fifth week of gestation, further development results in the appearance of

five vesicles or divisions of the brain

This representation of your body surface in the brain is called a

homunculus, which means "little man in the brain".

( the lemniscal pathways) One of the main concerns throughout the rest of the course will be

how information from peripheral sensory receptors travels in the brain to reach the cortex, because it is the cortex that endows us with the conscious perception of a stimulus.

What else did Michael Merzenich and his colleagues do

conducted the opposite experiment (Fig. 9A3). In this experiment, they again mapped the cortical representation of the hand and fingers, but then amputated one of the fingers (digit 3). A few months later they then remapped the representation of the hand. What they found was that the area where the amputated digit, digit 3, had previously been represented was now activated by either digit 2 or digit 4. In other words, the area where digit 3 had previously been represented was not silent, but rather the area had been partially invaded by the fibers that represented digit 2 and partially by the fibers that innerve digit 4; the representations of digits 2 and 4 expanded to fill the void that had previously been innervated by digit 3.

Each of these five subdivision undergoes

considerable growth before birth, but these five subdivisions form the basis of the fully-matured brain.

the fibers of the corticospinal tract descend from the

cortex and when they reach the lower portion of the medulla, they cross to the opposite side.

after the ascending fibers from the spinal cord synapse in the somatosensory nucleus in the medulla, the fibers

cross and ascend to the thalamus on the opposite side of the brain! This crossing, or decussation, occurs in the auditory, visual as well the motor lemniscus (described next). What this means is that the right side of your body is represented on the left side of your brain and the left side of your body is represented in the right side of your brain. If someone suffers damage, through a stroke or an injury, to the right somatosensory cortex, that person will lose sensation on the left side of their body.

The brainstem is the portion of the brain that lies between th

diencephalon and the spinal cord Several different cell groups in the brainstem give rise to important descending tract

There is another common feature that the somatosensory lemniscus shares with the motor lemniscus the corticospinal tract); specifically, they both cross

In the medulla

( the cerebellar channel) The two direct pathways are:

1) The vestibulocerebellar tract, a projection from secondary sensory cell groups in the medulla (the vestibular nuclei) that are innervated by the vestibular nerve of the inner ear; and 2) The spinocerebellar tract, a projection of second order neurons in the spinal cord to the cerebellum. These pathways or tracts, like many others in the brain, are named with the first term referring to the origin (spinal cord or vestibular nuclei) and the second to the termination of the pathway (cerebellum).

When the axons of primary sensory neurons enter the spinal cord, numerous collaterals branch off from the parent axon. The various branches are distributed to three functional channels:

1) a reflex channel; 2) a cerebellar channel; and 3) a lemniscal channel that relays sensory information to the cortex.

The forebrain now has two distinct portions

; 1) an endbrain (telencephalon), also called the cerebral hemispheres, and 2) a between brain (diencephalon), characterized by the outgrowth of the optic vesicles or future eyes. The midbrain continues to develop and remains as one of the primary divisions but is not subdivided. The hindbrain, however, is now composed of two regions: 1) the more anterior or rostral portion becomes the future pons and cerebellum, which collectively comprise the metencephalon, and 2) the more caudal portion is destined to become the medulla oblongata (mylencephalon).

What is the third major component of the vertebrate nervous system

A neuronal network interposed between the sensory nerves and motor nerves that is best characterized as an integration system. Thus, in its most basic form the nervous system can be though of as a three-neuron nervous system with a sensory neuron, an intermediate or coordinating set of neurons and a motor or effector neuron (Fig. 1).

What is the first major opponent of the vertebrate nervous system

A sensory component that serves to inform the organism about the conditions in the environment. Your eyes and ears are perhaps two of the most familiar and important examples of sensory structures that allow you to know what events are occurring around you.

The enlarged cranial portion is called the____brain and the narrower portion that runs down the hollow of vertebral column is the _____.

Brain, spinal cord

With these findings in mind, let us next turn to Tom, one of Ramachandran's patients who lost his arm just below the elbow.

Dr. Ramachandran examined Tom and conducted the following experiment with a Q-tip! First Ramachandran blindfolded Tom so that Tom could not see where Ramachandran was touching him. Then he took the Q-tip and started stroking various parts of Tom's body while asking Tom to tell him where he felt the sensation.

Three subdivisions of the endbrain

First, there is the olfactory bulb. Second, there is a relatively large area labeled the basal ganglia in Figure 3. This is a region of cell bodies and forms the bulk of the endbrain in submammalian animals. Third, the telencephalon is covered by a layer of cells called the cortex.

motoneuron are also known as the

Final common pathway

Why were there two maps instead of just one?

If you look again at the Penfield map of the somatosensory cortex, you will see that the hand area in the brain is flanked below the face area and above by the upper arm and shoulder area (Fig. 12). Input from Tom's hand area was lost after the amputation, and consequently, the sensory fibers originating from Tom's face, which normally activate only the face area in the cortex, now invaded the vacated territory of the hand and began to activate cells there. Therefore, when Ramachandran touched Tom's face, he also felt sensations in the phantom hand. But the invasion of the hand cortex also results from sensory fibers that normally innervate the brain region above the hand cortex (that is, fibers that originate in the upper arm and shoulder).

The type of information conveyed by this lemniscal system can best be visualized by preforming a simple test

If you put your hand in your pocket or pocketbook, there should be little difficulty identifying many objects by touch alone. Keys, pencils and tissue paper are easily recognized by their shape, texture, hardness, etc. Thus, the lemniscal tracts are highly specific with respect to place and modality and are endowed with an exquisite capacity for spatial and temporal discriminations. The everyday experiences of seeing, hearing, touching, and so on are possible only after our sensory receptors change light, acoustic or mechanical energy into nervous impulses and transmit these impulses over lemniscal systems to the cerebral cortex where the information is transformed such that a stimulus is perceived as possessing a particular quality, location and intensity rating.

(The lemniscal pathways) The large tract of axons that conveys information about a sensory modality (and motor fibers as well) is called a

Lemniscus

The fact that the representation of a particular portion of the body can be modified was shown elegantly by

Michael Merzenich and his colleagues.

The only avenue that the central nervous system has for influencing the muscles of the body is through the

Motoneurons

Such an aggregation of cell bodies, all of which receive the same inputs and all of which send their axonal projections to the same targets, is called a

Nucleus of the brain

The basic idea is that the state of any skeletal muscle is influenced, in part, by

Reflex pathways, that is by the inputs from the various proprioceptors that monitor muscle length and joint position.

the entire pathway from the first-order fibers to the cortex is

Somatotopically organized

How could a part of the body (the face) so far removed from the hand and arm come to innervate the part of the somatosensory cortex that normally represents the hand? How could the map of the body surface, the homunculus (the so called "little man in the brain"), be changed so dramatically?

The answer might lie, at least partially, in the homunculus itself. Recall that Merzenich showed that neurons in the cortex that represent one part of the body (digits 2 and 4 in the example above) apparently innervate the adjacent region of the cortex, the part normally activated by digit 3, after digit 3 had been amputated. Now look at Penfield's map of the body surface on the cortex (the homunculus) again (Fig. 10). The alignment of some parts of the body in the cortex is not the same as they occur on the body itself. In particular, notice that the hand area in the brain is flanked on one side by the face area, and on the other side by the upper arm and shoulder area. Thus, by sectioning the nerves from the arm and hand, the dominant input to the hand area was removed by Pons allowing the silent synapses from the face to be expressed (and after 11 years, presumably sprouting of neurons from the face region had also occurred, and the terminals of the sprouts from the face innervated the cortical region that previously represented the hand and arm). Thus, the map of the body surface had been changed. In this case one region of the cortex, which previously had represented the hand and arm, now came to represent the face (in addition to the normal representation of the face in the face region).

( the cerebellar channel) All sensory systems send fibers to the

The cerebellum

What is the cortex

The term cortex derives from the Latin word meaning bark, like the bark on a tree. It is basically a layer of cells, about 2-3 mm thick, that covers the entire telencephalon, just like bark tree provides a thin covering around a tree. We will have much to say about cortex in later chapters. Additionally, the diencephalon is divided into two regions: 1) a dorsal (upper) area called the thalamus and 2) a ventral (lower) region below it called the hypothalamus (Fig. 3 and 4). Collectively, the telencephalon and diencephalon comprise the forebrain

The vertebrate has how many subdivisions

The vertebrate brain has five subdivision which are most easily visualized during forebrain, and B: that is filled with cerebrospinal fluid. The hollow interior will become the four major ventricles in the mature brain. C: The brain at a later developmental stage. Notice that the forebrain is now 198 development

The third major component in motor neurons is the cerebellum and basal ganglia

These structures do not themselves send fiber tracts to innervate motor neurons but rather innervate the cells that give rise to the descending fiber tracts and thereby affect their activity and indirectly influence the activity of motorneurons.

the endbrain or telencephalon can be further subdivided into

Three separate subdivisions

What did Michael Merzenich and his colleagues do

What they did is to map the representation of the hand and fingers of a monkey with microelectrodes, as shown in Fig. 9A. They then let the monkey recover and after recovery the monkey was trained to selectively use only digits 2 and 3 to perform a task for which they received a food reward. After several weeks of training, they again mapped the representation of the fingers in the same monkey. What they found is that the representation of the of the digits, digits 2 and 3, that were used more extensively than the other digits expanded in while the representation of the other digits contracted, and became smaller (Fig. 9A2). This showed that cortical maps are dynamic and can adjust depending upon the amount of use, or really the amount of sensory stimulation each area experienced over time.

Since the monkey's paralyzed arm was not sending messages to the brain, you would not expect to record any

activity from the hand or arm area of the somatosensory cortex when the monkey's useless arm or hand was touched. There should be a big patch of silent cortex corresponding to the affected hand, and this was indeed the case. That is, when the hand or arm was stimulated, no activity could be evoked in the hand or arm region of the somatosensory cortex, or in any other region of the cortex. However, and this was the surprise, the hand and arm regions of the somatosensory cortex were activated when the monkey's face was stimulated! So were neurons that normally represented the face, but these were expected to be activated by facial stimulation. It appeared that sensory information from the monkey's face not only went to the face area of the cortex, as it would in a normal animal, but it also had invaded the territory of the paralyzed hand!

While the lower vertebrates are scarcely more

advanced behaviorally than the invertebrates, the reptiles, birds and mammals exhibit behavioral patterns of ever-increasing complexity and plasticity. Their brains are correspondingly more highly developed as reflected in the increase in total brain size, the development of the cerebral hemispheres and, in mammals, the progressive increase and amount of complexity of the cerebral cortex.

One indication of the importance of the cerebellum is that

although it accounts for about 10% of the brain's volume, it contains over 50% of the neurons in the brain!

In the Merzenich experiments described above, one part of the cortex that had previously represented one digit was induced, by

amputation, to represent adjacent digits, digits that represented a neighboring regional portion of the body surface (the fingers). Other experiments revealed something similar in principle, but even more remarkable in detail: that a region of the cortex, which represented the hand, could be induced to represent an entirely different part of the body, in this case the face! It was this experiment, conducted by Tim Pons and his colleagues at the National Institutes of Health, that caught Dr. Ramachandran's attention.

A phantom limb is an

arm or a leg that lingers indefinitely in the mind of patients long after it has been lost in an accident or removed by a surgeon. Some patients wake up from anesthesia and are incredulous when told that their arm or leg had to be sacrificed, because they still vividly feel its presence. They often feel that they can still move the limb in a normal manner, to reach for something with the missing arm or walk with the missing leg. Moreover, some of these patients experience excruciating pain in the phantom arm or hand or fingers, so much so that they contemplate suicide. The pain is not only unrelenting, but also untreatable; no one has the foggiest idea of how it arises or how to deal with it.

the somatosensory nucleus in the medulla receives inputs from the

ascending axons of the peripheral sensory nerves. The fibers from the somatosensory nucleus in the medulla then continue, without synaptic interruption, as a great ribbon of fibers that all synapse in a common target, the somatosensory nucleus of the thalamus. The cells of the somatosensory nucleus of the thalamus then send their third-order fibers to a restricted area of the cerebral cortex, appropriately called the somatosensory cortex.

What is the significance of the grooms structural developments.

becomes clear only when changes in the internal organization of the nervous system are also considered. It is this internal organization that provides the structural foundation for the control of behavior. Indeed, an understanding of the "wiring diagram" of the central nervous system is an essential ingredient for any attempt to understand vertebrate neurobiology. In the following section we will explore in the way in which the sensory, intermediate and motor components of the vertebrate nervous system are organized and linked.

sensory fibers from the thumb travel next to those from the

index finger and so on. The sensory fibers from the fingers in turn travel next to the fibers from the palm of the hand, which travel next to the fibers from the wrist and so on. This arrangement is maintained from the spinal cord to the cortex. The net result of this orderly arrangement of fibers is to re-create a representation of "you", that is your body, on the cortex and in the somatosensory pathway leading to the cortex. Thus the body surface is mapped on the brain.

Since the corticospinal tract is critically important for

initiating and performing voluntary movements, especially fine movements, people who suffer a stroke or injury to the right motor cortex display paralysis on the left side of the body, and those whose left motor cortex is damaged suffer paralysis on the right side of the body.

Both the brain and spinal cord are composed almost entirely of what type of neurons

intermediate neurons (intermediate neurons are those that do not directly innervate a sensory receptor nor do they send their axons to innervate an effector).

During phylogeny the most dramatic and important advances have involved the

intermediate or integrative component.

That is basically what the cerebellum does for motor activity;

it coordinates activity so that motor patterns operate smoothly.

Both the cerebellum and basal ganglia are

large structures that do NOT have any direct motor pathways. Rather, they both have profound influences on motor function through their innervation of the cells that actually give rise to the descending motor pathways that innervate motoneurons.

As animals have evolved, their brains became

larger and more complex, allowing for greater sophistication and modification of behavior.

The cerebellum , which means

little brain", is about the size of your fist, and sits on the top (dorsal surface) of the pons

The influences on the motoneurons are mediated through

local reflex channels and from the descending motor pathways. This means that the activity of a motoneuron, at any particular moment, is determined by the sum total of the various excitatory and inhibitory influences that are acting upon it.

Although the corticospinal originates from widespread areas of the cortex, a particularly important component originates from a restricted region of the cortex called the motor cortex or sometimes the primary motor cortex (Figs. 2 and 3). This portion of the cortex is called the

motor cortex" because weak electrical stimulation of its neurons quickly evokes movements in the muscles innervated by those neurons (remember that point, it is significant!). Electrical stimulation of other parts of the cortex do not evoke such discrete movements.

The nervous system developed in

multicellular animals to coordinate the activities of the various tissues in the body

The cerebellum is composed of three functional and anatomically distinct regions The second is

newer and is composed of a portion on the midline called the vermis (worm). The vermis is the spinocerebellum because it is the region that receives the projections from the spinocerebellar tracts. The vermis, in turn, separates two large lateral parts, the cerebellar hemishpheres that comprise the bulk of the cerebellum. The cerebral hemispheres are known as the cerebrocerebellum because of their massive connections with the cerebral cortex

The cell bodies that give rise to descending motor pathways or tracts originate from

nuclei, i.e., cell groups, in the brainstem and from cells in the cortex

(Reflex channel) Another familiar example is the withdrawal

of your hand when something hot is touched. The response (withdrawal) is also automatic; you do not have to think about whether or not you should pull your hand away. This reflex was also generated by a pre-wired circuit in the spinal cord.

Ramachandran continued this procedure until he explored Tom's complete body surface. When he touched Tom's chest, right shoulder, right leg or lower back, Tom felt

sensations only in those places and not in the phantom. But Ramachandran also found a second laid out map of the missing hand- tucked onto Tom's left upper arm a few inches above the line of amputation (Figs. 11 and 12). Stroking the skin surface on this second map (stroking Tom's upper arm) also evoked precisely localized sensations on the individual fingers:

An important feature of all neuronal systems is the specialized sensory receptors for

sensing the external world and the preferential pathways for conveying this information to the rest of the nervous system.

Pons and his colleagues performed an experiment on monkeys in which they

severed all the nerve fibers, sensory and motor, that projected from one arm into the spinal cord. Eleven years after the surgery, Pons and his colleagues anesthetized the animals, opened their skulls and recorded from the somatosensory cortex with microelectrodes.

What is actually represented in the homunculus, however, is not the

skin itself, but rather is the density of the sensory receptors in your skin, becasue it is the receptors whose axons enter the spinal cord and form the somatosensory lemniscus. But the sensory innervation of the skin is different in various parts of the body. For example, your fingers and lips are much more densely innervated with sensory receptors than is the skin on your back, which is why fingers and lips are the most sensitive parts of your body.

( Reflex channel) Interneurons are

small neurons whose cell bodies are in the spinal cord and whose axons do not project out of the cord, but rather only make local synaptic connections with one or more motoneurons or other neurons in the cord

The cerebellum is composed of three functional and anatomically distinct regions The first is the

small, vestibular part of the cerebellum that is phylogenetically the oldest part and is sometimes referred to as the "balancing brain". The portion of the cerebellum that controls vestibular function has a name, the flocculonodular lobe, but the name is unimportant for our purposes.

By systematically stimulating small regions of the motor cortex, investigators found that the motor cortex is

somatotopically arranged in a way very similar to the somatosensory cortex; that is, the cortical cells that give rise to the descending fibers activating the different parts of the body lie in the same relation to one another as do those body structures

There is a curious feature of all lemniscal pathways; they all cross

somewhere in the brain.

(Reflex channel) The classic example of a reflex connection is the

stretch reflex, as elicited when a doctor taps your knee during a medical examination. The tap on your knee stretches the quadriceps muscle in your leg, which is sensed by the stretch receptors in the muscle spindles (Fig. 5). The stretching of the spindle generates an action potential in the sensory nerve which then travels to the spinal cord and makes an excitatory connection with the motoneuron that projects back to the quadriceps muscle. The excitation of the quadriceps, which is an extensor muscle, causes the quadriceps to contract, thereby lifting (extending) your leg. Of course you did not have to think about any of this because the reflex is generated by a prewired circuit in the central nervous system, and will occur whenever the appropriate stimulus is applied (stretching of the quadriceps muscle in this case).

(Reflex channel) Many of the primary sensory fibers that enter the spinal cord make direct

synaptic connections with motoneurons and indirect connections through interneurons

It was not only touch that occurred in the phantom; Tom also experienced

temperature. When Dr. Ramachandran placed a drop of warm water on Tom's face, he felt it there immediately and also said that his phantom hand felt warm. When the water accidentally trickled down Tom's face, he exclaimed with considerable surprise that he could actually feel the warm water trickling down the length of his phantom arm. Tom demonstrated this by using his normal hand to trace out the path of the water down his phantom. This is all truly remarkable-a person systematically mislocalizing a complex sensation, such as a "trickle" from his face to his phantom hand

The total number of brain cells, for example, in a higher primate is estimated to be on the order of

tens of billions, of which no more than a few million are motor neurons. Thus, there are tens of thousands of intermediate neurons for every one motor neuron. This figure underscores the idea that the vertebrate central nervous system is not strictly a structure for receiving sensory information nor initiating motor activity, but rather for integrating and processing sensory information and expressing the sum total of this integrative activity in the form of appropriate motor activity.

In vertebrates the intermediate neurons are protected and are enclosed in

the bony cranium and the vertebral column.

( the lemniscal pathways) The sensory information is conveyed in

the brain by large and prominent bundles of axons that comprise tracts in the central nervous system.

Those influences, however, can be over-ridden by

the strong influences of the corticospinal tract and/or fine tuned by the influences of the other descending pathways. For example, your stretch reflexes maintain the contraction of your leg muscles when you are standing, which is why you can stand for long periods without thinking about it. But if you decide to, you can lift your leg and walk, which basically is due to the action of the corticospinal tract over-ridding spinal reflex activity. Similarly, if you are standing and lose your balance momentarily, you probably will not fall down, but rather your posture will be adjusted by the activity of the vestibulospinal tract.

the greatest amount of cortical space is devoted to

those neurons that project to the parts of the body, such as the hands and fingers, which are capable of finely graded movements. Thus, there is also a motor homunculus whose representation of the body is, like the somatosensory homunculus, distorted because the homunculus actually represents the relative number of motor fibers that innervate each of the muscles in the body.

During the first month of human development, the brain exhibits

three distinct dilations or primary divisions. These are the forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon).

People with cerebellar damage display a dramatic

tremor when they try to move. When asked to touch the tip of their nose with their finger, they display uncoordinated tremors as their finger is moved to their nose, with the tremor increasing in amplitude the closer the finger gets to the nose. They often cannot actually touch their nose, but rather their finger is moved past their nose (past pointing) or they even poke themselves in the eye. Intentional movements become unsteady and jerky, whereas the same movements in normal persons are steady and smooth. What seems to happen is that movements tend to be decomposed. That is, instead of occurring simultaneously in several joint, they take place in one joint at a time, and the velocity is uneven, sometimes too fast and sometimes too slow. The movements are all there, but are not coordinated and synchronized.

( the cerebellar channel) The projections are either through

two direct tracts of axons from a nucleus in the spinal cord or brainstem, or through a more complex, indirect system with several intervening synapses that are described in the later section on motor systems.

The neurons in the cerebellum have a

unique and highly orderly arrangement, which creates three layers of cells, each stacked upon the next. The orderly stacking of different types of cells is called a cortical arrangement, and thus one refers to the cerebellar cortex (as opposed to the cerebral cortex that covers the forebrain).

The second major component in motor neurons is the descending pathways that originate from

various parts of the brain and send their axons down the brain as distinct fiber tracts to synapse in the spinal cord (and cranial nerve nuclei) and influence the activity of the motoneurons