Neural Pathways

In vertebrates the two main cell types of nervous systems are neuroglia cells and nerve cells. Neuroglia are believed to perform a variety of nutritive and other housekeeping functions in nerve tissue, and nerve cells, or neurons, are the impulse-generating and impulse- conducting units proper.

Each neuron typically consists of a nucleus-containing cell body and of one or more filamentous outgrowths, or fibres, that extend away from the cell body.

Impulses usually originate at the terminals of fibres called dendrites, which then carry the impulses toward the cell body. Impulses travel away from a cell body through fibres called axons. Many dendrites and axons are short, but others have lengths of over a yard (of example, axons from the base of the spinal cord to the toes in man).

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The long axons and dendrites are enveloped by a Schwann sheath, a single layer of thin flat cells that supplies nutrients to a fibre and that also provides a pathway when a cut fibre regenerates. In certain cases the cells of a Schwann sheath wind around the nerve fibre several times, and the fatty contents of these wrapping layers then form a myelin sheath. A myelin wrapping is believed to serve as a kind of insulation, comparable perhaps to an insulating rubber envelope around an electric wire.

The fibres of two adjacent neurons are not in direct contact; their terminals come close together at a synapse, a microscopic space through which impulses are carried by chemical means.

In crude terms a whole nervous system can be envisaged as an intricate network of neurons, with fibres interconnecting functionally at numerous synapses. Many pathways in such a network form reflex arcs, composed of a sequence of neurons with specific functions.

Thus, sensory (or afferent) neurons in such an arc transmit impulses from a receptor to a modulator, and motor (or efferent) neurons transmit from a modulator effectors. Neurons in a modulator are interneurons. Groups of nerve fibres frequently traverse a body region as a single collective fibre bundle, or nerve. Nerves are designed as sensory, motor, or mixed depending on whether they contain sensory fibres, motor fibres, or both.

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The most primitive type of neuron arrangement known is a nerve net. Representing the only neural structures in, for example, certain coelenterates, such nets also form at least part of the nervous system of most other animals. In vertebrates, for example, nerve nets occur in the walls of the alimentary tracts.

Most animals in addition contain nerve cords, condensed regions of nets, and ganglia, dense accumulations of neurons and fibre terminals. Ganglia can be sensory, motor, or mixed according to the kinds of fibres they connect with.

Large ganglia usually contain functional subdivisions called nerve centers, specialized groups of interneuron’s that regulate specific activities. Very large ganglia usually also store information as memory and control intricate forms of behaviour. Large ganglia or groups of ganglia that integrate the main sensory inputs and motor outputs of animals constitute brains.

By far and most complex nervous systems are those of vertebrates. The main parts develop from a hollow dorsal neural tube, formed in the embryo as ingrowths from the ectoderm. The anterior portion of the tube enlarges as a brain, the posterior portion becomes the spinal cord, and nerves grow out from both. The fluid filled space in the tube forms brain ventricles interiorly and a spinal canal posterior. In the mature brain the major divisions are the forebrain, the midbrain and the hindbrain.

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Of two main subdivisions of the forebrain, the more anterior one is the epencephalon. It contains paired olfactory lobes, the centers for the sense of smell. In birds and mammals the epencephalon is enlarged greatly as a pair of cerebral hemispheres, which cover virtually all the rest of the brain.

These hemispheres contain the centers for the most complex sensory integration and for voluntary motor activities, and they also play the key roles in the control of memory and intelligence. A conspicuous set of nerve tracts, the corpus callosum, interconnects the two hemispheres.

Behind the telencephalon, the (unpaired) diencephalon contains the thalamus and hypothalamus. These lateral regions control numerous involuntary activities, and they also affect consciousness, sleep, food intake, and emotional states.

Ventrally the pituitary gland and dorsally the pineal body project from the diencephalon. The pineal-body forms a third eye on the top of the head in lampreys and tuataras but is hidden under the cerebral hemispheres in birds and mammals.

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The midbrain, or me encephalon, contains dorsally located optic lobes. In all vertebrates except the mammals these loDes contain the centers of vision, and the nerve tracts from the eyes terminate there. In mammals the optic nerve tracts continue to visual centers located posterior in the cerebral hemispheres. The original optic lobes here are little more than relay stations for visual nerve impulses.

The hindbrain consists of two subdivisions, an anterior metencephalon and a posterior myelencephalon, or medulla oblongata. Dorsally the metencephalon includes the cerebellum, a comparatively large lobe that coordinates muscle contractions as smoothly integrated movements.

For example, locomotion and balancing activities are regulated from this lobe. Ventrally the metencephalon contains a conspicuous bulge, the pons, in which the nerve between brain and spinal cord cross from the left side to the right side. Because of this crossover the left side of the brain controls activities on the right side of the body and vice versa.

The medulla oblongata, which contains the nerve centers controlling heartbeat, vasomotion, and breathing, continues posteriorly as the spinal cord. Twelve pairs of cranial nerves (only 10 pairs in fishes and amphibia) emerge from the brain, and most of them lead away from the medulla oblongata.

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The spinal cord gives rise to segmental spinal nerves (31 pairs in mammals) which pass to the trunk and the appendages. In each spinal nerve sensory fibres from the body enter the dorsal part of the spinal cord, and motor fibres leave from the ventral part. The cell bodies of the sensory’ fibres lie just outside the spinal cord.

Vertebrates also have a well-developed autonomic subdivision of the nervous system (ANS, distinct from the remainder, called the central subdivision, or CNS). Controlling all involuntary activities and containing only nonmyelinated nerve fibres, this autonomic system has its nerve centers in the spinal cord and the brain and in a series of these centers from all body parts that are not under voluntary control. (Among such fibres are, for example, those that form the stretch receptors in the large blood vessels near the heart).

From the autonomic centers lead away two functionally different sets of autonomic motor fibres. Fibres from the brain and the most posterior part of the spinal cord represents a parasympathetic outflow of the autonomic system; and fibres from the midpoint of the spinal cord from a sympathetic outflow.

Each organ or the body that is not under voluntary control receives fibres from both outflows” and these generally have opposing effects. If parasympathetic fibres stimulate it; or vice Versa. For example, the inhibitory fibres to the pacemaker of the heart (which travel in the cranial vagus nerves) are part of the parasympathetic outflow, and the accelerating fibres to the heart belong to the sympathetic outflow.

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Similarly, vasodilator fibres to blood vessels are parasympathetic, and vasoconstrictor fibers are sympathetic. All other organs that function involuntarily are likewise equipped with both braking and accelerating controls.

Each autonomic motor path to a given organ consists of at least two consecutive neurons, which synapse in an autonomic ganglion located somewhere along the path to the organ.

The nerve fibre leading to this ganglion is said to be preganglionic, and the fibre from the ganglion to the organ is postganglionic. In the sympathetic outflow the autonomic ganglia lie just outside the spinal cord, and on each side of the cord they are interconnected as an autonomic chain. These chain ganglia also have interconnections with the spinal ganglia Parasympathetic ganglia are more dispersed and are not arranged as chains.

The involuntary operations of the ANS are interrelated closely with the voluntary ones of the CNS. For example, within certain limits a man can alter his breathing rate voluntarily through CNS control, even though this rate is basically under involuntary ANS control. Conversely, numerous autonomic changes affect voluntary behaviour- for example; ANS-controlled hiccoughing limits CNS- regulated speaking.

Neural Impulses

A nerve impulse consists of a sequence of electrochemical reactions; during the passage of an impulse a wave of electric depolarization sweeps along a nerve fibre. After an impulse has passed the reaction balance returns to the original state readying the fibre for a new impulse.

A resting, nonstimulated neuron is electrically positive along the outside of its surface membrane and electrically negative along the inside. These electric charges are carried by ions that are part of, or are attached to, the two sides of the cell membrane of the neuron.

This membrane is so constructed that in the rest state it prevents the positive and negative ions from coming together. As a result an electric potential is maintained through the cell membrane; the membrane is said to be polarized electrically. When an impulse sweeps along a nerve fibre, the permeability of the membrane changes at successive points along the fibre.

As this happens at any one point, an avenue is created through which the positive and negative ions of an adjacent point can pass, thus depolarizing that region. In other words, one depolarized region drains electric charges from adjacent parts of the membrane, causing depolarization there, too.

In this manner the impulse itself produces the necessary conditions that allow it to advance farther, and it travels wavelike along a fibre. Some short time after an impulse has passed a given point the membrane at that point reacquires both its original permeability state and its polarization.

If fine wires and electric measuring equipment are connected to anerve, the passage of an impulse is recorded in such equipment by a flow of current of certain characteristics. By studying these action currents, or action potentials, of different nerves, it has been found that impulses differ is speed, strength, and frequency. In some nerve fibers, like those from the heart-rate center to the heart, impulses are fired continuously in aid succession.

Adjustment of heart rate occurs through frequency modulation heart rate changes with alteration in impulse frequency. In motor fibers to many glands, by contrast, the fibers are normally at rest and carry impulses only when secretions are to be produced. Each type of fiber has its own characteristic pattern of impulse transmission, and it has been found also that impulse speeds tend to be directly proportional to the thickness of a nerve fiber.

Moreover, speeds are influenced by the presence of absence of myelin sheaths. Myelinated CNS fibers conduct impulses at speeds up to 100 yd per sec. whereas nonmyelinated ANS fibers of comparable thickness conduct at about 25 yd per sec. at most.

How does an impulse get across a synapse? In certain cases it can be shown that when an impulse reaches an axon terminal, the terminal acts as an endocrine structure and secretes substance. This hormone diffuses through the synapse, and some reaches dendrite terminals of adjacent neurons. There the hormone can depolarize the dendrites in such a way that new impulses are initiated in them.

Four hormonal substances that function in this manner have been identified in vertebrates: serotonin, acetylcholine, adrenaline, and noradrenalin. One or the other of the last two is secreted by sympathetic postganglionic fibers, and probably also by at least some of the internerurons in brain and spinal cord (for example those in the ANS centers of the hypothalamus).

All such neurons are said to be adrenergic. Serotonin or acetylcholine is produced by sympathetic preganglionic fibers, all fibers of the parasympathetic system, and probably also by CNS fibers and CNS centers in brain and spinal cord. Such neurons are cholinergic.

The secretion pattern of these hormonal substances in the brain is not yet known very precisely, largely because of the so-called blood-brain barrier, a selective metabolic block between blood capillaries and nerve tissue. Probably maintained by neuroglia cells that surround the capillaries, this barrier let’s only oxygen and basic nutrients such as glucose and amino acids pass into the brain, and it lets only waste products leave.

The hormonal transmitter substances normally cannot diffuse in or out, hence investigation of their activity in the brain has proved difficult. Even so, through research done in conjunction with tranquilizers and drugs such as LSD, it has been found that the hormonal transmitters play a key role in regulating moods and emotional states both normally and in mental illness.

Thus, elevated moods and feelings of well-being are associated directly with high levels of noradrenalin and serotonin in the brain, whereas depressed states are associated with low concentrations of these substances. Some findings further suggest mental state might result from internally generated alterations in the chemical reaction patterns through which the transmitter substances are manufactured in the brain.

Synaptic impulse transmission by chemicals also has important consequences outside the brain. For example, nerve fibers as such rarely fatigue, but their transmitter- secreting terminals “tire” fairly easily. Moreover, since only axon terminals secrete hormones .and only dendrite terminals are sensitive to these substances, impulse conduction becomes unidirectional.

Neural Centers

In general, the activity of all types of neural modulator is based on two kinds of information: genetically inherited information and newly acquired information obtained through the sensory system.

Certain genetically determined neural pathways and patterns of neural activity are already established once an animal has completed its embryonic development. Even in a just-formed nervous system, therefore, relatively simple sensory inputs to the modulators often can evoke fairly complex outputs to the effectors, and behaviour can be correspondingly complex.

In many cases, indeed, sensory inputs are not required at all; neural centers are often spontaneously active. This is particularly true for those that control the most vital rhythmic processes of an animal. For example, the breathing center in the medulla oblongata sends out rhythmic motor impulses spontaneously.

This rhythm can be shown to persist even if the center is isolated surgically. Similarly spontaneous activity takes place in the heart-rate center and in many-possibly most-other neural control centers as well.

In effect, the genetic endowment ensures that, as soon as animal is completely developed, it has fully functional neural controls for at least those motor activities that are basically necessary for survival. To the extent that these activities are spontaneous, neural operations evidently do not involve complete reflexes; receptors and sensory paths do not participate directly.

To a greater or lesser degree, however, inherited neural activity is usually modified by sensory experience. Information about the current neural centers in the form of more or less complex sensory impulses. In such instances nervous activity is based on complete reflexes.

But here the modulating centers do not merely relay incoming sensory impulses to outgoing motor paths; even the simplest modulators are usually capable of reflex modification. The nature of this modification depends in some cases on inherited factors, in others on information acquired through past experience and stored as memory.

Among the simplest and most basic forms of modification are suppression and augmentation of reflexes. Suppression occurs when certain neurons inhibit others.

Some synapses are so organized that, when impulses arrive in incoming nerve fibers, the production of impulses in outgoing fibers becomes harder rather than easier. The opposite effect, augmentation, can be brought about by a summation of impulses.

In a synapse receiving many incoming impulses, each impulse individually might be too weak to produce an impulse in an outgoing fiber. However, the small quantities of chemical transmitters produced by the many incoming fibers can add together and become sufficiently powerful to initiate impulses in an outgoing fiber.

A more complex form of modulator activity based largely on summation and inhibition is channel selection. Even a simply organized modulator selects among many possible outgoing fibres and sends out impulses only over certain specifically chosen paths. Normally only appropriate effectors will then receive motor commands. As a result the effectors response of an animal can be adaptively useful and can actually contribute to steady-state maintenance.

Little is known as yet about the mechanism by which certain neural channels are selected in preference to others. In most cases preferred circuits become established during the embryonic development of the nervous system.

Thereafter given sets of sensory impulses to a modulator result in more or less fixed, predictable sets of motor impulses to effectors. Such neural activities are among those that have a largely genetic basis, and they govern most of the internal operations of most animals.

Preferred channels include, for example, sets of interneuron’s that are arranged as oscillator circuits, in which an impulse travels continuously over a circular route. Each time such an impulse passes a given synapse, a motor impulse to effectors might be initiated.

The rhythmic heart beat of an insect is controlled by an oscillator circuit of this type, in which nine circularly arranged neurons are embedded directly in the heart. Many other rhythmic, automatic activities are known to be governed by oscillator circuits.

A modulator often reacts to an incoming sensory impulse by sending out not just one but several selected motor commands. Here a simple external stimulus can lead to the completion of curring as a single, integrated pattern of activity, or programmed behaviour. A good example is the startle response in man. An unexpected blow directed at the head leads to a closing of the eyes, crouching stance, and a rising of hands to the face.

These several dozen separate reflexes occur simultaneously, as a unified “program”. Most programs of this type are largely inherited, and in most animals, invertebrates in particular, behaviour is very largely programmatic in this sense.

Moreover, the execution of such programs involves most of the neurons present in the nerve centers. In comparatively large- brained animals, the majority of neurons again probably form fixed inborn circuits, yet substantial numbers seem to remain available for the later development of new or modified circuits and new or modified circuits and new or modified (but not only) caphalopod mollusks, arthropods, and vertebrates, are capable of learning and of storing learned experiences as memory.

The simplest form of learning is probably habituation, or progressive loss of responsiveness to repeated stimulation. For example, most animals can out avoidance or escape activities in response to mild stress stimuli. However, if such stimuli are repeated many times in succession the response can gradually subside and ultimately often disappear.

It is not well known as yet how such habituation comes about. (Indeed, organisms without nervous systems can habituate to environmental stimuli; too, hence learning of this type probably operates on an intracellular, chemical level).

A more complex form of learning depends on conditioned reflexes. In so-called classical conditioning, two or more stimuli are presented to an animal simultaneously and repeatedly, until the animal learns to execute the same response to either stimulus. For example, if bright light is directed into the eyes the pupils will contract reflex.

If the light stimulus is given repeatedly and is accompanied each time by food, sound, or some other stimulus, then papillary contraction can eventually be made to occur by such a stimulus alone, without the bright light.

Evidently, the animal now associated the second stimulus with light, and it comes to have not only the inherited neural circuit to the eyes but also an additional one acquired by learning through experience. Either one alone or both together can then produce the papillary response.

Indeed, papillary retraining in man is probably possible even if the second stimuli are no more than a single spoken word, and if this word is spoken by the test subject him. The stimulus word might not even need to be spoken but could be merely an unvoiced thought.

Such a self- conditioned individual would then be able to contract his pupils at will. The occasionally recorded feats of human self-control over pain and other normally no volitional responses are undoubtedly based on self-conditioning of this sort.

Learning can also occur through operant conditioning, which differs from the classical type in that the animal participates actively and deliberately in the learning process. Thus if a certain activity at first happens to be carried out by chance and if this activity happens to have desirable consequences, then the animal can recreate these consequences by deliberately repeating the activity.

Learning by this means becomes particularly effective if it is “reinforced”, that is, if repetition of an activity entails material or psychological rewards. Much of the learning of vertebrates-mammals and man in particular-is based on operant conditioning of this sort.

Undoubtedly the most complex modulator activities are those that are involved in intelligence, personality, ability to think abstractly, and capacity to manipulate and control the environment. Depending extensively on memory and learning, such functions is developed to any notable degree only in the most advanced mammals.

But note again that, regardless of their relative complexities, all modulators depend on adequate information; apart from whatever built- in, inherited information they have available, modulators can act only on information that the neural receptors supply in the form of sensory impulses.

Neural Receptors

Receptor cells are either epitheliosensory or neurosensory. The first type is a specialized, no nervous epithelial cell that receives stimuli at one end and is innervated by a sensory nerve fiber at the other. The second type is a modified sensory neuron that carries a dendrite like stimulus-receiving extension at the one end.

At the other is an axon that synapses with other neurons. Both types of receptor cell can occur in clusters and together with accessory cells form sense organs. All receptor cells of invertebrates are of the neurosensory type. Vertebrates contain both neurosensory and epitheliosensory receptors.

Nervous impulses generated by receptors become perceptions only in the neural centers. In effect, eyes do not see and ears do not hear; eye-brain complexes are required for seeing, ear-brain complexes for hearing. In some instances the perceptions become conscious but more often they do not. When nerve impulses reach the brain from a blood vessel, for example, sensing takes place, but in this case the sensation does not become conscious.

Environmental change as such is known to be an important factor in the production of sensory impulses by receptor cells.

For when a given stimulus persists unchanged for a time, a sense dulls, or “adapts”. For example, we soon become relatively insensitive to the pressure of clothes, to a persistent odor, or to a taste. Pain is most difficult to adapt to, but odor perception dulls very easily. We cannot judge our own body odors, for example, since we live with them constantly and adapt to them continuously.

Different kinds of sense perceptions depend not so much on differences in impulses to the brain as on the different central connections of sensory fibers in the brain.

For example, if a fiber from a heat receptor and a fiber from a cold receptor could be cut and the cut ends were allowed to enervate the sense organs in switched order, then the animal would feel hot when the cold receptor were stimulated and cold when the heat receptor were stimulated. In other words, the quality of a perception depends on which of various brain centers receives signals.

Furthermore, correct localization of a stimulus similarly depends on the central connections. The anatomical distribution of receptors throughout the body is matched virtually point for point in the anatomical distribution of neural centers.

So long as these structural relations are preserved, impulses will be correctly interpreted as coming from particular body regions and particular receptors. That this is actually so has been demonstrated through experimental rearrangements of neural pathways.

One qualification should be added here, however. It is a fairly common experience that pain originating in an internal organ is often sensed as if it originated at some other region. For example, pain stimuli actually affecting the liver can be felt as pain in the shoulder region; and an ache in open tooth is often thought to come from the whole side of the head.

In such cases of referred pain, pain fibers that originate in different body regions lead to some general area in the brain. Impulses through one of the fibers can then stimulate a greater or lesser portion of that area. Pain sensations can be diffuse as a result, as if impulses actually arrived over more than one pain fiber.