Unit 3-Nervous System
Properties of Nerve
Nerve cells are highly adapted for rapid communication of information. The mechanisms by which they do so are not yet entirely understood, but certain aspects of these mechanisms are clear. Before considering these mechanisms, it will be helpful to consider the structure of the nerve cell and of the nerve.
1. Nerve Cell Structure:
The neuron, or nerve cell, consists of two parts. One is the cell body, which is much like the typical cell illustrated in Figure 4. Nerve cells differ from other cells in the parts which extend from the cell body. Some of these extensions are quite short and multiply branched; these are called dendrites. Others are single and quite long; these are called axons. An axon may extend from the brain to the lowest part of the spinal cord, a meter in a man of normal size. Axons may give off branches, called collaterals.
The structure of a neuron depends on its location and function. A neuron which carries sensory information to the central nervous system may have one very long dendrite, which looks like an axon. Both the long dendrite and the axon carry information from the sense organ to the central nervous system, making an afferent pathway. Neurons carrying information from the central nervous system, for example, motor neurons, have many short dendrites and one long axon. Information from the central nervous system reaches the effector organ by efferent nervous pathways. Long extensions from the neuron are usually, but not always, sheathed. The sheathing substance is called myelin, a fatty material. Surrounding the myelin is usually another cellular sheath called neurilemma. The cells of this sheath are called Schwann cells.
The two sheaths are discontinuous; that is to say, they surround the axon for a certain distance and stop. They begin again almost immediately, but between the sections of the sheaths, the axon is virtually unprotected. These unsheathed sites are called nodes of Ranvier. Figure X shows a typical motor neuron with dendrites, cell body axon and sheaths. Figure 92 shows the structure of a sensory neuron of the type that detects touch. Note that the long dendrite is sheathed as well as the axon.
2. Peripheral Nerve:
A peripheral nerve is a collection of nerve fibers, some of which are sheathed, others not; some of which carry afferent information, others efferent. All are bound together by connective tissue, perineurium. Because most nerves contain afferent and efferent fibers, they are said to be mixed. There are a few examples of purely afferent nerves and purely efferent ones which will be discussed later. Figure 93 shows a typical peripheral nerve. This nerve contains afferent and efferent fibers, some with, some without sheaths.
3. Conduction of the Nerve Impulse:
Almost all cells contain potassium ions in high concentration and few sodium ions. They are surrounded by a fluid which is low in potassium but high in sodium. Such differences in concentration mean that potassium tends to leave the cell. Sodium would tend to enter it too, but for the fact that the resting membrane is impermeable to sodium. Since potassium, which is positively charged, tends to leave the cell, and the negatively charged particles associated with it cannot because they are too large, the outside of the cell becomes positively charged with respect to the inside. Though only a few ions are involved, sizeable potentials are developed. In the normal resting cell, the potential is 0.070 V or 70 millivolts.
The voltage difference between outside and inside exerted across the cell membrane is called the membrane potential. It is seen in the axon as well as the cell body. When the membrane potential exists, the cell is said to be "polarized".
When the membrane is disturbed, the conditions which give rise to the membrane potential suddenly break down. The membrane becomes permeable to sodium, which now flows from outside to inside; potassium, replaced by sodium, now moves easily from inside to outside. The membrane potential is abolished, even reversed, and the polarization is replaced by depolarization.
The outside of the depolarized area is electrically negative to the outside of the polarized area near it. Because of this electrical difference, a current flows from the polarized to the depolarized part. This current flow through the polarized part disturbs its membrane, and it too becomes depolarized. The newly depolarized part, electronegative to the adjacent portion, sets up an electrical difference which depolarizes that portion. This process continuing along the nerve fiber is the nerve impulse.
From the above description, it would appear that the propagation of the nerve impulse is electrical in nature. This is probably, but not certainly, so. It can certainly be detected electrically. Before illustrating the electrical detection of the nerve impulse, we must emphasize that very soon after depolarization occurs, the fiber repolarizes. The cellular content, momentarily swamped by sodium after the breakdown of the membrane, is reconstituted while sodium is being expelled by the sodium pump and potassium returns to its normal position.
Figure 94 shows how a galvanometer attached to an axon displays the electrical changes described above. In Figure 94a, the galvanometer leads are both attached to the outside of the uninjured nerve. In Figure 94b, one lead is placed on an injured area of the nerve. It is important to emphasize that whether or not electrical events cause the nerve impulse to propagate, the speed of propagation is a property of the membrane, so the nerve impulse moves much more slowly than an electric field. Electromagnetic fields are propagated at 3 x l010 cm/sec. Electrons flow more slowly through wires, but still much more rapidly than the nerve impulse which in the fastest myelinated fibers moves at 12,000 cm / sec, and in the slowest myelinated fibers, 500 cm / sec. Though the reasons are not known, it seems that the conduction velocity is greatest in thick nerve fibers and least in thin ones.
Non-myelinated nerve fibers conduct the nerve impulse even more slowly, 100 cm / sec, than the slowest myelinated fibers, and some experiments suggest that there may be nerve fibers which conduct even more slowly, at about 5 to 10 cm / sec. The difference between myelinated and non-myelinated fibers is so great that it seems probable that the impulse "jumps" from one node of Ranvier to the next in myelinated fibers (saltatory conduction). The same principles described previously still apply, but depolarization and repolarization occur only at the nodes (Figure 95).
4. All-or-None Property of Nerves:
When a stimulus is applied to nerve, its response is a property of its membrane characteristics, not of the stimulus. No matter how great or small the stimulus, the speed and the size of the action potential are the same provided only that the stimulus is adequate to begin the depolarization. Lesser stimuli lead to no response, and greater stimuli lead to all the response of which the fiber is capable. This property of individual nerve fibers is called the all-or-none property. It must be emphasized here that this property is not characteristic of a group of nerve fibers, such as a mixed nerve, where each fiber in the group may have a different threshhold of excitation. In such a group, gradual increase in stimulus strength will lead to involvement of more and more fibers, until all are involved. After that point, stimulation produces no greater response.
5. Refractory Period:
In Chapter 5 we noted that muscle failed to respond to a second stimulus which occurred too soon after a first stimulus. The same is true of nerve, but the refractory period is quite short, about 1/1000 of a second.
6. Neuron Trains:
When sense organs are stimulated they activate their sensory neurons, and nerve impulses go toward the central nervous system. The axons of the stimulated neurons make contact with other neurons in the central nervous system by means of a microscopic structure called the terminal button, which conveys a signal to the next neuron.
Like the axon, the neuron has a threshhold. A signal from one terminal button is usually not enough to produce depolarization of the whole neuron. Most neurons in the central nervous system have several hundred to several thousand such connections. It is not, at this time, certain how many terminal buttons must be activated to activate any neuron. When enough are activated, however, the neuron discharges. Through its axon a new terminal button is activated, and if enough terminal buttons on a secondary neuron are stimulated, it too will be activated. A tertiary neuron may now be activated in accordance with the same principles, and so forth.
At the same time, any neuron in the neuron train may receive "activating information" from other neurons. It is certain, also, that neurons can receive information from other neurons which tends to prevent their activation, inhibition.
Thus the activity of the central nervous system is exceedingly adjustable. The motor neurons which are eventually activated after sensory information has been processed in the central nervous system are variable in number and in the muscle units which they control. The simple reflex arc, in which a single sensory neuron stimulates a secondary one which stimulates a motor neuron, simply does not exist, although some motor responses to some stimuli are more or less predictable.
The junction between the terminal button and the next succeeding nerve cell is called the synapse. The nature of synaptic transmission is not clear. Some have suggested that it is purely electrical. It may be so, but in many cases, synaptic transmission can be shown to be associated with the release of chemical substances which may depolarize or hyperpolarize a portion of the next neuron in the train. The substance most often considered responsible for synaptic depolarization is acetyl choline. Synaptic hyperpolarization (inhibitory) is usually considered electrical in origin, but inhibitory substances such as gamma amino butyric acid have been suggested.
Regardless of the mechanism of conduction, there does appear to be measurable delay at the synapse of about 1/1000 of a second. Responses involving many synapses tend to occur long after the stimulus. For example, a sensory input may involve 100 synapses in a train before a response occurs. The response therefore cannot occur before 0.100 seconds after the impulse.
Efferent nerve fibers carry information to effector organs. The information is now believed to be transmitted from the nerve to the effector organ by chemical substances. In the case of voluntary muscle, the junction of nerve and muscle at which chemical transmission of the nerve impulse occurs is a rather specialized structure called the neuromuscular junction. A diagram of the neuromuscular junction is shown in Figure 96.
When the nerve is stimulated, its end in the neuromuscular junction releases a little acetyl choline into the junction. If enough acetyl choline is released and picked up at specific sites on the muscular side of the junction, the muscle will depolarize, and contraction will follow. A second contraction depends on the destruction of the acetyl choline which provoked the first, which is usually accomplished by the enzyme cholinesterase (See Chapter 3).
There are a number of ways of influencing the neuromuscular junction. Some have medical application, while others are more likely to have military application. One medical application has been mentioned (Chapter 3). In myasthenia gravis, either too little acetyl choline is made, it is destroyed too quickly, or the receptor sites are relatively insensitive to it. This condition can be alleviated by administration of mild cholinesterase inhibitors, such as prostigmine or neostigmine.
Curare, an arrow tip poison discovered by South American Indians, appears to occupy the sites sensitive to acetyl choline but does not produce depolarization. Thus paralysis of voluntary muscle results. This drug is of great use in surgery, where it is employed to produce muscular relaxation. Large doses paralyze the muscles of respiration, so one should always be prepared to administer artificial respiration when the drug is used. Fortunately, curare is destroyed fairly rapidly so that its paralytic effects are temporary.
Most anti-cholinesterases are not medically useful, in particular the so-called "nerve gases". These substances destroy cholinesterase. Acetyl choline, when released at the neuromuscular junction can depolarize the muscle, but since the acetyl choline is not destroyed, the muscle does not repolarize. It is, therefore, unresponsive to further nerve stimulation and paralyzed. The development of methods to spread these substances over large populated areas may result in a revolution in the methods of warfare, or it may result in the extermination of animal life on this planet.
7. Some Properties of Neuron Train Response:
When a single sensory nerve fiber is stimulated through its sense organ, the impulse, arriving at a secondary neuron, does not usually stimulate it. It is said to be inadequate. If, however, a number of sensory fibers discharge at the same time on the same secondary neuron, the secondary neuron may discharge. This is called summation of inadequate stimuli in space, since the receptors are usually spatially separated.
Consider a neuron further down the train. A single sensory neuron may send its impulses to it by a very direct path. If the impulse occurs only once, there will be no secondary discharge. If there are some pathways involving only a few neurons, while others involve more, the impulses, arriving at different times, are still inadequate to excite. However, if stimulation is repeated, the late signals from the late stimuli, travelling through long ones, may all arrive together, and stimulation of the second neuron may result. By this means, the repetition of a single inadequate stimulus may lead to discharge of a secondary neuron. This process is called summation of inadequate stimuli in time.
A sensory input may activate all the neurons of some chains, leading to a motor response. Other pathways may also be involved, but not enough to discharge. A slight sensory input, not ordinarily enough to stimulate, may enter the partially involved, undischarged train, the sub-liminal fringe, and a response may occur to it. The slight sensory input is said to have been facilitated. See Figure 97.
A strong sensory input may activate many motor neurons. Another simultaneous strong sensory input may do the same. Some of the motor neurons are activated by both inputs. The contraction from the double input is, therefore, less than the sum of the contraction from each. The response is said to have been partially occluded. See Figure 98.
A continued sensory input may involve more and more motor neurons as it goes on, in a manner similar to that described under summation of inadequate stimuli in time. This increased involvement of motor neurons is called recruitment. Even after a sensory input stops, its traces may linger; that is to say that neuron trains, set in motion, will move. Thus the response may outlast the stimulus. The phenomenon is called after discharge.
The response to certain sensory inputs involves contraction of some muscles and the relaxations of others. This is called reciprocal innervation. An example of this is seen in walking. The extension of the thigh is followed by its flexion. While the extensor muscles are activated, the flexor muscles must be inhibited and vice versa. These adjustments are made in the central nervous system by neuronal activation and neuronal inhibition.
All of these properties of neuron trains were described along with many others by the English neurophysiologist Sherrington just after 1900. Sherrington's book, The Integrative Action of the Nervous System, can still be studied with profit.
From the above brief outline, the student may recognize that the motor response to sensation can be infinitely diverse. The diversity stems in large part from the complicated organization of the central nervous system in which the sensory input is processed. This will be considered in Chapter 5.
8. A Typical Reflex (Stretch Reflex):
One of the most reproducible reflexes is the stretch reflex, especially pronounced in the muscles which normally maintain posture. The knee jerk is a familiar example of this reflex. When the patellar tendon is tapped just below the knee cap the muscles of the front of the thigh are stretched a little. This stretching stimulates receptors in the muscles and the tendons. These receptors, called proprioceptors, transmit impulses to the processes of the sensory neurons serving the proprioceptors. The impulses now enter the cord by way of its dorsal root (See Chapter 5). Sufficient sensory impulses are usually generated to serve to excite a group of secondary neurons, which in turn act on motor neurons. Impulses from the vestibular nuclei of the medulla (Chapter 5) facilitate discharge of some motor neurons, which cause contraction of a group of motor units. The response to the stretching of the muscle is a shortening of the muscle.
The neuron train involved in this type of response is a short one. When the muscle is stretched, the sensory input arrives all together, and the motor output, not distorted by long trains, is synchronous, meaning the muscle contracts all at once. Such contractions give rise to a jerky movement. In contrast, asynchronous discharge leads to steady contraction.
Normally the sensory input of this reflex is asynchronous so that the muscular response is also asynchronous. Different sensors and different motor units are involved at different times. Thus, posture is easily maintained against gravity.
Continue to Chapter 5.