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(2) Motor, conveying impulses to muscles and exciting them to contract.

(3) Secretory, the stimulation of which calls forth the activity of a gland.

(7) Inhibitory, when they check or prevent some activity by the impulses which they carry.

(o) Vasomotor nerves, which regulate the contraction of the muscular coat of the blood vessels.

(8) Trophic, thermic, electric nerves are also to be named, the two former being of doubtful existence, and the latter being only found in those animals which are capable of emitting electric discharges, such as electric fishes.

III. INTERCENTRAL NERVES act as bonds of union between the several ganglion cells of the nervous centres, which are connected, in a most elaborate manner, one with the other. The terminals of these fibres are possibly both receiving and directing agents, and the delicate strands of protoplasm communicating between them probably convey impulses in different directions, but of this we can have no definite knowledge, although such a supposition would aid us in forming a mental picture of the manner in which the wonderfully complete intercentral communications are accomplished.

MODE OF INVESTIGATION. In order to understand the functions of the different nerves a knowledge of their central connections and their peripheral distribution is necessary. But anatomical research, unaided by. experimental inquiry, does not suffice to determine their function.

The procedure adopted in testing the function of a nerve is the following: The nerve is exposed and cut, and it is observed whether there be any loss of sensation or muscular paralysis in the part to which it passes. The end connected with the centres is spoken of as the central or proximal end, and that leading to the distribution of the nerve is called the peripheral or distal end. Each of these cut ends is then stimulated, and the results are observed. If the nerve be purely motor, stimulation of the proximal end will yield no result, but when the distal end is irritated, movements follow. If, on the other hand, it be a sensory nerve, stimulation of the distal end gives no result, and that of the proximal end produces signs of pain.

CHEMISTRY OF NERVE FIBRES. The axis cylinder of nerves is probably composed, as already mentioned, of protoplasm; further than that nothing is known of its chemical properties. The medullary sheath yields certain substances which are related to the fats, and can be extracted with ether and chloroform. Among these is the peculiar compound nitrogenous fat, lecithin, containing phosphorus, also cholesterin, cerebin, and kreatin.

ELECTRIC PROPERTIES OF NERVES. Like muscle, nerves may be regarded as having a state of rest and a state of activity, but the two states are not obvious in the same striking way as they are in muscle, nor do we know much of the physical properties of nerve. While at rest, however, it shows electric phenomena similar to those which have already been described as belonging to muscle tissue. These electrical currents are contemporaneous with the life of the nerve, and they undergo the same variation as occurs in muscle when the nerve passes into the active state ; that is, when it transmits an impulse.

The so-called natural current of nerve is practically the same as that of muscle, passing in the nerve to the central part from the cut extremities of the fibre ; that is to say, the current passes through the galvanometer from the electrode leading from the middle of the nerve, to that applied to the extremity. The electromotive force of a small nerve is much less than that of a muscle. In a frog's sciatic it has been estimated to be 0.02 of a Daniell cell. The natural current of the frog's nerve is said to increase in intensity in proportion to the increase in temperature up to about 20° C., after which it decreases.

Experiments on nerve currents must be carried on with all the precautions mentioned in speaking of muscle currents, and with the non-polarizable electrodes there figured (page 448).

THE ACTIVE STATE OF NERVE FIBRES. Nerves pass into a state of activity in response to a variety of stimuli, but their active condition cannot be readily recognized, because the only change we can detect in the nerve is that which takes place in the electric state. If it be connected with its terminals, we learn when a nerve is carrying an impulse from the results occurring in them on stimulation. In the case of an afferent nerve, we get evidence of a sensation, and when the nerve is efferent, for example, bearing impulses from the centres to the muscles, we judge of the state of activity of the nerve by the muscle contraction. For experimental purposes we use the nerve and the muscle of a frog. This nerve-muscle preparation is made from the leg of a frog : the sciatic nerve is carefully prepared from the thigh and abdominal cavity without being dragged or squeezed, and the gastrocnemius is separated from all its attachments except that to the femur, about two-thirds of which bone is left, so that the preparation may be fixed in the clamp. In fact, the method used for the direct stimulation of muscle is also employed for the study of the excitability of nerve fibres.

NERVE STIMULI. Besides the normal physiological impulse which comes from the cells in connection with the nerve fibres, a variety of stimuli may excite their active state. These nerve stimuli differ little from those which are found to affect muscle, when applied directly to that tissue. They may be enumerated as follows:

1. Mechanical Stimulation.—Almost any mechanical impulse, applied to any part of a nerve, causes its excitation. The stimulus must have a certain degree of intensity, and definite, though it may be of very short duration. If mechanical stimuli be frequently applied to a nerve in the same place, the irritability of the part is soon destroyed ; but if fresh parts of the nerves be stimulated, at each application the nerve passes into a state of tetanus, as shown by the contraction of the muscle to which it is supplied.

2. Chemical Stimulation.—Loss of water by the tissue of the nerve, whether this be caused by evaporation, or facilitated with

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blotting paper, exposure over sulphuric acid, or the addition of solutions of high density, such as syrup, glycerine, or strong salt solution. The application of strong metallic salts or acids; or alcohol and ether, also a solution of bile irritates nerves; weak alkalies, except ammonia, which has no effect on nerve, although it acts on muscle when applied directly to that tissue.

3. Thermic stimulation occurs when sudden changes are brought about, approaching either of the extreme temperatures at which the nerve can act; i. e., near 5° or 50° C.

4. Electric stimulation is by far the most important for physiologists, being the most easily applied and regulated, and the least injurious to the nerve tissue. As was mentioned with respect to muscle, any sufficiently rapid change of intensity in an electric current passing through a nerve causes the molecular changes we call excitation, as shown by the muscle contracting, and the natural electric currents of the nerve undergoing variation. The less the absolute intensity of the current, the greater the effect caused by any given change in intensity. The muscle of a nervemuscle preparation contracts, when a weak constant current, say from a single small Daniell cell, is suddenly allowed to pass through the nerve. This is done by placing a part of the nerve in the circuit, which is made complete, by closing a key, when the stimulation is to be applied. This form of stimulation is called a making shock. While the current is allowed to pass through the nerve, little or no effect is produced, if the battery be quite constant. On breaking the circuit, by opening the key, the current suddenly ceases, and another contraction occurs ; this is called the breaking shock. At each making and breaking of the constant current, a stimulus is applied to the nerve, and transmitted to the muscle, and it has been found that a weaker current suffices to bring about a contraction when applied to the nerve, than when applied directly to the muscle.

If a strong constant current be allowed to pass through a considerable length of a nerve for some little time, and the circuit be then suddenly broken, instead of a single contraction, tetanus of the muscle results. This breaking tetanus (Ritter's tetanus) is easily produced when the positive pole or anode is next the muscle.

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Sometimes, in particular conditions of the nerve, and with certain strengths of stimulation, a making tetanus also occurs, but more rarely and only when the negative pole is next the muscle.

When a constant current, such as we get directly from a Daniell cell, is used, that part of the nerve between the stimulating points, through which the current passes, is found not to be equally affected throughout its entire length, but one single point is stimulated whence the impulse spreads. This point may be where either of the poles is in contact with the nerve ; and, further, the stimulus starts from a different pole, according as the circuit is made or broken. With a making shock the stimulation takes place at the negative pole or cathode, and with a breaking shock at the positive pole or anode. That is to say, the point where the current leaves the nerve is affected at the make, and the point where the current enters the nerve is affected at the break of the current.

It has been found that, other things being equal, the making shock is a more powerful stimulus than the breaking shock; i. e., a weak current will sooner cause a contraction when the circuit is made than when it is broken.

This fact, that the impulse starts from the anode in a breaking shock, is proved by means of the breaking tetanus just alluded to. It has been found that when the anode is next to the muscle the breaking tetanus is more marked and lasts longer than when the anode is further from the muscle than the cathode. When the cathode is nearer to the muscle than the anode, section of the nerve between these points during stimulation stops the contraction at once, and no breaking tetanus occurs, because the point from which the stimulus comes is cut off from the muscle. Intra-polar section has no effect if the anode be next the muscle, and the tetanus proceeds in a normal way, because the active pole remains in continuity with the muscle. That the stimulus occurs at the cathode in making a current, may be demonstrated by the fact that it takes a certain measurable time for the impulse to travel along the nerve. If the cathode be placed as

. far as possible from the muscle and the anode quite near it, the contraction after a breaking shock, when the stimulus starts from

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