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.
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: -
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.
Loss of water by the tissue of the nerve, whether this be caused by evaporation, or facilitated with 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.
Thermic Stimulation occurs when sudden changes are brought about, approaching either of the extreme temperatures at which the nerve can act; i. e., near 50 or 500 C.
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 nerve-muscle 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.
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 the anode, will occur sooner than that which follows the making shock, when the stimulus starts from the cathode, because the impulse has a less distance of nerve to traverse in the former case.
In most experiments on nerve, a constant current, i. e., one coming directly from a battery, is seldom used, because there is no ready means of regulating or varying the strength of the stimulation. The instantaneous current induced in one coil of wire - the secondary coil - by the making or breaking of a current passing through another coil - the primary coil - is more effective and suitable for physiological purposes. It must be remembered, however, that the induced current is both a rise and fall of electric current, 1. e., a make and break; but the duration of the two changes is so small (circa,. 00004") that they only act as a single stimulus. As there is no current in the secondary coil while a constant current is kept passing through the primary, of course the induced current cannot be used for experiments relating to the making and breaking shocks. The strength of the induced current being approximately in inverse proportion to the square of the distance between the two coils - moving the secondary away from the primary coil gives a ready means of varying and regulating the strength of the stimulus, without any special care being devoted to the exact strength of the element used.
Du Bois-Reymond's Inductorium is the instrument commonly used in physiological laboratories. In it the secondary coil can be moved away from the primary on a graduated slide, and the primary current may be made to pass through a magnetic interrupter so as to cause a rapid succession of breaks and makes, and thus give a series of stimulations, one after another, which is necessary to produce tetanus. A drawing and further description of the instrument will be found at pages 453, 454.