« PreviousContinue »
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, i. 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 produce tetanus. A drawing and further description of the instrument will be found at pages 453, 454.
VELOCITY OF NERVE FORCE. It has already been stated that nerve fibres are capable of conducting impulses in either direction-from or to the nervous centres. The position and character of the terminal organs
determines the direction in which the nerve impulse usually produces results. In the ordinary peripheral nerves there are generally both kinds—efferent and afferent fibres, carrying impulses in different directions.
When we reflect that the passage of an impulse along a nerve is brought about by a molecular change in the axis cylinder, we are at once struck with the rapidity with which impressions are transmitted from one part of the body to another. This velocity is, however, only relatively great. When we compare it with the velocity of the electric current or of light, we at once see how much slower the rate of nerve impulse is, and that it may be compared with rates of motion commonly under our observation. To take every-day examples-viz., nine metres per second is about the rate at which a quick runner can accomplish his 100 yards; race horses can gallop about 15 metres a second for a mile or so; a mail train at full speed travels about 30 metres a second, and the velocity of nerve force has been estimated to be in cold-blooded animals 27 metres per second; and in man about 33 metres per second. So that the intercommunications between man's brain and the various parts of his body only travel about the same rate as an express train, and about twice as fast as the quickest horse can gallop.
Different methods may be employed for the measurement of the rate of transmission of nerve force. The simplest is, with a good myograph, such as described in Chap. xxv, p. 462, to make a muscle draw two curves, one over the other, in one of which the stimulation is applied to the nerve close to the muscle, and in the other as far as possible away from it. The difference in duration of the latent period in the two curves, shown by the tuningfork tracing, corresponds to the time taken by the impulse to travel along the part of the nerve between the two points of stimulation, the length of which can be directly measured ; and hence the velocity of the impulse estimated.
Utilizing the fact that the extent of the deflection of the needle of a galvanometer is in proportion to the duration of a current of known strength passing through it for a short time, an accurate measurement of the difference in time of remote
and near stimulation of a nerve may be made. By a special mechanism the time-measuring current is sent through the galvanometer at the same moment that the stimulating current goes through the nerve, and the instant the muscle begins to contract, it breaks the current passing through the galvanometer, so that this time-measuring current lasts only from the moment when the nerve is stimulated until the muscle begins to contract.
THE ELECTRIC CHANGE IN NERVE. Negative Variation.-—The natural current of a nerve, like that of muscle, undergoes a diminution at the moment the nerve is stimulated ; this is termed the negative variation. It occurs with any other form of stimulation as well as when an electric shock is used, so it is not dependent on an escape of the stimulating current. In the case of a single stimulation, the negative variation is so rapidly over-lasting only .0005 sec., that the inertia of the needle of the galvanometer prevents the change in the current being indicated. In tetanus, however, it makes a decided impression on the galvanometric needle. The strength of the negative variation depends on the condition of the nerve and the strength of the stimulus ; being stronger when the nerve is fresh and irritable and has a good natural current, and when a strong stimulus is applied.
The negative variation of the natural currents passes along the nerve from the point of stimulation in both directions, just as does the nerve impulse; and with a galvanometer the electric change may be traced from the nerve to the muscle. It has also been shown that the negative variation travels along the nerve at the same velocity as the impulse ; namely, about 27 metres per second. Further, this rate is said to be influenced in the same way by the passage of a constant current through the nerve (to be presently described) as is the impulse derived from stimulus. These points seem to lead to the belief that the nerve impulse and the negative variation are closely related. This peculiar electric change and its accompanying impulse pass along the nerves as a kind of wave of activity, the speed and duration of which we know to be 27 metres per sec. and .0005
of a sec. respectively; the length of the wave we therefore calculate to be about 18 millimetres.
ELECTROTONUS. If one of two wires leading to a galvanometer be applied to the centre, and the other to the end of a nerve, so as to indicate the natural current, and at the same time another part of the nerve be placed in the circuit of a constant current from a battery, when the circuit of the constant (now called polarizing) current is completed, a change is found to take place in the natural current. This is called electrotonus. Instead of the
Diagram to illustrate Electrotonus. N. N'. Portion of Nerve. G. G'. Galvanometers. D. Battery from which polarizing cur. rent can be sent into nerve by closing key K. The direction of the polarizing and
electrotonic currents is indicated by the arrows, and is seen to be the same. natural currents from the centre to the end of the nerve, a current is found to pass through the entire length of the nerve in the same direction as the polarizing current from the battery. This electrotonic current is not proportional to the strength of the natural currents, and is to be recognized when the latter are no longer to be found. It is stronger with a strong polarizing current, and is most marked in the immediate neighborhood of the poles, fading gradually away as one passes to the remoter parts of the nerve. The electrotonic state is not to be attributed to an escape of the constant polarizing current, because it decreases gradually with the waning of the physiological activity of the nerve, and ceases at the death of the nerve long before the tissue has lost its power of conducting electric currents. It has been shown that a ligature applied to the nerve so as to destroy its physiological continuity, but not its power of carrying electric currents, prevents the passage of the electrotonic current to the part of the nerve which is thus separated.
The condition of the portion of the nerve near the anode is found to differ somewhat from that near the cathode, and hence it is found convenient to speak of the region of the anode being in the anelectrotonic, and that of the cathode being in the catelectrotonic condition. A certain time appears to be required for the production of electrotonus ; in a current of less duration than .0015 of a second we are unable to detect the electrotonic state. The negative variation must, therefore, have passed away before the electrotonus has commenced.
IRRITABILITY OF NERVE FIBRES. The irritability of nerves varies according to certain conditions and circumstances. While uninjured in the body, the irritability of a nerve depends upon
1. A supply of blood sufficient to supply nutriment, and to carry off any injurious effete matters that may be produced by its molecular changes.
2. A suitable amount of rest. Prolonged activity causes fatigue and loss of irritability, no doubt from the same causes mentioned as bringing about fatigue in muscles. The chemical changes taking place in nerves have not yet, however, been made out with any degree of accuracy.
3. Uninjured connection with the nerve centres. When a spinal nerve is cut, the part connected with the periphery rapidly undergoes degenerative changes which seem to depend upon faulty nutrition, since they are accompanied by structural changesfatty degeneration. This appears to commence in a very short time after the section—often in about three to five days. The part of the nerve remaining in direct connection with the cord retains its irritability for a very much longer time.