Principles of General Physiology

As already pointed out in Chapter III., it is not necessary to suppose that this membrane is in the form of a distinct film, or separate phase, which could be picked off. Being formed by condensation of constituents present in either or both of the two phases, cell protoplasm and surrounding liquid, of which it is the contact surface, it may be looked upon as belonging, in a certain sense, to both. We may notice that Mines (1912, p. 230) comes to similar conclusions in explanation of the results of his work on the effect of ions on the electrical charge of surfaces.

Our consideration of Nernst's theory of excitation may be best concluded with the words used by Keith Lucas (1912, p. 524): "It is not a complete theory, ready for acceptance, but it is an indispensable guide to the strengthening of our experimental data, and so to the ultimate elaboration of a hypothesis, which shall be free from those difficulties which are at present so obvious." The nature of the local excitatory process is especially in need of further investigation. It seems, however, from what has already been done, that the final solution will be on the lines of that proposed by Nernst.

Structure of Nerves. — The fact is familiar that some nerve fibres are encased in a sheath of considerable thickness. This consists chiefly of lecithin and similar lipoids, containing the radicles of unsaturated fatty acids and, therefore, staining with osmic acid. The function of this " medullary sheath " is problematical. It is obviously not necessary as an insulator, since many nerve fibres are devoid of it, and we can see, by taking such a case as that of the superior cervical ganglion, that the non-medullated fibres are isolated from each other. Aledullated fibres leave the spinal cord and proceed to this ganglion, where they form junctions, "synapses," with another set of neurones, whose fibres are not medullated. In this ganglion, therefore, non-medullated fibres of various functions are mixed together, but isolated functionally. These various kinds of fibres are vasoconstrictors, dilators for the iris and nerves to the different secretory glands.

Although the medullary sheath is formed by cells independent of the neurone itself, it must have an intimate connection with it, since, on cutting the axi-. cylinder free from the nucleated cell body to which it belongs, the medullary sheath undergoes degeneration along with the axis cylinder. When nerve fibres regenerate by growing out again from the cells, the remains of the old sheath^ seem to act as guides for the new fibres, which grow down into them.

It has been suggested that this myclin sheath may serve as a source of nutrition to the fibre. It is very doubtful, as we have seen, whether there is any metabolism in the nerve fibre to require food. After treatment with fixing reagents, the contents of the axis cylinders appear as a number of ^laments, " nntro-fibrils," as they have been called. There is no actual proof of their presence in the living state, and they are, in all probability, produced by the action of the reagents used. Mott (1912) finds no indication of their presence in the living nerve cell.

On the contrary, Carlson (1911) has brought forward evidence to show that the axis cylinder has the properties of a liquid, just as protoplasm in general. Certain animals, such as the slug^ exhibit great changes in their length ; the nerve fibres must be stretched in the long form of the slug, since they are not folded up in the shortened condition of the animal. Now, Carlson finds that, if the pedal nerves are excited close to the pedal ganglia with the animal artificially stretched and again when unstretched, the time which elapses before the foot muscle contracts is greater in the former case. It is important to note that the degree of stretching was not such as to affect the excitability of the nerve, which was tested by using submaximal stimuli, and the same height of contraction found in both cases. If the nerve is stretched too much the muscle enters into contraction, and also the extent of the stretching was about that of the normal crawling movements. What are we to conclude from the result obtained ? It is evident that the nerve itself was actually stretched and not merely uncoiled, since the latter would have had no influence on the time of conduction. It is difficult to understand how any substance but one having the characters of a liquid could be increased in length without showing any result beyond an increase in time of conduction proportional to the increase in length. Carlson, in fact, shows that the rate of conduction is unaltered. Since increase in length implies decrease in diameter, the result indicates that the rate of conduction is independent of the sectional area of the axis cylinder.

<ii>thlin (1913) finds that both the axis cylinder and the medullary sheath are doubly refracting. That of the former is of the kind shown by fibres consisting chiefly of protein, such as muscle fibres and connective tissue, but comparatively slight. That of the latter is similar to that of liquid crystals of the glycero-phosphatides. It was also shown by polarisation methods that the apparently non-medullated fibres of many invertebrates have a sheath of glycero-phosphatides and that the Crustacea show a gradually increasing development of medullary sheath from the lower to the higher members of the class.

The Mature of the Nerre Impulse. — It may be useful to collect together the evidence obtained so far on this question. That it is a reversible, physico-chemical process, not associated with loss of material on account of metabolic reactions, is indicated by the following fa On the other hand, the existence of fatigue in the absence of oxygen points to a minimal consumption of material for energy purposes, although it seems to me that the evidence on this question is not so decisive as it should be, and that further investigation is necessary before it can be interpreted in the sense mentioned. / A distinction must be made, as we saw, between the local process at the spot excited, a process which is not propagated, and the propagated disturbance set up when the former exceeds a certain magnitude. /A stimulus must, therefore, possess a certain minimal amount of energy in order to excite, and it seems that this is required to effect the local change. No further supply of energy appears to be necessary in the progress of the wave along the nerve fibre. In the natural connection of the nerves with their cell bodies, the energy required to start the initial process is, no doubt, supplied by the cell body, in which oxidation processes of a recognisable degree are known to occur.

The conditions in the nerve fibre which are apparently concerned in the process are the polarisable membranes, of colloidal structure, and electrolytes, present in the complex, liquid, colloidal system of the axis cylinder. Although the evidence seems to preponderate on the side of the physicochemical theory, it must not be forgotten that certain phenomena are not easy to explain on this view. Keith Lucas has pointed out that the propagation of disturbances along wires or similar channels may be of two kinds : (1) Like that of sound waves, or the passage of an electric potential difference along a condenser system, such as a submarine cable ; this is purely physical, and does not involve production of energy as it travels. (2) A chemical system, such as a train of gunpowder. In this case, energy is evolved as the disturbance is transferred from one point to another, and products of change are given off. Now, Adrian's results, showing that a disturbance recovers its magnitude in a normal region, after having been reduced in a narcotised one, suggest a process more like the second one. If a sound wave, for example, is reduced in magnitude by passing thi-ough a region in which sound is conducted badly, say cotton wool, the energy of its vibration is diminished, and there is nothing to increase it again when it returns to a medium conducting well. On the other hand, if the train of gunpowder be made very narrow at one part, so that the energy of the disturbance is much less as it traverses this part, the original energy is regained when the dimensions of the train become similar to what they were original!}'. As long as the disturbance passes through the narrowed part at all, the original magnitude is regained in the normal part. Of course, the process in nerve cannot be regarded as being so simple as this ; there are evidently physico-chemical processes connected with the movement of ions and the presence of semi-permeable membranes, and we are at present in the dark as to the way a reaction associated with the giving off of chemical energy conies into relation with the former.

It is evident, however, that if, in excitation, the membrane ceases to be semipermeable, so that the internal electrolytes diffuse out, some supply of energy may be necessary in order to restore the original state of the system. The chief function of the tissue known as muscular is that of producing movements by shortening or change of tension. This aspect of its activity will be considered in the next chapter. Regarded as excitable tissues, muscles show very much the same characteristics as nerves. They can be set into activity by the direct application of a stimulus, which sets up a wave of excitation, which travels at a slower rate than that in nerve. -

The Refractory Period is longer than in nerve, and can be particularly well seen in the muscle of the heart. Muscle shows the " all or none " phenomenon, as shown especially by the work of Keith Lucas (1909). In the state of excitation there is evidence of increased permeability of the muscle cell, evidence of a more direct nature than in the case of nerve. The observations of Lillie (1911) on the larva of Arenicola have been already referred to (pages 138 and 139). Some experiments in which substances such as bile suits. saponin, and sodium oleate, which are known to make the cell membrane permeable, were found to cause quick, vigorous twitches of frog's muscle, are reported in the same paper.

The same fact is shown by the increased electrical conductivity of striated muscle in a state of excitation, as in the experiments of M'Clendon (1912, 2). This means that the membrane Incomes permeable to ions to which it was impermeable while unexcited. The state of polarisation, in other words, ceases to exist. In general, the remarks made above with respect to the similar change in nerve apply also to muscle. A further fact which indicates increased permeability is that found by Siebeck (1913). Potassium chloride enters more rapidly into excited muscles than into resting ones.

If the change of semipermeability into permeability is essential to the act of excitation, it will readily be seen that, while this state lasts, there will be a "refractory period." There is evidence also that the duration of the electrical change coincides very closely with that of the refractory state, as would be expected to be the case if this electrical change were due to the disappearance of the state of impermeability to the ions of one sign of charge, with the consequent depolarisation at the membrane.

In muscle, however, we find an additional factor, that of contraction, by which energy is given out. Along with this, phenomena are shown by muscle which nerve does not show. These will be treated of more fully in the next chapter ; but there are four properties of muscle which are connected with this factor that should be mentioned here. They are latent period, metabolism, heat production, and fatigue. Latent Period.— The state of excitation indicated by the electrical change, commences at such a short interval after the application of a stimulus, that it is difficult to be certain that they are not simultaneous. There is, on the contrary, an interval which can easily be measured before the state of contraction begins. If the electrical change were unknown, it would appear that nothing was taking place in the latent period before contraction.

It appears, then, that there is, in muscle, an extra mechanism superadded to the simple excitation process, namely, that giving rise to the contractile effect. There is direct evidence that the propagated disturbance, with its electrical change, can continue in muscle which has been treated in such a manner as to show no trace of contraction. HartPs experiments are the most convincing. If a part of a muscle be immersed in distilled water, it will be found to be incapable of contraction when stimulated, although this waterlogged part will still conduct a disturbance to the normal part. Noyons (1908 and 1910) has shown that certain drugs will abolish the beats of the heart of the frog and tortoise, while leaving the electrical change still strong* And Mines (1912, 2) has shown that skeletal muscles of the ray, treated with a dilute solution of ether, completely lose their power of contractile response to strong electrical stimuli, while retaining that to acid, alkali, or potassium salts. Presumably, the loss to electric stimuli was due to failure of conduction of a propagated disturbance. In the same paper, Mines refers to his observations that the conduction of the excitation process in heart muscle is arrested by trivalent cations, whereas the contractile process is not so affected (see Fig. 172 below).

Owing to the fact that this contractile process is one attended with the performance of external work, we find phenomena not present in nerve — consumption of oxygen, giving off carbon dioxide, production of heat, fatigue, and so on, all of which belong to the subject of the next chapter. In order, however, to throw light on the process of inhibition, it is necessary to make use of the states of contraction and relaxation as indications of what has happened.

We have already seen that there is a distinction between striated, skeletal, " voluntary " muscles, with their rapid contraction, and the smooth, "involuntary" muscles of the viscera and blood vessels, with their slow rate of contraction. The latter are, in their natural, unstimulated condition, in a state of partial contraction, so that two sets of nerves are required, one set to increase the activity, which may therefore be called "excitatory," the other set to decrease it, "inhibitory" nerves. The voluntary, skeletal muscles are, if unstimulated, completely at rest. They are supplied with one set of nerves only, those causing excitation, the other being needless. If continued tonic contraction is required, it must be kept up by continued innervation from the nerve centres ; so that, to inhibit this state of contraction, influences must be brought to bear on the nerve centres themselves to stop their activity. It is unnecessary to remark that the excitatory and inhibitory centres of the smooth muscles are also liable to similar exciting and inhibiting influences. There are, then, at least two kinds of inhibition, one exercised on muscle itself directly, the other on nerve cells, when these are in a state of activity. Again, the inhibitory nerves of smooth muscle arise from centres in the nervous system, and these centres, if in a state of activity, can be inhibited by the play upon them of nerve impulses from other sources. We have thus, in Sherrington's phrase, an "inhibition of inhibition," that is, a central inhibition of nerve activity which was producing inhibition in peripheral organs. We shall find evidence of the actual occurrence of this phenomenon in the case of vasomotor reflexes.

The two excitable substances already discussed, muscle and nerve, are not the only members of the class. It might be supposed that a nerve acting on a muscle merely formed some kind of direct connection therewith, but a simple experiment shows that there is something between them, itself an excitable substance. Let us take two nerve-muscle preparations and arrange the nerves of both on similar electrodes in series in the same circuit, so that they can be excited with the same strength of stimulus. On the one nerve, between the seat of excitation and the muscle, we place a pair of non-polarisable electrodes, through which we send a galvanic current of sufficient strength to block the nerve impulses on this side. Both nerves are thus excited, one muscle only. After a time, the muscle becomes fatigued and ceases to respond. At this moment, the galvanic current causing the block is cut off ; the muscle on this side goes into tetanus. In this way, it is seen that the fatigue was not localised in the nerve trunk. The next step is to apply electrodes directly to the muscle which had ceased contracting ; it is found to be able to respond vigorously. So that the seat of the fatigue is not in the actual contractile substance of the muscle. The unavoidable conclusion is that there is some intermediate substance, more easily fatigued than either nerve or muscle.

The action of the arrow poison, curare, affords similar evidence. If the nerve only is immersed in a solution of this drug, it is not paralysed. If the muscle is immersed, excitation of the nerve has no effect upon it ; but it is not because the muscle itself is paralysed, since placing the electrodes on it produces contraction. Under the microscope, there is to be seen, where the nerve enters the muscle fibre, or rather comes into connection with it, what appears to be a specialised structure, the " end-plate " ; but that this is not the substance for which we are seeking is clearly shown in several ways. Adrenaline, as we shall see in more detail in Chapter XXIV., is a secretory product of the suprarenal bodies and has the property of exciting organs supplied by sympathetic nerves, and in precisely the same way as excitation of these nerves themselves. When, therefore, it is applied to arteries innervated by vaso-constrictor nerves from the sympathetic, these arteries contract. On the other hand, if applied to arteries not supplied by sympathetic vaso-constrictor nerves, no contraction results. It does not, accordingly, produce its effect by direct action on the muscle cells. If the nerves are cut and allowed to degenerate, the constrictor effect of adrenaline is undiminished. Now there is every reason to believe that the visible nerve endings in muscle degenerate with the nerve fibre.

Langley (1906, p. 179) finds that the nerve endings on the sartorius muscle of the frog disappear in six weeks after section of the nerve to the muscle. There is, moreover, no histological evidence of any difference between the fibres in the trunk of the nerve and their endings on the muscle. It is evident that the intermediate substance, on which adrenaline acts, lies on the muscle side of the place of entry of the nerve fibre. Elliott speaks of it as the "myo-neural" junction (1905, p. 43<j).

Further light is thrown on the question by Langley's work on the antagonism between nicotine and curare (1906). Nicotine, in fairly large doses, acts like curare in preventing excitation of a motor nerve from reaching the contractile substance of the muscle. In the fowl, 10 to 15 mg. suffices. The first effect of the injection is to cause contraction of the muscles ; but the remarkable thing is, that, after a dose which paralyses the nerve action, direct application of the drug to tinmuscle itself still causes tonic contraction. Further, this effect is abolished by curare. There is, in fact, a quantitative antagonism between the two substain e& If nicotine be given after a dose of curare sufficient to paralyse the effect of nerve stimulation, a tonic contraction is caused. Repeated doses of nicotine finally paralyse the structures at first excited by it, although the muscle is still excitable to electrical stimulation ; this is a further proof of some intermediate substance. As we have seen, curare acts on something on the muscle side of the nerve ending and nicotine must also act on the same substance. This constituent of the neuromuscular system, which is not the contractile substance of the muscle nor the excitable substance of the nerve, is called by Langley the "receptive substance." It receives the stimulus from the nerve and transmits it to the contractile mechanism of the muscle.

We may now consider the evidence brought by Keith Lucas from a different point of view. In making experiments on the excitation of muscles with condenser discharges to find the constant called by Waller the "characteristic," Lucas (1906, 1) found that there were two distinct optimal stimuli, in one of which the rate of incidence is represented by 37 to 63 and in another of which it is represented by 1,780 to 19,300. After moderate doses of curare, these are both left present, although that with the higher optimal rate of incidence of energy shows signs of abolition, which is complete with large doses. It appears that we have to do with something analogous to Langley's receptive substance or Elliott's myo-neural junction.

In further investigation, Lucas (1906, 2) found that the end of the sartorius muscle which is free from nerves shows only one optimum, represented by 20 to 36. The trunk of the sciatic nerve also has an optimal rate represented by 41 to 233 only. Muscle fibre, free from nerve endings, has, therefore, an excitable substance (a) of low optimal rate. The nerve trunk has one (y) of slightly higher value. In the middle of the sartorius there are at least two, detectable by the use of the condenser ; there is the muscle itself as above (a) and another (£$) of an extremely high optimal stimulus, on the muscle side of the curare block. In later work (1906, 3 and 1907, 1) it was found better to use currents of varying strengths and durations in place of the condenser discharges, and curves were drawn correlating the current strength just sufficient to excite with the current duration. In this way, the three substances above mentioned were found in the middle region of the sartorius. The current strength used was, in all experiments, such that its necessary duration never exceeded 0-02 second.

It was pointed out above (page 394) that the logarithm of the constant 6 of Hill's modified Nernst formula of excitation is a function of - ,, which is itself a measure of the rate at which the diffusion of the ions concerned in excitation takes place. It is natural to suppose that the rate of incidence of energy in the optimal stimulus will be related to this factor, and Keith Lucas calculates (1910, p. 245) the values of log 6 for various excitable substances as follows : —

These are arranged in order of rate of diffusion of ions. Compare these with the values of the current durations at which the current strength reaches its smallest values, that is, the optimal rate of incidence of stimulus : we have : — Some interesting conclusions are drawn by Keith Lucas from these figures. If the ions concerned in the excitatory process were the simple inorganic ones, K.% Ca", Cl', etc., the variations in rate could not exceed 10 to 1, whereas between substance /3 and ventricular muscle there is a ratio of 4,000 to 1. The temperature coefficient is also higher than that for a simple ionic velocity. Similarly, when the calcium of the Ringer's solution is replaced by an isotonic amount of sodium, the value representing the rate of movement of the ions in excitation (k) decreases ten times, whereas the ratio of the velocities of Na- to Ca" is as 44 to 53 only. There is evidently some factor not as yet completely accounted for, and the question requires further investigation.

Lapicque, who has worked out the theory of electrical excitation and come to certain conclusions similar to those of Lucas, but without arriving at a mathematical form of his hypothesis, has made a hydrodynamic model (1909) on which many of the facts can be demonstrated ; the movement of ions is imitated by that of water. Lapicque has introduced a constant which he calls " chronaxie," relating to the rate of movement of the ions concerned in excitation, which constant is of similar significance to Waller's " characteristic " and the logarithm of the constant 6 of Hill's modified Nernst formula. Lapicque and Legendre (1913) find that there is a relationship between this rate of movement and the diameter of the various nerve fibres in the same animal. The larger the fibre, the faster the movement. Thus the clironaxie, which is the reciprocal of the rate, is 0-0003 second in the case of the motor nerve to the gastrocnemius, whose fibres have a diameter of 0-02 mm. and 0-02 second in the motor fibres to the stomach, with a diameter of 0-002 mm. It might, perhaps, be expected that the rate of movement would be greater in a large fibre, if the ions in the middle have to reach the membrane on the outside of the axis cylinder in the same time as those in a small fibre. But this is purely hypothetical. W. W. Waller (1914) finds that the optimal time value (Lapicque's " chronaxie ") for excitation of sudo-motor nerves is ten times that for motor nerves to muscle.

Mode of Connection between Nerve and Muscle. — The balance of evidence is decidedly in favour of the view that there is merely close contact at the end-plate of the nerve ; there is, apparently, no continuity of cell substance, but a membrane is interposed. When a nerve fibre degenerates, the process does not proceed beyond the end-plate. There is, no doubt, in excitation, a change at this " synaptic " membrane, by which the transfer takes place. The receptive substance of Langley must be supposed to lie on the muscle side of the membrane and be confined to the region in the neighbourhood of the connection with the nerve fibre. It is not necessary to regard this substance as a constituent radicle of the protoplasmic " molecule " of the muscle cell.

As we shall see later, a similar problem arises as to the transmission of excitation from the branches of one nerve cell, or neurone, to the cell body of another. We shall find that Sherrington's conception of a "synaptic membrane " is most in accordance with experimental facts. The name is applied to any process by which an action in progress is stopped by the application of some influence from without. It is not used when a process, such as a nerve impulse or a muscular Contraction, excited by a momentary stimulus, runs a definite time course and then ceases spontaneously.

The result itself can evidently be brought about in different ways, according to the particular mechanisms involved ; so that it does not seem correct to speak of a general theory of inhibition. Consider a smooth muscle cell in a state of natural tonus : excitation of a certain nerve fibre, connected to this cell, puts an end to the contractile process and the muscle cell relaxes. A nerve cell may be in a state of excitation due to changes within itself, perhaps brought about by chemical influence outside, but the effect upon it of stimulation of a certain nerve fibre may be to quell the state of excitation or to make it inaccessible to the action of the chemical stimulant. Suppose, however, that the state of excitation is brought about by the reception of continuous stimuli brought in by nerve fibres making synaptic junctions with the cell in question. It is clmr that the excitatory state of the cell could be abolished by making the synaptic

FIG. 109. PLKXCS OF NKRVES AROUND A SMALL ARTERY OF THE SVI.KKN. Oolgi's method. a, Artery, surrounded hy branched nerve fibres, which end freely. y, Nerve from which the plexus arises. r, Pulp substance. The broad whit* space around the artery is the Malpi^hian substance. In places nerve fibres are seen passing to the pulp. membrane, of which we spoke above, impermeable to the excitatory process arriving by the nerve fibres ; in other words, a block might be produced. In the case of the nerve cell, therefore, at least two different modes of inhibition are possible : the influences exciting it may be cut off, or the cell itself made incapable of responding to them, although they may be duly received. As another instance, we might take the ventricular muscle of the heart. It is conceivable that its natural beats might be stopped in several ways. The muscle itself might be made temporarily inexcitable, the rhythmic stimuli coming from the auricle might be prevented from reaching the ventricle by the production of a block on their course, or the contractile mechanism alone might be put out of gear, leaving the excitatory process intact, and so on.