Life in Motion/Lecture IV

by John Gray McKendrick

LECTURE IV
Action of a nerve—Rapidity of nerve-current—Nature of nerve-current—Analogy with electric current—Nerveless animals—Heat in muscle—Muscles liberate mechanical energy and heat—Chemical changes in muscle.

We have now seen that the living matter forming a muscle is irritable or excitable, that is to say, it responds or reacts to a stimulus. We have also learned that the muscle shows its response or reaction by a contraction. Lastly, we have found that the natural stimulus that sets the muscle into action is something that happens in a nerve. Let us to-day, in the first place, study more carefully than we have yet done what occurs in a nerve.

A nerve, like a muscle, is composed of living matter, and this living matter, like all living matter, is irritable; but it does not show its irritability in any way evident to our senses. Suppose I irritate a little bit of nerve, which I have every reason to think is still alive, it shows nothing. The electric shocks sent to it produce no evident result. We must not assume, however, because we see nothing following the irritation of a nerve, that nothing happens. There may be changes in the nerve for all we know to the contrary, and the fact that changes do occur in the nerve would be evident if the nerve had still been connected with a muscle, because then, as you now know, irritation of the nerve would have been followed by contraction of the muscle. We would have seen the muscle move, and that would have been a proof that something really occurred in the nerve at the point of irritation, and that something passed along the nerve to the muscle. But perhaps you think I am going a little too fast. You may say that it is possible that irritation of the nerve at one point causes an instantaneous change throughout all the nerve, and that nothing really passes along it. Now this is a question that we can only settle by experiment.

Suppose that we irritate a nerve close to where it enters a muscle, the muscle will not contract at the instant the stimulus is applied to the nerve. There is always a loss of time. We may suppose that this time may be divided into three portions. First, a period in which changes occur in the nerve; second, a period in which changes occur in the muscle (the latent period); and third, a period occupied by the contraction of the muscle. Now suppose that instead of irritating the nerve close to the muscle, we irritate it at a point farther off, say two inches from the muscle. If, then, the times of the latent period and of the contraction remain the same, and if something travels alons: the nerve from the distant irritated point, the muscle should contract a little later than when the nerve is irritated close to the muscle. Reasoning in this way, Professors von Helmholtz and du Bois Reymond, now a good many years ago, devised methods by which this experiment may be made.

Let us try an experiment or two to illustrate this method. One of the most ingenious and simplest instruments for the purpose is the spring myograph of Professor du Bois Reymond, which I now show you. It consists of a smoked-glass plate, which is driven in front of the recording marker of the myograph by the recoil of a steel spring C. Underneath the frame carrying the glass plate are two binding screws at F, to one of which is attached an arm of brass 1, which can so move horizon- Fig. 54.—Diagram showing arrangement of apparatus in the experiment of measuring the rapidity of the nerve-current. For description see text. tally as to establish metallic contact between the two binding screws marked 1, 2. By means of these screws the myograph is interposed in the circuit of a galvanic element and the primary coil I of an induction machine, and the brass arm is so placed as to connect both binding screws, thus completing the circuit. From underneath the frame carrying the smoked-glass plate there descends a small Fig. 55.—The spring myograph of du Bois Reymond. flange, which (when the glass plate, by releasing a catch not seen in the figure but close to C, is driven across by the spiral spring from left to right) pushes the brass arm aside, and thus breaks the circuit of the primary coil. When this occurs an opening shock is sent from the secondary coil II to a commutator, E, an instrument by which electric currents may be transmitted to the nerve, either to a point close to the muscle at A, or at a distance from it, B.

Now we have the apparatus arranged so as to send the shock to the nerve at a point close to the muscle A; the muscle contracts, and draws by means of the marker, on the smoked surface of the glass, the curve seen at A in the lower part of the diagram. This leaves the horizontal line (which would be drawn by the marker were the muscle at rest) at A. We shall, in the next place, arrange for another experiment, in which the nerve will be stimulated at a distance from the muscle, at the point B, in the upper part of the diagram. This we do by again pushing the smoked glass plate back to its first position, closing the primary circuit by the brass arm at the binding screws, and reversing the commutator so as to send the shock along the wires to B. Touch the spring; the plate again darts across, breaks the circuit, and the muscle again contracts, but this time it describes on the smoked surface the curve B, seen to the left of A, in the diagram. You observe this curve leaves the horizontal line at B, that is, a little later than when the nerve was stimulated close to the muscle (Fig. 54).

It follows, therefore, that the distance on the horizontal line from A to B represents the time occupied by the transmission of the nervous impulse from B to A of the nerve. We measure the rate at which the glass plate was travelling by bringing to bear on it a marker connected with one of the prongs of this tuning-fork, and we cause the fork to vibrate at the moment when the glass plate dashes past the markers (Fig. 55). The time waves thus accurately measure the rate of movement of the glass plate, and consequently the minute interval of time between A and B.

This experiment proves that when a nerve is irritated at any point some kind of change is then produced, and that a change is propagated with a certain velocity along the nerve. This something we call a current, for want of a better term; but it is not a current but something sent on from point to point. It travels slowly compared with the velocity of light or of electricity. In the nerves of the frog the velocity is about eighty-seven feet per second, and in higher animals of constant temperature, such as in man, it only reaches a speed not exceeding three hundred feet per second.

The real nature of the change in a nerve-fibre during the transmission of the "current" is unknown. A nerve is both a receiver and a conductor of impressions. It can be stimulated at any part of its course, and from the stimulated point something is propagated along the nerve. Many explanations have been offered, but none is satisfactory. Naturally one thinks of the passage of electricity along a conductor, but, as I have said, the current is incomparably slower than the passage of electricity even along a nerve. The appearance of a nerve-fibre with its axis-cylinder is not unlike a wire insulated by some substance like silk or gutta-percha. Wires which direct the electrical change are insulated for the purpose of preventing the electricity from passing from one wire to another. We have no evidence that the nervous change can pass from one nerve-fibre to another. We know also that when an electric current passes along one wire it may produce currents, so-called induction currents, in adjacent wires; but there is nothing analogous to this in nerves. We do not know of induced nerve-currents. Each nerve-fibre appears to conduct its own change or current.

The phenomenon is more like that of a rapid series of chemical changes passing quickly along a tract, as when a train of gunpowder slowly burns, or when a long thin band of cfun cotton, such as we have here, is seen to burn slowly from end to end. But the analogy is not complete. The train of gunpowder and the band of gun cotton disappear and leave nothing behind, but the nerve-fibre remains. It must be said that the evidence we possess of chemical changes in the nerve-fibre is very meagre, no doubt because of their comparative insignificance. Still, small as the change is, it is sufficient to set off the highly unstable material in a muscular fibre and to produce chemical changes attended by the liberation, as we have seen, of mechanical energy. The change in a nerve-fibre can also produce changes in other organs. If the nerve-fibre reaches the cells in the spinal cord it may set up changes in these which result in a trans- mission of nerve-currents or shocks along other fibres, as in the phenomena of reflex action. Again, if the fibre passes to the brain, it excites changes in nerve-cells connected in some way with consciousness, and we thus come to know of something which affected the fibre at its commencement. Thus when light falls on the eye it affects the nervous structure called the retina; the retina is connected with the brain by nerve-fibres which are affected by the changes occurring in the retina, and nerve-currents travel along these nerve-fibres to the brain. In the brain they set up changes in nerve-cells which result in the consciousness of light, that is to say, we have a sensation which we call light. In all these instances, the nature of the change in the nerve-fibre and the mode of its transmission are the same. The results are different because the fibres end in different kinds of terminal structures.

Thus an electric current travelling along a wire may do very different things according to the nature of the apparatus at the end of the wire. Here is a wire conducting a powerful current. At this point, we cause it to branch out so as to divide the current into a number of streams. You see here the current decomposing water, there magnetising soft iron, here again doing the mechanical work of turning a wheel. In like manner, there is contraction of a muscle if the nerve ends in a muscle, change in the calibre of a blood-vessel if the nerve ends in that structure, secretion from a gland if the nerve is connected either with the vessels or the cells of a gland, an electric discharge or shock if the nerve terminates in the electric organ of an electric fish, and a feeling or sensation if the nerve-fibre goes to a sentient brain.

But if nerves are of so much importance, you will naturally ask how motions are produced in animals that have no nerves. Many animals show not a trace of nervous structures and yet they move. Again, the hearts of some animals beat with great regularity and still no nerve-fibres exist in their tissue. Such nerveless structures respond to a stimulus. Give them a shock of electricity and they contract. There can be no doubt that in such cases the contraction of one part produces some kind of disturbance, it may be electrical, which is propagated to adjoining parts, and acts upon these as a stimulus. Thus a kind of wave of contraction passes through the structure. Something of the same kind has been observed in muscle; but to this we shall return when we come to the consideration of the electrical phenomena in muscle.

We have now studied muscle as a producer of what we may call mechanical energy. At this stage, I shall leave the order in the syllabus and take up a subject mentioned in connection with the fifth lecture, namely, the production of heat by a muscle. We detect heat usually by a thermometer; but the heat we must look for in a muscle is so small in quantity as to oblige us to use a more delicate method. It is well known that minute quantities of heat may be detected by the use of what are called thermal piles. To understand the principle involved in the working of a thermal pile, look at this simple experiment. I have here a number of strips of the metals iron and copper soldered together. The points at which the two metals are fixed together are called junctions. The apparatus is put into connection with a galvanometer, and you will observe the coils of the galvanometer contain only comparatively few coils of wire. Such a galvanometer is said to be of low or small resistance, and it is well adapted for Fig. 56.— Thermal junctions of iron, i, and copper, c. g, galvanometer; a a, junctions that may be pressed together. such an experiment as we are now about to make. Now if I heat one set of the iron and copper junctions by simply pressing them together at a a (Fig. 56) while the other set is kept cool, a current of electricity is generated which passes round the coils of the galvanometer and causes a deflection of the needle. A very small difference in the temperatures of the two sets of junctions is quite sufficient to produce a current.

This arrangement is made more sensitive by having a large number of thermo-electric junctions constituting a thermo-electric pile. I shall connect this pile with our galvanometer, and you see it is so sensitive that, if I hold my hand near it, the heat radiating from my hand at once causes a movement of the galvanometer needle and a corresponding movement of the spot of light on the scale. When I heat the other set of junctions, you observe the movement of Fig. 57—Small thermal pile. Observe the junctions of bismuth and antimony. The distance from a to b is one-fourth of an inch. the spot of light is in the opposite direction, because the current passes in the opposite direction through the coils of the galvanometer.

Let us now examine the muscle by a thermo-electric arrangement. I have two very small thermal piles, and I connect them together, so that if I heat one by bringing my hand near it, the spot of light from the galvanometer moves to the right, and if I heat the other it moves to the left. We now place this one, which causes a deflection to the right when it becomes hot, in connection with a muscle to which a weight is attached, and we place the nerve of the muscle over the two wires from the secondary coil of our induction Fig. 58.—Diagram showing the arrangement of the apparatus in the demonstration of the heat of muscular contraction, a, case with sides of thick plate glass; b b′, thermal piles; b, galvanometer element; d, key; e, primary and f secondary coil. The contents of the glass case are seen on a large scale in next figure. The galvanometer has a low resistance. machine. To keep off all radiant heat as much as possible, we shall enclose the whole apparatus in a square chamber, the walls and roof of which are made of plate glass one inch in thickness. Notice the position of the spot of light. I now open the key so as to tetanise the muscle, and you notice that at once the spot of light moves to the right, proving that the thermal pile with which the muscle was in contact has become hotter, or, in other words, that the muscle in the tetanic state has become hotter. Fig. 59.—a, thermal pile touching muscle; b, other thermal pile; w, weight keeping muscle on the stretch. It is not so easy to show that even a simple contraction produces heat, because such a movement of the gastrocnemius of a frog is associated only with a rise of temperature of from one-thousandth to onefive-thousandth of a degree centigrade. Tetanus of frog's muscle gives from fourteen to eighteen-thousandths of a degree centigrade. No doubt each contraction of the muscles of higher animals, such as those of man, produces or is associated with more heat, but still the amount for each individual contraction is not much. But by the accumulation of small amounts, a large amount is formed, and there can be no doubt that a large proportion of the heat of our bodies is derived from the muscles.

That muscles produce heat is consistent with our daily experience. The more actively we work our muscles the hotter we become. When we wish to become warm we run, or leap, or dance, and in doing so we exercise our muscles.

We must now take a more scientific view of this matter. Muscles liberate energy as mechanical energy and as heat. The mechanical energy does work by moving one part of the skeleton upon the other, or by lifting a weight, as when I lift this book. Heat is another mode of energy, and we have seen that it is also liberated. Now a steam-engine does the same kind of thing. It expends or produces mechanical energy, and it also becomes hot. But we know that energy never comes out of nothing. You cannot get it for nothing. You can only get it by the expenditure, or, if you put it in another form, by the disappearance, of another mode of energy. This is one of the greatest thoughts of modern times—this thought of the persistence of energy. We cannot create or destroy matter. We can only transform it. In like manner, we cannot create or destroy energy. We can only transform it. Our steam-engines, and gas-engines, and hot-air engines are all transformers of energy. None of them makes it; they receive it from something, and they pass it on in other forms.

Take a steam-engine. It works by the steam expanding by heat and moving the piston. The hot steam comes from the boiler containing the water. The water is heated by the fire of the furnace. In the fire combustion or burning is taking place. The fuel, consisting chiefly of matters rich in carbon, is burnt, that is to say, the carbon unites with the oxygen of the air to form carbonic acid gas; but in this chemical operation which we call burning or oxidation, heat is set free, and the heat is the energy that drives the piston. The piston moves and drives all kinds of machinery, that is to say, the heat that moves the piston, through the medium of the steam, is transformed partly into what the engineer would call mechanical energy. It is this mechanical energy that does work. All the energy, however, set free by oxidation from the fuel is not transformed into mechanical energy. A part, a large part, as much as eighty-eight per cent of it, is set free simply as heat, which, I need hardly say, is of no use to the engineer. The same kind of reasoning guides engineers in the construction of all kinds of engines, and they are always striving to get as much as possible of the energy of the fuel transformed into mechanical energy.

Now turn to our muscle. Is it also a transformer of energy? If it is capable of manifesting mechanical energy, as undoubtedly it is in doing the work of lifting a weight, and if it becomes hot, these two energies, mechanical and thermal, must come from somewhere. Living though it be, it can no more create energy than the metallic, dead steam-engine can do.

Can we show that the muscle is also the seat of chemical changes? If we can do so, we may find that there are operations going on in a muscle that are comparable to the combustions or burnings in the furnace of a steam-engine. Let us try. Here is a bit of fresh muscle. I test it with red litmus paper, a well-known test for alkalies, and you observe the paper becomes slightly blue. We find then that the reaction, as it is termed, of a quiet muscle is alkaline. The muscle is alkaline on account of certain alkaline salts of soda present in it. Let us now test a similar muscle that has been tetanised since the beginning of the lecture. You see the red litmus shows nothing. There is no blueness, as in the other case, and we conclude that the muscle is not now alkaline. But we now test it with a bit of blue litmus paper, which is the method employed by chemists to detect acids, and you see it is reddened. This reddening shows the presence of an acid substance, and careful chemical research has proved that the acidity is due to a kind of lactic acid, an acid closely allied to the acid that we find in sour milk, hence called lactic (from lac, milk) acid. Here is evidence then of one chemical change produced by or connected with activity of muscle.

We are all familiar with the fact that living things breathe, and that breathing is the taking in of oxygen and the giving out of carbonic acid gas. When we inhale air in inspiration, the air, which is a mixture of two gases, oxygen and nitrogen, passes into our lungs through passages or tubes that become narrower and narrower, until they end in little sacs or dilatations known as the air-cells of the lungs. On the walls of the air-cells are networks of minute blood-vessels in which the blood flows, and it is here that respiratory exchanges occur between the air and the gases that exist in the blood. Oxygen gas passes into the blood and carbonic acid gas passes out. It is not necessary to demonstrate to you that oxygen gas is necessary for breathing. We all know that this gas must be present in any atmosphere fit for breathing, and that if an animal is placed in an atmosphere containing no oxygen, or if it is placed in a vacuum, it very quickly dies. The best test for carbonic acid gas is lime-water, which becomes turbid when the gas is led through it or shaken up with it. The chemists have given me this jar of carbonic acid, and you see Fig. 60.—Breathing into lime-water. Faraday's method as shown in the Chemistry of a Candle. when I shake up some of this clear lime-water with it, how white and milky it becomes. We can readily, by this test, show you that carbonic acid is produced by breathing, and it is interesting at a Christmas lecture at this Institution to employ the simple method used thirty years ago by Faraday in his celebrated course on the chemistry of a candle. I have two bottles, one containing lime-water and the other common water, and, by the arrangement of the tubes, when I inspire I draw air through the water, and when I expire I blow air through the lime-water. You see the water remains clear but the lime-water becomes turbid in a few minutes. It becomes turbid by the carbonic acid in the breath combining with the lime in the lime-water so as to form carbonate of lime, which is not readily dissolved, and consequently gives the white appearance to the lime-water. Carbonic acid gas, then, comes from our lungs.

The air in the lungs, as I have said, receives the carbonic acid from the blood. Those who are unfamiliar with physiology can hardly conceive of the blood as containing a large amount of gas. Take a hundred cubic centimetres of blood: this quantity may contain about sixty cubic centimetres of gas, and perhaps two-thirds of this consists of carbonic acid, the other one-third being oxygen. Although it contains this large amount of gas, blood does not effervesce in the air, because the pressure of the air on its surface prevents it from escaping at ordinary temperatures, but if we allow blood to run into a vacuum, as I do when I allow it to pass into this large jar, you see how it effervesces.

The question that next arises is, Where does this carbonic acid come from? The blood, as you know, is sent out through the arteries by the force of the heart-beat; these arteries become smaller and smaller until they end in networks of fine vessels called capillaries, many of which are not wider than the one-two-thousandth of an inch; and from these capillaries the veins originate that carry the blood back to the heart. Capillaries exist in greater or less number in almost all the tissues, and it is by the blood circulating in these that the tissues are nourished. Under the pressure in these minute vessels, fluid matter oozes through their walls and bathes all the neighbouring tissues. This fluid holds in solution the matters needed for nourishing the tissues, and it also contains gases in solution. Thus the fluid is both a nutritive and respiratory medium; by it the tissues are nourished, and by it they breathe. Each little element of tissue needs oxygen, and it produces carbonic acid gas. The blood in passing through the tissues thus loses, to some extent, the one gas, oxygen, while it gains the other, carbonic acid. It is thus changed from arterial or bright scarlet blood to dark venous blood. Here axe two jars, one containing oxygen the other containing carbonic acid. I add a little blood to each and shake it up with the gas. See the magnificent scarlet of the one and the dark purple of the other. The blood thus made venous is carried back to the heart by the veins, and is then sent to the lungs. Here it gets rid of a good deal of its carbonic acid, and gains more oxygen, and it is thus reconverted into arterial blood, to be again distributed through the body.

Now the tissues in which the consumption of oxygen and production of carbonic acid go on with greatest rapidity are the muscular tissues, and the more mechanical energy a muscle expends in doing work, the more oxygen it needs and the more carbonic acid it produces. The venous blood flowing out of a muscle is always rich in carbonic acid. Here are two muscles, both in an atmosphere of oxygen. Each is in a little tube inverted over mercury. The one has been at rest. the other has been tetanised at intervals by an arrangement I need not at present describe. You observe that the mercury has risen in each case. I add a little lime-water to the tube containing the muscle that has been working hard, and you see how muddy it Fig. 61.— Muscle in tube of oxygen over mercury. a, platinum wire fused into end of tube and connected with small hook, from which a frog's limb is suspended; c, toe of limb ; b, trough containing mercury. A small amount of mercury is on the side of the tube between mercury in b and toe at c, so that induction shocks sent in by x and y readily tetanise the limb. The limb receives tetanising shocks at intervals of thirty seconds, and the experiment may go on for sixty or eighty minutes. at once becomes. The same experiment with the resting muscle does not show the same degree of muddiness, indicating that the resting muscle has not produced so much carbonic acid as the working muscle. Still it is interesting to observe that even the resting muscle breathes. It is a little living thing, taking in oxygen and giving out carbonic acid. When it is obliged to work hard it breathes faster, that is to say, it produces more carbonic acid, and uses up more oxygen. This is exactly what is consistent with experience. Active muscular exercise, as in running, causes an increased consumption of oxygen, and an increased elimination from the blood of carbonic acid.

I imagine some of you may be asking the question. Will a muscle contract only in an atmosphere containing oxygen? Let us appeal to experiment. In this jar, is a muscle still capable of contracting, and yet there is practically no air here, as it has been nearly all removed by an air-pump. In this other jar you see a muscle contracting in an atmosphere of nitrogen, and in this third jar a similar muscle contracting in an atmosphere of hydrogen. You observe this fourth one contracting even in a jar of carbonic acid. It is quite true a muscle will not live long in these circumstances, but even in an atmosphere containing no oxygen a muscle will go on producing carbonic acid. Now as carbonic acid is a compound made up of carbon and oxygen, it is evident that the muscle must have got oxygen from somewhere. The only thing we can say about this is that the muscle takes it from matters in its own substance that contain oxygen.

We have learned, then, that an active muscle becomes acid, that it uses up oxygen, and that it produces carbonic acid. Other chemical changes happen in a muscle that I will not attempt to demonstrate, as the methods by which this can be done require time and many refined appliances not suitable for the lecture-room. I shall merely mention them. Thus a peculiar kind of starch (glycogen), formed in the liver, is carried to the muscles by the blood, and is there consumed. We do not know, however, how the muscle uses the glycogen, whether it uses it directly or whether it first splits it up and then uses some product of its decomposition. So long ago as 1845, von Helmholtz pointed out that by exercise the substances that can be dissolved out of muscle by water are diminished, while those soluble in alcohol are increased, indicating that the one set of substances was used up, while the other set was probably produced by muscular activity. It is also well known that very complex bodies containing nitrogen are found in extracts of muscle, and it is highly probable that these are in a way waste products—substances that have resulted from the breaking down of the matter of the muscle. These facts that I have laid before you all point one way. They all tell of chemical changes in muscle, and they all support the statement that the harder the muscle has to work, the greater is the activity of the chemical phenomena happening in it. Let us put this in more correct scientific language. The setting free of energy by the muscle,—as mechanical energy when it moves, and as heat when it becomes warm,—is associated with, and is likely the result of, the chemical processes happening in it.