|Muscles and Nerves
Intensity of Diathesis in Living Substance
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Though physiologists have already discovered a vast amount about this path, I do not propose to go into much detail. I have noticed that in science, problems remain unsolved more often because irrelevant facts have attracted too much attention than because relevant ones have remained undiscovered. The irrelevant facts lead to irrelevant questions and in the search for answers to these, the original problem tends to be overlooked. For no problem is this more true than for the Problem of Interaction where, as I have said already, there is such a deplorable tendency to hope that a solution will pop out of some future discovery while scientists are thinking about something else, and where it is only too often forgotten that one will learn little if one does not even trouble to think out what one wants to know. So let us keep reminding ourselves that the important thing to do in this field is to find the relevant questions and the most precise way of formulating them. It will soon appear that a few meagre facts about the path for the diathesis in living substance lead to some very puzzling questions. If further known facts can provide answers, these facts should be considered after the questions have been asked and not before.
In immediate control of the small switches in the crane cabin are the muscles of the craneman's right hand. These muscles are the operating elements of the relay of which the small switches are the controlled element. Each muscle consists of a number of muscle fibres. The energy to work them does not concern us. We know that plenty is available in the surrounding tissues, and that fact may suffice. The detailed sequence of the transformation that the energy undergoes before it appears as the mechanical energy of muscular contraction would make a long story anyhow.
In control of each muscle fibre is a minute device called an endplate. This is the termination of a nerve fibril on the muscle fibre. Small though it is, the endplate is quite a complicated piece of mechanism. It liberates a chemical substance called acetylcholine, and it is thought that this undergoes a chemical change (one authority says that it detonates) in close proximity to the muscle fibre when this is required to contract. Thus the combination of endplate and muscle fibre forms a relay. The energy in the acetylcholine is the operating energy for this relay and the energy expended by the muscle fibre is the controlled energy. I am told that the operating energy is much less than the controlled energy, so the principle of tapering energy requirement is observed here.
The detonation of the acetylcholine is, in turn, controlled by an impulse that arrives through the nerve fibril, which is also called a neuraxon. The mechanism of detonation is not fully understood and is not relevant here. The detonating impulse may be an electric current, for such current does flow in the neuraxon. But whatever the impulse is, it arrives at the moment when the muscle fibre is required to contract, and it brings with it the operating energy for the detonation of the acetylcholine. Thus the energy in this chemical substance is the controlled energy of the relay constituted by the combination of neuraxon and endplate.
The neuraxon is a part of a nerve cell called a lower motor neuron. The main body of this cell is in the spinal chord, and this neuraxon is there in close contact with another neuraxon. This latter forms part of another nerve cell called an upper motor neuron, the main body of which is in the part of the brain known as the motor cortex. The place where the two neuraxons are in contact is called a synapse.
There are at least two good reasons for the supposition that a synapse conforms to our definition of a relay. The first is that an impulse can travel through it in one direction only, and irreversibility has already been mentioned as a characteristic feature of a relay. The second reason is that at a synapse, an impulse may be delayed; and it is another characteristic of a relay that it can be designed to delay the passage of diathesis through it. Many relays used in engineering are so designed. So it is difficult to imagine how any device that does not conform to our definition of a relay could behave as a synapse does. It is therefore reasonable to assume (and I think that most physiologists do assume) that the operating energy of a synapse is distinct from its controlled energy. Both are, of course, very small, and there is no difficulty in accounting for the necessary supply from the surrounding tissues.
Whether there is any taper of energy requirement in a synapse does not seem to be known. It is quite possible that the operating energy that reaches the synapse through the neuraxon of the upper motor neuron is of the same order of magnitude as the controlled energy that is supplied through the neuraxon of the lower motor neuron to the endplate. Whether it is so or not is not relevant to our present investigation.
There is a great number of synapses in the motor cortex of the brain and they are interconnected by neuraxons in a most complicated manner, but I do not propose to discuss these complications here. If I did, I should not clarify the Problem of Control, but only obscure it. The complicated interconnections of synapses, the branches and loops along which they lie, the spurs that join them here and there to the efferent nervous systems, may all have to be studied some day by those who would solve the Problem of Control, but they must not provide an excuse for dodging the problem. Suffice it that many synapses are in series — or, as I would say, in cascade — along the path of an impulse to a muscle fibre. Whether or not there be taper of energy requirements between these, the energy taken by the operating element of a synapse in the cortex must be very minute, for a synapse is only a part of a nerve cell and the number of nerve cells in the cortex is prodigious. The operating element is again only a part of a synapse and the energy required for its operation must be minute indeed. This should be remembered by those who naively suppose that our problem is to find the source of the energy that works a path for diathesis.
Energy is needed, as for every physical process, but there is no doubt as to its source. The tissues of the brain contain an ample store of energy in chemical form.
Merely to clasp the handle of one of his switches, the craneman must do many things. Just one of them is to crook a finger. This single performance calls many muscles into play. There are muscles to raise the finger, muscles to curve it, muscles to restrain and steady the movement at each of its stages, muscles to prevent other parts of the hand from making unwanted movements. Each of these muscles must, at every moment, exert a nicely graded effort. During the process of crooking a finger, any given muscle must pull a little harder at one moment and relax a little at the next.
This nice grading of effort calls for a stupendous amount of co-ordination. The muscle consists of a bundle of fibres and each of these works on the "all or nothing" principle: it exerts either its full effort, or none. When a muscle pulls more strongly, it is because a larger number of its component fibres is contracting; and when it pulls less strongly, the number of contracting fibres is smaller. So the simple act of crooking a finger requires that a vast number of distinct muscle fibres in a vast number of muscles shall each perform its function of contracting in the correct sequence with accurate timing and with perfect co-ordination. Here an "element of drill" really does enter into the system.
It is the same for every other part of the crane-man's body while he is clasping a switch handle. It is the same for the muscles of his remaining fingers, for the muscles in the palm of his hand, for those in his arm, for those of his foot as he steps forward to reach a switch. If the path for the diathesis that lies between the crane cabin and the motors consists of some half dozen parallel channels, the path that lies between the craneman's brain and his hand consists of millions. The imagination recoils before the complexity of the drills performed by the operating elements of all these synapses. What the audience can see when a juggler is performing with a hat, a walking stick and an egg is a crude operation compared with what is happening out of sight inside the juggler's body. The process of muscular contraction is clearly not one about which we may be content to think along anthropomorphic lines.