Читать книгу Philosophical Foundations of Neuroscience - P. M. S. Hacker - Страница 23

The spinal cord can operateindependently of the enkephalon

It had been noted since time immemorial that cutting off the head of a snake did not stop its movements in response to a touch for some days. However, a thorough study of the ability of the spinal cord to mediate the contraction of muscle and movement in the absence of the enkephalon was not made until the investigations of Alexander Stuart (1673–1742). In his Croonian Lecture to the Royal Society of London in 1739, Stuart described experiments in which he first cut off the head of a frog and then used a blunt instrument to bring pressure on the medulla, resulting in the movement of the limbs. From these experiments he concluded that compression had forced the animal spirits out of the spinal cord into the nerves to the muscles.⁶³ In this way Stuart thought that he had provided experimental evidence for the flow of animal spirit from the spinal cord to the muscle as the agent which initiated contraction.

The problem of how animals can continue to function at some level in the absence of the enkephalon was taken up next by Robert Whytt (1714–1766) in his works Essays on the Vital and Involuntary Motions of Animals and Observations on the Sensibility and Irritability of the Parts of Man and Other Animals, written about 1751 in Edinburgh.⁶⁴ Whytt was not able to accept the mechanical principle that both Descartes and Willis had advanced: namely, that the reflex does not require the intervention of a soul to initiate it. He wrote: ‘The motions performed by us in consequence of irritation, are owing to the original constitution of our frame, whence the soul or sentient principle, immediately, and without any previous ratiocination, endeavours by all means, and in the most effectual manner, to avoid or get rid of every disagreeable sensation conveyed to it by whatever hurts or annoys the body.’ It is important to note that this conception of the soul is more Cartesian than Aristotelian in inspiration. For it is a principle of sentience (and so of consciousness). Whytt anticipated Sherrington on the stretch reflex: ‘Whatever stretches the fibres of any muscle so far as to extend them beyond their usual length, excites them into contraction about in the same manner as if they had been irritated by any sharp instrument, or acrid liquor.’⁶⁵

Idea of a spinal cord reflex

The idea of a ‘soul’ or sentient principle operating in the nervous system that remains after loss of the enkephalon was taken up by Prochàska. He resurrected the notion of the ‘Sensorium commune’ which had been associated with the lateral ventricles in the ventricular doctrine, but was now attributed by Prochàska to the brain and spinal cord.⁶⁶ This work of Prochàska, in clearly associating sensory reception and motor action at the spinal cord level greatly helped in the development of the idea of a reflex. This connection is made explicit in his comment that: ‘As, therefore, the principal function of the Sensorium commune consists in the reflection of sensorial into motor impressions, it is to be observed, that this reflection takes place whether the mind be conscious or unconscious of it.’⁶⁷ But he too failed to resolve the problem.

Galvani’s discovery of animal electricity makes redundant the supposition of a cortical store of animal spirits for motor action

The problem of how the brain and spinal cord participate in the integrative action of the nervous system which accompanies behaviour arose, as we have seen, with the discovery of nerves and their origins in the brain and spinal cord by Galen and his followers. They retained the Aristotelian notion of vital pneuma generated in the heart, but were now required to modify Aristotle’ s ideas by their discovery of spinal nerves and the importance of the integrity of these nerves for motor behaviour. Galen suggested that when vital pneuma enters the brain it is converted into psychic pneuma. Thence it passes down both the cranial nerves and spinal cord and out of the spinal nerves to activate muscles. Galen conceived of psychic pneuma as a fluid that either flowed along hollow tubes in the nerves or provided the substrate for the flow of a potency, somewhat akin to the modern conception of an action potential. Descartes, as we have seen, refined Galen’ s conception, describing the vital pneuma as composed of fine blood particles which, on reaching the brain, are converted into even finer particles. He named these ‘animal spirits’. The discovery by Willis of reflexes raised the problem of how animal spirits might participate in the integrative actions of the nervous system that accompany reflexes. This difficulty was exacerbated by the realization that reflexes could be performed by decorticate animals. For in this case how could the production of animal spirits, dependent on the integrity of the brain, participate in reflex generation by spinal cords with their attached motor nerves in the absence of the brain? The solution was provided by Luigi Galvani (1737–1798). He showed that nerves could conduct electricity in a way similar to that whereby metallic wires conduct voltaic electricity, and that the potential for generating this electricity in the nerves could be found in the nerves themselves.⁶⁸ His key experiment to show that nerves conduct electricity was to suspend a frog’ s exposed spinal cord/leg preparation in a sealed jar by means of a wire passed through the spinal cord and then through a seal at the top of the jar; lead shot was present in the bottom of the jar. A wire was then strung across the ceiling to pick up the charge from a frictional machine and convey it to the wire from which the spinal cord was strung. This apparatus unequivocally demonstrated that when the machine sparked, the legs twitched. From this Galvani concluded that the spinal cord and its attached nerves conduct electricity.⁶⁹ These discoveries showed that there was no need for a store of animal spirits in the brain derived from psychic pneuma and used by the spinal cord and its attached nerves to control organs. Both spinal cord and nerves possessed the ability to generate the electricity needed to initiate reflexes independently of the brain.

Bell and Magendie: identification of sensory and motor spinal nerves

We have seen the confusion that was still present in the eighteenth century concerning reflexes and the extent to which Whytt had to invoke a conception of the soul in relation to the function of the spinal cord in order to explain reflex action. This difficulty was to remain until a demarcation emerged between the sensory and motor functions of the posterior and anterior roots of the spinal cord at the beginning of the nineteenth century. This was due to Charles Bell (1774–1842), who identified the anterior roots as motor, and to François Magendie (1783–1855), who was responsible for the idea that the posterior roots are sensory. Much controversy attended the attribution of these discoveries.⁷⁰

It is striking that the arguments for the claim that the anterior roots are motor in function did not involve any reference to the action of either the soul or a sensorium commune. The reason why debate on these issues came to be based on experimental observations alone, without any such reference, is clear. Bell’ s and Magendie’ s experiments did not involve disruption of the spinal cord or removal of the brain, so questions as to how reflexes could be elicited without the brain did not arise. Rather, their research centred on the effects of cutting nerves that lead from the brain and spinal cord to the peripheral parts of the body. Bell had satisfied himself by dissection that the anterior and posterior roots were continuous with particular columns of the spinal cord that were connected to the brain. So there was no conflict between the idea that the soul resides only in (or interacts only with) the brain and the fact that severing the roots produced the effects observed. In an incisive passage, Bell seems also to have understood correctly the integrative power of the spinal cord in decapitated animals: ‘the spinal marrow has much resemblance to the brain, in the composition of its cineritious and medullary matter. In short its structure declares it to be more than a nerve, that is, to possess properties independently of the brain.’⁷¹ In this passage the requirement of a soul or a sensorium commune in the spinal cord is abandoned, even though animals without brains are being considered.

Bell does not seem to have made any reference to the posterior roots possessing a sensory function. This may be explicable by the fact that most of his work was carried out on stunned rabbits. It was Bell’ s contemporary, Magendie, who first distinguished between motor and sensory nerves in relation to the anterior and posterior roots. In its proceedings for 1822, the French Academy of Science announced: ‘Monsieur Magendie reports the discovery he has recently made, that if the posterior roots of the spinal nerves are cut, only the sensation of these nerves is abolished, and if the anterior roots are cut, only the movements they cause are lost.’⁷² These experiments established the sensory nature of the posterior nerves.

The experiments of Bell and Magendie provided the grounds for what became known as the Bell–Magendie hypothesis of spinal roots, which is best stated in Magendie’ s words: ‘it is sufficient for me at present to be able to advance as positive, that the anterior and posterior roots of the nerves which arise from the spinal marrow, have different functions, that the posterior appear more particularly destined to sensibility, whilst the anterior seem more especially allied to motion’.⁷³

Marshall Hall: isolating sensation from sensing in the spinal cord

Bell and Magendie had avoided becoming caught up in the controversies as to whether the spinal cord contained a soul capable of initiating motion independently of the cerebrum. This, as we have seen, was because the experiments they performed involved only cutting the spinal nerves. Nevertheless, the apparent problem remained of how sensibility (i.e. the ability to feel a sensation) could be associated (as it seemed that it had to be associated) with the isolated spinal cord. This problem was largely dissolved by Marshall Hall (1790–1857) in the 1830s. He gave a full communication to the Royal Society in 1833 entitled ‘On the reflex function of the medulla oblongata and medulla spinalis’, in which he concluded that ‘there is a property of the sentient and motor system of nerves which is independent of sensation and volition; – a property of the motor nerves independent of immoderate irritation; – a property which attaches itself to any part of an animal, the corresponding portion of the brain and spinal marrow of which is entire’.⁷⁴

The spinal cord as a reflex centre – the true spinal marrow: Hall makes the supposition of a spinal soul redundant

By 1837 Hall had given an account of the spinal cord as containing a reflex centre that operated in a non-sentient and non-volitional manner by contrast with the nerves of sensation which pass up to the brain and the motor nerves of volition that pass down from the brain. These conclusions were revolutionary inasmuch as they stated clearly that sensory nerves exist that do not produce sensations and that motor nerves exist that do not merely mediate volitional acts. So reflex acts do not require a nervous arc from muscle to brain and then from brain to muscle, as Charles Bell had thought. Rather, the reflex arc required

1 a nerve leading from the point or part irritated to and into the spinal cord marrow,

2 the spinal marrow itself,

3 a nerve or nerves passing out of or from the spinal marrow; all in essential relation or connection with each other.⁷⁵

This work laid the foundations for, and in some respects anticipated, Sherrington’ s work later in the century. Following Hall, the notions of a spinal soul and a spinal Sensorium communis were, by and large, abandoned. In 1831 Johannes Muller confirmed the Bell–Magendie law experimentally.

In 1890 Michael Foster (1836–1907) published the fifth edition of his great work A Textbook of Physiology, in which he gave a succinct account of the relationship between spinal reflexes and the brain. Yet even in the last decade of the nineteenth century the idea of a spinal cord soul still lingered and was considered by Foster in his Textbook to be worthy of consideration, as in his comments that:

We may thus infer that when the brainless frog is stirred by some stimulus to a reflex act, the spinal soul is lit up by a momentary flash of consciousness coming out of the darkness and dying away into the darkness again; and we may perhaps further infer that such a passing consciousness is the better developed the larger the portion of the cord involved in the reflex act and the more complex the movement.⁷⁶