Proprioception

“ The nerves of the senses are not polar, do not therefore excite the brain, nor does this again affect the motor system. The muscle therefore arrives at a state of non-tension; it becomes relaxed, and along with it necessarily the organs of sense, which are thrown into activity by muscular motion. The arms and fingers, whose business is to touch, sink down; the feet which move, and thereby warm and animate the body, are slackened and bent together; the body is in the recumbent posture ”

“ The Spinal Reflex Arcs of the Hind Limb. - When the skin of the limb is stimulated the flexion-reflex already described is evoked. The reflex is excited by nocuous stimuli such as a prick or squeeze applied to the skin anywhere in the limb, but most easily when applied to the foot. Electrical stimuli wherever applied evoke the same reflex. Similarly electrical stimuli applied to any afferent nerve of the limb evoke this reflex, whether the afferent nerve be from skin or from the muscles. ”

“ The external stimulus acts upon the organ of sense, and the impression is transmitted along the centripetal way to the nerve centre—the corresponding motor impulse is elaborated, and is transmitted along the centrifugal path to the organ of motion, provoking a movement. Although the arc represents diagrammatically the mechanism of reflex spinal [Pg 223] actions, it may still be considered as a fundamental key explaining the phenomena of the more complex nervous mechanisms. Man, with the peripheral sensory system, gathers various stimuli from his environment. ”

“ But they have all failed to notice with what peculiar intensity muscular movements call surface-sensibilities into play, and how largely the mere discernment of impressions depends on the mobility of the surfaces upon which they fall.
Our articular surfaces are tactile organs which become intensely painful when inflamed. Besides pressure, the only stimulus they receive is their motion upon each other. To the sensation of this motion more than anything else seems due the perception of the position which our limbs may have assumed. ”

“ All unconscious acts are reflex acts, as when the hand is drawn away from a hot iron. If an impulse is sent along one of the sensory fibers, it enters the cord through the posterior horn, where its nerve cell is found. Then, through some connection between the nerve cell of the sensory fiber and that of the motor fiber the impulse is transmitted to the motor cell and another impulse is sent out of the cord along the motor fiber of the nerve to the muscle. One of the commonest reflexes is the knee-jerk. ”

“ Hence it was clear that responsive centres exist in the spinal cord capable of receiving a sensory message and of transmitting a motor impulse in reply—a function hitherto supposed to be reserved for the brain. Further studies went to show that such phenomena of reflex action on the part of centres lying outside the range of consciousness, both in the spinal cord and in the brain itself, are extremely common; that, in short, they enter constantly into the activities of every living organism and have a most important share in the sum total of vital movements. ”

“ And what is very remarkable is, that, in speaking of these movements which arise by a sensation being as it were reflected from the central apparatus into a limb—as, for example, when one's finger is pricked and the arm is suddenly drawn up, the motion of the sensory nerve travels to the spine and is again reflected down to the muscles of the arm—Descartes uses the very phrase that, we at this present time employ. ”

“ The back portion of the spinal marrow and the nerves that run from it are employed in sensation, or the sense of feeling. These nerves extend over the whole body, but are largely developed in the net-work of nerves in the skin. The front portion of the spinal marrow and its branches are employed in moving those muscles in all parts of the body which are controlled by the will or choice of the mind. These are called the nerves of motion. ”

“ Numerous other reflex centers are described by physiologists, but enough has been said to emphasize the great importance of the spinal cord as an independent nerve center, besides its function as a conductor of nervous impulses to and from the brain.
280. The Brain as a Reflex Center. The brain, as we have just stated, is the seat of consciousness and intelligence. It is also the seat of many reflex, automatic, and coordinating centers. These give rise to certain reflex actions which are as entirely independent of consciousness as are those of the spinal cord. ”

“ The most complex habits, as we shall presently see more fully, are, from the same point of view, nothing but concatenated discharges in the nerve-centres, due to the presence there of systems of reflex paths, so organized as to wake each other up successively—the impression produced by one muscular contraction serving as a stimulus to provoke the next, until a final impression inhibits the process and closes the chain. The only difficult mechanical problem is to explain the formation de novo of a simple reflex or path in a pre-existing nervous system. ”

“ This movement of the whole limb is what we think of when the joint's nerves are excited in that way; and its place is so much more important than the joint's place that our sense of the latter is taken up, so to speak, into our perception of the former, and the sensation of the movement seems to diffuse itself into our very fingers and toes. But by abstracting our attention from the suggestion of the entire extremity we can perfectly well perceive the same sensation as if it were concentrated in one spot. ”

“ The answer is, that it is nothing more nor less than the antecedent mental image of the movement to be executed. We constantly receive nerve-impulses by the sensory nerves which have their source in the skin, joints, tendons, and muscles; these impulses fuse into an aggregate sensation which informs us about the varying positions of our limbs. Like other sensations, this leaves behind it a mental image, and it is this mental image, brought up as the last result of the play of association, which immediately precedes the external movement and constitutes the "feeling of innervation." ”

“ Those nerves, which serve the senses, principally arise from that part of the brain, which is lodged in the head; and those, which serve the purposes of muscular motion, principally arise from that part of the brain, which is lodged in the neck and back, and which is erroneously called the spinal marrow. The ultimate fibrils of these nerves terminate in the immediate organs of sense and muscular fibres, and if a ligature be put on any part of their passage from the head or spine, all motion and perception cease in the parts beneath the ligature. ”

“ Even a weak stimulation of the noci-ceptive arcs arising in the foot often suffices to lower or abolish the knee-jerk or the reflex extensor tonus of the elbow or knee. If various species of reflex are arranged, therefore, in their order of potency in regard to power to interrupt one another, the reflexes initiated in receptors which considered as senseorgans excite sensations of strong affective quality lie at the upper end of the scale, and the reflexes that are answerable for the postural tonus of skeletal muscles lie at the lower end of the scale. ”

“ In the case of table-rapping there are movements of the hands, in our case there are those of the head. Our head, balanced as it is upon the cervical vertebral column, is continually in a state of unstable equilibrium and therefore peculiarly susceptible to movement-impulses of every kind. But I could induce not only movements of the head, but also of the arms and legs, and this by having the subject assume a posture which enabled him to hold arms or legs in as unstable a position as possible. ”

“ From the earliest ancestors of ours which had feet, down to the present day, the movement of the feet must always have accompanied the contraction of the muscles; and here, if anywhere, habit's hereditary consequences ought to be found, if the principle that habits are transmitted from one generation to another is sound at all. [11] No sooner then should the brain-centres for perceiving muscular contractions be excited, than those other centres functionally consolidated with them ought to share the excitement, and produce a consciousness that the foot has moved. ”

“ IT is a novel proposition that the position of the head in respect to the body or of the shoulders in reference to the back, that the carriage of the whole body in walking and the attitude of a person in conversation, should be governed in an important measure by the form of the cranium. It will also, doubtless, be regarded as a bold assertion to say that all these positions and attitudes and even the gait of individuals are largely modified, even in many instances controlled, by the normal position of the eyes in respect to the cranium. ”

“ For movement of a vertebra away from the side on which you stand, place your arms and hands in such a position that the pisiform bone of adjusting hand, both elbows, and both shoulder joints (shoulders being dropped loosely forward) will fall in the same plane and that the plane of direction in which the vertebra is to be moved. In other words, let the force be applied in a line straight from your body through the vertebra. Always shift your feet to a proper position from which to direct the movement. ”

“ If the spinal cord is divided in the middle of the back, for example, the skin of the feet may be cut, or pinched, or burned, or wetted with vitriol, without any sensation of touch, or of pain, arising in consciousness. ”

“ We shall later see good reason to deny the existence of the latter feeling. We have accounted for the better part at least of the resident feelings of motion in limbs by the sensibility of the articular surfaces. The skin and ligaments also must have feelings awakened as they are stretched or squeezed in flexion or extension. And I am inclined to think that the sensations of our contracting muscles themselves probably play as small a part in building up our exact knowledge of space as any class of sensations which we possess. ”

“ I do not assert that competent physiologists ever believe that the seat of motility is elsewhere than in the muscles; what they mean is, doubtless, that the centre, from which the stimulus issues which will excite the muscles, is in the spinal chord. But how easily the ambiguous language leads to ambiguity of conception may be seen in a hundred examples; and it may be, to a great extent, avoided by rigorously demarcating the phenomena of Sensibility, Neurility, and Contractility, as the actions of three different organs: nerve-centres, nerves, and muscles. ”

“ We cannot touch an object without employing our muscles by extending the organs of locomotion, as hands or feet, though the object may touch us independently of our self-activity. So pressure is apprehended by us as stress or strain; the muscles of the ear are strained when we intently listen; the eye is especially under the control of a system of muscles, so that it becomes the special organ for the cognition of motion. ”

“ Unhappily there are but few parents and teachers who have ever formed any definite conception of the importance of proper early training, and that simply because they never understood its importance; nor is it possible that they ever should, until they have some knowledge of the structure and functions of the body which they have to train and develope. We propose, therefore, in this chapter, to give a general idea of the structure and functions of the spine, as well as a brief description of some of the curvatures to which it is subject.
The distinctive character of man is the erect posture. ”

“ 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. ”

“ To accomplish a true reflex movement, a whole mechanism is necessary, set up in the spinal cord or the medulla. To choose voluntarily between several definite courses of action, cerebral centres are necessary, that is, crossways from which paths start, leading to motor mechanisms of diverse form but equal precision. ”

“ Mobility of the spine is of first importance, for in health there is motion between adjacent vertebrae. Lack of movement may be caused by muscular tension, by stretching of ligaments, or by a union of the parts due to bony deposit. ”

“ In relation to the brain, the spinal cord is a great mixed motor and sensory nerve, but, in addition to this, it is also a distinct nervous centre, in which originate and terminate all those involuntary impulses which exert so potent an influence in the preservation and economy of the body. ”

“ All your reasoning, and all your marshalled facts, are swept away by the irresistible evidence of human patients with injured spinal cords, whose legs have manifested reflex actions, and who nevertheless declared they had no sensation whatever in them. ”

“ Reflex arcs from widely different parts conjoin and pour their influence harmoniously into the same muscle. The motor neurones of a muscle of the knee are the terminus ad quern of reflex arcs arising in receptors not only of its own foot, but from the crossed fore foot and pinna and tail, also undoubtedly from the otic labyrinth, olfactory organs and eyes. ”