and John Dennison2
Department of Psychological Medicine, University of Otago School of Medicine, Wellington, New Zealand
Department of Anatomy, Otago Medical School, Dunedin, New Zealand
Concept of spontaneous movement
Distinction from reflex movement
Position sense and position representation
Sensations of nervous activation of muscle, and their memory images
Sense of motion and movement imagery both derive from organ sensations
Also, tactile sensations and touch representations from the hand’s tactile organ are derived from organ sensations
You may already have noticed that so far I have completely ignored a group of phenomena that have close connection with our awareness of physicality, namely our entire apparatus for movement. I have avoided this until now, because it is quite complicated. How can we understand that any comparative mollusc learns to master its muscular system so completely—as is indeed the case—if we avoid consideration of its innate abilities? We now examine this point more closely.
It is known that humans like all vertebrates still bear the clearest traces of their lineage from the invertebrates, in that the spinal cord preserves a segmentation, designated by the numbered vertebrae. We must also remember to take this metameric structure into account along with that ‘motion machine’ [Ed] which supports the brain. This operates by reflex mechanisms of the spinal cord, which, right from the start, bring sensory and motor apparatus of the same transverse level into mutual relationship. This is probably why a gentle prick applied to the plantar surface of the toes immediately elicits a plantar flexion reflex. This metamerism not only determines the structure of the spinal cord, but also, as shown in Pflüger’s ‘irradiation of reflexes’ [Ed], recruits larger muscle areas, according to the stimulus strength, even almost all muscle groups, involving connections which are presumably built into the grey matter of the spinal cord. These reflexes have no immediate link to awareness; they are innate, taking place even in the absence of consciousness, as in deep sleep or coma. However, later, when awake, we become aware of them. That mollusc, by comparison, receives information from reflex movements based on innate characteristics of its body. Without immediately prejudging more detailed statements, we can call these messages ‘sensations of movement’ [W], and their residual memory images ‘representations of movement’ [W] or motor memory images. Now, let us try to understand those ‘sensations of movement’ [W] in greater detail.
In reflex movement, a whole series of different sensations arise. First, let us denote the joint which is moved, by g. Joint sensation depends on the excursion of a given joint movement, and is associated with a certain skin sensation h, because, on the flexor side of the joint, skin surfaces are shortened to a specific extent, and approach one another; on the extensor side, on the other hand, surfaces are stretched and move apart. Thus there is a constant relationship between g and h, which applies if the joint reaches the same position passively. However, muscles are also demonstrably involved, since during flexion they are relaxed on the flexor side, and elongated and stretched on the extensor side, the opposite occurring during extension. Observations from pathology demonstrate that these muscle sensations m are independent of—and can sometimes occur in the absence of—skin and joint sensations. The highly specific ratio of these three sensations to one another, that is, g:h:m, we call ‘position sense’ [W], l, so that l = g:h:m.
We can call memory images formed from this ‘position representations’ [W] L. The same position sense must arise when the joint is moved passively, and is in no way linked exclusively to reflex action.
A subject’s position representations can be examined clinically, because a person passively adopts some arbitrary position for his joints. A healthy person is then in a position to either imitate these positions, or to make some alteration; at least we recognize that he has full control over the position of his limbs. This investigation is particularly relevant for easily moveable joints of fingers and toes. Successful testing of position sense depends on total exclusion of the subject’s own activity, that is, elimination of his own motor impulses. This, of course, is not equally possible for all individuals, because it presupposes the person to have a degree of control over his movements.
Muscle sensation has previously, and incorrectly, been given a dominant role in position sense, and therefore presence of position representations as a unique sense has been called the ‘muscle sense’ [W]. We prefer not to use this word, since it can lead to further misunderstanding.
Suppose now that position sense l is generated by reflex action; this creates the sensation of movement b; but another feature must be added to what has gone before: It may be nothing more than sensation linked with nerve cell activity z, which can be taken as the origin of nervous activation of muscle at the moment of the reflex. Muscle contractions which occur at this moment trigger certain muscular sensations m 1; and these stand in constant relationship with activity in nerve cells z, thus m 1:z. We can then designate the message which reaches consciousness, as the sense of nervous activation i, and then i = z:m 1. The movement sensation b, taken as a whole, then contains both components of the reflex movement—the sense of nervous activation—and the position sense—and these, as is readily apparent, stand in constant relation to each other: b = i:l.
Muscle sensation m 1 can also be tested directly, by electrical stimulation of the muscle. In this case, the most that can be determined is the current strength that can be perceived, and whether changes in this measure are perceptible. The resulting joint movement requires a separate position sense, perceived like any other in normal awareness. However, a regular connection between muscle sensation and position sense does not exist in this experiment, because isolated contraction of one muscle never occurs normally, the experimental conditions thus being a novelty for conscious awareness.
Sensations of movement which reach consciousness in this way, and which, through their content, always recur in like manner, develop into movement representations, as robust features of consciousness, which we can denote as B. Although they give rise to memory images limited to reflex movements, definite patterns of muscle activation are nonetheless represented, since reflex movements are undoubtedly coordinated, in the sense described by Duchenne. According to Duchenne’s classification of muscle coordination, as impulsive, collateral, or antagonistic, reflex movements are impulsive and collateral. Reflex movements are not lacking in usefulness, and can be considered more clearly as protective reactions, which defend against a stimulus, or remove part of the body from the vicinity of a stimulus. These two occasions can be regarded as the most important preconditions leading to spontaneous movements. A person recalls that during reflex movement a message reaches consciousness not only from the movement itself, but also from the sensation e, which elicited the reflex. The memory image E of this stimulus, whether it be tactile or directly painful, will consequently remain associated with the movement representation B. We will then only speak of spontaneous movement, when the memory image E evokes the movement representation B through the association pathway EB in such a manner that the movement actually takes place. A necessary assumption is that a centrifugal pathway p stretches from B to the nerve cells which went into action previously during the reflex response. This pathway has, in fact, been demonstrated: It is the pyramidal tract.