Nervous System

Reflex inhibition has its seat in the grey matter of the spinal centre. It acts at some point in the reflex arc up-stream from the all-or-none motor unit (motor nerve-fibre with its group of muscle-fibres). The inhibitory reflex is like the excitatory capable of very delicate grading ; its effect in diminishing or precluding this or that amount of an excitatory reflex is effected entirely by reg ulating the number of the motor units thrown out of action or kept out of action. Intensity of contraction or of relaxation is thus in both cases a question simply of number of motor-units reacting. The reflex centre continues discharging impulses for a certain time after its exciting stimulus has ceased. This of ter discharge succeeding a strong stimulus may persist even for several seconds.

Refractory Phase.

Besides characters common to all or many spinal reflexes certain spinal reflexes have features peculiar to themselves or exhibited by them in degrees not obvious in other re flexes. One of these features is refractory phase. The scratch reflex exemplifies this. In the dog, cat and many other animals the hind limb often performs a rapid scratching movement, the foot being applied to the skin of the shoulder or neck as if to groom the hairy coat in that region. This movement is in the intact animal under control of the brain, and can be executed or de sisted from at will. When certain of the higher centres in the brain have been destroyed, this scratching action occurs very readily and in an uncontrolled way. When the spinal cord has been severed in the neck this scratching movement of the hind limb can be elicited with regularity as a spinal reflex by merely rubbing the skin of the side of the neck or shoulder, or applying there a weak electric current to the skin. In this reflex the stim ulus excites afferent nerves connected with the hairs in the skin and these convey impulses to the spinal centres in the neck or shoulder segments, and these in turn discharge impulses into nerve fibres entirely intraspinal passing backward along the cord to reach motor centres in the hind limb region. These motor centres in turn discharge centrifugal impulses into the muscles of the hind limb of the same side of the body as the shoulder which is the seat of irritation. The motor discharge is peculiar in that it causes the muscles of the hind limb to contract rhythmically at a rate of about f our contractions per second, and the discharge is peculiar further in that it excites the flexor and extensor muscles of the joints alternately so that at the hip for instance the limb is alternately flexed and extended, each single phase of the move ment lasting about an eighth of a second. Now this rhythmic discharge remains the same in rate whether the exciting stimulus applied to the skin be continuous or one of many various rates of repetition. Evidently at some point in the reflex arc there is a mechanism which after reacting to the impulses reaching it remains for a certain brief part of a second unresponsive, and then becomes once more for a brief period responsive, and so on. And this phasic alternation of excitability and inexcitability re peats itself throughout the continuance of the reflex. The phase of inexcitability is termed the refractory phase. Its seat lies in the spinal centre. A similar element almost certainly forms part of the co-ordinating mechanism for many other cyclic reflexes, including those of the stepping of the limbs, the movement of the jaw in mastication, the action of the eyelids in blinking, and per haps the respiratory movements of the chest and larynx.

Fatigue.

Nerve trunks do not easily tire out under stimula tion even most prolonged. Reflex actions on the other hand relatively soon tire. Some are more resistant, however, than are others. The flexion-reflex may be continued for ten minutes at a time. As a reflex tires, the muscular contraction which it causes tends to become less intense and less steady. The relatively rapid onset of fatigue in reflexes is counterbalanced by speedy recovery in repose. A long flexion-reflex, when from fatigue it has become weak, tremulous and irregular, will recommence after 3o seconds' repose with almost the same vigour and steadi ness as if it had not recently been tired out.

The natural movements to which the artificially provoked re flexes seem to correspond do not demand prolonged motor activ ity, or when they do, demand it in rhythmic repetition with in tervening pauses which allow repose.

Reflex Postures.

But there are certain reflexes which do persist for long periods at a time. These are reflex postures. The hind limbs of the "spinal" frog assume an attitude which is reflex, for it ceases on severance of the afferent spinal roots. This atti tude is one of flexion at hip, knee and ankle, resembling the well known natural posture of the frog as it squats when quiet in the tank. Similarly in the "decerebrate" dog or cat certain muscles, e.g., the limb extensors, exhibit a persistent contraction. These tonic reflexes are related to attitudes. In the dog and cat they are exhibited by those muscles whose action antagonizes gravity in postures which are usual in the animal, thus the extensors of the knee and hip and shoulder and elbow are in tonic contraction during standing. The postural contraction is accompanied by electrical "action currents" like those of other contraction, though weak and indicative of self-smothering from the rhythms being asynchronous in the contributory fibre-groups. The postural con traction of the extensor muscles in the maintenance of the erect posture is traceable to a reflex, called the stretch-reflex. A stretch, e.g., by a pull upon the tendon, lengthening the muscle by even so little as 1%, excites reflex contraction of some of its fibres. The passive stretch is a mechanical stimulus to some of the pro prioceptive organs (perhaps the muscle-spindles) in the muscle. The reflex is therefore unobtainable when the afferent nerve-fibres of the muscle itself have been severed. The essential centre for the stretch-reflex is spinal, but mid-brain and cerebellar centres much reinforce it. In the erect posture the head, neck, tail and jaw would droop and the limbs fold up under their own weight and that of superincumbent parts, were not the "antigravity" muscles checked from yielding by the stretch-reflex which their own passive yield induces in themselves. Hence the reflexly main tained "standing" is a multiple stretch-reflex. Considerable lati tude of actual pose is allowed to the individual parts because each muscle involved acts for itself and develops and regulates its own stretch-reflex.