The so-called "flexion-reflex" of the limb is one of the most accessible of the local reflex reactions which can thus be studied with an isolated portion of the spinal cord as its centre.
Let it be supposed that the limb observed is the hind limb. The three main joints of the limb are the hip, the knee and the ankle. Each of these joints is provided with muscles which flex or bend it, and others which extend or straighten it. It is found that the reflex throws into contraction the flexor muscles of each of these joints. It matters little which of all the various afferent nerves of the limb is stimulated, whichever of these the afferent nerve may be, the centrifugal discharge goes to practically the same muscles, namely, always to the flexors of the joints.
Not only does the reflex action not discharge motor impulses into the nerves of the extensor muscles, but if the spinal cord happens to be discharging impulses into these nerves when the reflex is evoked this discharge is suppressed or diminished (in hibited). In this way the latter muscles are prevented from impeding the action of the contracting flexors. This inhibition prevents other reflexes from upsetting for the time being the due action of the flexion-reflex, for it renders the muscles opposing that reflex less accessible to motor discharge through the spinal cord whatever the quarter whence incitation to that discharge may come.
A feature of this reflex is its graded intensity. A weak stimulus evokes in the flexor muscles a contraction which is weak and in the extensor muscles a relaxation which is slight. In the case of the muscle-fibre any strength of stimulus if sufficient to excite it at all excites the fibre totally and evokes its full contraction (Keith Lucas) and this latter remains the same in amount so long as the mechanical and nutritive conditions of the fibre remain constant. Each muscle-fibre reacting on this "all-or-none" principle and each nerve-fibre similarly responding on that same principle (Kato), different amounts of contraction yielded by the muscle under different strengths of stimulation of its nerve signify dif ferences simply in the number of muscle-fibres which the stimulus excites. In other words the weaker stimuli excite fewer of the
nerve-fibres innervating the muscle. When the tension developed by the muscle, e.g., against a stiff spring, reaches the maximal ob tainable from it, that fact tells us that all the muscle-fibres are then in contraction ; when the tension development is less than maximal some of the muscle-fibres are not in action. A stimulus to the nerve which evokes maximal tension from the muscle is one which succeeds in exciting all the nerve-fibres innervating the muscle, and any stimulus which evokes less than maximal tension is leaving some of the nerve-fibres "idle." The tension developed by a muscle under reflex excitation falls short of its maximal tension, even when the afferent nerve is stimulated maximally. The reflex activates a proportion only of the total aggregate of the muscle's fibres, and therefore a pro portion only of the spinal motor neurones innervating the muscle. But within the range of the limit imposed by the maximal number of muscle-fibres which it can activate, the grading of the number activated in accordance with the grading of the strength of stim ulation of the afferent nerve is very delicate, much more so than in the case of the motor nerve itself. The actual fractional pro portion of the muscle's total aggregate of motor units which an afferent nerve, maximally stimulated, can activate, differs from one afferent nerve to another, and for the same afferent nerve in different experiments. A subconvulsive dose of strychnine raises the size of the fraction. The threshold value of stimulus for the reflex is usually higher than for the contraction of the muscle from the motor nerve. Spinal reflex action no more regards "muscles" than does a motor centre of the cerebral cortex. Both ignore muscles as entities and have in view movements purely. A weak flexion-reflex activates a few fibres of each of the several flexor muscles of the limb. The reflex treats a complex composed from some (i.e., those of similar threshold for the particular afferent nerve) motor-units of hip-flexor, of knee-flexor and of ankle-flexor as functionally more homogeneous than is the total group of fibres composing any one of these muscles alone.