ON PLASTIC TONUS AND PROPRIOCEPTIVE REFLEXES
Authors: Sherrington C. S. 
Published: December 2014

1. Certain reactions (the “lengthening reaction” and the “shortening reaction) which confer upon the extensor muscle of the knee a quality of plasticity both in the spinal and especially in the decerebrate animal are dependent on the proprioceptive arc of the muscle itself, and all other spinal afferents than those of that arc are inessential to them.

2. The “lengthening reaction” is as follows. When the tonic muscle is stretched it assumes, by virtue of its proprioceptive arc, a new tonic length which is approximately that to which it has been stretched. In this reaction the behaviour of the muscle bears resemblance to the behaviour of preparations of unstriped muscle as noted by v. Uexküll (Sipunculus retractor preparations), Grutzner, Magnus, and others.

3. Conversely, when in the tonic preparation of the knee extensor the muscle is shortened, either passively or by active contraction of the muscle itself, the muscle retains approximately the shortened length thus given it. This is the “shortening reaction.”

4. The vastocrureus muscle, when de‐afferented by severance of the particular afferent spinal roots through which its afferent fibres reach the spinal cord, reacts in the decerebrate animal differently from the muscle with afferent nerve intact. It shows from the earliest period examined after section of its afferents, namely, 2 hours, up to the latest period examined, namely, 140 days, certain defects. Among these defects are the following. The de‐afferented muscle is toneless, and yields no trace either of “shortening reaction” or of “lengthening reaction.” Manipulation of it fails to evoke either contraction or inhibition of the fellow muscle of the opposite limb. Under decerebrate rigidity, its reflex contraction, excited from whatever source, instead of being prolonged beyond the duration of the external stimulus, ceases immediately on withdrawal of that stimulus, the muscle then lapsing at once into full relaxation. In response to slowly repeated reflex stimuli, the de‐afferented muscle exhibits a coarsely clonic instead of a steady tetanus. Its failure in duration of contraction is due to absence of any “shortening reaction” coming to reinforce and maintain the reflex excited through the extrinsic afferent arc. This defect in the reflexes of the de‐afferented muscle bears out what has been previously indicated, that proprioceptive reflexes normally fuse with other reflexes as adjuvant to them.

Further, in the de‐afferented muscle a reflex contraction excited from an extrinsic arc cannot be cut short by a forced stretch of the contracting muscle, as is the case with extensor muscles still possessed of their normal afferent nerve fibres. This indicates, as has been pointed out previously, that a function of proprioceptive reflexes is to produce a compensatory reaction cutting short a reflex, and restoring the status quo ante existing before that reflex set in.

The de‐afferented muscle presents no tonic reflexes, and this is in accord with what has been previously pointed out, namely, that proprioceptive reflexes tend especially to be tonic in character.

The reflex contractions of the de‐afferented extensor muscle appear to suffer fatigue sooner than those of the muscle with afferent nerve intact. Its reflex contractions wane under prolonged stimulation earlier and more abruptly than is the case with muscle still possessed of afferents.

5. The proprioceptive apparatus of the vastocrureus and other extensor muscles seems specially adapted to stimuli of mechanical quality; the apparatus reacts both to stretch of the muscle and to the converse of that, and its response is different in the two cases.

6. The proprioceptive reflex apparatus of the vastocrureus muscle yields four reactions, two reacting on the muscle itself and two on its fellow muscle of the opposite limb. These, reactions are coupled in pairs which are not interchangeable. For one pair of these reflexes the adequate stimulus is shortening, it matters not whether passive or active, of the muscle: this stimulus induces shortening of the tonic length of the muscle itself, and the reflex relaxation (more rarely reflex contraction) of the fellow muscle of the opposite limb. For the other pair of reflexes the adequate stimulus is stretch of the tonic muscle: this stimulus causes reflex lengthening of the muscle itself, and reflex contraction of the fellow muscle of the opposite limb.

Of these four reactions, those with contralateral effect are indubitably reflex; the present investigation has found no valid reason against supposing that those with ipselateral effect are truly reflex also: one relation in which they stand to the reflex nervous system emerges with clearness, namely, that the integrity of the intrinsic reflex arc of their own muscle is absolutely essential to their production.

7. The plastic tonus of the extensor muscles is autogenous, being in each muscle dependent on afferent nerve fibres from that muscle itself.