Postural Reflexes

POSTURAL REFLEXES (from English postur- pose, position), a term proposed by Sherrington to denote reflexes that ensure the maintenance of a certain position (the maintenance of attitude) of the entire organism or of one of its parts (of one limb or one of its segments). The concept of P. r. is opposed to the concept of reflexes whose effect is movement (reflexes of movement). The difference, however, is not absolute: "between a briefly lasting P. r. and a slow motor reflex the difference is only a difference in degree" (Sherrington). The stimuli that cause P. r. come either from proprioceptors or from the labyrinth. Proprioceptors are centripetal conductors that come from the organs of movement, mainly from the muscles themselves. The stimulation that occurs in a given muscle due to its contraction produces postural contraction in the same muscle, resp. in its synergists, and leads to fixation of the achieved pose. Thus, autoregulation of muscle tone is achieved ("autogenetic reflexive tone"). Thus, the tone of extensor muscles is maintained by their own afferent nerves. In the case of decerebrate rigidity, which consists in a sharp increase in extensor tone, if the afferent conductors of the extensors are cut, the rigidity disappears. As special reactions in this group, Sherrington distinguishes the extensor and flexor reactions, which are well observed in decerebrate rigidity on a preparation on which all afferent fibers are cut, with the exception of the afferent nerves of the knee extensors. If on such a preparation, contraction of the m. biceps is caused by stimulation of the peripheral segment of the nerve innervating it, then after the cessation of stimulation, the lower leg does not return to its initial position of full extension, but remains somewhat flexed: the extensor reaction, which is a reflex, since the section of the dorsal roots of the extensors abolishes it. By changing the conditions of the experiment appropriately, a flexor reaction can also be detected in the extensors: the lower leg will freeze in the extension position. The labyrinth also has the same significance for postural reflexes, which Sherrington also considers as a proprioceptive organ. The systematic study of P. r., especially proprioceptive reflexes from the neck muscles and labyrinthine reflexes, and clarification of their significance in the statics of the body (reflexes of position and adjusting reflexes) was carried out by Magnus and his school (see Magnus-Klein reflexes). In clinical practice, P. r. are understood as reflexes whose strengthening is characteristic of extrapyramidal rigidity in paralysis agitans, postencephalitic parkinsonism, pseudosclerosis, Wilson's disease, etc. This includes P. r. ("reflexes de posture") described by Foix and Thevenard (Ch. Foix, A. Thevenard): if in a patient lying on his back, the lower leg is flexed at the knee joint and held for some time in the position of extreme flexion, then upon release it extends insufficiently and does not give the energetic extension recoil that is characteristic of normality. The same phenomenon is observed in the flexors of the forearm and in the dorsal flexors of the foot (see Westphal's symptoms, syndrome). Other designations for phenomena of this group: fixation rigidity, adaptation rigidity. Along with typical changes in tone (rigidity or plastic hypertension), characterized by continuous and uniform resistance of muscles to stretching, the increase in P. r. is a very important diagnostic sign of extrapyramidal diseases. Essentially, the increase in tone in extrapyramidal rigidity can be largely reduced to an increase in P. r., by virtue of which the muscle continuously strives to maintain the state of shortening or lengthening in which it is found at each given moment of passive movement.