The Activity of Muscle Receptors in the Kitten

Abstract

Abstract In the present paper which is a continuation of earlier publications on the postnatal development of postural reflexes, the activity of muscle receptors is studied in kittens from birth to 45 days of age. It is found that the discharge of receptors in the gastrocnemius muscle in response to stretch is phasic in character in the newborn and young kitten, while tonic responses are obtained from more proximal muscles around the hip and spine. Even the selective strongly tonic effect of succinylcholine iodide on muscle spindles is missing. Similarly, at that time the response of the Golgi tendon organs is phasic. With increasing age of the kitten more sustained discharges appear though in the beginning the rate of firing is low in the gastrocnemius muscle. Evidence is presented to show that the appearance of tonic discharges is in some way related to parallel changes in the electrical properties of the nerve fibres and the mechanical properties of the muscle, but further work is needed to elucidate the full significance of this finding. It is also shown that gamma control of muscle spindles in the gastrocnemius can be demonstrated 17–20 days after birth. Evidence is further presented to the effect that gamma control of muscle spindles in proximal muscles appears earlier, and that the gamma control of spindles in distal muscles is preceded by development of an alpha mechanism of spindle control. The obvious significance of these findings for the postnatal appearance of tonic stretch reflexes and decerebrate rigidity is discussed.