Responses of muscle receptors in the kitten.

Responses have been recorded in single functional afferents supplying muscle receptors of the soleus muscle in kittens aged between 1 and 23 days. Recordings of nerve volleys in whole dorsal or ventral roots in response to muscle nerve stimulation showed that conduction velocity for afferent and motor fibres was similar and increased from 9 m s-1 in a 2-day-old animal to 33 m s-1 in a 23-day-old animal. Of a total of 215 single functional axons isolated from 28 animals, 82 showed responses typical of tendon organs and 103 resembled muscle spindles, leaving 30 unidentified units. Both muscle spindles and tendon organs were characterized by having an over-all low firing rate compared with responses of adult receptors. The most immature receptors also lacked a maintained response to the hold phase of a ramp-and-hold stretch. Spindles in animals over the whole range of ages, including the youngest animals could be shown to be supplied with a fusimotor innervation. On a few occasions it was possible to isolate single fusimotor axons to a spindle; such axons were sometimes found to conduct impulses at a speed characteristic of non-myelinated fibres. Evidence was obtained for the existence of a beta-(skeletofusimotor) innervation of spindles. On four occasions it was possible to isolate single functional beta-axons in the ventral root. The incidence of beta-innervation appeared to be higher than in the adult.