Responses of muscle receptors in the kitten to succinyl choline

Responses of soleus muscle afferents were studied in anaesthetised kittens, after intravenous injection of succinyl choline (SCh), a drug which in adult spindles, at low doses, produces a contracture preferentially of bag1 intrafusal muscle fibres and so mimics the effect of dynamic fusimotor stimulation. Following injection of SCh in kittens aged between 1 and 57 days the integrated discharge recorded in large portions of dorsal root during muscle stretch showed a smaller increase than in the adult. Recordings from functionally single afferents of muscle spindles showed that in the youngest animals SCh induced a resting discharge from all spindles, which were normally silent. The response to a ramp-and-hold stretch showed an abrupt rise in firing rate at the start of the stretch followed by a slow decline. The abrupt fall at the end of the ramp, typical of the adult dynamic response, was almost absent in spindles of animals 2-5 days old. In the younger animals SCh allowed primary and secondary spindle endings to be distinguished on the basis of their response to stretch. Conduction velocity of developing Group I nerve fibres was found to increase from 9.5 ms-1 on day 2 to 51.6 ms-1 on day 57, an increase of 0.77 ms-1 per day, while Group II increased from 5.0 ms-1 to 23.2 ms-1 over the same period, an increase of 0.33 ms-1 per day. The two groups appeared to be separate even in the youngest animals. It is concluded that although spindles in the newborn kitten respond vigorously to SCh, the pattern of discharge is quite different from that seen in older animals.