The Motor Innervation of Newborn Kitten Muscle Spindles

In neonatal kittens an appreciable number of soleus spindle afferents did not respond when the ventral root filaments were stimulated at strengths sufficient to recruit fusimotor axons (Gregory and Proske, 1986). This may imply that the fusimotor innervation is not yet functionally developed, as Gregory and Proske suggest. However, Milburn (1984) did find some fusimotor endings in peroneal muscles of newborn kittens which had some of the post-synaptic features of fusimotor endings in adult cats (see Boyd and Gladden, 1985, Part 1 review). We are presently engaged in a more systematic review of the development of fusimotor innervation, using the tenuissimus muscle. By birth in these muscles also post-junctional elaboration was already well advanced at some fusimotor endings, while post-junctional folding was absent or rudimentary at extrafusai motor endings.