THE STRUCTURAL AND FUNCTIONAL-DEVELOPMENT OF MUSCLE-SPINDLES AND THEIR CONNECTIONS IN FETAL SHEEP

In this paper we have studied the structural and functional developmen t of hindlimb muscle receptors and the connections of their afferent f ibres in fetal sheep (n = 26) from 67-143 days of gestation (term = 14 6 days). By recording extracellular discharges in dorsal root ganglia (L7, S1) we have shown that muscle spindle afferents first respond to a ramp-and-hold stretch at mid-gestation (similar to 75 days). Silver- stained preparations of muscle spindles revealed that afferent fibres are just beginning to form annulospiral windings at this age. It there fore appears that the annulospiral formation is not a necessary requir ement for the generation of the response. By 87-92 days some receptors had developed a discharge at resting muscle length. Discharges were g enerally more robust and easier to elicit and static and dynamic compo nents could be identified in the response to stretch. Although static sensitivity was generally low it was more evident than dynamic sensiti vity, By 107-115 days it was possible to clearly distinguish between m uscle and tendon afferents and to tentatively classify muscle response s as originating from primary or secondary afferent spindle endings. W ith increasing gestational age there was a progressive increase in the length and complexity of the spindle innervation in parallel with the maturation of functional activity. Biocytin injections into the dorsa l root ganglia revealed afferent projections to the motoneuron pools b y 67 days. Silver-staining of muscles showed that innervation of extra fusal fibres was also present by this age. We therefore conclude that the neural pathways necessary for reflex activity involving muscle spi ndles are present and functional from early in gestation and could con tribute to early fetal movements.