PRENATAL DEVELOPMENT OF SOMATOSENSORY PRIMARY AFFERENT CONNECTIONS INTHE SHEEP

A summary is presented of recently published studies on the structural and functional development of cutaneous and muscle receptors and the connections of their afferent fibres in fetal sheep (n = 26) aged betw een 67 and 143 days gestation (term, 146 days). In these studies it wa s shown that primary afferent fibres projected to, and made synaptic c onnections with, dorsal horn neurons in lumbosacral spinal cord by 56- 61 days gestation. Sensory innervation of the skin occurred later by a bout 75 days gestation and, at this age, stimulation of the skin first activated cutaneous afferent fibres and evoked a discharge in dorsal root ganglion and dorsal horn neurons. Muscle stretch first activated muscle spindles and evoked a discharge in dorsal root ganglion cells b y about 75 days. Prior to this (by about 67 days) primary afferent fib res had begun to innervate motoneuron pools in the spinal cord, and mo tor nerves had begun to innervate muscle fibres. Both muscle spindle a nd cutaneous innervation were relatively simple at mid gestation indic ating that the structure of sensory receptors need not be complex in o rder to generate a response. Neural pathways necessary for reflex acti vity involving muscle spindles are therefore present and functional by mid gestation as are cutaneous pathways projecting from the skin to t he spinal cord.