The structure, function and metamorphic reorganization of somatotopically projecting sensory neurons inManduca sexta larvae

Abstract

1. A set of innervated sensory hairs that cover the abdominal surface ofManduca larvae is described (Fig. 1). 2. Intracellular recordings from intersegmental muscle motorneurons reveal that they receive different inputs from these sensilla, depending upon the location of their target muscle. In general, sensilla on the same side of the body excite, and those on the opposite side inhibit these motorneurons (Figs. 2, 3). Motorneurons receive input from sensilla on several segments. 3. The branching patterns of the sensory neurons within the CNS were revealed by filling individual sensilla with cobalt. There is a strong correlation between the branching pattern of a sensory neuron and the peripheral location of the hair it innervates (Figs. 4–9). 4. This precise topographic map is maintained across various borders on the body surface. Thus anterior sensory neurons in one segment branch in a similar area as afferents from posterior sensilla in the next anterior segment (Fig. 10). Similarly, sensilla on either side of the dorsal and ventral midlines send their axons to the midline of the ganglion, where their processes overlap (Fig. 10). 5. Ten large hairs are identifiable on each segment of the 5th instar larva, and their sensory neurons branch in a characteristic region of the CNS (Fig. 11). These hairs are among only 16 present on each abdominal segment of the 1st instar animal (Fig. 12). 6. During the larval-pupal transition many of the hairs are lost, but a subset of the sensory neurons remain to innervate the pupal gin-trap sensilla. The central processes of these afferents increase in extent during this developmental period (Fig. 13).