Somatotopic mapping of sensory neurons innervating mechanosensory hairs on the larval prolegs of Manduca sexta.

The abdominal prolegs are the principal locomotory appendages of the larval tobacco hornworm, Manduca sexta. The prolegs bear numerous mechanosensory bristle sensilla, each innervated by an afferent neuron that arborizes within the central nervous system (CNS). Based on their positions on the proleg, we have divided the sensilla into planta hairs (PHs), lateral hairs (LHs), and medial hairs (MHs). Previously, we found that PH afferents produce monosynaptic excitatory postsynaptic potentials (EPSPs) in proleg retractor muscle motoneurons, the size of which depends on the position of the hair in the PH array. In this paper we examined the central arbors of the proleg afferents to determine whether there was an anatomical correlate to the pattern of synaptic strengths. We found that the afferent arbors are arranged somatotopically within the CNS in a pattern similar to that for bristle afferents elsewhere on the abdomen; i.e., the anterior-posterior and medial-lateral position of a hair on the proleg was reflected in the location of the afferent arbor along the corresponding axes within sensory neuropil. All afferents terminated within a similar ventral region of neuropil. The arbors of PH, MH, and to a lesser extent, LH afferents, were enlarged as compared to afferents innervating hairs elsewhere on the abdomen. This feature, combined with the dense innervation of the proleg, causes the proleg region to be relatively overrepresented in sensory neuropil. We also examined the afferents innervating a pair of ventral midline hairs (VMHs) present in each abdominal segment, which, unlike the other afferents, showed segment-specific central arbors. We conclude that the somatotopic mapping of afferent arbors may contribute to the specificity of synaptic connections in this system.