Neuroanatomical and neurophysiological aspects of vibrational processing in the central nervous system of semi-terrestrial crabs

Abstract

1. The response properties and morphology of five types of vibration sensitive interneurons in the central nervous system of the fiddler crab,Uca minax, were determined by extracellular and intracellular recording and staining techniques. 2. When presented with artificial calls (pulsed tone bursts), two neuron types (Phasic-I and Phasic-II) respond to pulse onset with a brief burst of one to five spikes. Two (Tonic-I and Tonic-II) emit a sustained discharge during the duration of every pulse, and one, (Inhibited) shows a spontaneous discharge which is either reduced in rate, or suppressed altogether, for the duration of the call. 3. Phasic-I and Phasic-II units function as temporal filters, failing to follow stimuli beyond certain pulse repetition rates with a 1∶1 relationship between stimulus and response. The high frequency cut-off point is dependent upon pulse duration and is greater for Phasic-II than Phasic-I units (Fig. 6). 4. Tonic-II, but not Tonic-I units, function as low-pass frequency filters. They are excited by stimuli between 100 and 300 Hz and inhibited at frequencies between 350 and 1000 Hz (Fig. 1). Both Tonic-I and Tonic-II units code the temporal fine structure (pulse repetition rate, pulse duration) of artificial calls. 5. The natural mating calls of both conspecific and heterospecific crabs elicit responses from all units which are predictable, based upon responses to synthetic calls. All the major temporal features of the mating calls are encoded by the combined activity of the neurons. 6. The morphology of three of the five unit types (Phasic-I, Phasic-II and Inhibited) in the ventral nerve cord is described. All are multisegmental and unilateral, with proximal branches located within each thoracic hemi-ganglion (Fig. 8). The cell bodies lie in the fifth thoracic segment. The units differ with respect to their branching pattern within each hemi-ganglion and in the size of the cell body. 7. In the brain all unit types project into the tritocerebrum. Phasic-I cells also project into the protocerebrum and optic ganglia. The primary projection is within the dorso-medial aspect of the tritocerebrum. Projections here are bilateral (Fig. 9). 8. The structural and functional implications of these findings are discussed. Of particular interest is a comparison with analogous system in the acoustically sensitive insects.