Mechanoafferent neurons innervating tail of Aplysia. I. Response properties and synaptic connections

Mechanical, chemical, or electrical stimulation of the tail elicits a short-latency (less than 1 s) tail-withdrawal reflex that is graded with the intensity of the stimulus. The tail-withdrawal reflex is not elicited by stimulation of parts of the body outside of the tail region. Mechanoafferent neurons innervating the tail are located in a small subcluster within a large, homogeneous group of medium-size (40-80 μm) cells on the ventrocaudal (VC) surface of each pleural ganglion. The tail sensory neurons within this large VC cluster are activated by tactile pressure or by electrical stimulation of discrete regions of the tail. They adapt slowly to maintained stimulation and sometimes respond to stimulus offset as well. Both mechanical and eletrical stimuli produce responses that are graded with the intensity of the stimulus. Cells in the VC cluster appear to be primary mechanoreceptors because they have axons in peripheral nerves (including nerves innervating the tail), they exhibit action potentials lacking prepotentials in response to tactile stimulation, and these action potentials are still produced by cutaneous stimulation when peripheral and central chemical synaptic transmission is blocked. Stimulation of fields all over the body surface evokes synaptically mediated hyperpolarizing responses in individual mechanoafferent neurons that may represent afferent inhibition. Hyperpolarizing responses lasting many seconds can be produced by brief cutaneous stimuli. The mechanoafferent neurons innervating the tail region make strong monosynaptic connections to tail motor neurons in the ipsilateral pedal ganglion, and through these connections this subpopulation of the VC neurons appears to make a substantial contribution to the short-latency tail-withdrawal reflex. In addition, the combined excitatory receptive fields of these mechanoafferents match the excitatory receptive field of the tail-withdrawal reflex. Mechanoafferent neurons in the VC cluster that have receptive fields on other parts of the body (outside the excitatory receptive field of the tail-withdrawal reflex) have not been observed to make monosynaptic connections to the tail motor neurons. The neurons innervating the tail are reliably found in a discrete region within the larger VC cluster. In addition to this gross somatotopic organization, there is evidence of a finer level of somatotopic organization between the position of the excitatory receptive field on the tail and the position of the cell soma in the tail subcluster. Because of the high correlation among the position of the cell body in the VC cluster, the presence of an excitatory receptive field on the tail, and the presence of a monosynaptic connection to identified tail motor neurons, it is possible to identify tail mechanoafferent neurons without having to stimulate the tail or even to include the tail in the preparations.