Neurolunch: ‘Sensory Neuron Branching at Paddlefish Electroreceptors,’ Sept. 18

Neurolunch presents David Russell on “Sensory Neuron Branching at Paddlefish Electroreceptors” on Thursday, Sept. 18, at noon at the Life Sciences Building 201.

Abstract: The spatial organization of ampullary electroreceptors on the skin of paddlefish (Polyodon spathula) prompts questions about the site(s) where action potentials are initiated in sensory neuron axons. Since each myelinated afferent receives synaptic excitation from the receptor cells of a cluster of 20-30 sensory epithelia (the receptive field), is there convergence onto a single spike initiation zone, or are there multiple spike origins? We addressed these questions, and in general characterized the terminal branching of primary afferents at electroreceptors, using immunofluorescent labeling of marker proteins of myelinated axons, and also DiI tracing of axons. The antibodies were to neurofilament-H for localizing sensory neuron axons, to myelin sheath proteins, or to voltage gated Na channels for identifying nodes, plus others for labeling receptor cells. We observed that near the center of an electroreceptor, an afferent branches into a radial tree of ~3 myelinated sub-branches, each of which forks until myelination stops at a heminode on every short distal segment. Beyond each heminode, the axons branch further into bundles of fine neurites, which continue 0.15 mm to innervate 2 or 3 adjacent sensory epithelia, one bundle to each. The neurites ramify over the basolateral surfaces of receptor cells, making synaptic boutons. Because each heminode receives synaptic excitation from 2 or 3 sensory epithelia, but an afferent has 10-15 heminodes for its receptive field of 20-30 sensory epithelia, the afferent branching pattern is a mix of the two theoretical extremes cited. If morphological heminodes are identified as spike initiation sites, then these results raise general issues of how spikes from multiple origins can be compressed into a single output spike train along an afferent’s axon conveying electrosensory information to the brain.