Proprioceptors, bristle receptors, efferent sensory impulses, neurofibrils and number of axons in the parapodial nerve of the polychaete Harmothoë

Abstract

In the second segmental (parapodial) nerve of the polynoid worm Harmothoë there are three (fast motor) axons of 6 to 8μ, six of 4 to 6μ, fifty-seven of 2 to 4μ, 176 of 1 to 2μ, and about 2100 of less than 1μ. Most axons over 2μ are unilemnal, all those over 1μ and many of the smaller ones have a neurofibril which consists of densely packed neurotubules, running through the axoplasm. The large number of axons is in reasonable agreement with the number of peripheral sensory cells. Recordings from axons of bristle receptors show them to be highly sensitive, rapidly adapting mechanoreceptors. Units which could act as proprioceptors have been identified with sensory cells having elongated dendrites lying across the ventral surface of the parapodium; they respond with adapting trains of impulses to appropriate strains in the cuticle. Touch receptors are scattered over the body surface. The bipolar neurons of the parapodial cirri are inferred to be chemoreceptors. Impulses emerge from the ventral cord in each segment on both sides when bristle receptors on any part of the worm are touched. These impulses are inferred to be efferent in the sen­sory axons of the bristle receptors. They emerge from a synaptically connected system which runs throughout the worm and the excitation within this system cannot be labelled as beginning or terminating in any one segment. This non-specific system synapses with the giant fibres at many rapidly adapting synapses and excitation in it activates and necessarily accompanies the sequence of locomotory movements.