Hyperpolarizing responses to stretch in sensory neurones innervating leech body wall muscle.

Abstract

The membrane properties, morphology and physiological responses of peripherally located sensory neurones that innervate body wall muscle of the leech Hirudo medicinalis have been investigated using intracelllar recording and dye injection techniques. The peripheral neurones and their dendrities were visualized directly in whole mounts of the body wall by intracellular injection of horseradish peroxidase or Lucifer Yellow. They lie along the course of segmental nerves between the layers of longitudinal and oblique body well muscle and within the sheath of the nerve. They have a distinctive morphology with two expanded, fan-shaped dendrities arranged in series separated by the cell body and a 300 .mu.m long cylindrical process. Both dendrities are associated with longitudinal muscle of the ventral body wall but with separate bands of muscle fibres. The axons project into the ventral nerve cord and arborize within the ipsilateral half of the segmental ganglion. No processes extend across the mid-line of the ganglion or enter the connectives to neighbouring ganglia. ''Resting'' membrane potentials recorded from the peripheral cell body or from the axon as it entered the segmental ganglion ranged from -30 to -70 mV. The transmembrane potential recorded depended on the amount by which the body wall was stretched: the most hyperpolarized values were recorded from the most stretched preparations. Although the peripheral cell body can generate overshooting action potentials these are not actively to the CNS. Rather, imposed voltage changes spread decrementally along the axon Input resistances measured in the cell body ranged from 14 to 26 m.OMEGA.. The space constant, estimated from the spread of hyperpolarizing current injected into the cell body, was 2.4 mm. The response of the neurones to change in length of the longitudinal muscle recorded from the axon near its terminal arborization within the ventral nerve cord is a graded DC signal: the neurones thus relay information to CNS synapses in analogue form. Spiking activity recorded extracellularly in the anterior segmental nerve root in resonse to stretch of the body wall is due to activation of touch mechanosensory cells that innervate the skin. Unlike stretch receptors innervating skeletal muscle in vertebrates or arthropods, the leech neurons respond to stretch of the body wall muscle maintained hyperpolarizing potentials and to release of stretch with depolarization. Both hyperpolarizing and depolarizing responses depended on the amplitude of the final displacement. Depolarizing responses to ramp release also consisted of an initial dynamic component depenent on the rate of release from stretch.