Extension-sensitivity in the crayfish abdomen: II. The tonic cord stretch reflex

Control of abdominal extension in the freely moving intact crayfish cherax destructor. II. Activity Of the superficial extensor motor neurones

The activity of the superficial extensor motor neurones was recorded during slow abdominal extension in the crayfish Cherax destructor. Postural extensions were evoked by lowering a platform from beneath the suspended crayfish. During extensions where the abdomen was physically blocked from achieving full extension, the largest superficial extensor motor neurone (SEMN6) fired at a higher rate than during unhindered extensions. Blocking a segment neighbouring that being examined also increased SEMN6 activity, demonstrating an intersegmental spread of the reflex. The increase in SEMN6 firing rate occurred in the absence of activity in the sensory neurone of the tonic muscle receptor organ, demonstrating that the tonic sensory neurone is not necessary for load compensation during these abdominal extensions in C. destructor. The findings support earlier evidence suggesting that other receptor systems can mediate load compensation in the abdomen of the crayfish.

The edge cell, a possible intraspinal mechanoreceptor

In the lateral edge of the "white matter" in the lamprey spinal cord, there is a group of nerve cells referred to as edge cells. The results of a combined physiological, light microscopical, and electron microscopical study suggest that these cells serve as intraspinal mechanoreceptors. Edge cells are depolarized on stretch of the lateral margin of the spinal cord, and they have nestlike ramifications in this region oriented in a rostrocaudal plane. These cells exhibit a close structural similarity with the crayfish stretch receptor.

Mechanosensitive neurons in the spinal cord of the lamprey

'Fictive locomotion' in the lamprey in vitro spinal cord-notochord preparation can be entrained by side to side movements of the spinal cord-notochord which mimic swimming movements even after transection of dorsal and ventral roots. This study provides direct evidence of mechanosensitive neurons intrinsic to the spinal cord. Neurons with axons located in the lateral aspect of the spinal cord discharge in response to moderate bending movements of the spinal cord. Movements of this amplitude will inevitably occur during normal swimming.

Entrainment of the spinal pattern generators for swimming by mechano-sensitive elements in the lamprey spinal cord in vitro

Imposed sinusoidal bending of a mobile region of the curarized spinal cord/notochord preparation of the lamprey results in phase-locking (i.e. 'entrainment') of the 'fictive swimming' motor pattern (recording in ventral roots) to the bending movements. This entrainment phenomenon occurs both with intact ventral and dorsal roots and with all roots cut (i.e. a completely isolated spinal cord). It is proposed that mechano-sensitive elements within the spinal cord contribute in part to the entrainment.