Extension-sensitivity in the crayfish abdomen
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In the isolated crayfish abdominal cord, the “spontaneous” discharge of postural motor neurons was found to depend (tonically) on cord length. This tonic cord stretch reflex inhibits flexor and excites extensor motor neurons and recruits additional extensor neurons.
The necessary and sufficient stimulus for the reflex in a particular ganglion is stretch of any part of the more posterior cord segments; stretch applied to anterior segments is without influence.
The reflex is present over the range of cord lengths obtaining in the intact animal.
The discharge rate of the tonic stretch receptor is shown to be also influenced. Evidence is presented that this is by way of the motor neuron which shares innervation of the superficial extensor musculature and the muscular part of the stretch receptor organ.
The existence of the tonic cord stretch reflex is taken as added evidence of the proprioceptive role of the cord stretch receptor system. The potential positive feedback nature of the reflex, its relation to other known proprioceptive reflexes, and its possible functional role are discussed.
KeywordsMotor Neuron Stretch Receptor Postural Motor Sufficient Stimulus Cord Length
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