Receptors sensitive to muscle length in the horseshoe crab

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Abstract
Sensory activity was recorded extracellularly from fine branches of the internal pedal nerve in the femur of the horseshoe crab walking leg. Increased receptor spike frequency was associated with an increase in the length of the tibial flexor muscle produced by extension of the femero‐patellar and patello‐tibial joints. Active flexor contraction led to a reduction in spike frequency, and this reduction was smaller when contraction was isometric. Studies which combined methylene blue staining of putative receptor neurons, local mechanical stimulation and physiological recording suggest that multipolar neurons located near the tibial flexor insertions are muscle length receptors.