Sensory and Motor Interaction in the Locomotor Reflexes of Crabs

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Abstract
Proprioceptive reflexes involved in control of the mero-carpopodite (MC) joint in walking legs of Cancer magister are mediated by four groups of sensory cells: two cell groups (myochordotonal organs), located proximally in the leg segment, which are associated with a receptor muscle and come under efferent control, and two cell groups (MC organs) located distally which signal joint position and movement but are independent of efferent control. Resistance reflexes, which oppose passive movements of the joint, are evoked by both types of proprioceptive organs. Reflexes evoked by the MC organs are maximally effective at the extremes of joint position. Reciprocal inhibition of antagonistic muscle groups appears to occur entirely at synapses within the central nervous system. Efferent control of the receptor muscle comes under the influence of both the myochordotonal and MC organs. The reflexes evoked by the MC organs on the receptor muscles are similar to those which impinge on the main flexor. The myochordotonal organ excites the receptor muscle on joint movements away from the rest position. Interaction of input from the two types of sense organs provides the system with a dynamically determined reference point for the basic postural position. This may be altered by efferent control of the receptor muscle. Reflexes evoked by both types of proprioceptive organ spread to the other walking legs, but are more labile than reflexes to the same leg.