Load compensation in the crayfish abdomen

Abstract

1. Unit discharge of the superficial extensor motor neurons (SEMNs) and the tonic sensory neuron (SRI) of the muscle receptor organ (MRO) was monitored during extensions of the loaded crayfish abdomen. Extensions were evoked by stimulation of command fibers in the circumesophageal connectives. 2. The excitatory SEMNs increased their discharge when extending against a load. 3. During most extensions of the loaded abdomen the SR1 was silent. When the SR1 was active, its activity was accompanied by excitation of SEMNs No. 4 and No. 2 as proposed by Fields (1966). 4. In 6 of the 7 preparations in which the load was restricted to a single joint (A2–A3) the SR1 which spanned that joint was silent although the SEMNs increased their discharge in response to the load. 5. It is concluded that in extensions driven by single command fibers the principle load sensors in the abdomen are not the MROs. The load compensating increase in SEMN discharge is the result of excitation of an unidentified load sensitive sensory system.