Sensory-motor interactions in antennal reflexes of the American lobster

Abstract

1. The behavior, proprioceptors, musculature, and innervation of the distal three segments of the antenna of the American lobster,Homarus americanus, were studied. 2. The unrestrained lobster moves the two distal joints of the antenna in two ways: 1) simultaneous extension or flexion at both joints, or 2) movement of the distal segment by itself. 3. Sensory information about the movements at the two joints is provided mainly by two proprioceptors, the MCF chordotonal organ spanning both joints and the CF myochordotonal organ spanning the most distal joint. 4. The MCF chordotonal organ is an unusual proprioceptor in that some of its units respond similarly to movement at either joint. 5. Movement at the two distal joints is controlled by four muscles, one pair of flexors and one pair of extensors. 6. The innervation pattern involves seven excitatory motor neurons: one independent motor neuron to each of the four muscles, one common motor neuron to the flexor muscles, and two common motor neurons to the extensor muscles. 7. Induced joint movements generate resistance-reflex responses: flexion drives the two independent extensor motor neurons E4 and E5 and one of the two common extensor motor neurons EC1; extension drives the independent motor neurons F4 and F5. The response is the same to movement at either joint. 8. Direct mechanical stimulation of the MCF chordotonal organ also generates resistance reflexes: stretch drives the flexor motor neurons F4 and F5; relaxation drives extensor motor neurons E4, E5, and EC1. These responses are similar to those produced by induced joint movement. 9. The functional significance of the relationship between the common proprioceptor and the motor neurons is discussed.