Reflex actions of one proprioceptor on the motoneurones of a muscle receptor and their central modulation in the shore crab.

1. Reflex efferent control of a muscle stretch receptor by a joint proprioceptor of the same limb was studied in an isolated CNS preparation from the shore crab. The influence of 'fictive locomotor' activity on this interjoint reflex was also examined. 2. The thoracic-coxal muscle receptor organ (TCMRO) and the coxo-basal chordotonal organ (CBCO), which monitor movement and position of the first and second joints of the posterior leg, were isolated together with the whole thoracic ganglion complex. The TCMRO, functionally analogous to a mammalian muscle spindle, has two receptor motoneurones. RM1 innervating the receptor muscle alone and RM2 which also supplies the 'extrafusal' promotor muscle. The CBCO is a typical arthropod elastic strand organ, with many sensory neurones but lacking an efferent supply. The TCMRO was fixed at its mid-length, and stretch-hold-release stimuli were applied to the CBCO. Efferent activity was recorded from the cut nerve roots of the four basal limb muscles and intracellularly as excitatory junction potentials (EJPs) from the receptor muscle. 3. A dynamic increase in the frequency of action potentials in RM1 occurred on both stretch and release of the CBCO. During the hold phase the RM1 activity declined from the dynamic response but remained elevated compared to the resting tonic discharge. RM2, identified by EJPs occurring 1:1 with a unit in the promoter nerve, responded in a similar way. 4. One or more promotor motoneurones were usually co-activated with the two receptor efferents in response to input from the CBCO. In a typical example the average spike frequency of RM1 rose from 0 to 27 Hz during the dynamic phases (stretch and release) of the CBCO stimulus, falling to 2.5 Hz during the hold phase, while the corresponding promotor spike frequencies were 25 and 7.5 Hz, respectively. The other three muscle nerves recorded from generally also showed reflex driving by the CBCO. 5. The totally isolated thoracic ganglion could produce a rhythmic, bursting motor output in the absence of any sensory input. During this centrally generated activity the receptor motor innervation was strongly co-activated with the promotor bursts, and the reflex input from the CBCO was overridden or modulated in a phase-dependent manner. 6. The proximally directed interjoint reflex to the receptor muscle probably functions to maintain the tension on the sensory endings of the TCMRO, and so enable them to respond effectively at all times to movements of the basal leg joint.(ABSTRACT TRUNCATED AT 400 WORDS)