Reflex pathways responsive to depression of the locust coxotrochanteral joint.

Resistance reflexes in arthropods are negative feedback reflexes broadly analogous to the vertebrate stretch reflex (Bush, 1962). Such reflexes may well be concerned with the maintenance of stable posture as a prerequisite for motor behaviour. Proprioceptors in the arthropod limb monitoring position and movement include muscle receptor organs and the chordotonal organs (see Mill, 1976, for reviews). In insects, an additional class of joint receptor has been described: the strand receptors (Braunig and Hustert, 1980; Braunig et al. 1981; Braunig, 1982). The cell bodies of strand receptor primary afferent neurones are located within the segmental ganglia rather than at the peripheral sense organ. The coxotrochanteral joint of the locust is equipped with two strand receptors (SRs) and one muscle receptor organ (MRO), which all respond to depression of the joint. Selective stimulation of each of these receptors can generate a resistance reflex in trochanteral levator motoneurones (Braunig and Hustert, 1983). In arthropods, local proprioceptive reflexes are mediated by direct connections from afferent neurones to motoneurones (Blight and Llinas, 1980; Burrows, 1975, 1987a; Pearson et al. 1976; Skorupski, 1991). In addition, in the locust, both tactile and proprioceptive afferents are mapped onto a layer of spiking local interneurones, that can, in turn, influence motoneurones directly and indirectly through a layer of nonspiking local interneurones (Burrows, 1987a,b\ Burrows et al. 1988; Laurent and Burrows, 1989; Siegler and Burrows, 1983). Motoneurones thus receive both monoand polysynaptic primary afferent input. This scheme derives largely from studies on distal afferents, but less is known about the reflex organization of the proximal leg joints. The morphology, innervation and reflex actions of coxal proprioceptors are documented (Braunig, 1982; Braunig et al. 1981; Braunig and Hustert, 1983), but their central synaptic pathways are not yet known. In this paper we present evidence that coxotrochanteral strand receptor