Reflex role played by efferent control of an invertebrate stretch receptor.

Abstract

The postural extensor muscles of a crayfish abdominal segment exhibit a ''resistance reflex'' that is initiated by stretch of the parallel muscle receptor organs; its efferent limb involves only one of the 5 motoneurons innervating the extensors. Of the other 4 motoneurons, two innervate extensors only, and two others innervate the receptor muscle as well. It has been shown that electrical stimulation of single central ''command'' interneurons may selectively excite those motoneurons innervating the receptor muscle, those motoneurons innervating only the extensors, an inhibitory neuron to the receptor cell. Pathways thus exist for initiating movements with or without adjustment of the proprioceptive servo. This control circuit is compared with that provided by the gamma loop for mammalian muscles. Intersegmental coordination is assisted by the spread of a reflex whereby one stretch receptor selectively inhibits its anterior neighbors.