Afferent control of walking movements in the stick insectCuniculina impigra

Abstract

1. When a rampwise stretch of large amplitude is applied to the prothoracic femoral chordotonal organ of an active, decerebrate stick insect, usually the first muscles to be activated are the ‘stance phase muscles’, the flexor tibiae, the retractor unguis, and the retractor coxae. Later during the stimulus when the chordotonal organ signals a fairly flexed joint position, the activity of these muscles declines and the ‘swing phase muscles’, the extensor tibiae and the protractor coxae, become active. This sequence takes place in an active animal and is referred to here as response type 1. The same sequence, but with weaker reactions, may occur in an inactive animal (response type 2). There is a gradual transition between these response types and a pure resistance reflex like that shown by a resting, intact animal (response type 4 in an inactive animal; response type 5 in an active animal). 2. Stimulation of the campaniform sensilla or increasing the angle of the subcoxal joint can also elicit response type 1 and 2. 3. During response types 1 and 2 the electrical activity of the muscles and motor neurons involved appears the same as at the end of a stance phase and the transition to a swing phase. 4. In response types 1 and 2 the FETi soma is first hyperpolarized and then strongly depolarized. The common inhibitor (CI) soma responds with a brief depolarization at the onset of stimulation and a longer lasting depolarization at stimulus offset. 5. The existence of response type 2 indicates that the pattern generator for foreleg walking can be in a state in which it is not generating a walking pattern by itself, but can be induced to produce part of a step by appropriate stimulation. Thus, under these conditions it acts like a reflex center.