Motor Programme Switching in the Crayfish Swimmeret System

Intracellular and extracellular recordings have been made from neurones of the swimmeret system in the semi-isolated abdominal ganglion of the crayfish during rhythmic activity. Extracellular recordings commonly reveal a motor programme (MP1) consisting of low-amplitude symmetrical power and return stroke activity with phase-constant posterior-to-anterior intersegmental coordination. Occasionally a different motor programme (MP2) is expressed. MP2 has higher amplitude episodic activity, with return stroke duration greater than power stroke, and with latency-constant anterior-to-posterior or near synchronous intersegmental coordination. Preparations may switch spontaneously between the two motor programmes. Intracellular recordings show that interneurones whose membrane potentials oscillate during MP1 and which can reset its rhythm usually cease to oscillate during MP2. During production of MP1, current injected into any one of a small number of interneurones can induce MP2. The polarity of current required is usually such as to drive the membrane potential towards the level normally associated with return stroke during MP1. During MP1 many motor neurones receive synaptic input with approximately sinusoidal waveform. During MP2 they may receive an episodic input with approximately sawtooth waveform, and/or input consisting of large, unitary EPSPs. The unitary EPSPs drive a ‘bursty’ mode of MP2 activity that is sometimes seen. The bursts of unitary EPSPs in MP2 appear to derive from a different source to that of the sinusoidal input in MP1. These sources are probably caudally-conducting through-interneurones and non-spiking local interneurones respectively. Thus experimental perturbation of a single neurone can induce a motor programme switch such as to change the activity of some hundreds of neurones in at least three ganglia. Neurones with this property would be convenient targets for controlling influences in the intact animal.