Discharge Patterns of Neurones Supplying Tonic Abdominal Flexor Muscles in the Crayfish

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Abstract
1. The discharge patterns of tonic flexor motoneurones in the crayfish abdomen have been investigated by simultaneous recording from several nerve roots. The five flexor motoneurones supplying each segment are serially homologous, smaller units have higher discharge frequencies, and the excitability of a given unit is generally higher in more caudal ganglia. 2. Even the smallest axons are capable of generating substantial muscle tension at their spontaneous discharge frequencies. Tension development is extremely tonic. Single motor impulses are without effect, latencies are long, and the frequency/tension relation is steep. 3. The inhibitory axon to each flexor discharges during ‘extension’ reflexes, but has no visible effect upon relaxation time, or upon the response to subsequent excitation. Inhibitory impulses do not relax previously unexcited muscle. 4. Phase histograms for the discharge of pairs of homologous or non-homologous efferent axons across a single ganglion, within the same root, and between adjacent ganglia have revealed several coupling mechanisms. One is mediated by short time-constant electrotonic junctions between bilaterally paired inhibitory axons. The second involves weaker, more generalized interactions of longer time-constant between non-homologous axons. The third is brought about by common presynaptic sources of excitation. Inputs tend to be common for homologues in adjacent ganglia rather than for non-homologues in a single ganglion, a finding consistent with the selectivity shown by central interneurones in their effects upon motoneurones.