Innervation of the Abdominal Intersegmental Muscles in the Grasshopper

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Abstract
1. Extracellular records from the motor neurones to the median internal dorsal muscles (abdominal intersegmental) of Melanoplus differentiate have been made simultaneously with intracellular records from the muscle fibres. 2. Four types of PSP have been recorded intracellularly: type 1, a large PSP with a regenerative response and a small overshoot; type 2, a PSP of between 5 and 15 mV; type 3, a PSP of between 1 and 5 mV; type 4, a hyperpolarizing PSP of up to 5 mV. 3. Although there is variation in the size of the spikes recorded en passant, the four types of PSP can generally be correlated with large, medium, small and very small spikes. 4. Evidence is presented to demonstrate that there are two motor units for each type of PSP in the dorsal intersegmental muscles. Each muscle appears to have a mixture of muscle fibres of each unit. 5. This is consistent with the anatomical finding of eight motor neurones supplying the muscles.