Properties of Action Potentials from Insect Motor Nerve Fibres

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Abstract
The properties of nerve action potentials in small insect motor nerves were studied using extracellular recording electrodes. A length of nerve was lifted out of solution and recordings were made with respect to the solution either from an intact nerve (triphasic recording) or from near a cut end of the nerve (monophasic recording). In a cockroach nerve, the number of spontaneously active fibres was small enough that corresponding nerve fibres could be identified in each preparation by their action potential amplitude and their pattern of activity. Under controlled conditions, the absolute amplitudes of either monophasic or triphasic records were reproducible and could be used to calculate fibre diameter. The calculations were confirmed from histological sections of the nerve. Conduction velocity varied approximately as the 0·78 power of fibre diameter in a cockroach nerve and as 0·7 power of fibre diameter in a locust nerve. These values are considerably larger than the square root relation predicted if membrane properties are independent of fibre diameter. Membrane properties probably vary with fibre diameter since the action potential duration increases dramatically for fibres below 5 μ in diameter. For the cockroach nerve systematic structural differences between fibres of different sizes are also seen with the electron microscope and the relation of these to the functional differences is considered.