Electrical Characteristics of the Membrane of an Identified Insect Motor Neurone

1. The electrical properties of the membrane of an identified locust motor neurone, the fast extensor tibiae in the metathoracic ganglion, have been investigated to determine: the distribution of excitable and inexcitable membrane; the impulse initiation zone; and the conduction velocity of the spike in the ganglion and in the axon. 2. The waveform of extracellularly recorded spikes indicates that the transition from inactive to active membrane occurs along the region of the neurite which bears many arborizations within the neuropile. 3. Measurements of the delay between orthodromically or antidromically evoked spikes, recorded at the soma and other points along the neurite, place the impulse initiating zone close to the transition between active and inactive membrane. 4. Within the ganglion, the spike is conducted at different velocities over different parts of the neurite. The average velocity within the ganglion is, however, only about a seventh of that in the axon (0-54 m.s" against 4-1 m.s").