Reticulospinal influences on segmental neurons inLampetra fluviatilis

Abstract

By recording the activity of single Muller's axons extra- and intracellularly, the electrical properties of fibers of the descending tract were determined during stimulation of the vestibular system, optic nerve, and spinal cord. The amplitude of the action potentials (APs) is 60–80 mv, their duration 3 msec, the highest frequency of responses to rythymic stimulation 170–250/sec, and the velocity of conduction of excitation is 5.4±0.4 m/sec along fibers of the ventromedial tract and 2.4±0.4 m/see in the lateral tract. These velocities remain unchanged over segments of the spinal cord up to 20 cm long. In response to stimulation of the descending tract by single pulses, only 1 AP is generated in Muller's axons whatever the strength of stimulation. In response to synaptic stimulation of Muller's neurons, discharges of regular frequency and duration are generated in axons of the descending tract just as in the neurons themselves. Transmission of excitation from Muller's axons to segmental motoneurons can take place with segmental delay of less than 1–3 msec, indicating the existence of mono-synaptic connections between these structures. Motoneuron activity in response to stimulation of the descending tract consists of generation of 1–6 APs at intervals of 7–20 msec. The suggestion that there are connections between one motoneuron and several Muller's axons is discussed.