Development of isometric tension in simian extraocular muscle.

1. Isometric tension resulting from electrical stimulation of the abducens nerve was measured in the lateral rectus muscle of anaesthetized monkeys.2. The lateral rectus showed no increase in maximum tension at stimulus frequencies greater than 400/sec.3. However, at frequencies between 400 and 600/sec, the rate of change of tension increased so that maximum tension was achieved more rapidly.4. Rates greater than 600/sec caused no further increases in the rate of change of tension and actually resulted in a decrease of maximum tension.5. Since, in addition, single fibre recordings from ocular motor nerves reveal maximum discharge rates of less than 600/sec, units in the abducens nucleus that fire at greater than 600/sec are likely interneurones.6. Stimulation at rates of 30/sec, the frequency at which feline slow or slow twitch fibres produce fused contraction, produced no measurable tonic component of tension. Therefore, the force contributed by slow muscle fibres probably plays an insignificant role during eye movements.