Responses of muscle spindles in a tortoise.

Responses were recorded from muscle spindles in the extensor digitorum longus muscle of the freshwater tortoise Chelodina longicollis. Most spindles showed no resting discharge unless the muscle was stretched up to a millimetre beyond the minimum body length. The firing rate then increased by 2-5 imp./sec/mm extension. All spindles were relatively insensitive to the rate of change of muscle length. The dynamic index was small and increased in direct proportion to the rate of stretch. Stimulation of selected filaments of the nerve produced a response from the spindle that was attributed to intrafusal contraction. A comparison of the response of the spindle to muscle stretch with the response during simultaneous stretch and stimulation of a motor filament revealed two distinct types of behaviour. In the first, a dynamic effect, motor stimulation greatly increased the response of the spindle to the velocity component of stretch. Other filaments had a static action: an increase in firing of the spindle with the muscle at constant length, but little change in the response to the velocity component of stretch. Stimulation of up to 7 different filaments produced an effect on a spindle. When the motor effects for a given spindle could be clearly characterised, they were all static or all dynamic. Both static and dynamic effects on the one spindle were seen only rarely.