Synaptic activity in motoneurons during natural stimulation of muscle spindles.

Synaptic activity evoked in cat motoneurons during stretch stimulation of muscle spindles in the homonymous muscle has been studied by intracellular recording. A class of miniature excitatory postsynaptic potentials evoked by such physiologic stimulation results from activity in single group la spindle afferent fibers, and the criterion for identification is a predictable pattern of rhythmic occurrence of the synaptic potentials. Statistical examination of the amplitude distributions of this class of miniature synaptic potentials suggests that some group Ia fibers liberate a relatively large number of quantal excitatory postsynaptic potential units per fiber impulse.