Intracellular potentials from intrafusal muscle fibers evoked by stimulation of static and dynamic fusimotor axons in the cat.

1. Membrane potential changes of single intrafusal muscle fibres were intracellularly recorded in spindles of cat's tenuissimus muscle on stimulating single static or dynamic fusimotor axons.2. One third of responses elicited on stimulating static fusimotor axons were action potentials while the remainder were junction potentials. Repetitive stimulation of static axons eliciting junction potentials produced summation, facilitation and in some instances the appearance of propagated potentials.3. All the responses evoked on stimulating dynamic fusimotor axons were junction potentials which summated but never produced propagated potentials during repetitive stimulation.4. Most of the impalements leading to junction potentials were located in the transition zone between intra- and extra-capsular regions of spindle poles.5. The relation between extracellular and intracellular potentials elicited by stimulation of a fusimotor axon (static or dynamic) makes it possible to assign a physiological nature to the junction potentials recorded intracellularly and to exclude effects attributable to injury.6. A coupling between junction potential and local contraction is indirectly inferred from the frequencygrams which have been previously obtained during single shock stimulation of dynamic fusimotor axons.