VOLTAGE-GATED SODIUM CHANNEL LOCATION IN A SOMATIC MECHANORECEPTOR: THE MUSCLE SPINDLE. Michael Chua and Carlton C. Hunt. Department of Cell & Molecular Physiology, University of North Carolina, Chapel Hill.
Muscle spindle Ia afferent nerves respond to mechanical stimuli by firing actions potentials with a frequency and pattern that is a complex function of stimulus waveform and intensity. This process occurs as a result of mechanical stimuli being transmitted by intrafusal muscle fibers to Ia afferent terminals, which transduce deformations into receptor potentials. Receptor potentials can initiate action potentials at a site that has not been well defined but is presumed to be rich with voltage-gated sodium channels. In order to better understand the process that encodes mechanical stimuli into action potential trains we studied the distribution of voltage-gated sodium channels immunocytochemically in cat muscle spindles. The primary antibody (AP1380-10.1, kindly provided by Dr. Rock Levinson, University of Colorado), binds to vertebrate voltage-gated sodium channels. Small muscle bundles containing both extrafusal neuromuscular junctions and muscle spindles were labeled. A high density of voltage-gated sodium channel labeling was observed at the sub-synaptic membrane of neuromuscular junctions as well as at the nodes of Ranvier of motor axons, confirming the results of others. In muscle spindles we found that the sarcolemmas of intrafusal muscle fibers were well labeled, consistent with conduction of action potentials in these specialized muscle fibers. However, the terminal Ia afferent axons, which are unmyelinated and wrap around the intrafusal muscle fibers, had no discernable labeling. These terminals are known to produce receptor potentials in response to mechanical deformation. Myelinated regions of the same axons displayed strong labeling at nodes of Ranvier. Such nodes close to sensory terminals are sites where action potential firing is likely to be initiated.