Postsynaptic currents in phasic and tonic muscle fibers of the rat extraocular muscle

Abstract

Focal extracellular recordings were made of postjunctional currents produced at synapses of the inferior rectus eye muscle fibers by the spontaneous release of quanta of transmitter. These consisted of miniature endplate currents, or MEPC, in phasic fibers and miniature postjunctional currents, or MPJC, in tonic fibers. Open time of ionic channels (τchan) was also registered. In tonic fibers, MPJC lasted considerably longer than MEPC did in phasic fibers: rising time, decay time, and τchan in the former measured respectively 2.5, 4–5, and 2.2 times higher than in the latter. Acetylcholinesterace (AChE) inhibition produced a much greater (4.4-fold extension of current decay in phasic than in tonic fibers, where a 1.8-fold increase was seen, thereby reducing the gap between the decay time of currents in these fibers to a difference of 1.6 times. The more protracted decay of MPJC in tonic fibers compared with MEPC in phasic fibers is determined by the lower functional activity of AChE as well as the higher value of τchan. Duration of MEPC and magnitude of τchan in the "slow" phasic fibers of rat skeletal muscles fell well below the same parameters measured in the tonic fibers of the ocular muscle.