Role of Abortive Spike on Encoding Mechanism in Frog Muscle Spindle

Publisher Summary This chapter presents experiment to assess the hypothesis that the abortive spikes as a trigger may be summated with the generator potential to initiate impulses on the sensory nerve terminal, where inhibition may also occur by after-hyperpolarization. Amplitude and interval histograms of abortive and/or propagated spikes are constructed from trains of spontaneous discharges recorded from isolated and semi-isolated single-type spindles in frog sartorius muscle at steady states of various degrees of extension. The amplitude distribution of the spikes occurring within 70 msec following orthodromic propagated spikes, in which after-hyperpolarization was conspicuous, included a group distinctly larger in comparison with that occurring later than 70 msec. This implies that the larger abortive spikes may remain insufficient to reach the threshold for triggering propagated spikes during the after-hyperpolarization. Microapplication of tetrodotoxin produces a semi-blocked condition, in which propagated spikes disappears but abortive spikes survives. Amplitude histograms of the abortive spikes during this condition indicate that antidromic impulses are unable to invade the nerve terminal. Stretching the muscle spindle in this condition gives rise to a burst discharge of abortive spikes and half-sized action potentials.