Journal of Neurophysiology, Vol 53, Issue 1 76-88, Copyright © 1985 by APS
Action potential patterns of isolated frog muscle spindle in response to sinusoidal stimulation
H. Querfurth
Signal transfer in the isolated frog muscle spindle is investigated using
the linear frequency domain analysis technique. Sinusoidal stretches of
different amplitudes (20-120 micron) and frequencies (0.1-120 Hz) were
applied at different levels of static prestretch, ranging from resting
length (L0) up to L0 + 400 micron, so that the frequency-response
characteristics were measured at different operating points within the
dynamic range. The neuronal responses were recorded from the first node of
the afferent stem fiber with a modified air-gap technique. By this means,
subthreshold receptor potentials, prepotentials preceding the impulse, and
the propagated action potentials were recorded simultaneously, thus
providing a detailed insight into the encoding process. There is a
well-defined dynamic range of receptor responses. At L0, the encoding site
is depolarized to its firing level and discharges spontaneous
stimulus-independent impulses. The upper limit is given by the saturation
of the receptor potential, which keeps the depolarization maximum below the
level of sodium inactivation. Therefore a "depolarization block" or
"overstretch" does not exist in the muscle spindle; i.e., the receptor
retains its ability to encode information over a large range of dynamic and
static displacements. Since the dynamic curves of the receptor potential
are not symmetrical about their static operating point, the impulse pattern
remains modulated throughout the dynamic range, even if small sinusoids are
superimposed on a large static prestretch. The afferent discharge pattern
is mainly regulated by the modulated AC component of the receptor
potential. At low stimulus frequencies (less than 1 Hz) the receptor
potential modulates almost linearly about the mean membrane voltage, so
that the evoked discharge pattern displays a smooth analog signal, which is
close to sinusoidal. Increasing the static prestretch increases both the
peak response and the modulation depth of the impulse pattern. In the
intermediate frequency range (1-10 Hz), the cycle histogram disintegrates
into discrete peaks separated by empty bins, because the nonlinear receptor
potential elicites firmly phase-locked action potentials during its fast
depolarization transient. Raising the prestretch level improves the
precision of phase locking and increases the number of spikes elicited per
cycle.(ABSTRACT TRUNCATED AT 400 WORDS)
