Journal of Neuroscience, Vol 5, 1434-1445, Copyright © 1985 by Society for Neuroscience
Afferent perturbations during "monopodal" swimming movements in the turtle: phase-dependent cutaneous modulation and proprioceptive resetting of the locomotor rhythm
PR Lennard
Locomotion consists of a repeating series of movement cycles (locomotor
rhythm) with an orderly activation of musculature during each movement
cycle (intracycle motor pattern). The effects of sensory stimulation, on
both the intracycle motor pattern and the locomotor rhythm, were examined
during electrically elicited swimming movements of a single turtle
hindlimb. The resulting "monopodal" swimming was not subject to
movement-related reflexes from other limbs or postural constraints, and
provided a sensitive system for analyzing the effects of transient sensory
perturbations. During "monopodal" swimming, cutaneous and extensor
muscle-nerve stimulation (single 0.1- to 0.3-msec electrical pulse) had
similar phase-dependent effects on the swim cycle in progress. Stimuli
delivered during the powerstroke (limb retracting) shortened the period of
the cycle. Stimulation during the returnstroke prolonged the cycle. Changes
in cycle period were accompanied by in- phase adjustments of the EMG burst
duration or interburst interval which was being expressed at the time of
stimulus delivery. The in- phase adjustment of each muscle served to
maintain the timing relationships between muscles, and resulted in the
preservation of the intracycle motor pattern. Cutaneous and muscle-nerve
stimulation had dramatically different effects on the locomotor rhythm.
Cutaneous nerve stimulation produced period changes in poststimulus cycles
which led to a temporary phase shift of the swimming rhythm. This temporary
modulation suggests that cutaneous afferents do not have direct access to
the timing circuitry of the central nervous system locomotor network.
Muscle-nerve stimulation only altered the period of the cycle in progress
at the time of stimulus delivery, and thus permanently reset the locomotor
rhythm. This permanent phase shift suggests that muscle afferents have
direct access to a central timing network which controls the locomotor
rhythm.
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