Extraocular motoneuron stimulation frequency effects on motor unit tension in cat.
The contractile characteristics of 47 twitch and 3 nontwitch lateral rectus motor units in 11 cats were examined using two different stimulation paradigms derived from observed motoneuron firing frequencies in alert animals. The twitch units showed an average twitch tension (46.0 +/- 8.1 mg), contraction time (6.15 +/- 0.26 ms) and median fusion frequency (170 Hz) consistent with previous studies, kt value, defined as the slope of the relation between a series of constant frequency tetanic stimulation trains (lasting 200 ms) and tetanic tension, correlated with maximum tetanic tension (r = 0.984, p < 0.05). ktps value, defined as the slope of the relation between tetanic tension and a series of constant frequency stimulation trains (lasting 1975 ms) that immediately followed a 25-ms high-frequency burst (pulse/step paradigm), was similarly correlated (r = 0.853, p < 0.05) with maximum tetanic tension (x = 256.5 +/- 35.2 mg). ktps values were lower that kt values for each unit, but the units did not change their position in the numerical hierarchy. Eighty-four percent of the motor units produced different maximum tetanic tensions (x = 24 +/- 3%), when comparing pulse/step to constant frequency stimulation, but only 14% of those units produced a greater maximum tetanic tension during pulse/step stimulation. In contrast, 46% of the motor units contracted with more force during the step phase of the pulse/step paradigm than with constant frequency stimulation when the stimulation rate was below 120 Hz: 24% of the units contracted with less force. In addition, pulse/step stimulation frequency ranges above 120 Hz (step phase) were ineffective in increasing tension while higher frequencies continued to elicit tension increases during constant frequency stimulation. The impact of these tension variations on eye movement is discussed.
Shall
M S
MS
Department of Anatomy, Medical College of Virginia, Virginia Commonwealth University, Richmond 23298-0709, USA.
Wilson
K E
KE
Goldberg
S J
SJ
D000818:Animals; D002415:Cats; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology; D009801:Oculomotor Muscles / Q000502:physiology*