Motor Unit Recruitment in a Distributed Model of Extraocular Muscle.
Department of Psychology, University of Sheffield, Sheffield S10 2TP,
APStracts 3:0065N, 1996.
SUMMARY AND CONCLUSIONS
1. Eye position commands are frequently treated as a lumped, single-valued
variable that is linearly related to eye position. As a step towards
investigating how system-level linearity might be achieved despite non-linear
components, a distributed model of motor units in the abducens nucleus and
lateral rectus muscle was constructed. 2. Parameters in the model were
estimated using data from three main sources: (i) measurements of length-
tension curves for eye muscle in people: (ii) electrophysiological recording
of ocular motoneuron properties in monkeys; and (iii) investigations of
oculomotor unit properties in cat. Units (n = 100) in the distributed model
were assigned equal strengths, and for a given fixation command the force
developed by each unit was calculated, and the sum of unit forces compared
with the active force in the entire muscle as measured experimentally. 3. The
properties of the active units were then adjusted to reduce the size of any
resultant error, in a manner related to gradient descent methods for neural-
net training. Distributed models were 'trained' in this fashion for a series
of eye positions drawn at random from the oculomotor range until performance
stabilised. The goal of the training procedure was to obtain a good match
between the output of the model and the experimental data on muscle force as a
function of eye position. 4. Plots of trained motor-unit strength against
ocular motoneuron threshold revealed a U-shaped pattern, with the strongest
units being recruited at both extremes of the oculomotor range, and the
weakest units recruited in the middle. The pattern remained qualitatively
unaltered over a range of assumptions about the distribution of ocular
motoneuron parameters, and the relation between motoneuron firing rate and
unit force. 5. The right hand limb of the U-shaped pattern is similar to that
observed in spinal motoneurons, where stronger units tend to have higher
recruitment thresholds. The left hand limb may reflect the two specialisations
of eye muscle: the functional need for very precise control of eye position in
the middle of the oculomotor range, and the use of multiply innervated muscle
fibres to provide ripple free control of eye position at low firing
Received 2 October 1995; accepted in final form 12 March 1996.
APS Manuscript Number J655-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 1 April 96