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45
EMG activity increased, whereas the anterior
temporalis EMG activity decreased
7
.
The mastication muscles have intrinsic
abilities to contract and relax. This is achieved
when an impulse is transmitted along the
motorneurone and reaches a group of
muscle fibers. The control of muscle activity
is not clear but it has been suggested that
the gamma efferent system is permanently
active, though it does not necessarily set up
movement, and that the gamma discharge
keeps the alpha cells reflex in preparation
for the reception of impulses arriving from
the cortex or for the receipt of afferent
impulses from the spindles. It is possible
that all but the fastest voluntary movements
are controlled by the link between the
gamma efferents, the spindle afferents and
the alpha motoneurones. This combined
output produces the required contraction
or inhibi t ion of the musc les, wi th the
neuromuscular system keeping, as it were, a
check on itself. Thus, impulses initiated in the
motor cortex signaling voluntary movement
synapse with the gamma motorneurone.
They travel to the spindle and become
integrated with the afferent output from the
spindle receptors. The combined output
passes back to the alpha motoneurone and
thence to the extrafusal fibers when the
relevant muscle contraction takes place. If
the afferent fibers returning from the spindle
are cut, the muscle fails to contract but the
gamma motorneurones continue to fire at
an increase rate. Therefore, the spindle
afferents exert an inhibitory action on the
gamma motoneurones but remain excitatory
to the alpha motorneurones
9
.
In previously study, the absence of visual
cues demons t rate the imprec i s i on of
maintaining a free-movement finger position.
However, the efficiency of jaw positioning
is not determined by visual feedback and
it probably relies on the well-developed
proprioceptive inputs
8
. The neuromuscular
s y s t em has t h r ee d i v i s i ons : ( 1 ) t he
pontomedullary from the lower centers,
(2) the mesencephalic from the midbrain,
(3) the cortical by voluntary control. It not
only controls posture and tone but also the
movement.
(1) The pontomedullary control includes
three nuclei in the pons and medulla:
the vestibular nucleus receives impulses
from the inner ear and maintain the
muscle tone and posture, the lateral
reticular nucleus is facilitatory and sends
a constant discharge of gamma motor
efferents to the muscle spindles involved,
and the medial reticular system receives
impulses from the cerebral cortex to
provide indirect cerebral inhibition.
(2) The mesencephalic nuclei controls
musc l e t one i n two ways , one i s
tectospinal necleus to catch the impulses
from eyes and ears to increase muscle
iomedical Engineering Association
The Influence of Visual Feedback on the Velocity of Mandibular Movement