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CONTROL OF BODY
MOVEMENT AND POSTUREIntroduction
oCoordinated movement in a body is a complicatedprocess and involves nerves muscles and bones.
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The skeletal muscle contraction in the body is controlled through the motorunit which includes a single motor neuron and all the muscle fibres suppliedby it and all the motor neuron for a given muscle make up a motor neuron pool.
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When all the motor neuron in a pool get activated at one time leads to acoarse movement.Therefore the skeletal muscle activity depends ultimatelyupon the pattern and rate of discharge of motor neuron.
Inputs(information)to each spinal motor neuron end directly on alpha motorneurons which innervate the extrafusal muscle fibre which form the finalcommon pathway over which the neural control system coordinates theactivity of skeletal muscle fibres.
skeletal muscle contraction skeletal musclerelaxation
Therefore alpha motor neuron is the final common pathway serving bothAs integrating centreand efferent pathway.
Alphamotorneuron
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Many other inputs exert their effects via interneuronsor via gamma motorneurons which innervates the intrafusal muscle fibre.
ipsilateral inputs contralateral inputs
(from same side of (from opposite side of body)body)
Stretch reflex,tone crossed extensorAnd posture(muscle reflexSpindle)
Flexion reflex,noxious tendon reflexStimuli(skin)
Relax antagonist recurrent inhibition through(muscle spindle) renshaw cell
Tendon reflex
final common pathway
Major inputs to the cell body of a single spinal motor neuron
Spinal alpha
motor neuron
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Voluntary movement is planned within brain and the commands are sent tomuscles via descending pathways or motor pathways as follows:
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IN BRAIN AND SPINAL CORD
anterior descending pathway lateral pathways
concerned with the control of muscles concerned with the controlof trunk and proximal parts of limbs. of distal group of muscles.
control posture regulating system control fine,skilledfor postural adjustments and gross movements.movements.
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Levels of motorcontrol system
Highest level:The cerebral
cortex
Middle level:
subcorticalcenters
Brain stem andspinal cord
.levels of organization of neural system controlling body movement
Sensory motorcortex
1.Subcortical nuclei2.Brain stem.
Association cortex
Other brain areas
Spinal cord
Muscle fibres Receptors inmuscles,tendons,joints,
skin.
Motor neurons
Basal gangliacerebellum
Via descending pathway
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Cerebral cortex generate idea for voluntary movementand issue motor commands.
Subcortical centres i.e convert complex plans and
sensory motor cortex, ideas generated in cerebralCerebellum,basal ganglia, cortex into number of smallerSubcortical nuclei. motor programmes which
determines the pattern of neuralactivation required to performthe movement.
Brain stem and spinal cord major relay stationfor all motorcommands.smaller motor programmesgenerated at subcortical level
results into execution of movement.at this level.
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In this block diagram is shown that after the initial motor programme is
Implemented and the action has begun,those brain regions concerned withMotor control continue to receive a constant feedbackofupdatedinformationFrom the receptors in periphery in the form of peripheral sensory input aboutthe movements taking place.
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Body posture is controlled by brain and reflex mechanism. Reflex mechanisms are concerned with maintaining upright static postureand that of maintaining balance.
Postural reflexesThese are the group of reflexes which maintains body position andequilibrium either during rest or during movement by changing thedistribution of muscle tone in limbs and trunks.
Components of reflex arc in postural reflex: eyes
vestibular appratus
proprioceptors
alpha motor neuron
In the above flowchart afferent pathwaysare the inputs received fromEyes,the vestibular appratus and proprioceptors which are relayed to the
Higher integrating centre i.e brain stemand spinal cordwhich further resultsInto execution of motor task via efferent pathwayor motor pathway.
Afferent pathway
Integrating centre
Efferent pathway
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Postural reflex has two adjustment components:
Static reflexes phasic reflexes
It involves sustained contraction Involve transient movements onof musculature for upright base provided by the static reflexesbalanced position and maintain to the body.a stable possible background forvoluntary activity.
A major factor in control of posture is variation in the threshold of the spinalStretch reflex.this in turn is caused by:
1. Changes in excitability of motor neurons;and2. Changes in the rate of discharge in the gamma motor neuronsto muscle
spindles.
Gamma motor neurons activity is regulated to a large degree by descendingtracts from number of areas in the brain.
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Thus the sensitivity of muscle spindle and hence the threshold of spinalStretch reflex in various parts of the body can be adjusted and shifted toMeet the needs of postural control.
Postural reflexs are integrated at various levels in CNS from the spinal cordto cerebral cortex
Levels of integration Principle function
1. Spinal cord Control of spinal reflexes
2. medulla Regulation of heart,respiration and antigravityreflexes.
3. midbrain Regulation of rightning reflexes
4. midbrain,thalamus Regulation of locomotor reflexes
5. hypothalamus,limbic
system
Emotional functions
6. Cerebral cortex Initiates involuntary movements,emotions andmemory involved in conditioned reflexes
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