6.control of muscle tone
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Control of Muscle Tone
By Dr D Fisher
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Cerebellar Awareness'
After MS stimulation (stretch) APs are conductedalong the afferent fiber (Ia)
It enters into the spinal cord and divides into
several collaterals. Some of these collaterals synapse on the cell
bodies of neurons which ascend to thecerebellum (anterior and posterior
spinocerebellar tracts). Thus, at all times the cerebellum is aware of the
state of stretch in muscles, in other words theTONE of muscles.
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Cerebellar Awareness'
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Coactivation of Gamma Efferents
Whenever a motor command descends from the
motor cortex and synapses on neural cell bodies
which innervate muscles, collaterals from these
descending fibers also synapse on thecorresponding cell bodies (gamma efferents)
which innervates the ends of the intrafusal
muscle fibers.
This is important so that as the extrafusal
muscle fibers contract and shorten, the intrafusal
also shorten and remain taunt.
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Coactivation of Gamma Efferents
This enables the MS to always respond to
stretch even immediately after contraction
of a muscle. In other words the coactivation of gamma
efferents avoids 'silent periods' which
would occur if the intrafusal muscle fibers
did not contract simultaneously with theextrafusal muscle fibers.
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Coactivation of Gamma Efferents
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The Golgi Tendon Organ
The Golgi tendon organ is a receptor
found within the tendons of muscle.
It detects tension >100 grams in the
tendon.
It is innervated with a 1b afferent fiber.
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The Golgi Tendon Organ
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Summary
Golgi tendon organs detects tension in thetendon.
Afferent neurons conduct action potentialsto the spinal cord.
Afferent neurons synapse with inhibitory(inter) association neurons (secretesGABA) which in turn synapse with alphamotor neurons.
Inhibition of the alpha motor neuronscauses muscle relaxation, relieving thetension in the muscle.
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Control ofSkeletal Muscle
The prefrontal area has association areas
for the motivation and foresight to plan
and initiate movements.
In the premotor area motor functions are
organized before they are initiated in the
Motor Cortex.
The motor cortex (primary motor cortex) is
found on the precentral gyrus.
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Homunculus
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Homunculus
Definition: It is the literal representation ofconnected parts of the human body on thesurface of the brain.
The surface of the postcentral gyrus providesthe literal somatosensory homunculus,
while the surface of the precentral gyrusprovides the literal motor homunculus.
Please note that the sensory homunculus isalmost a mirror image of the motor homunculus.
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The Somatic MotorSystem
The Somatic motor system (SMS) can be
divided roughly into 3 components:
The Pyramidal System
Extrapyramidal System
Cerebellar System
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The Pyramidal System
(direct system) Involved with fine, discrete and precise
voluntary control of movement.
The command arises from the precentralgyrus (where the somatic motor cortexresides).
Fibers from the cortex descend to the
spinal cord where they synapse on theanterior horn motor neurons (alpha andgamma).
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Descending Spinal Pathways
Direct Control muscle tone
and conscious skilledmovements
Direct synapse of uppermotor neurons ofcerebral cortex withlower motor neurons inbrainstem or spinal cord
Tracts
Corticospinal
Lateral
Anterior corticobulbar
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The Extrapyramidal System
(indirect system)
Its main function is to provide course
background voluntary movement.
Arises from various sites on the cerebralcortex.
Interconnected with the pyramidal system.
The EPS is directly connected to theBasal Ganglia
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Descending Spinal Pathways
Indirect Synapse in some
intermediate nucleus
rather than directlywith lower motorneurons
EPS Tracts Rubrospinal
Vestibulospinal
Reticulospinal
Testospinal tract
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The Cerebellar Component
Function: To enable smoothcoordinated movement
plays an important role in themaintenance of posture andequilibrium.
Sends fibers via the cerebellar-spinal
tract to modulate the activity of thealpha (lower motor neurons) as well asthe gamma motor neurons.
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The Cerebellar Functions
The cerebellum receives information from a
wide array of senses:
Pressure and touch
Positional receptors: Spindle and Golgi tendonorgans
Eyes and ears
A copy of the command signal is sent from the
cerebral cortex to the cerebellum, where itacts as a comparator.
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Cerebellar Comparator
Function
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The Basal Ganglia
What is the Basal Ganglia?
Paired masses of gray matter in each cerebral
hemisphere
Largest Nuclei Corpus Striatum:
Caudate nucleus
Lentiform nucleus:
Putamen and globus pallidus
Claustrum and amygdaloid
Substantia nigra, subthalamic nucleus and red
nucleus.
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Neural Connections of the Basal
Ganglia
The basal ganglia are interconnected by
many fibers.
They have fibers connected to the:
Cerebral cortex (ascending fibers)
Thalamus and Hypothalamus Descending fibers to Gamma motor fibers
(sp.Cord)
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Clinical Manifestation
Abnormal body movements: Tremor (uncontrollable shaking)
Involuntary movements of the skeletal muscles
Paralysis Akinesis: (destruction of the caudate(most affected site in strokes) results in paralysis inthe opposite side of the body).
Globus pallidus: mostly concerned with muscle tonefor specific body movements.
Lesion in the subthalamic nucleus hemiballisms,jerky movements, spontaneous movements of thearms (affects the extremities legs and arms)
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Other Clinical Manifestations
Parkinsonism:
Concentration of dopamine in certain of
the basal ganglia is reduced. Treatmentinvolves giving the patient L-dopa which
passes the B-B Barrier.
Alzheimers:
Related to the degeneration ofsubstantia
innominata (basal ganglia nuclei)