2668554 control of posture and movement
TRANSCRIPT
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Control of Posture
and Movement Somatic motor activity
depends on the pattern andrate of discharge of spinal
motor neurons.
These neurons, the finalcommon paths are bombarded
from array of pathways.
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y p y
The inputs converging on the motor neurons subserve
three semi distinct functions through the:
1. Pyramidal System ( cort ico spinal pathways )= they bring about voluntary activity
Fig. 12-1
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2. Extrapyramidal System - theyadjust
body posture to provide stable
background for movement.Concerned
with grosser movements andposture
3.Cerebellum –
coordinating and
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Control of Axial and distalMuscles
Medial or Ventral Pathways andneurons are concerned with control
of muscle of the trunk and proximal portions of the limbs
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Lateral pathways are concerned
with the control of muscles in the distal portions of the limbs
Axial muscles are concerned withpostural adjustment and grossmovements
Distal limb muscles are thosethat mediate fine, skilledmovements
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CORTICOSPINALTRACT
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Anatomy and Function
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The fibers that cross the midline
in the medullary pyramids andfrom the lateral corticospinaltract make up about 80% of thefibers in the corticospinal
pathway.
20% of the fibers make up theanterior or ventral, corticospinaltract
The lateral corticos inal tract is
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Cortical Motor Areas
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30% of the fibers making up thecorticospinal tracts come fromthe motor cortex
30% comes from the premotor cortex
40% from the parietal lobeespecially the somatic sensory area
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The cortical representation of
each body part is
propo rt ionate in s ize to the ski l l with which the part is
used for fine, voluntarymovement.
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Effects of Section or Destruction
of Pyramidal System
A. Role in Movement
Effects of Section or Destructionof the Lateral Corticospinal Tract
loss of ability to grasp small
objects between two fingers andto make isolated movements of the wrist
can still use the hand in a gross
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These deficits are consistent with loss of
control of distal musculature of the limbs ,
which is concerned with fine skilled
movements
Lesions of Ventral Corticospinal Tract
produce axial muscle deficits that cause difficulty withbalance, walking and climbing
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B. Effects on Stretch Reflexes
prolonged hypotonia and flacidityrather than spasticity
Damage of the lateral
corticospinal tract produces
Babinski sign: dorsiflexion of the great toe and fanning of the
other toes when the lateral aspect of the sole of the
foot is scratch
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POSTURE-REGULATING
SYSTEMS ( Extrapyramidal
Mechanisms)
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When the neural axis istransected, the activities below
the section are cut off orreleased from the “control of higher brain centers” and often
appear to be accentuated
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Levels o f In tegrat ion
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SPINAL INTEGRATION
Spinal Shock –
results fromtransection of the cervical spinal cord
all spinal reflexes are depressed
duration of the shock depends uponthe degree of encephalization
frogs and rats – lasts for minutes dogs and cats – lasts for 1-2 hours monkeys – lasts for days humans – minimum of 2 weeks
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The recovery of the reflex
excitability may be due to:
* development of denervation
hypersensitivity to the mediatorsby the remaining spinalexcitatory endings
* sprouting of collaterals fromexisting neurons
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The first reflex response to appear as spinal
shock wares off is sl ight contract ion of the
leg f lexors and addu ctors in response to
nox ious st imu lus
Responses of Chronic SpinalAnimal
* Magnet reaction (positivesupporting reaction)
* Autonomic reflexes – reflexcontraction of full bladder and rectum
* Sexual reflexes
* Mass reflex - evacuation of bladder
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II. MEDULLARY COMPONENTS
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Hindbrain and spinal cord areisolated from the rest of the
brain by transection of the brainstem at the superior border of the pons. Procedure is called
Decerebration
Decerebrate rigidity develops as
soon as the brainstem istransected
It is found to be spastic due to
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Facilitation is due to two
factors:
increased general excitability of
the motor neuron pool
increase in the rate of discharge
in the gamma efferent neurons
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Characteristics of
Decerebrate Rigidity
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1. Decerebrate Posture –
“ Caricature of the normal standing position”
– neck and limbs extended, back arched,
tail elevated.
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2. Tonic Labyrinthine Reflexes
no righting reflexes are present,and the animal stays in positionwhere they are put
rigidity in the limbs varies withposition
if the animal is placed on its back
extension of all 4 limbs ismaximal
as the animal is turned to eitherside, rigidity decreases
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3. Tonic Neck Reflexes
Rigidity changes with headmovement
head turned to one side limbs on
that side (jaw limb) become morerigidly extended, while thecontralateral limb become less
flexion of the head causes flexionof the forelimbs and extension of thehindlimbs
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III. MIDBRAIN COMPONENTS
Midbrain Animal – produced bysection of the neural axis at the
superior border of the midbrain
Chronic midbrain animal can riseto the standing position, walk,
and right themselves
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Manifestations:
A. extensor rigidity – whenanimals lies quietly on its back
B. Righting reflex – to maintain
the normal standing positionand keep head upright
1. head righting reflex
2. neck righting reflex 3. body righting reflex
C. grasp reflex
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IV. CORTICAL COMPONENTS
Decortication (removal of
the cerebral cortex) produceslittle motor deficit.
Decorticate Animal
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Effects of Decortication
1. decorticate rigidity occurs only whenanimal is at rest
2. Placing and Hopping reactions aredisrupted
Hopping movements – keep the limbsin position to support the body whenanimal
standing is pushed laterally
Placing reactions – place the footfirmly on the supporting surface