2668554 Control of Posture and Movement

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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.



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



2. Extrapyramidal System - theyadjust

body posture to provide stable

background for movement.Concerned

with grosser movements andposture

3.Cerebellum –

coordinating and



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



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



CORTICOSPINALTRACT



Anatomy and Function



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



Cortical Motor Areas



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



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.



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



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



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



POSTURE-REGULATING

SYSTEMS ( Extrapyramidal

Mechanisms)



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



Levels o f In tegrat ion



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



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



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



II. MEDULLARY COMPONENTS



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



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



Characteristics of

Decerebrate Rigidity



1. Decerebrate Posture –

“ Caricature of the normal standing position”

– neck and limbs extended, back arched,

tail elevated.



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



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



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



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



IV. CORTICAL COMPONENTS

Decortication (removal of

the cerebral cortex) produceslittle motor deficit.

Decorticate Animal



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

2668554 Control of Posture and Movement

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