lect 1 descending motor improved pathways

7/29/2019 Lect 1 Descending Motor IMPROVED Pathways

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Descending motorpathways / tracts:1) PYRAMIDAL / CORTICO-SPINAL.

2) EXTRA-PYRAMIDAL / EXTRA CORTICO-

SPINAL. (All the descending motorpathways, other than pyramidal are calledextrapyramidal).


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PYRAMIDAL TRACTS: Consist of 1 million fibers.

Mostly myelinated fibers, but slow conducting.

Tract originates from cerebral cortex.

30% of fibers originate from primary motor area.

Another 30% from pre-motor area & supplementarymotor area.

40% from somatic sensory area of cerebral cortex.


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All these fibers converge through corona-radiata

towards internal capsule.

In internal capsule, the tract occupies the genu (thebent portion) & anterior 2/3 ofposterior limb.

Then tract passes into midbrain, where it occupiesmiddle 3/5 of cerebral peduncle / crus cerebri.

When tract passes through the internal capsule, fibersfor cervical parts of body are in genu & fibers for lowerparts of body are in posterior limb of internal capsule.


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Corona radiata

Genu ofint. capsule

has fibers for cervical

parts of body

Ant. 2/3 ofpost. Limb

ofint. cap has fib for

Lower parts of body

Transverse ponto-

cerebellar fibers break thetract into small bundles

In cerebral peduncle,

fibers for cervical parts

are medial

In cerebral peduncle,

fibers for lower parts

are lateral

Fibers converge through

towards internal capsule

PONS

MID BRAIN


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In cerebral peduncle, fibers for cervical parts of bodyare medial & fibers for lower parts are lateral.

From midbrain, tract enters the pons, where tract isbroken into small bundles by transverse ponto-cerebellar fibers.


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When these bundles enter medulla,

these gather / unite along the anteriorborder of medulla to form the pyramid (aswelling).

The tract is named pyramidal because ofpyramid formation along anterior borderof medulla.


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PONS

MEDULLA

PYRAMID


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In lower part of medulla, 80% of fibers

cross over to opposite side, to form

MOTOR DECUSSATION.

(Sensory decussation: fibers from

nucleus gracilus & cuneatus).


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LOWER MEDULLA

SPINAL CORD

80% fibers cross

MOTOR DECUSSATION

Lat cortico-spinal tract

Ant cortico-spinal tract


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Fig 55-4, Guyton


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After motor decussation, crossed fibers enter lateralwhite columns of spinal cord to form lateral cortico-spinal tract.

Fibers of tract which remain uncrossed, enter anteriorwhite column of spinal cord to form anterior cortico-spinal tract.

Fibers ofanterior cortico-spinal tract terminate inventral horn of cervical & upper thoracic segments ofspinal cord.

These fibers terminate onto motor neurons ( aplha &

gamma).

It is thought that these fibers are from supplementarymotor area, for control of attitudinal or positionalmovements,e-g, in climbing posture.


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Fig 55-4, Guyton


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Fibers oflateral cortico-spinal tract

terminate onto the motor neurons in

ventral horn of spinal cord at variouslevels.

From cortico-spinal tract as a whole,

45% fibers terminate in cervicalsegments,

35% in lumbar

20% in thoracic segments of spinalcord.


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Alpha / gamma

motor neuron

3% fibers,

no inter-neurons

BETZ CELLS

SPINAL CORD

Other fibers,

inter-neurons

CERVICAL

THORACIC

LUMBAR

45%

20%

35%

SPINAL CORD


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Most of cortico-spinal fibers for their

termination first synapse with inter-neurons which in turn synapse with motor

neurons (alpha & gamma).

Only fibers which arise from Betz cells

(Giant pyramidal cells), which form only

3% fibers, terminate directly into motor

neurons.


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CORTICO SPINAL TRACTCORTICO BULBAR

FIBERS

BRAIN STEM

CRANIAL NERVE NUCLEI

CORTEX

SPINAL CORD


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Certain branches of cortico-spinal tract go todifferent brain parts:

To caudate & lentiform nuclei.

To red nucleus.

To olivary nuclei & reticular formation.

There are also cortico-bulbar fibers whichcome along cortico-spinal tract. In the brainstem these fibers cross over to opposite side toterminate with motor neurons in nuclei ofcranial nerves.


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lect 1 descending motor improved pathways

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