lect 2 descending motor improved pathways

7/29/2019 Lect 2 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 & supplementary motorarea.

40% from somatic sensory area of cerebral cortex.

All these fibers converge through corona-radiata towardsinternal capsule.

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

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

When tract passes through the internal capsule, fibers forcervical parts of body are in genu & fibers for lower parts ofbody 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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PONS

MEDULLA

PYRAMID


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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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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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CORTICO SPINAL TRACTCORTICO BULBAR

FIBERS

BRAIN STEM

CRANIAL NERVE NUCLEI

CORTEX

SPINAL CORD


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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.

When these bundles enter medulla, these gather /unite along the anterior border of medulla to form thepyramid (a swelling). The tract is named pyramidalbecause of pyramid formation along anterior border of

medulla. 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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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 arefrom supplementary motor area, for control ofattitudinal or positional movements,e-g, in climbingposture.


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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 cervical segments,35% in lumbar& 20% in thoracic

segments of spinal cord.


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Most of cortico-spinal fibers for their

termination first synapse with inter-

neurons which in turn synapse with motorneurons (alpha & gamma).

Only fibers which arise from Betz cells

(Giant pyramidal cells), which form only3% fibers, terminate directly into motor

neurons.


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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 brain

stem these fibers cross over to opposite side toterminate with motor neurons in nuclei ofcranial nerves.


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Fig 55-5, Guyton


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lect 2 descending motor improved pathways

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