lect 1 descending motor improved pathways
TRANSCRIPT
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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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