descending tracts a brief overview for students of MBBS 1st prof.

1. Slide 1
BY: DR. ANJU JHA
Slide 2
Descending motor pathways
Lateral system pathways-Corticospinal tract,
corticobulbar tract, Rubrospinal tract
Medial system pathways-Vestibulospinal tract,
Reticulospinal tract.
Slide 3
 The descending tracts, concerned with the various
motor activities of the body.
 Theses are formed by motor nerve fibers arising from
the brain.
 These tracts descend into brain stem and spinal cord.

2. Slide 4
Traditional division of these tracts :
 Pyramidal tracts.
 Extra pyramidal tracts.
Slide 5
Division by the location in spinal cord:
 Lateral system pathway
 Medial system pathway.
Slide 6
Pyramidal tracts:
 Also known as cortico-spinal pathway.
 Constituted by the axons that transmit motor signals
from cortex to spinal cord.
 Origin of fibers of cortico-spinal tract :
 Primary motor cortex (area 4)-30%.
 Pre-motor area(area8) and supplementary motor area-
30%.
 Somatic sensory areas 40%.

3. Slide 7
Slide 8 Course and termination
Origin from cerebral cortex
Descends as part of corona radiata
Pass through posterior limb of the internal capsule
Through brainstem, forming pyramids in the medulla.
In lower part of medulla about 80% fibers of each pyramid
decussate in the midline
This decussation results in formation of two separate tracts.
Lateral corticospinal tract and Anterior corticospinal tract
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4. Slide 10
Lateral corticospinal tract:
 Constituted by 80% fibers which have crossed at
pyramid.
 These fibers descend through full length of spinal
cord.
 Location of the tract in spinal cord is posterior part of
lateral funiculus.
 Fibers terminates in the internuncial neurons of the
spinal grey matter.
Slide 11
 The internuncial neurons carry impulses to motor
neurons situated in the ventral grey horn.
 Some fibers terminates directly on ventral horn cells.
 The axons of ventral motor neurons supply the skeletal
muscles directly by passing through ventral root of
spinal cord.
Slide 12
 The neurons giving origin to the fibers of pyramidal
tract along with their axons constitute upper motor
neurons.
 The ventral motor neurons in the spinal cord along
with their axons constitute lower motor neurons.

5. Slide 13
Slide 14
Anterior corticospinal tract:
 Formed by 20% uncrossed pyramidal fibers.
 These fibers descend down through the anterior white
funiculus of the same side.
 These fibers do not reach further then mid thoracic
region.
 On reaching appropriate level of the spinal cord the
fibers cross the midline.
 They reach grey matter on opposite side of the cord.
 They terminate in similar manner as lateral
spinothalamic tract fibers.
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Salient features:
 Fibers of corticospinal tract are unmyelinated at birth.
 Myelination begins in the second postnatal week and
completed by 2yrs.
 80% fibers are small diameter (1-4µm)
 20% fibers are of large diameter (11-22µm).
 Large diameter fibers originate from cells of Betz
present in primary motor cortex.
 Large diameter fibers function is reduced in old age
resulting in to automatic shaking movements of old
age.

6. Slide 16
Function:
 Control of voluntary fine skilled movements of the
body though corticospinal tract.
Slide 17
Corticobulbar Tract:
Origin:
 These fibers arise from the primary cortex, frontal
lobe.
 The tract descends through the corona
radiata and genu of the internal capsule with a few
fibers in the posterior limb of the internal capsule
down to the midbrain.
 The corticobulbar fibers exit at the appropriate level
of the brainstem to synapse on the lower motor
neurons of the cranial nerves.
Slide 18
 In addition endings in these motor neurons, fibers of
the corticobulbar tract also end in the sensory nuclei
of the brainstem including gracile nucleus, cuneate
nucleus, solitary nucleus, and all trigeminal nuclei.

7. Slide 19
Functions
 The muscles of the face, head and neck are controlled
by the corticobulbar system, which terminates
on motor neurons within brainstem motor nuclei.
 The corticobulbar tract directly innervates the nuclei
for cranial nerves V, VII, XI, and XII.
 The corticobulbar tract also contributes to the motor
regions of X in the nucleus ambiguus.
Slide 20
Slide 21
Extrapyramidal tracts:
 Rubrospinal tract.
 Vestibulospinal tract.
 Reticulospinal tract.
 Tectospinal tract.
 Olivospinal tract.

8. Slide 22
Slide 23
Rubrospinal tract
 Origin : nerve cells in Red Nucleus.
 Path : fibers cross the midline and descend as
rubrospinal tract through pons and medulla and
terminate in anterior grey column of the spinal cord.
 Present in lateral column of spinal cord.
 Function: facilitate the activity of flexor muscles.
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