NEUROANATOMY: Descending Pathways

AFALLA, ARTETA

Dr. John Vincent Estrada

DESCENDING PATHWAYS

Two clinically most important descending fiber tracts:1. Corticospinal Tract2. Corticobulbar tract

Areas in the cerebral cortex that are involved in the descending pathways:

1. Primary Motor Area (Brodman area 4 in the Precentral gyrus)

2. Premotor Area (Brodman area 6; rostral/ anterior to area 4)

Supplementary Motor Area – most medial aspect of Brodman area 6

3. Some motor neurons of the Postcentral gyrus (Brodman areas 3, 1, 2)

MOTOR NEURONS

Lower Motor Neurons Motor neurons in the anterior (ventral) gray horn of

the spinal cord. Also refer to the motor component of the cranial

nerve nuclei (ex: Occulomotor nuclei).

Upper Motor neurons Found in cerebral cortex and basal ganglia. Referred to as the corticospinal and corticobulbar

tracts.

The motor neurons situated in the anterior gray columns of the spinal cord send axons to innervate skeletal muscle through the anterior roots of the spinal nerves. These motor neurons are sometimes referred to as the lower motor neurons and constitute the final common pathway to the muscles

The lower motor neurons are constantly bombarded by nervous impulses that descend from the medulla, pons, midbrain, and cerebral cortex as well as those that enter along sensory fibers from the posterior roots. The nerve fibers that descend in the white matter from different supraspinal nerve centers are segregated into nerve bundles called the descending tracts. These supraspinal neurons and their tracts are sometimes referred to as the upper motor neurons, and they provide numerous separate pathways that can influence motor activity.

CORTICOSPINAL TRACT

Concerned with skilled movements of the distal muscles of the limbs/ extremities.

Origin:1. One-third (1/3) of fibers originate from Brodman

area 42. Another third from area 6 and Supplementary Motor

area3. Remaining third from area 312 of Postcentral gyrus

Upper motor neurons that influence the head are represented in the lateral part of the motor areas while the lower extremities are represented in the superomedian part of the motor areas.

More upper motor neurons that influence the head than in the lower extremity

Perineum and abdomen and Lower extremities are small compared to representation of the head

Fibers of the corticospinal tract arise as axons of pyramidal cells situated in the fifth layer of the cerebral cortex (Fig. 4-21). About one-third of the fibers originate from the primary motor cortex (area 4), one-third originate from the secondary motor cortex (area 6), and one-third originate from the parietal lobe (areas 3, 1, and 2); thus, two-thirds of the fibers arise from the precentral gyrus, and one-third of the fibers arise from the postcentral gyrus.

CORTICOSPINAL TRACT PATHWAY

Basal motor neurons, specifically Pyramidal cells (Betz cells) in layer 5

↓Arises the fibers of the corticospinal tract

↓Posterior limb of the Internal Capsule in the Basal ganglia

↓Crus cerebri of the midbrain

↓Anterior part of the pons

↓In the medulla, the fibers form the pyramids in the anterior

aspect

* The pyramids of the medulla are composed of fibers of the corticospinal tract.* This is why the corticospinal tract is also called the Pyramidal tract.

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TRANSER GROUP NAME

NEUROANATOMY | Descending PathwaysLATERAL CORTICOSPINAL TRACT

Lower part of medulla:90% of fibers crossover/decussate to the opposite side

(Decussation of pyramids= gross manifestation)↓

Lateral Corticospinal tract↓

Descends in the lateral funiculus of the white matter.↓

Along the descent, fibers synapse with interneurons in Laminae 4, 5, 6, 7 and 8

↓Interneurons will synapse with motor neurons in lamina 9

↓Motor neurons in lamina 9, especially alpha fibers, controls

the distal muscles of the extremities

VENTRAL CORTICOSPINAL TRACT

Lower part of medulla:10% will not crossover in the pyramids, they just descend

ipsilaterally in the anterior funiculus↓

Ventral Corticospinal tract[Supplies upper limbs only (reaches only up to cervical and

upper thoracic)]↓

At their levels of termination, they decussate to the opposite side.

↓Synapse with motor neurons in lamina 9

CLINICAL SIGNIFICANCE OF CORTICOSPINAL TRACT

It controls the skilled movements of the extremities. Any damage in this will cause paralysis in the body part.

How does paralysis occur about? Occur because of stroke or cerebral hemorrhage

Example: rupture of the Lateral Striate artery (artery of cerebral hemorrhage)

Passes very close through the posterior limb of internal capsule.

Hematoma will exert pressure in the corticospinal tract, then the entire contralateral side will be paralyzed.

When damage is at and above the decussation it is contralateral. Below the decussation, manifestation is ipsilaterally.

CORTICOBULBAR TRACT

Fibers arise from the Frontal Eyefield(Brodman area 4 and 8 in the Middle Frontal gyrus that

controls eye movement)↓

Fibers also pass posterior limb of the internal capsule↓

In the brainstem, the fibers take a different route:They synapse to nuclei cranial nerves 3, 4, 5, 6, 7, 9, 10, 11

and 12

Cranial nerve nuclei are innervated bilaterally. In patients with stroke, they can still move their eyeballs

because of the bilateral innervation.

Two exemptions: Lower facial nucleus

Controls the muscles below the eyelids. Hypoglossal nucleus

Both receive fibers from the opposite side that are heavier or more in amount in the same side.

Lesions of the 2 nuclei will be manifested on the opposite side because of heavier innervations.

Example: movement of the buccinator muscle.

Quote for the night (dahil gabi ko ito ginawa)“Sa akin ka nakahawak pero sa kanya ka naman nakatingin…”

-- mouse, nagseselos sa monitor

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TRANSER GROUP NAME

NEUROANATOMY | Descending PathwaysCORTICOSPINAL TRACT

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