Damage to the pyramidal tract anywhere along its trajectory gives rise to a set of
symptoms called upper motor neuron syndrome or pyramidal syndrome
The pyramidal tract consists of the corticospinal and corticobulbar tracts. These are
bundles of axons that travel from the cerebral cortex and terminate either in the
brainstem (corticobulbar) or spinal cord (corticospinal) and are involved in control of
motor functions of the body.
-The corticobulbar tract conducts impulses from the brain to the nuclei of the
cranial nerves.
-The corticospinal tract conducts impulses from the brain to the spinal cord.
1
, Corticospinal tract
=Over half of the corticospinal tract fibers originate in the primary motor cortex
(Broadmann’s area 4) of the precentral gyrus. The remainder arise from the premotor
and supplementary motor areas (area 6) or from the parietal lobe (areas 3, 1, 2, 5
and 7)
-The primary motor cortex neurons contributing to the corticospinal tract are located
mostly in cortical layer 5. Layer 5 pyramidal cell axons synapse directly onto motor
neurons in the ventral horn of the spinal cord as well as onto spinal interneurons.
-About 3% of the corticospinal neurons are giant pyramidal cells called Betz cells, which
are the largest neurons in the human nervous system.
2
,Somatotopic organization of the motor cortex
-The primary motor cortex is somatotopically organized (that is, adjacent regions on the
cortex correspond to additional areas on the body surface)
-The cortical motor map is classically named motor homunculus
-Somatotopic organization is not confined to the cortex – motor pathways maintain a
rough organization along their entire length
3
,-Axons from the cerebral cortex enter the upper portions of the cerebral white matter
(corona radiata) and descend toward the internal capsule
Internal capsule
-n addition to the corticospinal tract, the cerebral white matter conveys information
between different cortical areas and between cortex and deep structures (such as basal
ganglia, thalamus and brainstem). These white matter pathways form a fanlike
structure as they enter the internal capsule, which condenses down to fewer and fewer
fibers as connections to different subcortical structures are made.
-The internal capsule in best appreciated in horizontal brain sections – looks like a letter
“V” with the point facing inward.
-! Thalamus and caudate nucleus are medial to the internal capsule and globus pallidus
and putamen are lateral to the internal capsule
-It has three parts: anterior limb, posterior limb and genu
Corticospinal tract in the
internal capsule
-The corticospinal tract lies in the
posterior limb of the internal capsule.
-There is a somatotopic map: motor
fiber for the face are most
anterior, and those for the arm
and leg are progressively more
posterior.
-Despite the somatotopic
arrangement, the fibers of the internal
capsule are compact enough that
lesions at this level often produce weakness of the entire contralateral body; however,
occasionally, capsular lesions can also produce more selective deficits.
Corticospinal tract - midbrain
4
, -Exiting the internal capsule, the corticospinal tract continues into the midbrain cerebral
peduncles.
-The white matter is located in the ventral portion of the cerebral peduncles and is called
basis pedunculi
-The middle one-third of the basis pedunculi contains corticobulbar and corticospinal
fibers with the face, arm and leg axons arranged from medial to lateral,
respectively.
Corticospinal tract – pons, medulla
-The corticospinal tract fibers next descend through the ventral pons, where they form
somewhat scattered fascicles
-These fascicles collect on the ventral surface of the medulla to form the medullary
pyramids – for this reason the corticospinal tract is sometimes reffered to as the
pyramidal tract
Corticospinal tract – medulla
5
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