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Fibers of the corticospinal tract arise as axons of pyra midal cells situated in the fifth layer of the cerebral
cortex (Fig. 4-21). About a third of the fibers originate
from the primary motor cortex (area 4), another third
from the secondary motor cortex (area 6), and the final
third 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.
These fibers do not control motor activity but influence
sensory input to the nervous system. Because electrical
stimulation of different parts of the precentral gyrus
produces movements of different parts of the opposite
side of the body, we can represent the parts of the body
in this area of the cortex. Note that the region controlling the face
is situated inferiorly, and the region controlling the
lower limb is situated superiorly and on the medial sur face of the hemisphere. The homunculus is a distorted
picture of the body, with the various parts having a size
proportional to the area of the cerebral cortex devoted
to their control. Interestingly, most corticospinal fibers
are myelinated and are relatively slow-conducting,
small fibers.
The descending fibers converge in the corona radi ata and then pass through the posterior limb of the
internal capsule. Here, the fibers are organized so that
those closest to the genu are concerned with cervical
portions of the body, while those situated more poste riorly are concerned with the lower extremity. The tract
then continues through the middle three fifths of the
basis pedunculi of the midbrain. Here, the fibers con cerned with cervical portions of the body are situated
medially, while those concerned with the leg are placedneuroanatomy made easy,introduction to neuroanatomy,snell neuroanatomy,neuroanatomy easy,Neuro anatomy by brainlss medicos,neuroanatomy by anatomy academy,Dr Najeeb,neuroanatomy in 7 days,neuroanatomy in hindi,#neuroanatomy,corticospinal ract,humunculus,selfless medicose neuroanatomy,corticospinal tract,corticospinal tract dr najeeb,corticospinal tract physiology,lateral corticospinal tract pathway,corticospinal tracts,corticospinal tract guyton,ninja nerd

laterally.
On entering the pons, the tract is broken into many
bundles by the transverse pontocerebellar fibers. In the medulla oblongata, the
bundles become grouped together along the anterior
border to form a swelling known as the pyramid. At the junction of the medulla
oblongata and the spinal cord, most of the fibers
cross the midline at the decussation of the pyramids
and enter the lateral white column of thespinal cord to form the lateral corticospinal tract
(Fig. 4-20). The remaining fibers do not cross in the
decussation but descend in the anterior white column
of the spinal cord as the anterior corticospinal tract
(Figs. 4-20 and 4-21). These fibers eventually cross the
midline and terminate in the anterior gray column of
the spinal cord segments in the cervical and uppe156
The corticospinal tracts are not the sole pathway
for serving voluntary movement. Rather, they form
the pathway that confers speed and agility to volun tary movements and is thus used in performing rapid
skilled movements. Many of the simple, basic volun tary movements are mediated by other descending
tracts.
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CHAPTER 4 Spinal Cord and Ascending, Descending, anol Intersegmental Tracts
regions informed about the cortical motor activity.
Once alerted, the subcortical regions may react
and send their own nervous impulses to the 0c and
7 motor neurons by other descending pathways.
Reticulospinal T racts
Throughout the midbrain, pons, and medulla oblon gata, groups of scattered nerve cells and nerve fibers
eXist that are collectively known as the reticular forma tion. From the pons, these neurons send axons, which
are mostly uncrossed, down into the spinal cord and
form the pontine reticulospinal tract (Fig. 4-22). From
the medulla, similar neurons send axons, which are crossed and uncrossed, to the spinal cord and form the
medullary reticulospinal tract.
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thoracic regions.
The lateral corticospinal tract descends the length of
the spinal cord; its fibers terminate in the anterior gray
co