The Frontal lobes - Mahidol University

19/07/54
Functional areas in the frontal lobes
The motor system is the part of the central nervous system that is involved with
movement. It consists of the pyramidal and extrapyramidal system.
The motor pathway also called pyramidal tract or the corticospinal tract start in the motor
center of the cerebral cortex.
There are upper and lower motor neurons in the corticospinal tract.
The motor impulses originates in the Giant pyramidal cells or Betz cells of the motor area i.e.
precentral gyrus of cerebral cortex. These are the upper motor neurons (UMN) of the
corticospinal tract. The axons of these cells pass in the depth of the cerebral cortex to the
Corona radiata and then to the Internal Capsule passing through the posterior branch of
internal capsule and continue to descend in the Midbrain and the Medulla Oblongata. In
the lower part of Medulla oblongata 80 to 85% of these fibers decussate (pass to the
opposite side) and descend in the White matter of the Lateral funiculus of the spinal cord
on the opposite side. The remaining 15 to 20% pass to the same side. Fibers for the
extremities (limbs) pass 100% to the opposite side. The fibers of the corticospinal tract
terminate at different levels in the Anterior horn of the Grey matter of the spinal cord.
Here the Lower Motor Neurons (LMN) of the corticospinal cord are located. Peripheral motor
nerves carry the motor impulses from the anterior horn to the voluntary muscles.
Pyramidal motor system:
Corticospinal tracts
tracts is a collection of
axons that travel between the cerebral cortex
of the brain and the spinal cord.
The corticospinal tract mostly contains motor axons.
It actually consists of two separate tracts in the
spinal cord: the lateral corticospinal tract and the
anterior corticospinal tract.
An understanding of these tracts leads to an
understanding of why for the most part, one side of
the body is controlled by the opposite side of the
brain.
The corticobulbar tract is also considered to be a
pyramidal tract, though it carries signals to motor
neurons of the cranial nerve nuclei, rather than the
spinal cord.
The neurons of the corticospinal tracts are referred
to as pyramidal neurons. The name comes from the
shape of the corticospinal tracts, which somewhat
resemble pyramids as they pass through the
medulla.
The corticospinal tract is concerned specifically with
discrete voluntary skilled movements, especially of
the distal parts of the limbs. (Sometimes called
"fractionated" movements)
The motor pathway
The corticospinal tract originates from pyramidal cells in layer V of the
cerebral cortex.
About half of its fibres arise from the primary motor cortex. Other
contributions come from the supplementary motor area, premotor
cortex, somatosensory cortex, parietal lobe, and cingulate gyrus. The
average fiber diameter is in the region of 10μm; around 3% of fibres are
extra‐large (20μm) and arise from Betz cells, mostly in the leg area of the
primary motor cortex.
Upper motor neurons
Upper motor neurons
The neuronal cell bodies in the motor cortex, together with their axons
that travel down through the brain stem and spinal cord are commonly
referred to as upper motor neurons. It should be noted however, that
they do not project to muscles, and thus the term 'motor neuron' is
somewhat misleading.
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