MODULE 5: DESCENDING
MOTOR PATHWAYS
Remember:
o Sensory input is distributed at the level of entrance (segmental
organization) to neighboring segments (intersegmental
organization) then to brainstem centers (suprasegmental
organization) and finally to the thalamus and cerebral cortex.
o The motor system has progressed the same way:
Small spinal neurons connect motor neurons segmentally
and intersegmentally to balance excitation/inhibition
Suprasegmental input from brainstem centers (vestibular
nuclei + reticular formation) provide balance, posture, and
smooth coordination.
Voluntary control of the muscles originates AT the cerebral
cortex and is directly related to spinal motor neurons
through CORTICOSPINAL TRACTS, but also through
brainstem centers.
Brain mass in the motor system is MUCH more than the
brain mass in the somatosensory system.
Corticospinal tracts
The origin of the corticospinal tracts in the cerebral cortex is in the
PRECENTRAL GYRUS (primary motor cortex), as well as other areas of the
frontal cortex (supplementary motor areas). They also originate from the
POSTCENTRAL gyrus (main sensory part of cerebral cortex)
The area of cortex dedicated to each part of the body depends on the
NUMBER of innervations that are there instead of the size of the body
part.
The corticospinal tract then goes through the white matter internal
capsule middle part of the cerebral peduncles (midbrain) down into
the basilar pons pyramids of the medulla. CHECK SOMATOTROPIC
ORGANIZATION OF THIS (in the internal capsule, cerebral peduncles, and
lateral funiculus)
Base of medulla:
o Corticospinal tract (aka pyramidal tract, since it arises from
pyramidal cells of the cerebral cortex [mostly giant pyramidal cells
called Betz cells]) splits into two divisions that go into the spinal
cord:
Lateral corticospinal tract:
Decussates at the motor decussation in the
transition area between medulla and C1 of spinal
cord
Ends in the lateral part of anterior horn in the
enlargments – these are concerned with DISTAL
LIMB MUSCULATURE.
MOTOR PATHWAYS
Remember:
o Sensory input is distributed at the level of entrance (segmental
organization) to neighboring segments (intersegmental
organization) then to brainstem centers (suprasegmental
organization) and finally to the thalamus and cerebral cortex.
o The motor system has progressed the same way:
Small spinal neurons connect motor neurons segmentally
and intersegmentally to balance excitation/inhibition
Suprasegmental input from brainstem centers (vestibular
nuclei + reticular formation) provide balance, posture, and
smooth coordination.
Voluntary control of the muscles originates AT the cerebral
cortex and is directly related to spinal motor neurons
through CORTICOSPINAL TRACTS, but also through
brainstem centers.
Brain mass in the motor system is MUCH more than the
brain mass in the somatosensory system.
Corticospinal tracts
The origin of the corticospinal tracts in the cerebral cortex is in the
PRECENTRAL GYRUS (primary motor cortex), as well as other areas of the
frontal cortex (supplementary motor areas). They also originate from the
POSTCENTRAL gyrus (main sensory part of cerebral cortex)
The area of cortex dedicated to each part of the body depends on the
NUMBER of innervations that are there instead of the size of the body
part.
The corticospinal tract then goes through the white matter internal
capsule middle part of the cerebral peduncles (midbrain) down into
the basilar pons pyramids of the medulla. CHECK SOMATOTROPIC
ORGANIZATION OF THIS (in the internal capsule, cerebral peduncles, and
lateral funiculus)
Base of medulla:
o Corticospinal tract (aka pyramidal tract, since it arises from
pyramidal cells of the cerebral cortex [mostly giant pyramidal cells
called Betz cells]) splits into two divisions that go into the spinal
cord:
Lateral corticospinal tract:
Decussates at the motor decussation in the
transition area between medulla and C1 of spinal
cord
Ends in the lateral part of anterior horn in the
enlargments – these are concerned with DISTAL
LIMB MUSCULATURE.
