2. Descending (motor) Tracts
âąAre the pathways by which motor signals are sent from the brain to lower motor neurons.
âąThe lower motor neurons then directly innervate muscles to produce movement.
ïPyramidal tracts
ïExtrapyramidal tracts
3. Pyramidal Tracts
âąThese tracts originate in the cerebral cortex, carrying motor fibers to the spinal cord and brain
stem.
âąThey are responsible for the voluntary control of the musculature of the body and face.
âąThere are no synapses within the descending pathways.
âąThe neurons synapse with a lower motor neuron. Thus, all the neurons within the descending
motor system are classed as upper motor neurons.
âąTheir cell bodies are found in the cerebral cortex or the brain stem, with their axons remaining
within the CNS.
4. Pyramidal Tracts
âąCorticospinal tract â supplies the musculature of the body (limbs & trunk).
âąCorticobulbar tract â supplies the musculature of the head and neck.
ï¶Originate from:
ï±Premotor cortex (area 6): responsible for generating a plan of movement.
ï±Supplementary motor cortex (area 6): programs complex motor sequences,
and is active during âmental rehearsalâ of a movement.
ï±Primary motor cortex (precentral gyrus, Brodmann area 4): responsible for
execution.
ï±Primary somatosensory cortex
5. Corticospinal tract
âąThe major function of this pathway is fine voluntary motor control of
the limbs.
âąThe main effector or motor neurons (efferent neurons) for voluntary
movement lie within layer V of the primary motor cortex and are a
type of giant pyramidal cell called Betz cells.
âąThe cells have their bodies in the cerebral cortex.
âąThe nerve axons travel from the cortex through the posterior limb of
internal capsule (located between the thalamus and the basal
ganglia).
âąThe neurons pass through the crus cerebri of the midbrain,
the pons and into the medulla.
âąThe axons form two prominences called the pyramids of the
medulla.
6. Corticospinal tract
âąIn the most inferior part of the medulla, the tract divides into two:
âąThe fibers within the lateral corticospinal tract decussate (cross over to the
other side), resulting in muscles being controlled by the opposite side of the
brain. They then descend into the spinal cord, terminating in the ventral
horn (at all segmental levels).
âąThe anterior corticospinal tract remains ipsilateral, descending into the
spinal cord. They then decussate and terminate in the ventral horn of
the cervical and upper thoracic segmental levels.
7. Corticobulbar tract
âąThe corticobulbar tracts arise from the lateral aspect of the primary motor
cortex. They receive the same inputs as the corticospinal tracts. The fibers
converge and pass through the internal capsule to the brainstem.
âąThe neurons terminate on the motor nuclei of the cranial nerves. Here, they
synapse with lower motor neurons, which carry the motor signals to the
muscles of the face and neck.
âąInvolved in control of facial and jaw musculature, swallowing and tongue
movements.
8. Corticobulbar tract
âąMany of these fibers innervate the motor neurons bilaterally.
ï¶Exceptions:
ïUpper motor neurons for the facial nerve (CN VII) have a contralateral innervation. This only
affects the muscles in the lower quadrant of the face â below the eyes.
ïUpper motor neurons for the hypoglossal (CN XII) nerve only provide contralateral innervation.
Hinweis der Redaktion
Bilaterally: For example, fibres from the left primary motor cortex act as upper motor neurones for the right and left trochlear nerves.