The document summarizes the descending motor tracts that transmit signals from the brain to lower motor neurons and muscles. It focuses on the pyramidal tracts, including the corticospinal tract which controls voluntary limb movement, and corticobulbar tract which controls head and neck muscles. The corticospinal tract originates in motor areas of the cortex and projects through the brainstem and spinal cord. The corticobulbar tract projects to motor nuclei controlling cranial nerves.

1. Descending Tracts:
Pyramidal tract

2. Descending (motor) Tracts
•Are the pathways by which motor signals are sent from the brain to lower motor neurons.
•The lower motor neurons then directly innervate muscles to produce movement.
Pyramidal tracts
Extrapyramidal tracts

3. Pyramidal Tracts
•These tracts originate in the cerebral cortex, carrying motor fibers to the spinal cord and brain
stem.
•They are responsible for the voluntary control of the musculature of the body and face.
•There are no synapses within the descending pathways.
•The neurons synapse with a lower motor neuron. Thus, all the neurons within the descending
motor system are classed as upper motor neurons.
•Their cell bodies are found in the cerebral cortex or the brain stem, with their axons remaining
within the CNS.

4. Pyramidal Tracts
•Corticospinal tract – supplies the musculature of the body (limbs & trunk).
•Corticobulbar tract – supplies the musculature of the head and neck.
Originate from:
Premotor cortex (area 6): responsible for generating a plan of movement.
Supplementary motor cortex (area 6): programs complex motor sequences,
and is active during “mental rehearsal” of a movement.
Primary motor cortex (precentral gyrus, Brodmann area 4): responsible for
execution.
Primary somatosensory cortex

5. Corticospinal tract
•The major function of this pathway is fine voluntary motor control of
the limbs.
•The 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.
•The cells have their bodies in the cerebral cortex.
•The nerve axons travel from the cortex through the posterior limb of
internal capsule (located between the thalamus and the basal
ganglia).
•The neurons pass through the crus cerebri of the midbrain,
the pons and into the medulla.
•The axons form two prominences called the pyramids of the
medulla.

6. Corticospinal tract
•In the most inferior part of the medulla, the tract divides into two:
•The 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).
•The 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
•The 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.
•The 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.
•Involved in control of facial and jaw musculature, swallowing and tongue
movements.

8. Corticobulbar tract
•Many of these fibers innervate the motor neurons bilaterally.
Exceptions:
Upper motor neurons for the facial nerve (CN VII) have a contralateral innervation. This only
affects the muscles in the lower quadrant of the face – below the eyes.
Upper motor neurons for the hypoglossal (CN XII) nerve only provide contralateral innervation.