Clinical Neuroscience Week 5 – Descending Motor Pathways (Hard)

Descending Motor Pathways

• Descending pathways carry motor signals from the brain to the muscles.
• Major descending pathways include pyramidal (corticospinal) tracts and extrapyramidal tracts.

Corticospinal Tract (CST)

• The CST is responsible for voluntary movements.
• It consists of two main parts: lateral and anterior (ventral).
• Pathways: cortex → corona radiata → internal capsule → midbrain → spinal cord → α and γ motor neurons → skeletal muscle.
• ~90% of fibers cross midline (decussate) at the medulla forming the lateral corticospinal tract impacting lumbar and sacral regions for limbs.
• ~10-20% of fibers remain ipsilateral forming the anterior corticospinal tract (impacting cervical and thoracic spinal regions)
• Other brain regions contributing to the CST include supplemental motor area, premotor cortex, and parietal lobe.
• For a stroke not impacting the CST, ipsilateral weakness can result, but most often won't be clinically significant.

Anterior/Ventral Corticospinal Tract

• This tract impacts primarily cervical and thoracic regions.
• Fibers do not cross the midline in the medulla, instead continuing ipsilateral through the spinal cord.
• Pathway: cortex → corona radiata → internal capsule → midbrain → spinomedullary junction → α and γ motor neurons → skeletal muscle.

Corticobulbar Pathway

• Homologous to the CST, but terminates in cranial nerve motor nuclei (CNs 5, 7, 9, 10, 11).
• It accompanies the CST through the internal capsule and cerebral peduncles.
• It mainly controls voluntary movements of the head and neck.

Corticobulbar Tracts

• These fibers originate in the motor cortex and control cranial nerve nuclei.
• These tracts provide bilateral input except for CNs 7 (facial) and 12 (hypoglossal).
• CN VII (facial) and XII (hypoglossal) muscles mainly receive contralateral input (unilateral)
• Damage to a specific corticobulbar tract usually doesn't have significant clinical implications, due to compensations from the contralateral tract.

Extrapyramidal Pathways

• These pathways support movement and locomotion and originate in the brainstem.
• Includes:
  • Reticulospinal: influences locomotion and posture. Originates in the reticular formation of the pons and medulla.
  • Rubrospinal: controls mainly flexor muscle movements. Originates at the red nucleus of the midbrain and mainly travels ipsilateral.
  • Vestibulospinal: adjusts posture with respect to gravity. Originates from vestibular nuclei in the pons.
  • Tectospinal: involved in head orientation in response to auditory stimuli. Originates in the superior colliculus of the midbrain.

Spinal Cord Injury

• Complete SCI: signals cannot pass the lesion.
• Incomplete SCI: some signals still pass the lesion.
• Injuries can affect strength, sensation, muscle tone, and reflexes below the site of injury bilaterally.
• Lesions will result in diminished or absent signals to the muscles

Clinical Application: Spasticity

• Spasticity is involuntary muscle resistance caused, typically, by damage in the corticospinal tracts.
• The extrapyramidal pathways continue providing excitatory input.
• Treatments are often multi-factorial.

Stroke

• Middle Cerebral Artery (MCA) stroke often impacts upper extremities more than lower, and contralaterally.
• Anterior Cerebral Artery (ACA) stroke usually results in more lower extremity weakness than upper.

Central vs. Peripheral Lesions

• Central Lesion: The brain's signal to the muscles is blocked (damage to the CST). Muscle weakness and/or spasticity are typical responses.
• Peripheral Lesion: The signal can get to the spinal cord, but muscle is affected; weakness is common. Muscles distal to the lesion typically degenerate and atrophy.

Age-Related Muscle Changes

• Aging results in decreased type II muscle fibers, motor neurons, and an overall loss of muscle with increased intramuscular fat.

Fatigue

• Central fatigue involves the brain's decreased capacity to stimulate muscles (related to a stroke for example).
• Peripheral fatigue involves the muscles themselves losing function.

Summary: Tracing the CST

• Starts at the primary motor cortex, traverses the corona radiata, internal capsule, midbrain, medulla, spinal cord and ends at the targeted muscle.