L55. Neuroscience: Pyramidal Tract Pathways

• The pyramidal tract is named after axons in the pyramid of the medulla and is also known as the upper motor neuron (UMN).
• It consists of the corticospinal and corticobulbar tracts.
• The corticospinal tract's axons primarily terminate in the spinal cord, potentially synapsing with sensory neurons, interneurons, and lower motor neurons.
• The corticobulbar tract's axons primarily terminate in the brainstem on motor neurons of cranial nerve nuclei.
• The pyramidal tract provides central control for initiating and learning skilled motor movements, and affects myotatic reflexes, muscle tone, and cutaneous reflexes.
• Pyramidal tract cells are located in layer V of the neocortex and are new phylogenetically, existing only in mammals.
• Myelination is largely completed by postnatal year 2 but continues slowly to age 12 or beyond.
• 20-24% of pyramidal tract fibers originate from Brodmann's area 4 (primary motor cortex) and 3-4% come from Betz cells.
• 30% of fibers originate from area 6, includes premotor area (PMA), anterior to area 4 and guides movement through sensory information.
• The SMA (supplementary motor area) which is medial and involved in planning that also makes up area 6.
• 30% originate from areas 3, 1 and 2 (primary sensory cortex) and 15% from areas 5 and 7 (superior parietal lobule).

Mapping of the Motor Cortex

• Stimulation of area 4 evokes discrete, isolated movements on the opposite body side due to pyramidal tract decussation.
• Contraction of specific muscles relates to the stimulation site in area 4, leading to the concept of the "motor homunculus".
• Area 4 and other regions of the brain have somatotopic organization.
• Stimulation of area 4 causes flick-like flexions or extensions involving few muscles.
• Mapping was done in the 1930s by Dr. Wilder Penfield on awake patients, and more data is gathered via fMRI and PET scans.
• Area 4 organizes spatial and temporal patterns of specific muscle contractions but doesn't plan or initiate movements.
• Planning of movements (and recall of programs to move) potentially occurs in Brodmann's area 6.

Somatotopic Organization of Area 6

• Stimulation of area 6 or other areas may result in patterned movements involving many muscles.
• Planning of movement (and recall of programs to move) probably occurs in area 6.
• Area 6 on the medial surface, called the supplementary motor area (SMA), has a different somatotopic map than the surface of area 6, known as the premotor area.
• Area 6 is comprised of the premotor area (PM) on the lateral surface and SMA on medial cortical surface projecting to area 4.

Pyramidal Tract Pathway

• Cell bodies are in the gray matter of the cortex, then become part of the corona radiata that funnels into the internal capsule (diencephalon).
• In the midbrain, axons are present in the middle of the cerebral peduncle, then as fascicles in the basilar pons, then in pyramids of the medulla.
• Finally, the axons are separated into the lateral and anterior corticospinal tracts in the spinal cord.
• Corticobulbar axons synapse in cranial nerve nuclei (V, VII, IX, X, XII).
• The PT fibers originate in layer 5 of the cerebral cortex through the ipsilateral cerebral hemisphere as corona radiata (radiating crown), then converge in the genu and posterior limb of the internal capsule.
• The corticobulbar tract is located primarily in the genu, while the corticospinal tract is in the posterior limb of the internal capsule.
• Regarding the diencephalon (thalamus), corticospinal fibers travel in the posterior limb of the internal capsule, synapsing directly or indirectly on spinal motor neurons.
• Corticobulbar fibers travel in the genu of the internal capsule (Face), synapsing in certain cranial nerve nuclei.
• The corticospinal fibers pass through the posterior limb between the lentiform nucleus (putamen and globus pallidus) and thalamus.
• Corticobulbar fibers run to the face and are present in the genu (knee) of the internal capsule.
• The posterior limb contains thalamocortical axons from the spinothalamic and DC-ML pathways.
• The lateral lenticulostriate artery (LSA), from M1 Middle Cerebral Artery branch, supplies posterior limb of internal capsule, especially lateral superior part.
• It's the chief cause of classic stroke: hemiparesis, hemihypesthesia, dysarthria, plus sometimes abnormal movements.
• Collaterals of the fibers terminate in the basal ganglia, thalamus, red nucleus, and reticular formation.
• Caudal midbrain is sectioned at an angle so corticospinal are actually shown in part of the basilar pons
• In the pons, the pyramidal tract (corticospinal) splits into separate bundles in the basilar pons, coalescing in the caudal pons.
• At mid pons, corticobulbar axons synapse on the neurons in motor nucleus of V and at caudal pons, corticobulbar axons synapse on the neurons in motor nucleus of VII.
• At the medulla level, axons form the medullary pyramid.
• Around 75-90% of PT fibers cross the midline and descend in the lateral funiculus as the lat. corticospinal tract at the medulla/spinal cord boundary.
• In the mid medulla level, the hypoglossal nucleus (CN XII nucleus) receives more crossed than uncrossed corticobulbar fibers so there is mild deviation of the jaw and tongue to the contralateral side of the lesion if there is a lesion of the corticobulbars.
• The uncrossed 10-25% descends in the ant. funiculus as the ant. corticospinal tract (ACST) that crosses the midline but is not important clinically.
• Lateral corticospinal fibers in the spinal cord are distributed: 55% to cervical, 20% to thoracic and 25% to lumbosacral spinal levels, while ACST fibers terminate only at cervical and upper thoracic spinal levels.
• Corticospinal fibers terminate in the intermediate gray matter on interneurons that synapse on motoneurons.
• A few corticospinal axons terminate directly on spinal motoneurons resulting in contraction of muscle fiber.

Pyramidal Tract or Upper Motor Neuron Damage

• Results in Increased Deep Tendon Reflexes (DTRs), Loss of Cremasteric/Abdominal Reflex, Clonus, Spastic Paralysis, Hypertonia and Positive Babinski Sign
• Positive Babinski Sign (abnormal plantar reflex) To check you stroke lateral sole→ Toes Flair, Big Toe Moves Up, positive sign is normally present in infants
• Hoffman sign is tested by tapping the nail or flicking the terminal phalanx of the third or fourth finger causing flexion/adduction of the terminal phalanx of the thumb.

Corticobulbar Tract

– PT fibers terminate in brainstem sensory relay nuclei, reticular formation, and some cranial nerve motor nuclei (motor V, motor VII, nucleus ambiguus, spinal accessory nucleus and hypoglossal motor nucleus).

• Descending corticobulbar fibers in a single tract, are generally distributed bilaterally except to V and XII nuclei which receive more crossed input.
• Unilateral lesions result in transient, mild deviation of the jaw (UMN to CN V nu.) and tongue to the contralateral side of the lesion (UMN to CN XII nu.).
• Unilateral lesions can result in lasting paralysis or weakness on the lower face (UMN to ventral facial nu.) unless there is recovery from the stroke.
• Bilateral lesions result in pseudobulbar palsy (inability to chew, speak, swallow, and breathe) and ultimately death.
• Axons originate from the lateral aspect of the inferior precentral gyrus where there is representation of the face.
• Axons are located in the genu and posterior limb of the internal capsule.
• Axons are closest to the midline of the middle 2/3rds of cerebral peduncle in the rostral midbrain.
• In the caudal midbrain, axons are more distant from the corticospinals.
• In the rostral pons, the corticobulbars are medial to medial lemniscus.
• There is bilateral but stronger contralateral input to the motor nucleus of V lesion of the corticobulbars to motor nucleus of V, which results in slight deviation of the jaw to the side contralateral to the lesion.
• The pathway is difficult to see beyond the pons in the caudal pons exhibiting bilateral innervation to the dorsal part of motor VII, facial nu.
• Only the contralateral part synapses with the ventral section.
• There is bilateral input to the nucleus ambiguous (CN IX & X) in the medulla.
• There is also bilateral input but stronger contralaterally input to the hypoglossal nucleus, damage to the corticobulbars results in tongue deviation to the side contralateral to the lesion upon protrusion.
• Bilateral innervation to Ambiguus and Spinal Accessory Nuclei (not clinically important because most lesions are on 1 side).
• Transient mild deviation of the jaw and tongue to the contralateral side of the lesion.
• Lesions of the Motor nu. of VII is very important clinically
• Damage causes paralysis or weakness on the lower face where wrinkling of the forehead when looking up still works.

Diseases of the Motor System

• Poliomyelitis caused by a virus that kills motor neurons in the spinal cord but is almost eradicated by use of the Salk vaccine in 1950s.
• Hereditary spastic paraplegia is an inherited disease that causes axonal degeneration of the lateral corticospinal tract and axons in fasciculus gracilis
• Multiple sclerosis is a demyelinating disease affecting many types of axons, plaques, or lesions in the optic nerve, brain stem, basal ganglia, and spinal cord are examples.
• Myasthenia Gravis is an autoimmune disease that results from antibodies that block nicotinic acetylcholine receptors at the junction of the nerve and muscle, commonly affecting eye muscles.
• Amyotrophic Lateral Sclerosis (ALS, Lou Gehrig's disease) is possibly caused by a mutation in SOD or other genes.

Explore the functions of motor cortex areas 4 and 6, including the motor homunculus. Compare fMRI and PET with FDG to Wilder Penfield's methods. Understand the impact of damage to Brodmann's area 6 on motor skills such as coordination.