Motor pathway

Question 1

Where does motor initiation occur?

Answer 1

Cortex

Question 2

Describe organization of LMN.

Answer 2

LMNs are motor neurons of brainstem and spinal cord. They release acetylcholine and are organized in columns: medial=trunk, lateral=distal limbs, dorsal=flexors, and ventral=extensors.

Question 3

Define motor unit.

Answer 3

A motor unit are all muscle fibers innervated by a LMN.

Question 4

Define size principle.

Answer 4

LMNs are recruited in order of size and force. S-units->FR-units->FF-units.

Question 5

Define upper motor neuron.

Answer 5

Motor neurons above brainstem. Usually refer to corticospinal and corticobulbar tracts. Clinically, most often refer to lateral corticospinal tract.

Question 6

Compare and contrast the three muscle fiber types.

Answer 6

S-unit=slow/small unit. FR=fast, medium force, resistant to fatigue unit. FF=fast, strong force, fatigue-able unit.

Question 7

What are the 4 major control systems of spinal cord LMNs?

Answer 7

1. Reflex
2. Basal ganglia/cerebellum
3. Descending pathways: lateral corticospinal tract
4. Cortical centers: motor cortex=origin of commands to motor nucleus.

Question 8

Lateral corticospinal tract. Describe origins, location within internal capsule, and terminations.

Answer 8

Lateral CST originates from various regions within the cortex (~50% from primary motor cortex, area 4). Converges at corona radiata, passes through posterior limb of internal capsule, passes through crus cerebri, pons, and pyramids of medulla. Pyramidal decussation of ~90% of fibers, maintaining ATL (arms most medial, trunk, legs most lateral) organization of fibers. Fibers run down corticospinal tract and synapse at anterior horn grey matter. Also terminate at brainstem and collaterals to various parts of brain.

Question 9

How do basal ganglia and cerebellum affect movement? How do clinical manifestations of lesions at these sites differ from lesions to UMNs?

Answer 9

Basal ganglia and cerebellum indirectly affect movement. Lesions to these result in impairment of motor function. Lesions to UMNs result in both impairment of motor function AND muscle weakness.

Question 10

Describe corticobulbar tract. Origins, location within internal capsule, terminations, and CN nuclei it does or doesn’t innervate.

Answer 10

Corticobulbar tract descends anterior to corticospinal tract. It terminates on CN motor nerves, sensory relay nuclei, and reticular formation (most fibers end on interneurons in reticular formation). Occulomotor, trochlear, and abducens nerves DO NOT receive innervation from corticobulbar tract. Eye gets indirect innervation from corticobulbar tract via reticular formation.