Motor Systems

Question 1

Contrast or compare an S-unit, FR-unit and FF-unit and the 3 types of fibers.

Answer 1

S unit- small MN, type I muscle fiber, red fibbers, ++++ mitchondria, ++++ capillaries

FR unit- type IIa fibers, white fibers, +++ mitochondria, +++ capillaries

FF-unit- Large MN, type IIb fibers, white fibers, ++ mitochondria, ++ capillaries

Type I (red fibers, slow twitch, small force, resistant fatigue)
Type IIa (white fibers, fast twitch, moderate force, resistant fatigue)
Type IIb (white fibers, fast twitch, large force, fatigable)

Question 2

Define motor unit.

Answer 2

all the muscle fibers innervated by a LMN (motor neurons of brainstem and spinal cord)

Question 3

Define size principle.

Answer 3

LMNs are recruited in order of size and force

S units — (add)— FR units —- (add) — FF units

Question 4

Define upper motor neuron.

Answer 4

corticospinal and corticobulbar neurons

Question 5

Describe the organization of LMNs into “groups”

Answer 5

Arranged into columns:

Medially placed- axial (trunk) muscles
Laterally placed -distal limb mubscles
Dorsally placed- flexor muscles
Ventrally placed- extensor muscles

Question 6

Describe the components/functions of the 4-major “control systems” of the spinal cord LMNs

Answer 6

Reflex and pattern generators in the spinal cord

Descending pathways:

• Corticospinal tract (lateral) – all excitatory (glutamate is the transmitter)
• Vestibulospinal tract – postural adjustments/head movements
• Reticulospinal tract – locomotion and postural control
• Tectospinal tract – reflex turning of head to visual/auditory stimuli
• (Rubrospinal tract) – significance in humans?

Cortical centers:

• Association cortex – “decide” that a movement is needed
• Supplementary motor area – planning/learning complex movements
• Premotor cortex – devise a “plan” for the movement
• Motor cortex – origin of “commands” to motor neurons

Basal ganglia and cerebellum – selection/adjusting movements

Question 7

Describe the Lateral CST (the origins of its components, location within the internal capsule, and terminations

Answer 7

CST – multiple origins and terminations

CST has ~ 1 million fibers
-Primary motor cortex – only 50% of CST
(Betz cells – concentrated here, but only 3% of CST)
-Adjacent Frontal motor and Parietal areas contribute the rest of the CST fibers
(Sensorimotor cortex – both sides of the central sulcus)

CST projects to:

• spinal cord and brainstem
• collaterals (direct and indirect) to basal ganglia, thalamus, reticular formation, sensory nuclei

CST passes through posterior limb of internal capsule

Question 8

Describe the corticobulbar tract (its locations within the internal capsule, pattern of terminations and CN motor nuclei it does and doesn’t innervate)

Answer 8

Descend anterior to corticospinal tract

Corticobulbar (corticonuclear) tract

• Motor neurons of cranial nerves
• Sensory relay nuclei
• Reticular formation

May end on motor neurons, most end on interneurons of the reticular formation

Oculomotor, trochlear and abducens nuclei receive no direct corticobulbar innervation (test question!)