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Flashcards in Motor Systems Deck (8)
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Contrast or compare an S-unit, FR-unit and FF-unit and the 3 types of fibers.

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)


Define motor unit.

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


Define size principle.

LMNs are recruited in order of size and force

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


Define upper motor neuron.

corticospinal and corticobulbar neurons


Describe the organization of LMNs into "groups"

Arranged into columns:

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


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

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


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

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


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

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!)