2: Descending Motor Systems Flashcards
Compare and contrast medial and lateral motor systems: where does each run, and what areas and types of movements does each control?
MEDIAL
- -Ventral/ventrolateral funiculus
- -Axial & proximal limb function
- -Usually bilateral effects
- -MLF tracts stop at upper thoracic region (only -> neck movement)
- -Other tracts in cap of ventral horn run length of spinal cord (regulate muscle tone, crude movements)
LATERAL
- -Lateral funiculus
- -Limb function
- -More precise movements
List the descending tracts of the MEDIAL motor systems of the spinal cord that run in or near the MLF and the predominant functions of each. (4) BUT FIRST, list four features of these tracts.
Descend in MLF -> anterior funiculus, terminate in cervical/thoracic levels, control head and neck, terminate bilaterally
- Tectospinal tract:
- -Arises from superior colliculus
- -Crosses in midbrain (dorsal tegmental decussation)
- -Involved in head and neck movements to stimuli, especially visual and auditory - Medial vestibulospinal tract:
- -Arises from medial vestibular nucleus
- -Involved in movement of neck in response to vestibular stimulation (stabilizes head while body is moving) - Pontine (medial) reticulospinal tract:
- -From reticular formation of pons, specifically paramedian pontine reticular formation (PPRF)
- -Links head turning with horizontal eye movements
List the descending tracts of the MEDIAL motor systems of the spinal cord that do NOT run in or near the MLF and the predominant functions of each. (3)
NOT IN MLF (run near cap of ventral horn, extend length of spinal cord)
- Ventral corticospinal tract:
- -Arises from trunk area of motor cortex
- -Functions in voluntary control of axial muscle function - Lateral vestibulospinal tract:
- -From lateral vestibular nucleus
- -Runs IPSILATERALLY in ventral part of lateral funiculus
- -Functions as a strong activator of extensor motor neurons (-> vestibular righting reflex) - Medullary (lateral) reticulospinal tract:
- -Runs bilaterally in ventral part of lateral funiculus
- -Activates inhibitory interneurons/gamma motor neurons in spinal cord
- -Helps control gross movements and regulates muscle tone
List the general functions of descending motor tracts. (4)
Produce movement
Modify tone
Control autonomic funciton
Regulate sensory transmission
What characteristics of descending tracts make them necessary/useful for higher centers of the brain to control motor neurons? (3)
Some directly synapse on motor neurons
Most synapse on interneurons
Use motor patterns established in reflex systems in order to be efficient - can act through gamma loop
As a review, how does the gamma loop work? (3 steps)
- Gamma neurons activated -> contraction of intrafusal fibers
- Intrafusal fibers of muscle spindle shorten
- Reflex contraction of muscle & lengthening of antagonist muscles
What does the propriospinal tract contain? (3)
Interneurons connecting segments of the spinal cord
- -Involved in reflexes that spread over several segments
- -Aid in patterning of most normal movements
Very briefly, why do we have multiple motor pathways?
Phylogenetic hierarchy
- -Older tracts -> general effects, reflex responses; postures, tone, proximal limb/trunk movement
- -Newer tracts (like corticospinal tract) use older tracts to do new things
List the descending tracts of the LATERAL motor systems of the spinal cord and the predominant functions of each. (2)
- Rubrospinal tract:
- -Arises from red nucleus (input from cortex/cerebellum)
- -Crosses in midbrain at ventral tegmental decussation
- -Reaches all spinal levels
- -Terminates on interneurons
- -Functions in controlling movements of more proximal limb flexor muscles (crawling) - Corticospinal tract:
- -Arises from pyramidal neurons (largest = Betz cells) in motor cortex (precentral gyrus)
- -Traverse posterior limb of internal capsule -> cerebral peduncles of midbrain -> basal pons -> become pyramids of medulla
- -Terminate on…
a. Motor neurons to limb muscles -> control independent digits
b. 10% uncrossed -> anterior funiculus -> axial muscles -> crude movement
c. (MOST) Interneurons -> patterns of movement
d. Dorsal horn -> sensory transmission/reflexes
- -Functions in voluntary command of movement & regulation of sensory transmission through dorsal horn
What is a secondary function of the rubrospinal tracts?
Can function as an indirect corticospinal projection pathway
- -Control generalized, gross movements
- -Regulate gamma motor neuron activity and sensory transmission
- -Contain descending autonomic fibers and respiratory control fibers
NOT for highly skilled or refined movements
What are the functions of corticobulbar tracts? (5)
From cortex to brainstem
Functions:
- Affect motor cranial nerve nuclei -> VOLUNTARY control of muscles of the head
- EYE movement control
- Terminate in brainstem regions that then -> spinal cord (INDIRECT pathway from cortex to spinal cord)
- Terminate in SENSORY NUCLEI to influence sensory transmission
- Project to PONS for relay to cerebellum
- Terminate on AUTONOMIC CENTERS to influence respiration, etc
What type of movement will activate the LEAST amount of cortex? What areas of cortex would your example movement activate?
The simplest, repetitive movements (finger tapping)
Finger tapping -> hand area of primary motor cortex and adjacent somatosensory cortex
True/false: cortical motor neurons control the activity of single muscles
FALSE. They encode particular DIRECTIONS of movement (activate agonists, inhibit antagonists)
True/false: cortical motor neuron activity precedes movement by 20-30 ms
TRUE.
What area of the cortex is first activated in movement that requires a decision? What other things activate this area?
Premotor cortex, especially the Supplementary Motor Area
–Also activated in preparation for movements or during rehearsal of movement
