2: Descending Motor Systems Flashcards Preview

Neuroscience - Quizzes 1 - 3 > 2: Descending Motor Systems > Flashcards

Flashcards in 2: Descending Motor Systems Deck (24):
1

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

2

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
1. Tectospinal tract:
--Arises from superior colliculus
--Crosses in midbrain (dorsal tegmental decussation)
--Involved in head and neck movements to stimuli, especially visual and auditory

2. Medial vestibulospinal tract:
--Arises from medial vestibular nucleus
--Involved in movement of neck in response to vestibular stimulation (stabilizes head while body is moving)

3. Pontine (medial) reticulospinal tract:
--From reticular formation of pons, specifically paramedian pontine reticular formation (PPRF)
--Links head turning with horizontal eye movements

3

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)
1. Ventral corticospinal tract:
--Arises from trunk area of motor cortex
--Functions in voluntary control of axial muscle function

2. 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)

3. 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

4

List the general functions of descending motor tracts. (4)

Produce movement
Modify tone
Control autonomic funciton
Regulate sensory transmission

5

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

6

As a review, how does the gamma loop work? (3 steps)

1. Gamma neurons activated -> contraction of intrafusal fibers
2. Intrafusal fibers of muscle spindle shorten
3. Reflex contraction of muscle & lengthening of antagonist muscles

7

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

8

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

9

List the descending tracts of the LATERAL motor systems of the spinal cord and the predominant functions of each. (2)

1. 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)

2. 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

10

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

11

What are the functions of corticobulbar tracts? (5)

From cortex to brainstem

Functions:
1. Affect motor cranial nerve nuclei -> VOLUNTARY control of muscles of the head
2. EYE movement control
3. Terminate in brainstem regions that then -> spinal cord (INDIRECT pathway from cortex to spinal cord)
4. Terminate in SENSORY NUCLEI to influence sensory transmission
5. Project to PONS for relay to cerebellum
6. Terminate on AUTONOMIC CENTERS to influence respiration, etc

12

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

13

True/false: cortical motor neurons control the activity of single muscles

FALSE. They encode particular DIRECTIONS of movement (activate agonists, inhibit antagonists)

14

True/false: cortical motor neuron activity precedes movement by 20-30 ms

TRUE.

15

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

16

What results from a lesion in the supplementary motor area?

Abulia: prevents initiation of movement

17

Where is the internal capsule? What does it contain? Precisely where are its components located?

Internal capsule: medial in brain at level of thalamus and basal ganglia
Contains most corticofugal axons, but is mostly made of corticopontine fibers that relay to the cerebellum
-->Head muscles at genu (corticobulbar)
-->Arms and legs in posterior limb

18

Regarding corticobulbar fibers that control facial movement, where are they located? Differentiate the effects of lesions of these tracts on functionality of the lower face, upper face, and jaw muscles.

Located in precentral gyrus closest to lateral fissure -> internal capsule
--Lower face = crossed projections
--Upper face, jaw muscles = bilateral projections
----Lesion -> weakness on contralateral lower face, preservation of upper face function

19

What is hypermimia? Briefly, what can cause it?

Hypermimia: increased movement of the face to emotional stimuli

Upper motor neurons in anterior cingulate cortex -> emotional movements of the face
--Do NOT go through internal capsule, so internal capsule damage may -> hypermimia

20

What type of lesion would cause lower face weakness? What about weakness of an entire side of the face?

Lower face weakness = lesion ROSTRAL to level of facial nucleus

Entire side weakness = lesion to facial nucleus or nerve

21

To which areas of the brainstem does the cerebral cortex project as part of indirect corticospinal tracts? Why are these tracts important? Give an example.

Red nucleus, reticular formation, superior colliculus

Used to regulate muscle tone or activate motor patterns to stabilize body during movement
--Ex. pull up on a bar with biceps, gastrocs contract so you don't fall over

22

What happens when a patient has disruption of the corticoreticular pathway and therefore the medullary reticulospinal pathway?

Dramatic increase in spinal cord reflexes
--Medullary reticulospinal pathway is INHIBITORY

23

What features indicate an abnormal muscle stretch reflex? What can cause increased reflexes?

Involvement of multiple muscles (normally confined to one)
Reflex spreads bilaterally (normally unilateral)
Clonus

Cause: damage to medullary reticulospinal tract or

24

What results from a lesion of ONLY the corticospinal fibers?

Unilateral weakness with NO hyperreflexia