Descending Motor Neurons

Question 1

A lesion in a lower motor neuron would have what effect on each of the following categories:

strength

muscle tone

stretch reflexes

atrophy

other

Answer 1

strength: decreased

muscle tone: decreased

stretch reflexes: decreased

atrophy: severe
other: fasiculations

       fibrillations

Question 2

How would a lesion on the upper motor neurons effect the following categories:

strength

muscle tone

stretch reflexes

atrophy

other

Answer 2

strength: decreased

muscle tone: increased

stretch reflexes: increased

atrophy: mild
other: clonus (rapid series of muscle contractions that occur in response to sudden stretch of a muscle)

pathologic reflexes (Babinski sign)

Question 3

T/F: each muscle fiber type populates one motor unit, no mixing

Answer 3

True

Question 4

What are the characteristics of Type 1 muscle fibers?

Action:

Lipids:

Glycogen:

Ultrastructure:

Physiology:

Answer 4

Action: sustained force, weight bearing

Lipids: abundant

Glycogen: scant

Ultrastructure: many mitochondria

Physiology: slow twitch

“one (type 1) slow (twitch) fat (lipid rich) red (appearance) ox (oxidative, mitochondria-rich)”

Question 5

What might be the result of damage to the cerebellum or the basal ganglia in relation to the LMN?

Answer 5

may have involuntary movements, incoordination, difficulty initiating movement

(This question underlines the idea that those areas have no direct effect on the LMN; however, they are vital in design, choice, and monitoring of movement)

Question 6

Describe the corticospinal tract origin. Give a percentage for each area of the brain.

Answer 6

Primary motor area; precentral gyrus (area 4) : 40%

Somatic sensory area; postcentral gyrus (area 3,1,2): 25%

Premotor area, lateral surface (area 6): 20%

Supplementary motor area, medial surface (area 6): 10%

Superior parietal lobule (area 5 & 7): 5%

Question 7

Which side of the body would be effected if the left side of the primary motor cortex were damaged?

Answer 7

right side would be effected

Question 8

What part of the brain controls fine digital movements?

Answer 8

primary motor area

Question 9

What are some functions of the premotor area?

Answer 9

• plans movements in response to external cues (e.g. instructions)
• control of proximal and axial musculature (trunk, shoulder, hip)
• may assemble empathetic facial movements

Question 10

What might be the result of a lesion in the premotor area (lateral aspect of Area 6)

Answer 10

• moderate weakness of contralateral proximal muscles
• loss of ability to associate learned hand movements to verbal or visual cues

Question 11

What are some functions of the supplementary motor area

(medial aspect of Area 6)

Answer 11

Plans movement while thinking (interally paced)

• assembles (learns) new sequences (playing new music)
• assembles previously learned sequence (music scale)
• “imagines” movements

Question 12

Where do the fibers in the lateral corticospinal tract descend?

Answer 12

lateral funiculus

Question 13

Where do the lateral fibers of the CST cross?

Answer 13

decussation of the medulla

Question 14

What does the rubrospinal tract control?

Answer 14

shoulder and proximal arm musculature

• facilitates upper extremity flexor muscle tone
• believed to be small in humans, some question significance

Question 15

What does the reticulospinal tract control?

Answer 15

axial musculature - walking

Question 16

What does the vestibulospinal tract control?

Answer 16

axial musculature - balance

• mediates postural adjustment & head movements
• antigravity reflexes
• righting reflex (cats); righting reflex to head inversion

Question 17

What does the tectospinal tract control?

Answer 17

believed to be important in head turning reflexes in response to visual stimuli, unclear function in humans