Motoneurons and motor units

Question 1

What are the three types of physiological movement?

Answer 1

Voluntary
Relflexes (somatic)
Rhythmic motor patterns

Question 2

Where are motoneurons found?

Answer 2

• In the spinal cord as anterior horn cells

- In the brainstem as cranial nerves (3-7. 9-12)

Question 3

What are the two types of motoneurons?

Answer 3

Alpha and gamma

Question 4

What are the properties of alpha MNs?

Answer 4

• They innervate extrafusal muscle fibres
• FF type motoneurons (Type IIb)
• S type muscle fibres (Type I)

Question 5

What are the properties of gamma MN’s?

Answer 5

• Innervate intrafusal muscle fibres (Muscle spindles(peripheral elements) )
• They control the response of muscle spindles to stretch

Question 6

What is a motor unit comprised of?

Answer 6

• Cell body of (a) motoneuron
• Axon
• All NMJs
• All extrafusal muscle fibres innervated

Question 7

What is the innervation ratio?

Answer 7

Number of muscle fibres innervated by a single MN

Question 8

What are the physiological consequences of the size principle?

Answer 8

• S type motor units fire first, and S type muscle fibres are good for weak sustained contractions.
• As a result weaker contractions can be graded better
• However exercise is necessary to prevent atrophy of FF type units.

Question 9

What are the differences between S type and FF type motor units?

Answer 9

S type: many units; little to no fatigue; aerobic metabolism; long twitch time; small innervation ratio; weak powered.
FF type: few motor units; rapid fatigue; anaerobic metabolism; short twitch time; high innervation ratio; powerful

Question 10

What does the cross innervation expt show?

Answer 10

That trophic factors may be released at NMJ along with Ach.

Question 11

How is the force produced by a muscle increased?

Answer 11

• An increased frequency of AP’s arriving at NMJ

- Increase the number of motor units recruited.