Lecture 14

Question 1

Define:

Alpha motor neuron

Answer 1

A neuron innervating power producing extrafusal muscle fibers

Question 2

Describe:

Alpha motor neurons

(4 points)

Answer 2

• Axons innervate extrafusal muscle fibers
• Axons are much bigger, have quicker conduction times
• Larger than gamma motor neurons
• Innervate numerous extrafusal muscle fibers

Question 3

What is the difference between intrafusal and extrafusal muscle fibers?

Answer 3

Intrafusal muscle fibers lie inside extrafusal muscle fibres

Question 4

What is the function of an intrafusal muscle fiber?

Answer 4

Detects stretching

Question 5

What are intrafusal muscle fibers also known as?

Answer 5

Muscle spindle

Question 6

Describe:

Gamma motor neurons

Answer 6

• Wraps around center of intrafusal muscle fiber
• Sends information back to central nervous system
• Synapse with alpha motor neuron to coordinate contractions

Question 7

Where does information from the gamma motor neuron enter and exit?

Answer 7

Enter at the back of the spinal cord via the dorsal root - specifically the dorsal root ganglion
Leaves the central nervous system via the ventral root ganglion

Question 8

What does the sensitivity of the intrafusal muscle fibers depend on?

Answer 8

Fusimotor neurons

Question 9

What are fusimotor neurons also known as?

Answer 9

Gamma motor neurons

Question 10

What is the result of increased fusimotor drive?

Answer 10

Any small stretch in the intrafusal muscle fibers can be controlled via contraction of extrafusal muscle fibres

Question 11

Define:

Motor Unit

Answer 11

Alpha motor neurons and all the muscle fibers it innervates; a unit of force production within the CNS

Question 12

True or False:

Motor units are smaller than gamma motor neurons

Answer 12

False, motor units are much larger than gamma motor neurons

Question 13

What changes in motor units depending on the type of muscle that they are innervating?

Answer 13

Innervation ratio changes

Question 14

What does innervation ratio depend on?

Answer 14

Dependent on the muscle and how precise the muscle has to be

Question 15

True or False:

Higher innervation ratio allows for more precise motor control

Answer 15

False, low innervation ratio allows for more precise motor control

Question 16

Define:

Gamma motor neuron

Answer 16

Small neurons innervating intrafusal muscle fibers

Question 17

Describe:

Gamma motor neurons

Answer 17

• Determines sensitivity of intrafusal muscle fibers to stretch
• Smaller than alpha motor neurons
• Oriented in parallel to extrafusal muscle fibers

Question 18

What type of connections do gamma motor neurons form?

Answer 18

Polar-to-polar connections

Question 19

Define:

Polar-to-polar connections in gamma motor neurons

Answer 19

Connection to bone a and bone b along with its structure makes it optimally designed in order to detect stretch

Question 20

List:

3 types of extrafusal muscle fibers

Answer 20

1. Fast twitch, fatigable (FF) (Fast Motor Unit)
2. Fast twitch, fatigue resistant (FR)
3. Slow twitch, fatigue resistant (SR) (Slow Motor Unit)

Question 21

Describe:

Fast twich, fatigable (FF) (Fast Motor Unit)

Answer 21

• Characterized by highest conduction velocity (100 m/s)
• Large fiber diameter
• Innervate fast twitch muscle fibers (Type II)
• Produce lots of force, uses anaerobic glycolysis

Question 22

Describe:

Fast twitch, fatigue resistant (FR)

Answer 22

• Characterized by medium conduction velocity (60 m/s)
• Medium fiber diameter
• Innervate fast and/or slow twitch muscle fibers

Question 23

Describe:

Slow twitch, fatigue resistant (SR) (Slow Motor Unit)

Answer 23

• Characterized by slow conduction velocity (40 m/s)
• Small fiber diameter
• Innervate slow twitch muscle fibers (Type I)
• Slow oxidative, uses aerobic glycolysis
• Creates sustained force over time

Question 24

How do we read a muscle biopsy cross section?

Answer 24

Darkest colour is type I slow oxidative muscle type
Lightest colour is a fast twitch motor unit

Question 25

Why are slow oxidative muscle types darkest in colour on a muscle biopsy cross section?

Answer 25

As it is oxidative, it requires hemoglobin making it darker in color

Question 26

In a person with spinal cord injuries, what is percent composition of fast and slow motor units?

Answer 26

96% fast
4% slow

Question 27

Why do people with spinal injuries have such a high percentage of fast motor units?

Answer 27

They have limited motion, thus they only require fast motor units

Question 28

Describe:

Henneman Size Principle

Answer 28

Size of motor units increase with size of force production
* Less force = only slow motor units are used
* More force = fast motor units start to be used

Question 29

How are motor units clinically relative? Explain

Answer 29

Falls in older adults
* Individuals 30 years onward lost ~1%/year of total number of motor units with more rapid decrease after 60
* Older adults exhibit selective loss of fast motor units, which is vital for regaining balance is best done with fast motor units as it is fast-twitch

Question 30

What happens to motor units and muscle fibers with older adults?

Answer 30

• Large diameter motor units exhibit signs of demyelination and thus reduced conduction times
• Efficacy of muscle fiber, along with size of muscle fiber changes

Question 31

How are intrafusal and fast twitch muscles trained in older people?

Answer 31

Exercises such as wobble board and grid exercises