Properties of the Motor Unit

Question 1

List the components of a motor system.

Answer 1

1 - Motoneurones.

2 - Skeletal muscles.

3 - The NMJ.

Question 2

What are the functional units of the motor system?

Answer 2

Motor units.

Question 3

Define motor unit.

Answer 3

An alpha motoneurone plus all the extrafusal muscle fibres it supplies.

Question 4

What is the name of the tissue sheath enveloping a myofibril?

Answer 4

Endomysium.

Question 5

What is the name of the tissue sheath enveloping a fascicle?

Answer 5

Perimysium.

Question 6

What is the name of the tissue sheath enveloping a whole muscle?

Answer 6

Epimysium.

Question 7

Where, in relation to their respective muscles, do neurovascular bundles run?

Answer 7

In between and parallel to muscle fascicles.

Question 8

List 7 common assemblies of fascicles and give an example for each.

Answer 8

1 - Convergent (e.g. pectoralis major).

2 - Parallel muscles (sartorius).

3 - Circular muscles (e.g. orbicularis oris).

4 - Fusiform muscles (e.g. biceps brachii).

5 - Unipennate muscles (e.g. extensor digitorum longus).

6 - Bipennate muscles (e.g. rectus femoris).

7 - Multipennate muscles (e.g. deltoid).

Question 9

List and describe the segments of a sarcomere.

Answer 9

1 - A bands (full length of myosin).

2 - I bands (actin only).

3 - H zones (myosin only).

4 - M lines (the attachment site for adjacent myosin filaments).

5 - Z discs (the attachment site for adjacent actin filaments).

Question 10

Define lower motoneurone.

Answer 10

A somatic motor efferent with a cell body in either:

• Lamina IX of the spinal cord.
• A cranial nerve motor nucleus.

Question 11

List the types of lower motoneurone.

Answer 11

1 - Alpha.

2 - Beta.

3 - Gamma.

Question 12

What are the 3 classes of muscle fibres?

Answer 12

1 - Fast twitch.

2 - Intermediate twitch.

3 - Slow twitch.

Question 13

What will happen to a muscle that is reinnervated by a motoneurone of a different type than the motoneurone that innervated it originally?

Answer 13

The muscle will change type to match that of the new motoneurone.

Question 14

According to the dictat of the size principle, in what order are motor units with small cell bodies recruited?

Answer 14

• Motor units with the smallest cell bodies are recruited first.
• Motoneurones with the smallest cell bodies are recruited preferentially at the lowest threshold for firing action potentials.

Question 15

According to the dictat of the size principle, in what order are motor units with large cell bodies de-recruited?

Answer 15

In reverse order from that of their recruitment.

Question 16

According to the dictat of the size principle, in what order are motor units with large cell bodies recruited?

Answer 16

Larger motoneurones are recruited on a progressively incrementing scale of:

1 - Threshold for firing.

2 - Frequency of impulse initiation.

3 - Muscle force generation.

Question 17

Which spinal neural levels are responsible for the innervation of the diaphragm?

Answer 17

C3,4 & 5.

Question 18

Which spinal neural levels are responsible for the innervation of the biceps?

Answer 18

C5-6.

Question 19

Which spinal neural levels are responsible for the innervation of the wrists?

Answer 19

C8-T1.

Question 20

Which spinal neural levels are responsible for the innervation of the nipples?

Answer 20

T4.

Question 21

Which spinal neural levels are responsible for the innervation of the umbilicus?

Answer 21

T10.

Question 22

Which spinal neural levels are responsible for hip flexion?

Answer 22

L1-2.

Question 23

Which spinal neural levels are responsible for the innervation of the quadriceps?

Answer 23

L3-4.

Question 24

Which spinal neural levels are responsible for knee flexion?

Answer 24

S1.

Question 25

Which spinal neural levels are responsible for the innervation of the great toe?

Answer 25

L5.

Question 26

Which spinal neural levels are responsible for foot plantar flexion?

Answer 26

S1-2.

Question 27

Which spinal neural levels are responsible for urinary sphincter and anal sphincter tone?

Answer 27

S2-4.