5: Physiology - Motor units, muscle spindles, Golgi tendon organs and joint receptors

Question 1

The somatic motor system is made up of upper motor neurons and lower motor neurons.

What are their functions?

Answer 1

LMNs - initiate contraction of skeletal muscle

UMNs - modulate activity of LMNs

Question 2

Where in the spinal cord do LMNs emerge from?

Answer 2

Anterior horn

Question 3

What is the function of an alpha motor neuron?

Answer 3

Initiates contraction of skeletal muscle

Question 4

What is the function of a gamma motor neuron?

Which specific structure of skeletal muscle does it supply?

Answer 4

Acts as a proprioceptor to monitor muscle length and contraction

Muscle spindle

Question 5

How many spinal nerves are there at each level and in total?

Answer 5

30

C8, T12, L5, S5

Question 6

What is a muscle spindle?

Answer 6

Specialised stretch receptor which detects the degree of stretch of a skeletal muscle fibre

Question 7

What is a motor unit?

Answer 7

Alpha motor neuron and all the muscle fibres it supplies

Question 8

In terms of alpha motor neurons, what is a motor neuron pool?

Answer 8

All the motor units supplying a single named muscle

Question 9

What is the relationship between the firing rate of an alpha motor unit and the force of contraction?

Answer 9

Direct proportion

the greater the firing rate, the greater the force of contraction (up to a limit)

Question 10

What is the relationship between the number of activated motor units and force of contraction?

Answer 10

Direct proportion

the greater x, the greater y

Question 11

Where in the spinal cord are motor neurons supplying

a) proximal muscles
b) distal muscles

found?

Answer 11

a) Medial in anterior horn

b) Lateral in anterior horn

Question 12

Where are motor neurons supplying

a) flexor muscles

b) extensor muscles

found relative to each other in the anterior horn?

Answer 12

a) Posterior

b) Anterior

Question 13

Name some

a) alpha motor neuron
b) muscle fibre

factors which influence of rate and force of muscle contraction?

Answer 13

a) Frequency of impulses, summation of impulses

b) Size (hypertrophied, atrophied?), type (slow or fast twitch?)

Question 14

What word is used to describe the contraction of skeletal muscle caused by a single action potential?

Answer 14

Twitch

Question 15

What is required to produce sustained muscle contraction compared to a brief twitch?

Answer 15

Sustained firing of action potentials

Question 16

The greater the firing rate of alpha motor neurons, the greater the time period of muscle contraction.

What happens if the firing rate is excessively fast?

Answer 16

Tetany

Involuntary muscle contractions and cramps

Question 17

How many muscle fibres do

a) fine motor muscles
b) gross motor muscles have?

this is not a trick

Answer 17

a) Few muscle fibres (e.g extraocular muscles)

b) Many muscle fibres (e.g quadriceps)

Question 18

How does the size of an alpha motor neuron change depending on the size of its motor unit?

Answer 18

Bigger alpha motor neurons for bigger motor units

Question 19

How does the addition of small motor units affect the rate of increase of force of contraction?

Answer 19

If you add smaller motor units on, you add less force

opposite true for larger motor units

Question 20

Which size of motor units tend to be recruited in slow fatiguing exercise?

Answer 20

Small motor units

Question 21

Which size of motor units are activated by strenuous, fast fatiguing exercise?

Answer 21

Large motor units

to generate bigger increases in muscle contraction