Motor Units, Muscle Spindles, Golgi Tendon Organs and Joint Receptors

Question 1

From which 3 places can lower motor neurones (LMN) receive input?

Answer 1

1) Upper motor neurones 2) Proprioreceptors 3) interneurones

Question 2

Which kind of neurones innervate that muscle fibres?

Answer 2

α-motor neurones (α-MNs)

Question 3

Which kind of neurones innervate the muscle spindle (sensory)?

Answer 3

γ-motor neurones (γ-MNs)

Question 4

How do LMNs exit the spinal cord?

Answer 4

Ventral routes (or cranial nerves)

Question 5

What is the name of the nerve formed by the Ventral (anterior) and dorsal (posterior) roots?

Answer 5

Mixed spinal nerve

Question 6

Where is the greatest number of motor neurones in the spinal cord?

Answer 6

Cervical Enlargement (C3-T1) and Lumbar Enlargement (L1-S3)

Question 7

What is a motor unit?

Answer 7

An α-MN and all the muscle fibres it innervates.

Question 8

What is a motor neurone pool?

Answer 8

The collection of α-MNs that innervate a single muscle.

Question 9

What two mechanisms serve to increase force of muscle contraction?

Answer 9

1) Increased frequency of AP through α-MN

2) Recruitment of additional, synergistic motor units

Question 10

What 3 factors dictate the degree of neuromuscular activation?

Answer 10

1) Firing rate of LMNs
2) Number of LMNs that innervate a muscle (motor units)
3) Co-ordination of movement (integration)

Question 11

What 2 factors dictate the degree of force production?

Answer 11

1) Fibre size (hypertrophy)

2) Fibre phenotype (fast or slow contracting)

Question 12

What is the basis for the categorisation of muscle types?

Answer 12

The speed at which myosin ATPase splits ATP to provide energy for cross bridge cycling

Question 13

What is motor neurone recruitment?

Answer 13

the activation of additional motor units to accomplish an increase in contractile strength in a muscle. Slow motor units are recruited first, then faster, allowing graded development of muscle force.

Question 14

What is the myotatic reflex?

Answer 14

If the muscle is stretched this is detected but the muscle spindle which triggers contraction.

Question 15

What are the stages in the myotatic reflex?

Answer 15

1) Stretch of muscle spindle
2) Activation of Ia afferent
3) Excitatory synaptic transmission in spinal cord (glutamate)
4) activation of α-MN
5) Contraction of homonymous (stretched) muscle

Question 16

Why does the body ensure that extrafusal (muscle) fibres and intrafusal (spindle) fibres contract in parallel?

Answer 16

α- and γ-MNs are normally co-activated during voluntary movement to prevent the spindle from going slack while the muscle contracts.

Question 17

What are Golgi Tendon Organs?

Answer 17

Structures found in the junction between muscle and tendon which monitor changes in muscle tension.

Question 18

What is the purpose of Ia sensory fibres?

Answer 18

They carry information from the muscle spindle.

Question 19

What is the purpose of the Ib sensory fibres?

Answer 19

They carry information from the Golgi tendon organs.

Question 20

What is the purpose of a Golgi Tendon Organ?

Answer 20

These prevent the muscle from overload (e.g. in weight-lifting). Ib afferents enter the spinal cord and synapse on the α-MN of the muscle being stretched. It is the opposite of the myotatic reflex.