Muscle Spindles & Neuromuscular Junction

Question 1

What is a motor unit?

Answer 1

An alpha motor neurone and all the extrafusal skeletal muscle fibres it innervates.

Question 2

Where in the spinal cord are alpha motor neurones that control distal muscles?

Answer 2

Lateral

Question 3

Where in the spinal cord are alpha motor neurones that control proximal muscles?

Answer 3

Medial

Question 4

Which neurotransmitter is found in neuromuscular junctions?

Answer 4

Acetylcholine

Question 5

What is the motor end plate?

Answer 5

The region of muscle fibre that lies directly under the terminal portion of the axon.

Question 6

What is a neuromuscular junction?

Answer 6

The junction of an axon terminal with the motor end plate.

Question 7

Outline the mechanism of neuromuscular transmission.

Answer 7

1. Action potential opens the voltage gated Ca2+ channel, allowing Ca2+ ions to diffuse into the axon terminal
2. Ca2+ binds to proteins that enable vesicles containing ACh to fuse with the plasma membrane and release ACh into the cleft
3. ACh diffuses across the cleft and binds to cholinergic nicotinic receptors, opening ion channels
4. More Na+ moves in than K+ out, producing an end-plate potential
5. Action potential is propagated into the T tubules and spreads throughout muscle

Question 8

What do stretch receptors in the muscle spindle monitor?

Answer 8

Muscle length & rate of change of muscle length

Question 9

What are the muscle fibres within the spindle called?

Answer 9

Intrafusal fibres

Question 10

What are intrafusal fibres innervated by?

Answer 10

Gamma motor neurones, a type of lower motor neurone

Question 11

Name the 2 type of stretch receptor in the spindle.

Answer 11

Nuclear chain fibre

Nuclear bag fibre

Question 12

Where are fast type 1a afferent sensory nerves associated with on the spindle?

Answer 12

Middle third

Question 13

Where are slow conducting type 2 afferent sensory nerves associated with on the spindle?

Answer 13

Inferior and superior thirds

Question 14

What do golgi tendon organs monitor?

Answer 14

How much tension the motor units are exerting.

Question 15

What are golgi tendon organs?

Answer 15

Endings of afferent fibres that wrap around collagen bundles in the tendons

Question 16

How is the golgi tendon organ activated?

Answer 16

When the muscle is stretched, tension is exerted on the tendon. The tension straightens the collagen bundles and distorts the golgi tendon receptor endings, activating them.

Question 17

Give an example of a stretch reflex.

Answer 17

Knee jerk reflex.

Question 18

Describe the knee jerk reflex.

Answer 18

1. Patellar tendon is tapped, pushing the tendon and stretching the thigh muscles
2. Stretch receptors are activated, stimulating a burst of action potentials
3. Motor units are stimulated, causing the muscles to contract resulting in extension of the lower leg = knee jerk
4. This reflex is monosynaptic as interneurons are not involved

Question 19

Describe the withdrawal reflex.

Answer 19

Painful stimulation activates the flexor muscles and inhibits the extensor muscles of the ipsilateral (same side) leg. The leg moves away from the harmful stimulus.

Question 20

Describe the inverse stretch reflex.

Answer 20

1b fibres of the golgi tendon inhibit alpha motor neurones and so inhibit muscle contraction. This regulates muscle tension and protects the muscle from overload.

Question 21

What is clasp knife reflex caused by?

Answer 21

An upper motor neurone lesion

Question 22

What is the clasp knife reflex?

Answer 22

When one bends a limb, initially there is resistance. After a certain point, resistance falls dramatically. 1b fibres inhibit the alpha motor neurones.

Question 23

What is muscle tone?

Answer 23

The degree of contraction of a muscle

Question 24

What do muscles with high and low tone feel like?

Answer 24

High tone: firm + rigid

Low tone: Soft + flaccid

Question 25

What is hypertonia?

Answer 25

Abnormally high muscle tone. Increased level of alpha motor neurone activity keeps the muscle contracted. Disorder of descending pathway/upper motor neurones.

Question 26

What is hypotonia?

Answer 26

Abnormally low muscle tone. Weakness and absent reflex. Disorder of alpha motor neurones i.e. lower motor neurone disorder