Skeletal muscle

Question 1

What is a motor unit?

Answer 1

All the muscle fibres supplied by a single motor neurone. When the motor neurone is activated an AP is released causing contraction of all muscle fibres within that unit

Question 2

Where is the motor end plate found?

Answer 2

NMJ - On sarcolemma of muscle fibre

Contains acetylcholine receptors allowing for subsequent depolarisation

Question 3

How does Ca2+ concentration in the cell increase?

Answer 3

Depolarisation of motor end plate
Action potential travels along the sarcolemma and down the t tubules
Activates L type channels in the T tubules
There is signalling to the RyR channels which allow Ca2+ infulux into the sarcoplasm from the SR

Question 4

What channels remove Ca2+ when the muscle relaxes?

Answer 4

SERCA

They pump Ca2+ back into the sarcoplasmic reticulum

Question 5

Describe the process of excitation contraction coupling

Answer 5

Ca2+ binds to troponin C

Troponin I moves to expose the binding sites on the actin filaments

Myosin head binds to actin, bound to ATP

ATP -> ADP + Pi
This hydrolysis allows for the myosin head to tilit and re position

Pi is released from the myosin head, this allows the myosin head to reattach with a higher affinity to actin at a different binding site

Power stroke occurs as actin filaments are pulled to centre of the sarcomere
+ muscle shortens

ADP is then released. A new ATP molecule is required to restart this process.

Question 6

How are motor units recruited?

Answer 6

When motor units reach their threshold potential and action potential will be fired

This causes twitch of the muscle fibres in that motor unit ( cycle of contraction then relaxation)

Stimulus strength increases
so more motor units reach threshold potential which increases the twitch force

Question 7

When is a maximum response reached?

Answer 7

All motor units supplying a muscle are activated

Question 8

What is a tetanic contraction?

Answer 8

High stimulus frequency means the muscle cannot relax between stimuli

Question 9

What is an incomplete tetanus?

Answer 9

Occurs when the stimulus frequency is high but the muscle can partially relax between twitches

Question 10

What is EMG?

Answer 10

Measures electrical activity of skeletal muscle at rest and at contraction

Question 11

What is a surface EMG?

Answer 11

Electrodes placed on skin surface

• Poor use clinically
• Less invasive
• Only able to record superficial muscles
• Cannot distinguish activity of adjacent muscles
• May be affected by adipose tissue

Question 12

What is an intramuscular EMG?

Answer 12

Needle Electrodes are injected into the muscle

• Used clinically in diagnosis
• Invasive
• Can measure electrical activity of a single motor unit