(S1W2) Control of Muscle Activation part 2

Question 1

What happens after a single action potential?

Answer 1

• a twitch contraction which could last up to 0.2s
• there is a bit of a delay between receiving the action potential and the muscle actually producing any force

Question 2

What does an action potential cause to be released and how does that lead to a contraction?

Answer 2

• action potential causes calcium release
• at rest, tropomyosin is blocking the binding site between actin and myosin
• when calcium is released it binds to troponin, which moves the tropomyosin out of the way
• allowing the actin and myosin to link and pull against each other

Question 3

Is a single action potential realisitc for movement?

Answer 3

No a single action potential isn’t very realistic for many movements we’re consciously trying to control

Question 4

If there is a second action potential before the first muscle contraction has died down, can the action potentials build on top of each other to produce more force? And which type of summation is this?

Answer 4

yes and temporal summation

Question 5

What is a tetanic contraction?

Answer 5

• multiple action potentials at the same motor unit sum together
• the contraction force builds up until it reaches the point where all of the muscle fibres within that motor unit are being stimulated
• at this point we get this controlled, level plateau of muscle contraction and this is called a tetanus or tetanic contraction

Question 6

Can there be a mix of fast and slow twitch muscle fibres within the same motor unit?

Answer 6

No
* within each motor unit, the muscle fibres have the same mechanical properties
* we either have a fast or slow twitch motor unit
* so we either recruit the motor unit with fast twitch fibres in it or the motor unit with slow twitch fibres in it

Question 7

What are the three different types of motor units in terms of muscle fibre type?

Answer 7

• fast fatigue motor units - build up a lot of force very quickly but dies down very quickly as can’t maintain that level of force
• fast fatigue resistant motor units - middle ground, don’t build up quite as much force or as quickly but don’t die down as quickly, can maintain it better
• slow motor units - builds less force, more slowly, but can maintain than level of force for a long time

Question 8

Explain the “size principle”

Answer 8

• the recruitment of motor units within a muscle proceeds from small motor units to large ones
• slow motor units are usually small and fast motor units are usually big
• so proceeds from slow to fast
• because small, slow motor units have lower thresholds than fast ones
• so when stimulating a muscle, the slow motor units reach their threshold first, and fast motor units reach their thresholds last
• meaning the force builds up smoothly until the desired force is reached and then recruitment stops

Question 9

What are the advantages of the “size principle”

Answer 9

• recruitment can stop once desired force reached
• large forces aren’t produced when not required
• orderly recruitment reduces complexity because you are naturally building up the correct amount of force