Skeletal muscle: regulation of contraction and plasticity

Question 1

Describe the events that take place at the NMJ.

Answer 1

Question 2

Describe the events that take place after the generation of the EPP.

Answer 2

1. AP runs along sarcolemma and diffuses down T-tubules and reaches triads, where voltage sensors interact with calcium-release channels of the SR.
2. Ca2+ are released into the cytosol and bind to TnC subunit of troponin, causing a confirmational change in tropomyosin. This reveals myosin-binding sites on the actin filament.
3. Cross-bridge cycle is initiated.
4. Following muscle contraction, Ca2+ is pumped back into the SR to lower intracellular [Ca2+] and cause muscle relaxation.

Question 3

Define a twitch.

Answer 3

A brief contraction that occurs in response to a single AP.

Question 4

Why is the twitch much slower than the AP which triggered it?

Answer 4

The delay is caused by the time it takes for:

• SR Ca2+ channels to open
• Ca2+ ions to diffuse out of the SR and bind to TnC and shift Tropomyosin
• Cross-bridge cycle and tightening up of ‘slack’ elastic structures in the muscle before force can be recorded

Question 5

Why do fast twitch fibres contract faster than slow twitch fibres?

Answer 5

Their myosin has a faster ATPase activity and so can carry out more cross-bridge cycles per second compared to slow twitch fibres.

Question 6

Define a tetanus.

Answer 6

The sustained mechanical response to a series of APs.

When twitches occur in such quick succession, this is called ‘summation’

Depending on the rate/frequency of stimuli, tetani can be fused and thus produce a smooth contraction, or unfused.

Question 7

True or False: Each muscle fibre is contacted by only one motor neuron but each motor neuron innervates multiple muscle fibres.

Answer 7

True

Question 8

Define a motor unit

Answer 8

Motor unit = motor neuron + its muscle fibres

Question 9

What are the different types of muscle fibre?

Answer 9

• Slow (type I)
• Slow oxidative
• Fast (type II)
• Fast glycolytic
• Fast, oxidative and glycolytic

Different types are recruited for different activities

Question 10

How is muscle force regulated?

Answer 10

• by motor unit recruitment
• by ‘rate’ recruitment/frequency of stimulation - tentanic contractions
• Muscle hypertrophy (long term)

Question 11

What 4 factors affect force output?

Answer 11

1. muscle length (length-tension curve)
2. muscle shortening (force decreases as shortening increases)
3. muscle fatigue
4. muscle hypertrophy (increases with atrophy, decreases with pathology)

Question 12

How can different exercise training alter muscle output?

Answer 12

• Weight lifting - Hypertrophy of muscle fibres by increasing cross-sectional area of active muscle, thus increases force.
• Endurance training - improves oxidative metabolism by increasing capillary supply and number of mitochondria.
• Speed training - increases the diamter of fast (type II) muscle fibres.
• Electrostimulation - transformation of fast (type II) muscle fibres to slow (type I) muscle fibres.

Question 13

True or False: With the right training/exercise, you can increase the proportion of fast (type II) muscle fibres.

Answer 13

False

However, you can increase their diameter.

Question 14

Describe the plasticity of muscle in terms of:

1. Adaption to length changes
2. Disuse/denervation
3. Re-innervation
4. Regeneration

Answer 14

1. Muscle fibres will adapt to a new length by adding or deleting sarcomeres.
2. Atrophy of muscle fibres and fast, oxidative and glycolytic muscle fibres revert back to fast, glycolytic ones.
3. Motor axons grow back into muscle and randomly make contact with denervated fibres - may cause fibre type transformation due to the random nature of the re-innervation.
4. Replacement muscle fibres form following muscle damage.

Question 15

Outline how damaged muscle tissue regenerates.

Answer 15