Excitation-Contraction

Question 1

What is excitation?

Answer 1

• The process by which action potential in a motor neuron leads to depolarisation of muscle fibre membrane
• This depolarisation propagates along the sarcolemma and transverse tubules

Question 2

What are the key proteins and structures involved in excitation?

Answer 2

Motor neuron: action potential initiated here and travels down axon until it reaches neuromuscular junction
Neuromuscular junction: the synapse where motor neuron communicates with the muscle fibre. Release of acetylcholine from the motor neuron stimulates muscle fibre and initiates depolarisation
Transverse tubules: invaginations of the sarcolemma which penetrate deep into the muscle fibre allowing action potential to spread

Question 3

What is excitation-contraction coupling?

Answer 3

The process by which depolarisation of the muscle fibre leads to initiation of muscle contractions

Question 4

What are the key proteins and structures involved in excitation-contraction coupling?

Answer 4

Dihydropiridine receptor (DHPR): voltage sensitive protein located on T tubules that acts as a voltage sensor and undergoes conformational change upon depolarisation
Ryanodine receptor (RyR): calcium release channel located on sarcoplasmjc reticulum. Conformational change of DHPR activates adjacent RyR channels, leading to release of calcium ions from SR
Sarcoplasmic reticulum (SR): responsible for storing calcium ions. Release of calcium ions from SR into sarcoplasm triggers contraction

Question 5

What is contraction?

Answer 5

The process of force generation and shortening of the muscle fibre

Question 6

What are the key proteins and structures involved in contraction?

Answer 6

Actin and Mysosin: main contractile proteins
- binding of calcium ions to troponin leads to conformational change in troponin-tropomycin complex
- this exposes mysosin-binding sites on actin
- myosin heads bind to actin, forming cross bridges
- myosin hydrolyses ATP to provide energy for cross bridge cycling, leading to sliding of actin filaments along myosin filaments and muscle contraction

Question 7

What is relaxation?

Answer 7

The process by which muscle fibre returns to its resting state after contraction

Question 8

What are the key proteins and structures involved in relaxation?

Answer 8

Sarcoendoplasmic reticulum calcium ATPase (SERCA):
- ATP dependent pump located on SR membrane
- actively transports calcium ions back into SR from sarcoplasm
- reduces intracellular calcium concentration

Calcium binding proteins: calsequestrin
- aids in storage and release of calcium ions