CPPS 302 Excitation-Contraction coupling in skeletal muscle

What are the 4 main events in excitation-contraction coupling

1. Ach is released form the somatic motor neuron

2. Ach initiates an action potential in the muscle fiber

3. Action potential travels along the sarcolemma into the T-tubules and triggers Ca releas from the sarcoplasmic reticulum (SR)

4. Ca binds with TnC and initiates muscle contraction

Explain how somatic motor neuron initiates depolarization of a skeletal muscle fiber

1. An action potential reaches the motor neuron’s axons terminal

2. Voltage gated Ca channels open, allowing Ca to enter the neuron

3. Ca triggers exocytosis of vesicles containing Ach

4. Ach crosses the synaptic clef and binds to receptors on the muscle’s motor end plate.

5. These receptors open, allowing mainly Na+ to enter, causing depolarization of the muscle membrane.

How does Ach initiate an action potential in skeletal muscle fiber

1. Ach binds to receptors on the motor end plate

2. The receptors open, allowing mainly Na+ to enter, causing depolarization.

3. If depolarization reaches threshold, voltage-gated Na channels open, generating a muscle action potential

4. The action potential spreads along the sarcolemma and travels into the T-tubules.

• Main concept: Ach converts the nerve chemical signal into an electrical signal in the muscle fibre.

Why does muscle contraciton occur after the muscle action potential rather than immediately

Because excitation-contraction coupling takes time. The action potnetial must trigger Ca release and initiate cross-bridge activity before measurable tension develops.

How deos the duration of a muscle action potential compare to the duration of muscle contraction?

The muscle action potential is a very brief (about 2ms), while the resulting muscle twitch lasts much longer (tens to hundreds of milliseconds)

• Action potential is the electric trigger, while the muscle contraciton is the mechanical response that follows

What is the latent period in skeletal muscle contraction?

The short delay between the muscle action potential and the development of measurable tension, during which excitation-contraction coupling occurs

How does an action potential trigger Ca release from the sarcoplasmic reticulum

1. The action potential travels along the sarcolemma and into the T-tubules

2. Depolarization activates the DHP receptors in the T-tubule membrane

3. DHP receptors activate the RyR receptors on the SR membrane

4. Ryr channels open, releasing stored Ca from the SR into the sarcoplasm.

What are the roles of DHP and RyR receptors in skeletal muscle contraction

DHP = detecs changs in membrane voltage in the T tubules and activates the RyR

RyR = A Ca release channel on the SR that opens to release Ca into the muscle cell

How does Ca binding to troponin C initiate skeletal muscle contraction?

1. Ca released from the SR binds to troponin C (TnC)

2. This causes a conformational change in the troponin complex

3. Tropomyosin moves away from the myosin-binding sites on actin

4. Myosin can now bind to actin, allowing cross-bridge cycling and muscle contraction

What are the roles of the three troponin subunits in muscle contraction?

TnC: binds Ca to initiate contraction

TnL: inhibits actin-myosin interaction when the muscle is relaxed

TnT: Anchors the troponin complex to tropomyosin

How many isoforms does Troponin C have

Two: Low and High affinity

Explain the four main steps in the cross-bridge cycle during skeletal muscle contraction

1. Detachment: ATP binds to myosin, causing myosin to release actin

2. Cocking: ATP is hyrodyzed into ADP + Pi, providing energy to cock the myosin head into a high-energy position

3. Power stroke: myosin binds to actin, and Pi release triggers the power stroke, pulling actin towards the M lin e

4. ADP release: ADP is released, leaving myosin tightly bound to actin until another ATP binds.

Cycle repeats as long as Ca and ATP are available

Why is ATP necessary for muscle contraciton

ATP has two major functions during the cross-bridge cycle

• ATP binding: allows myosin to detach from actin

• ATP hydrolysis: provides energy to cock the myosin head, preparing it for the next power stroke

What is the sequence of events during excitation-contraction coupling?

1. Action potential

2. Ca release: sarcoplasmic reticulum releases Ca, increasing sarcoplasmic Ca concentration

3. Muscle tension: Ca binds troponin C, allowing cross-bridge cycling and force generation

4. Relaxation: Ca is pumped back into the SR, causing the muscle tension

Why does muscle tension peak after sarcoplasmic Ca concentration has aleready started decreasing?

Because force generation takes time. Even after Ca begins decreasing, cross-bridge cycling continues, allowing muscle tension to keep increasing temporarily.