Skeletal Muscle Contraction and Relaxation Notes

Skeletal Muscle Contraction

Muscle Fiber Structures

• Myofibrils: Contain actin and myosin, the contractile filaments responsible for muscle contraction.
• Sarcolemma: The muscle fiber cell membrane containing ion channels that facilitate muscle fiber depolarization.
• Sarcoplasmic Reticulum: Functions as the equivalent of the endoplasmic reticulum in muscle cells, storing calcium necessary for muscle contraction. It branches into cisterns to store calcium.
• Transverse Tubules (T-tubules): Invaginations of the sarcolemma that carry depolarization from the cell surface to the inner parts of the muscle fiber, enabling rapid and uniform contraction.

Four Steps Necessary for Skeletal Muscle Contraction

1. Neural Stimulation: Motor nerves, activated by the motor cortex of the brain or spinal cord (in the case of reflexes), send signals to the muscle.
2. Signal Transmission and Depolarization: The nerve signal is transmitted to the muscle, causing an electrical signal (depolarization) that leads to a muscle action potential.
3. Calcium Release: The action potential triggers the opening of calcium channels in the sarcoplasmic reticulum, releasing calcium into the sarcoplasm (cytoplasm of the muscle fiber).
4. Cross-Bridge Formation and Contraction: Calcium binds to contractile filaments, allowing the formation of cross-bridges between actin and myosin, which triggers muscle contraction.

Excitation-Contraction Coupling

The process from the stimulation of the muscle fiber to the conversion of the electrical action potential into mechanical contraction.

Motor Nerves and Motor Units

• Each skeletal muscle is innervated by at least one motor nerve, part of the efferent division of the nervous system.
• These nerves are under voluntary control, classified as part of the somatic nervous system, and are called somatic motor neurons.
• The axon of each motor neuron branches to innervate single muscle fibers. The synapse between the axon terminal and muscle fiber is the neuromuscular junction.
• A motor unit consists of a group of fibers innervated by a single neuron.
• Motor units allow for fine-tuned muscle contractions by activating different numbers of muscle fibers at different times.

Neuromuscular Junction

1. Action Potential Arrival: The motor cortex activates somatic motor neurons, and an action potential travels down the axon to the synaptic end bulb.
2. Calcium Influx: Depolarization at the synaptic end bulb opens voltage-sensitive calcium channels, allowing calcium to enter the cell.
3. Acetylcholine Release: Calcium influx triggers the movement of acetylcholine vesicles, which release acetylcholine into the synaptic cleft.
4. Receptor Binding: Acetylcholine binds to ion channels on the motor end plate of the muscle fiber, causing them to open and allowing sodium and potassium to pass through.
5. Muscle Fiber Depolarization: Sodium influx depolarizes the muscle cell, opening voltage-sensitive sodium channels and spreading the action potential across the muscle fiber.
6. Signal Termination: Acetylcholinesterase in the synaptic cleft breaks down acetylcholine, stopping the signaling for contraction.

Action Potential and Calcium Release

1. Depolarization Spread: Voltage-sensitive sodium channels open along the sarcolemma, causing an influx of sodium and further depolarization.
2. T-Tubule Activation: The depolarization wave reaches the T-tubules, which transmit the impulse to the sarcoplasmic reticulum.
3. Calcium Release: Calcium channels in the sarcoplasmic reticulum open, releasing calcium into the cytoplasm.
4. Cross-Bridge Formation: Calcium binds to troponin on the thin filaments, causing tropomyosin to shift and expose the actin-myosin binding sites, allowing cross-bridges to form.

Summary of Action Potential and Muscle Contraction

1. Action potential reaches the neuromuscular junction, opening voltage-sensitive calcium channels.
2. Calcium influx triggers the release of acetylcholine.
3. Acetylcholine binds to receptors, causing sodium influx and muscle cell depolarization.
4. Depolarization spreads via voltage-sensitive sodium channels along the sarcolemma and T-tubules.
5. Calcium channels in the sarcoplasmic reticulum open, releasing calcium into the cytoplasm.
6. Calcium binds to troponin, exposing actin-myosin binding sites.
7. Myosin binds to actin, forming cross-bridges and causing muscle contraction.

Cross-Bridge Cycle and ATP Involvement

1. ATP Binding: ATP binds to the myosin head, causing it to detach from actin.

2. ATP Hydrolysis: ATP is hydrolyzed into ADP and phosphate, repositioning the myosin head.

   ATP \rightarrow ADP + P_i

3. Cross-Bridge Formation: In the presence of calcium, myosin binds to actin.

4. Power Stroke: Myosin releases ADP and phosphate, changing shape and pulling the actin filament, shortening the sarcomere.

5. Cycle Repetition: A new ATP molecule binds to myosin, causing detachment, and the cycle repeats if calcium is present.

Rigor Mortis

• After death, ATP production ceases, and myosin cannot detach from actin, causing muscle rigidity.
• This condition lasts until enzymes break down muscle proteins.

Muscle Relaxation

1. Acetylcholine Removal: Acetylcholinesterase breaks down acetylcholine in the synaptic cleft.
2. Calcium Removal: Calcium transporters on the sarcoplasmic reticulum membrane use ATP to pump calcium back into the sarcoplasmic reticulum.
3. Binding Site Blockage: Tropomyosin returns to its original position, blocking the actin-myosin binding site.
4. Repolarization: Voltage-sensitive potassium channels open, releasing potassium and repolarizing the cell. The sodium-potassium pump uses ATP to restore ion balance.

ATP Usage in Muscle Contraction and Relaxation

• Powers the cross-bridge cycle for contraction.
• Used by calcium transporters to clear calcium from the cytoplasm, allowing relaxation.
• Used by the sodium-potassium pump to restore the resting membrane potential.