Muscle Contraction and Excitation-Contraction Coupling

Muscle Contraction: The Key

• The lecture focuses on how muscles contract, particularly skeletal muscles controlled by the somatic nervous system.
• The content on the first page of the chapter is crucial, forming the basis for the rest of the chapter.

Neuromuscular Junction

• Lower motor neurons release acetylcholine onto nicotinic receptors on skeletal muscle cells, exciting the cells and causing contraction.
• The neuromuscular junction is the chemical synapse between a lower motor neuron and a skeletal muscle cell.
• The motor end plate, located underneath the presynaptic terminal, is highly folded to increase the number of acetylcholine receptors, making the area very excitable.

Whole Muscle

• A whole muscle, such as a bicep, pectoral, or abdominal muscle, contracts by shortening and pulling on bone, causing movement.
• A whole muscle consists of thousands to hundreds of thousands of muscle cells.
• When these muscle cells shorten, the whole muscle shortens.

Sarcomere

• Sarcomeres shorten during muscle contraction, enabling muscle cells and the whole muscle to contract.
• The lecture will explain how the sarcomere goes from a relaxed to a shortened position.

Excitation-Contraction Coupling

• The lecture explains excitation-contraction coupling: the process where a muscle is excited and then contracts.
• The components include the neuromuscular junction, muscle cell T-tubule, sarcoplasmic reticulum (SR) with calcium, calcium channels, acetylcholine esterase, and nicotinic receptors.

Process of Muscle Contraction

1. Release of Acetylcholine:
   • Acetylcholine is released at the synaptic cleft.
2. Binding to Nicotinic Receptors:
   • Acetylcholine binds to nicotinic receptors.
3. Depolarization of Muscle Cell:
   • Nicotinic receptors are excitatory, causing depolarization of the muscle cell.
4. Action Potential Generation:
   • If the depolarization reaches threshold, the muscle cell generates an action potential.
   • This step is the 'excitation' phase.
5. Continuous Conduction of Action Potential:
   • The action potential travels along the sarcolemma and down the T-tubules via continuous conduction.
6. Opening of Calcium Channels:
   • The action potential causes calcium channels in the terminal cisternae to open.
7. Release of Calcium:
   • Calcium is transported out of the sarcoplasmic reticulum (SR) into the sarcoplasm.

Sarcomere Structure

• The sarcomere includes Z lines, thin filaments (actin), and thick filaments (myosin).
• Actin proteins polymerize to form actin filaments.
• Myosin is the main protein of the thick filament, featuring myosin heads.
• Tropomyosin wraps around the actin filament.
• Troponin is associated with the thin filament and attached to tropomyosin.
• The thin filament consists of actin, troponin, and tropomyosin, while the thick filament consists of myosin.

Role of Calcium

8. Calcium Binding to Troponin:
   • Calcium released from the SR diffuses to the sarcomere and binds to troponin.
9. Conformational Change in Troponin:
   • Troponin changes shape, becoming vertical after binding calcium.
   • Troponin pulls on tropomyosin, which moves to reveal active sites on the actin molecules.
   • These active sites are where myosin heads will attach.
10. Cross-Bridge Formation:
    • Myosin heads attach to the active sites on actin, forming a cross-bridge.
11. Power Stroke:
    • Myosin heads ratchet medially, pulling the thin filaments toward the middle of the sarcomere, resulting in the power stroke.
12. Sliding Filament Mechanism:
    • Thin filaments slide medially over the thick filament, shortening the sarcomere. This leads to muscle contraction.
    • This step is the contraction phase, where the muscle shortens because the sarcomere shortens, leading to the muscle cells and the whole muscle shortening.

Muscle Relaxation

13. ATP Binding to Myosin Head:
    • ATP molecules attaches to the myosin heads, allowing the heads to release the actin.
14. Sliding Back and Relaxation:
    • The thin filaments slide back laterally, away from the middle of the sarcomere, leading to muscle relaxation.

Role of Acetylcholinesterase

• Acetylcholinesterase is an enzyme needed for the reuptake of acetylcholine.
• It helps to remove acetylcholine from the synaptic cleft, preventing continuous action potential generation and muscle contraction.
• Without acetylcholinesterase, muscles would not relax, leading to asphyxiation and death due to the inability to exhale.
• Sarin gas inhibits acetylcholinesterase, causing continuous muscle contraction, bone breakage, and death.

Role of Calcium Pump

• The calcium pump transports calcium back into the sarcoplasmic reticulum (SR).
• This action lowers calcium levels in the sarcoplasm, allowing the muscle cell to relax.

Motor Unit

• A motor unit consists of the axon of a single motor neuron and the muscle fibers it innervates.
• $Motor Unit = Axon + Muscle Fibers$.
• A bundle of axons forms a nerve.
• Fewer muscle fibers in a motor unit allow for finer movements, such as those in the thenar muscle.
• More muscle fibers in a motor unit are seen in postural muscles, such as abdominal and erector muscles.

Recruitment of Motor Units

• To pick up heavier objects, more motor units must be recruited.
• Recruitment means more axons generate action potentials, causing more muscle cells to contract.
• More contracting muscle cells generate more tension in the muscle.
• The somatic nervous system is voluntary.
• Thoughts control the tension in muscles, such as running faster when late or generating appropriate tension to lift objects.