Muscle contraction is a complex process controlled by electrical signals and molecular interactions. At the core of this process is excitation–contraction coupling, which links the action potential from motor neurons to the mechanical shortening of muscle fibers. This intricate mechanism involves the release of calcium from the sarcoplasmic reticulum, the sliding of actin and myosin filaments within sarcomeres, and the careful regulation of ATP and calcium levels. Understanding this process is essential for studying muscle physiology, neuromuscular disorders, and the molecular basis of movement.
Excitation–Contraction Coupling
- Muscle contraction is initiated by the release of acetylcholine (ACh) from the axon terminals of alpha motor neurons.
- ACh produces a large EPSP in the postsynaptic membrane due to the activation of nicotinic ACh receptors.
- By the process of excitation–contraction coupling, this action potential triggers the release of Ca²⁺ from an organelle inside the muscle fiber, which leads to contraction of the fiber.
- Relaxation occurs when the Ca²⁺ levels are lowered by reuptake into the organelle.
Muscle Fiber Structure
- Muscle fibers are formed early in fetal development by the fusion of muscle precursor cells, or myoblasts. This fusion leaves each cell with more than one cell nucleus, so individual muscle cells are said to be multinucleated.
- Muscle fibers are enclosed by an excitable cell membrane called the sarcolemma. Within the muscle fiber are cylindrical structures called myofibrils, which contract in response to an action potential sweeping down the sarcolemma.
- Myofibrils are surrounded by the sarcoplasmic reticulum (SR), an extensive intracellular sac that stores Ca²⁺.
- Action potentials sweeping along the sarcolemma gain access to the sarcoplasmic reticulum deep inside the fiber by way of a network of tunnels called T tubules.
Molecular Basis of Muscle Contraction
- The myofibril is divided into segments by disks called Z lines.
- A segment composed of two Z lines and the myofibril in between is called a sarcomere. Anchored to each side of the Z lines is a series of bristles called thin filaments.
- The sliding of the filaments with respect to one another occurs because of the interaction between the major thick filament protein, myosin, and the major thin filament protein, actin.
- The exposed “heads” of the myosin molecules bind actin molecules and then undergo a conformational change that causes them to pivot. This pivoting causes the thick filament to move with respect to the thin filament.
Steps in Excitation–Contraction Coupling
1. An action potential occurs in an alpha motor neuron axon.
2. ACh is released by the axon terminal of the alpha motor neuron at the neuromuscular junction.
3. Nicotinic receptor channels in the sarcolemma open, and the postsynaptic sarcolemma depolarizes (EPSP).
4. Voltage-gated sodium channels in the sarcolemma open and an action potential is generated in the muscle fiber, which sweeps down the sarcolemma and into the T tubules.
5. Depolarization of the T tubules causes Ca²⁺ release from the SR.
6. Ca²⁺ binds to troponin.
7. Tropomyosin shifts position and myosin binding sites on actin are exposed.
8. Myosin heads bind actin.
9. Myosin heads pivot.
10. An ATP binds to each myosin head and it disengages from actin.
11. The cycle continues as long as Ca²⁺ and ATP are present.
Relaxation
1. As EPSPs end, the sarcolemma and T tubules return to their resting potentials.
2. Ca²⁺ is sequestered by the SR by an ATP-driven pump.
3. Myosin binding sites on actin are covered by tropomyosin.
