Excitation-Contraction Coupling in Skeletal Muscle Fibers

Motor Neuron and Muscle Fibers

• A single motor neuron, originating in the brain or spinal cord, transmits action potentials to numerous skeletal muscle fibers within a muscle.

Excitation-Contraction Coupling

• The sequence of events that translates action potentials in a muscle fiber into a contraction is termed excitation-contraction coupling.

Action Potential Propagation

• An action potential travels across the sarcolemma (muscle fiber membrane).

• It is rapidly conducted into the muscle fiber's interior via transverse tubules (T tubules).

Transverse Tubules (T tubules)

• T tubules are regularly spaced infoldings of the sarcolemma.

• They branch extensively throughout the muscle fiber.

Sarcoplasmic Reticulum (SR)

• T tubules make contact with the sarcoplasmic reticulum (SR), which is a calcium-storing membranous network, at numerous junctions.

Terminal Cisternae

• The SR forms sac-like bulges called terminal cisternae where it abuts the T tubule.

Triad Structure

• A triad consists of one T tubule and two adjacent terminal cisternae.

Protein Linkage and Calcium Release

• The membranes of the T tubule and terminal cisternae are linked by proteins that control calcium release.

Mechanism of Calcium Release

• As an action potential travels down the T tubule, it causes a voltage-sensitive protein to change shape.

• This shape change opens a calcium release channel in the SR.

• Calcium ions flood the sarcoplasm.

Role of Calcium Ions

• The rapid influx of calcium triggers the contraction of the skeletal muscle fiber.

• Calcium ions are responsible for coupling excitation to the contraction of skeletal muscle fibers.

Motor Neuron and Muscle Fibers

• A single motor neuron, originating in the brain or spinal cord, transmits action potentials to numerous skeletal muscle fibers within a muscle. These motor neurons are the final common pathway through which the nervous system influences skeletal muscle contraction. The number of muscle fibers innervated by a single motor neuron varies, influencing the precision of movement. Smaller motor units (fewer muscle fibers per neuron) allow for more precise control, while larger motor units generate more force.

Excitation-Contraction Coupling

• The sequence of events that translates action potentials in a muscle fiber into a contraction is termed excitation-contraction coupling. This intricate process involves the generation of an action potential, its propagation along the sarcolemma and T-tubules, calcium ion release from the sarcoplasmic reticulum, and the interaction of calcium with contractile proteins.

Action Potential Propagation

• An action potential travels across the sarcolemma (muscle fiber membrane). The sarcolemma contains voltage-gated sodium and potassium channels that facilitate the rapid depolarization and repolarization characteristic of action potentials.

• It is rapidly conducted into the muscle fiber's interior via transverse tubules (T tubules). This ensures that the action potential reaches all parts of the muscle fiber nearly simultaneously.

Transverse Tubules (T tubules)

• T tubules are regularly spaced infoldings of the sarcolemma. These invaginations significantly increase the surface area of the muscle fiber membrane, allowing for efficient action potential propagation.

• They branch extensively throughout the muscle fiber. Their strategic positioning ensures close proximity to the sarcoplasmic reticulum.

Sarcoplasmic Reticulum (SR)

• T tubules make contact with the sarcoplasmic reticulum (SR), which is a calcium-storing membranous network, at numerous junctions. The SR plays a critical role in regulating intracellular calcium ion concentration, which is essential for muscle contraction and relaxation. It is composed of a network of interconnected tubules and cisternae.

Terminal Cisternae

• The SR forms sac-like bulges called terminal cisternae where it abuts the T tubule. These cisternae serve as reservoirs for calcium ions.

Triad Structure

• A triad consists of one T tubule and two adjacent terminal cisternae. This structural arrangement facilitates the efficient communication between the sarcolemma and the sarcoplasmic reticulum.

Protein Linkage and Calcium Release

• The membranes of the T tubule and terminal cisternae are linked by proteins that control calcium release. These proteins include voltage-sensitive dihydropyridine receptors (DHPRs) in the T-tubule membrane and ryanodine receptors (RyRs) in the SR membrane.

Mechanism of Calcium Release

• As an action potential travels down the T tubule, it causes a voltage-sensitive protein (DHPR) to change shape. The DHPRs act as voltage sensors.

• This shape change opens a calcium release channel (RyR) in the SR. The RyRs are calcium channels that allow calcium ions to flow from the SR into the sarcoplasm.

• Calcium ions flood the sarcoplasm. The rapid increase in sarcoplasmic calcium ion concentration initiates muscle contraction.

Role of Calcium Ions

• The rapid influx of calcium triggers the contraction of the skeletal muscle fiber. Calcium ions bind to troponin, causing a conformational change that exposes the myosin-binding sites on actin filaments.

• Calcium ions are responsible for coupling excitation to the contraction of skeletal muscle fibers. Without calcium, the interaction between actin and myosin is blocked, and muscle contraction cannot occur.