The Process of Skeletal Muscle Contraction and Relaxation

The Process of Skeletal Muscle Contraction and Relaxation

10.4 Skeletal Muscle Fibers as Electrically Excitable Cells

Skeletal muscle fibers are specialized cells capable of responding to electrical stimuli, which is essential for muscle contraction and relaxation. This section outlines the key anatomical and physiological processes involved in muscle contraction, from the neuromuscular junction to the relaxation of the muscle fiber.

Anatomy of the Neuromuscular Junction

• Neuromuscular Junction (NMJ): The NMJ is the synapse between a motor neuron and a skeletal muscle fiber. It consists of the axon terminal of the neuron, the synaptic cleft, and the motor end plate of the muscle fiber.

• Axon Terminal: Contains synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).

• Motor End Plate: Specialized region of the muscle fiber's plasma membrane (sarcolemma) that contains ACh receptors.

• Synaptic Cleft: The small gap between the neuron and muscle fiber where neurotransmitters are released.

Events at the Neuromuscular Junction Leading to Action Potential

• When an action potential reaches the axon terminal, voltage-gated calcium channels open, allowing Ca2+ to enter the terminal.

• This triggers the release of ACh into the synaptic cleft.

• ACh binds to receptors on the motor end plate, causing sodium (Na+) channels to open and Na+ to enter the muscle fiber.

• The influx of Na+ generates an end-plate potential, which can trigger an action potential in the muscle fiber if the threshold is reached.

Excitation-Contraction Coupling

• This process links the generation of an action potential in the sarcolemma to the contraction of the muscle fiber.

• The action potential travels along the sarcolemma and down the T-tubules, leading to the release of Ca2+ from the sarcoplasmic reticulum.

• Ca2+ binds to troponin, causing a conformational change that moves tropomyosin away from actin's myosin-binding sites, allowing contraction to begin.

Sequence of Events in the Contraction Cycle

• Cross-Bridge Formation: Myosin heads bind to actin, forming cross-bridges.

• Power Stroke: Myosin heads pivot, pulling actin filaments toward the center of the sarcomere.

• Cross-Bridge Detachment: ATP binds to myosin, causing it to detach from actin.

• Reactivation of Myosin Head: ATP is hydrolyzed, re-cocking the myosin head for another cycle.

Equation for ATP Hydrolysis:

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

Process of Skeletal Muscle Fiber Relaxation

• ACh is broken down by acetylcholinesterase in the synaptic cleft, ending the signal to the muscle fiber.

• Ca2+ is actively transported back into the sarcoplasmic reticulum.

• Troponin and tropomyosin return to their resting positions, blocking myosin-binding sites on actin.

• The muscle fiber returns to its resting state, and contraction ends.