Question

Describe the process of skeletal muscle contraction, beginning with an action potential in a motor neuron and ending with the relaxation of the muscle. Your answer should include the following words: neuromuscular junction, T tubule, sarcoplasmic reticulum, calcium, thin filaments, binding sites, thick filaments, sarcomere, \(Z\) disc, and active transport.

Short answer

Skeletal muscle contraction begins with an action potential in a motor neuron, leading to the release of acetylcholine at the neuromuscular junction. This stimulates the muscle fiber membrane (sarcolemma) and spreads through the T tubules, triggering calcium release from the sarcoplasmic reticulum. The increase in calcium ions in the sarcoplasm allows calcium to bind to troponin on the thin filaments, exposing binding sites on actin. Myosin heads in the thick filaments attach to actin, and power strokes cause sarcomeres, which are delimited by Z discs, to shorten and generate muscle contraction. Active transport then returns calcium ions to the sarcoplasmic reticulum, causing muscle relaxation as tropomyosin covers the actin binding sites again.

Step-by-step solution

Action Potential in a Motor Neuron

An action potential is an electrical signal that travels along the motor neuron. When this signal reaches the end of the motor neuron, it causes the release of chemicals called neurotransmitters.

Neuromuscular Junction

The neuromuscular junction is a specialized synapse that connects the motor neuron to the muscle fiber. Here, the neurotransmitter responsible for muscle contraction, acetylcholine, is released into the synaptic cleft.

T Tubules and Sarcoplasmic Reticulum

The action potential generated by the neurotransmitter spreads along the muscle fiber membrane, called the sarcolemma, and through the T tubules. T tubules are invaginations of the sarcolemma that extend into the muscle fiber. These T tubules are in contact with the sarcoplasmic reticulum (SR), a specialized smooth endoplasmic reticulum that stores calcium ions.

Calcium Release

The action potential that travels along the T tubules triggers the release of calcium ions from the sarcoplasmic reticulum into the muscle cell's cytoplasm, called the sarcoplasm. The increase in calcium ions concentration is essential for muscle contraction.

Thin and Thick Filaments

Skeletal muscle fibers are made up of myofibrils which contain actin (thin filaments) and myosin (thick filaments). These filaments are organized into functional units called sarcomeres. The sarcomeres are delimited by Z discs, which are connected to actin filaments.

Calcium and Binding Sites

The increased concentration of calcium ions in the sarcoplasm binds to a protein called troponin, which is present on the thin filaments. This binding causes a conformational change, moving tropomyosin and exposing the binding sites on the actin filaments.

Sarcomeres Shorten

With the binding sites exposed, myosin heads in thick filaments attach to the actin and undergo a movement called the power stroke. This causes the sarcomeres to shorten, generating the force of muscle contraction. The Z discs approach each other, leading to muscle fiber shortening and muscle contraction.

Active Transport

As the action potential ceases, the calcium ions are actively transported back into the sarcoplasmic reticulum via active transport by a calcium ATPase pump. This process requires energy in the form of ATP (adenosine triphosphate).

Muscle Relaxation

As the concentration of calcium ions in the sarcoplasm decreases, calcium ions detach from troponin, allowing tropomyosin to move back over the binding sites on the actin filaments. The actin and myosin filaments return to their relaxed position, and the muscle relaxes.