Question

Describe the sliding filament model of muscle contraction within a sarcomere. Begin with the nerve impulse and end with the relaxation of the muscle. (pages \(261-63\) )

Short answer

The sliding filament model of muscle contraction begins with a nerve impulse reaching the neuromuscular junction, causing the release of acetylcholine and generating an action potential in the muscle cell. This leads to the release of calcium ions from the sarcoplasmic reticulum, which bind to troponin, causing tropomyosin to expose myosin-binding sites on actin filaments. Myosin heads then form cross-bridges with actin, undergoing power strokes, pulling the actin filaments and generating force, resulting in muscle contraction. To relax, the nerve impulse ceases, calcium ions are extracted from the sarcoplasm, and myosin-binding sites on actin are once again covered by tropomyosin, allowing filaments to return to resting positions.

Step-by-step solution

Overview of Sliding Filament Model

The sliding filament model of muscle contraction describes the process by which the interaction between actin and myosin filaments within a sarcomere promotes muscle contraction and relaxation. The main events in this model are the nerve impulse, calcium ion regulation, actin and myosin filament sliding, force generation, and muscle relaxation.

Nerve Impulse

A nerve impulse, or action potential, travels along a motor neuron and reaches the neuromuscular junction. The arrival of the nerve impulse at the junction triggers the release of the neurotransmitter acetylcholine, which binds to receptors on the muscle cell membrane. This binding causes an electrical signal, known as an action potential, to travel across the muscle cell membrane and into the interior of the muscle cell, called the sarcoplasm.

Calcium Ion Regulation

Inside the sarcoplasm, the action potential causes the release of calcium ions (Ca\(^{2+}\)) from the sarcoplasmic reticulum, a specialized structure within the muscle cell. The calcium ions bind to a protein called troponin, which is part of the thin actin filament. When calcium binds to troponin, it causes a conformational change that shifts the tropomyosin (another protein complex running over the thin filament), exposing the myosin-binding sites on the actin filaments.

Actin-Myosin Filament Sliding

With the myosin-binding sites exposed, the myosin heads from the thick filaments can bind to the actin filaments, forming cross-bridges between the thick and thin filaments. Myosin heads then undergo a power stroke, during which they pivot and pull the actin filaments towards the center of the sarcomere, causing the sarcomere to shorten. This sliding of actin and myosin filaments occurs simultaneously in multiple sarcomeres throughout the length of the muscle fiber, resulting in muscle contraction.

Force Generation

When the myosin heads undergo their power stroke, they pull the actin filaments towards the center of the sarcomere, generating force. The force generated by each individual sarcomere adds up, ultimately leading to the entire muscle contracting and exerting force on the attached structures, such as bones and tendons.

Muscle Relaxation

Muscle relaxation occurs when the nerve impulse ceases and calcium ions are actively pumped back into the sarcoplasmic reticulum, decreasing the concentration of Ca\(^{2+}\) within the sarcoplasm. The calcium ions unbind from the troponin, returning the protein to its original conformation and causing tropomyosin to once again cover the myosin-binding sites on the actin filaments. With the binding sites covered, the actin and myosin filaments can no longer form cross-bridges, and the muscle relaxes as the filaments return to their resting positions.

Key concepts

Sarcomere Anatomy

The sarcomere is the fundamental unit of a muscle's contraction and its intricate structure is essential for muscle function. Imagine it as a series of microscopic, neatly arranged compartments within a muscle fiber, each one packed with a precise arrangement of proteins. At the core are the actin and myosin filaments, the key players in muscle contraction.

The sarcomere is bordered by Z-discs, which anchor the thin actin filaments. Spanning between these limits, we find the thicker myosin filaments interlocked with actin. Each myosin filament is surrounded by six actin filaments, creating a hexagonal pattern that is mirrored throughout the muscle fiber. This architecture is crucial because it ensures that the force generated during contraction is uniform across the fiber, leading to smooth, coordinated movements.

Muscle Contraction Mechanism

Muscle contraction might seem complex, but it boils down to a beautiful molecular dance that occurs at the sarcomere level. When a muscle fiber is stimulated by a nerve impulse, this triggers a series of events, starting with the uncovering of binding sites on the actin filaments. This occurs when calcium ions bind to troponin, causing tropomyosin to move aside.

Then, energized myosin heads latch onto the exposed actin binding sites and pull the actin inwards in a power stroke. This is the crucial step that shortens the sarcomere. In effect, it's the repeated motions of these power strokes of countless myosin heads that make your muscles contract. ATP, the cellular energy currency, then binds to the myosin heads, causing them to release the actin and 're-cock' for another stroke, repeating the cycle as long as the stimulation continues and sufficient energy is present.

Neuromuscular Junction

Consider the neuromuscular junction the communication gateway between the nervous system and muscular system. It's where the end of a motor neuron reaches a muscle cell's surface, but they don't quite touch. Instead, they are separated by a tiny gap called the synaptic cleft.

Upon arrival of a nerve impulse, the motor neuron releases acetylcholine into the synaptic cleft, which then binds to receptors on the muscle cell's membrane. This triggers a cascade of events inside the muscle cell that leads to muscle contraction. The precision of this system ensures that muscles contract only when a signal is received, which provides the control necessary for coordinated movement.

Calcium Ion Regulation in Muscles

Calcium ions serve as the 'on' switch for muscle contraction. Stored within the sarcoplasmic reticulum, they're released into the sarcoplasm when a muscle is primed to contract. They bind to troponin which triggers the muscle contraction cycle by allowing the interaction of actin and myosin filaments.

For muscle relaxation, calcium ions are pumped back into the sarcoplasmic reticulum, a mechanism that depends on ATP. This withdrawal of calcium ions causes the troponin to revert to its original shape, consequently causing the tropomyosin to once again block the binding sites on the actin filaments. This elegant regulation of calcium ions ensures that muscle contraction occurs only when necessary and that it can be swiftly terminated when it's time to relax the muscle.