Mechanism of Skeletal Muscle Contraction

Introduction

• Skeletal muscle contraction is an energy-dependent process driven by ATP, where coordinated cross-bridge cycling produces force through filament sliding. Precise timing between electrical and mechanical events enables sustained contractions, allowing skeletal muscle to generate powerful and controlled movements.

• The action potential spreads from the sarcolemma into the fiber through T-tubules.
• This triggers release of calcium from the sarcoplasmic reticulum.
• Increased cytoplasmic calcium concentration initiates muscle activation.
• Calcium binds to troponin C and starts contraction.
• Electrical activity represents excitation, while shortening represents contraction. This linkage is called excitation–contraction coupling.

Excitation-Contraction Coupling

• Excitation–contraction coupling is the process by which electrical activity leads to muscle contraction through increased cytoplasmic calcium.
• Arrival of an action potential causes release of calcium from the sarcoplasmic reticulum.
• Elevated calcium enables interaction between actin and myosin filaments.
• Contraction refers to activation of cross-bridge cycling, which generates force within the muscle fiber.
• Muscle may generate force without shortening, as seen in isometric contraction.

Sliding-Filament Theory of Muscle Contraction:

• Muscle shortening occurs due to sliding of thin filaments over thick filaments toward the sarcomere center.
• The A band length remains constant during contraction.
• The I band and H zone decrease in width as filaments overlap.
• Z lines move closer, resulting in sarcomere shortening.

Molecular Mechanisms of Muscle Contraction

• Contraction depends on cyclic interaction between myosin heads and actin filaments. This process requires both calcium ions and adenosine triphosphate.
• Calcium exposes binding sites on actin by altering regulatory proteins.
• Myosin heads attach to actin, forming cross-bridges.
• Energy from adenosine triphosphate hydrolysis drives the power stroke. This pulls thin filaments toward the center of the sarcomere.
• Detachment of myosin heads also requires adenosine triphosphate.
• Repeated cycles of attachment and detachment produce force and movement.
• Chemical energy is converted into mechanical energy at cross-bridge sites. This coordinated process ensures efficient muscle contraction.

Role of Troponin, Tropomyosin and Calcium

• Calcium ions act as the key regulator linking electrical activity to contraction in skeletal muscle.
• In resting muscle, cytoplasmic calcium concentration is very low, around 10⁻⁷ moles per liter.
• Under resting conditions, troponin I and tropomyosin block myosin-binding sites on actin.
• Troponin T anchors the troponin complex to tropomyosin, maintaining this inhibitory position.
• The troponin–tropomyosin complex prevents unwanted interaction between actin and myosin.

Troponin–Tropomyosin Interaction

• Following an action potential, calcium is released from the sarcoplasmic reticulum.
• Cytoplasmic calcium rises markedly to about 10⁻⁴ moles per liter.
• Calcium binds to troponin C, producing a conformational change in the complex.
• This change displaces troponin I from actin and shifts tropomyosin away from binding sites.
• Myosin-binding sites on actin become exposed, allowing cross-bridge formation.
• Troponin T transmits these structural changes to tropomyosin along the filament.
• This coordinated shift enables effective interaction between actin and myosin.
• Regulation through thin filaments is termed actin-linked regulation.

Role of Calcium

• Increase in cytoplasmic calcium initiates contraction by removing inhibition on cross-bridge cycling.
• In skeletal muscle, a single action potential usually raises calcium enough to activate all binding sites.
• At high calcium levels, near 10⁻⁴ moles per liter, maximal force generation occurs.
• Further increase in calcium produces minimal additional force.
• Reduction in calcium concentration leads to detachment from troponin.
• Tropomyosin returns to its blocking position, preventing actin–myosin interaction.
• This results in cessation of contraction and onset of relaxation.
• In cardiac and smooth muscle, contraction strength depends more directly on calcium concentration.

Cross‑Bridge Cycle

• The cross-bridge cycle describes repeated interactions between myosin heads and actin filaments during contraction.
• Myosin heads attach to actin, forming cross-bridges and generating force.
• A power stroke pulls the thin filament toward the center of the sarcomere.
• Detachment of myosin requires binding of adenosine triphosphate.
• Hydrolysis of adenosine triphosphate re-energizes the myosin head for the next cycle.
• Cross-bridges operate asynchronously, ensuring smooth and sustained muscle contraction.

Events in the Resting Muscle

• In resting muscle, adenosine triphosphate is bound to the myosin head, preventing its attachment to actin.
• The myosin head possesses intrinsic adenosine triphosphatase activity.
• This enzyme hydrolyzes adenosine triphosphate into adenosine diphosphate and inorganic phosphate.
• Energy released from this reaction is stored within the myosin head.
• The myosin head undergoes rotation at the hinge region. It assumes a position approximately perpendicular to the thick filament.
• This configuration is directed toward actin but does not form a bond.
• The head is displaced slightly along the thin filament, aligning with a new actin site.
• This conformation is termed the energized state or cocked position of the myosin head.
• Energy remains stored as inorganic phosphate, similar to potential energy.
• Most cross-bridges remain in this energized state during rest. This maintains the muscle in a relaxed but ready condition.
• Continuous adenosine triphosphate hydrolysis ensures preparedness for rapid contraction when stimulated.

Events in the Stimulated Muscle

• Arrival of an action potential at T-tubules causes release of calcium ions from the sarcoplasmic reticulum.
• Calcium binds to troponin and exposes myosin-binding sites on actin filaments.
• The energized myosin head, carrying adenosine diphosphate and inorganic phosphate, shows high affinity for actin. It binds to actin, forming the actomyosin complex.
• Release of inorganic phosphate initiates the power stroke.
• The myosin head bends at the hinge region by about 45 degrees.
• This movement pulls the actin filament approximately 10 to 12 nanometers toward the sarcomere center.
• Following the power stroke, adenosine diphosphate is released.
• The myosin head remains firmly attached to actin in a rigid state.
• Detachment requires binding of a new molecule of adenosine triphosphate to the myosin head. This reduces the affinity of myosin for actin, causing separation of the cross-bridge.
• Hydrolysis of adenosine triphosphate re-energizes the myosin head.
• The head returns to its original cocked position, ready for another cycle.
• If cytoplasmic calcium remains elevated, binding sites stay exposed.
• Repeated cross-bridge cycles produce sustained contraction.
• The cycle continues as long as calcium concentration is high and energy supply is adequate.
• Contraction stops when calcium is removed and binding sites become blocked again.

Effects of Cross-Bridge Formation

• During contraction, each cross-bridge attaches to actin and pivots at the hinge region.
• Each thick filament contains approximately 500 to 600 myosin heads capable of rapid cycling.
• A single power stroke produces about 10 to 12 nanometers of filament displacement.
• Repeated cycles of attachment and pivoting pull thin filaments toward the sarcomere center.
• Collective action of multiple cross-bridges produces significant filament sliding. This results in shortening of the sarcomere and overall muscle contraction.
• Continuous stimulation allows repeated cycles, increasing the extent of shortening.
• In isometric contraction, filament sliding does not occur, but cross-bridge activity generates tension.

Role of ATP in Cross-Bridge Formation

• Adenosine triphosphate is essential for detachment of the myosin head from actin.
• Binding of adenosine triphosphate reduces affinity between myosin and actin, enabling separation.
• Adequate adenosine triphosphate maintains normal cycles of contraction and relaxation.
• In resting muscle, most cross-bridges remain detached, producing relaxation.
• However, a small proportion remain attached and contribute to muscle tone.
• Depletion of adenosine triphosphate prevents detachment of cross-bridges.
• This results in persistent attachment called a rigor cross-bridge.
• Sustained rigidity of muscles under this condition is termed rigor mortis.

Steps of Cross-Bridge Cycle

• Hydrolysis of adenosine triphosphate energizes the myosin head.
• In the presence of calcium, myosin binds to actin forming cross-bridges.
• Release of inorganic phosphate and adenosine diphosphate produces the power stroke.
• Binding of new adenosine triphosphate causes detachment of myosin from actin.

Changes During Contraction

• Z lines move closer toward the M line, shortening the sarcomere.
• The A band remains constant in length during contraction.
• The I band and H zone decrease in width.
• In maximal contraction, these regions may nearly disappear.

Summary of Skeletal Muscle Contraction

• Skeletal muscle contraction is a coordinated process involving calcium release, cross-bridge cycling, and adenosine triphosphate utilization. It converts chemical energy into mechanical force through regulated actin–myosin interaction.

Mechanism of Muscle Relaxation

• Muscle relaxation is an active, energy-dependent process requiring adenosine triphosphate.
• Calcium ions are actively transported from the sarcoplasm into the sarcoplasmic reticulum by calcium adenosine triphosphatase.
• In the absence of further nerve impulses, cytoplasmic calcium concentration falls rapidly.
• Reduced calcium availability limits binding to troponin.
• Tropomyosin returns to its original position and covers myosin-binding sites on actin. This prevents further interaction between actin and myosin filaments.
• Cross-bridge cycling stops, and force generation ceases.
• The muscle fiber returns to its resting state, completing relaxation.

Action Potential and Contractile Response

Action Potential

• The resting membrane potential of skeletal muscle is approximately −90 millivolts.
• The action potential originates at the end plate and propagates along the sarcolemma and T-tubules at about 5 meters per second. Its duration is about 2 to 4 milliseconds, with an absolute refractory period of 1 to 3 milliseconds.
• Depolarization occurs due to sodium influx, while repolarization results from potassium efflux.
• The action potential does not directly cause contraction of myofilaments.
• Instead, it increases cytosolic calcium concentration, which activates the contractile machinery.
• Mechanical contraction begins after the electrical event has ended, demonstrating excitation–contraction coupling.

Contractile Response

• A twitch contraction is the mechanical response of a single muscle fiber to one action potential. It includes a contraction phase followed by a relaxation phase.
• The contractile response begins about 2 milliseconds after the action potential. This delay is called the latent period, during which excitation–contraction coupling occurs.
• The latent period reflects several sequential events:

1. Conduction of impulse along the motor nerve.
2. Release and diffusion of acetylcholine at the neuromuscular junction.
3. Generation and spread of action potential along the sarcolemma and T-tubules.
4. Activation of the contractile apparatus.
5. Overcoming internal resistance of muscle fibers.

• Calcium released during a twitch activates the contractile machinery, but removal begins early. Therefore, the peak tension developed in a single twitch is relatively low.
• Contraction time is the interval from onset to peak tension.
• Relaxation time is the interval from peak tension to return to baseline.
• Twitch duration varies with fiber type, about 7 to 10 milliseconds in fast fibers and up to 100 milliseconds in slow fibers.
• Clinically, twitches are observed during tendon reflex testing.
• Spontaneous twitches may indicate lower motor neuron disorders.

Types of Contraction

• Muscle tension is the force generated by a contracting muscle, whereas load is the opposing force acting on it.
• Contraction occurs when muscle tension interacts with the applied load.
• Two main types of contraction are recognized: isometric and isotonic contraction.

Isometric Contraction

• Isometric contraction occurs when muscle develops tension without any change in its length. It is observed when the load exceeds the force generated by the muscle.
• Cross-bridges form and attempt to pull actin filaments, but no shortening occurs.
• Repeated cross-bridge cycling generates force without displacement.
• Since displacement is zero, external work done is also zero, although energy is consumed.
• The generated force is termed isometric tension. This type of contraction helps maintain posture and stabilize joints. It can be recorded experimentally using an isometric recording setup with fixed muscle ends.

Isotonic Contraction

• Isotonic contraction occurs when muscle length changes while maintaining nearly constant tension. It results from sliding of thin filaments over thick filaments, producing shortening of muscle fibers.
• Common examples include walking, running, and lifting objects. This type of contraction performs external work because force is applied with displacement.

Positive and Negative Works

• Positive work occurs when the muscle shortens while lifting a load against gravity.
• Energy is actively used to move the object upward.
• Negative work occurs when the muscle contracts while the load causes it to lengthen. This is termed eccentric contraction, such as controlled lowering of a weight or sitting down.
• During eccentric contraction, the external load exceeds muscle tension.
• The muscle resists stretching, preventing sudden or uncontrolled movement.
• Although lengthening occurs, cross-bridge activity continues to generate resistance.
• A stimulated muscle inherently tends to shorten unless opposed by a greater external force.
• In experimental conditions, one end of the muscle is fixed while the other is free to move.
• Contraction produces movement with relatively constant tension.
• Isotonic contraction is essential for performing daily activities requiring movement and mechanical work.

Processes in Isometric and Isotonic Contractions

• When lifting an object, muscle tension first increases without change in length, producing isometric contraction. This phase continues until muscle tension equals the opposing load.
• Once muscle tension exceeds the load, the muscle shortens and lifts the object. This phase represents isotonic contraction, where movement occurs.
• After lifting, maintaining the object at a fixed position requires isometric contraction again. Here, muscle tension balances the load without further shortening.
• During pushing against an immovable object, such as a wall, only isometric contraction occurs.
• In this case, muscle length remains constant despite active force generation. Thus, both contraction types occur sequentially in most voluntary movements.

Table 27.1: Differences between isometric and isotonic contractions.

Feature	Isometric Contraction	Isotonic Contraction
Muscle length	Remains constant	Changes (usually shortens)
Muscle tension	Increases without movement	Remains relatively constant
External work	No external work performed	Mechanical work is produced