Skeletal Muscle: Structure and Function

Overview

Skeletal muscle is a type of striated muscle tissue that is primarily responsible for voluntary movements, posture, and heat production in the human body. This study guide covers the structure, function, and physiology of skeletal muscle, including the neuromuscular junction, sarcomere, cross-bridge cycle, and contraction mechanisms.

Skeletal Muscle Functions

Main Functions of Skeletal Muscle

• Movement of Bones: Skeletal muscles attach to bones via tendons and contract to produce movement at joints.

• Maintain Posture and Body Position: Continuous muscle contractions help maintain posture and stabilize the body against gravity.

• Stabilize Joints: Muscles reinforce and stabilize joints, especially after disturbances, helping to return them to their original state.

• Heat Generation: Muscle contractions generate heat as a byproduct, which is important for maintaining body temperature (e.g., shivering).

Example: The muscles of the back and abdomen help maintain upright posture, while shivering involves rapid muscle contractions to generate heat in cold environments.

Skeletal Muscle Structure

Connective Tissue Organization

Skeletal muscle is organized into several layers of connective tissue that provide support, protection, and pathways for nerves and blood vessels.

• Epimysium: Dense irregular connective tissue that surrounds the entire muscle.

• Perimysium: Fibrous connective tissue that surrounds groups of muscle fibers, forming a fascicle.

• Endomysium: Areolar connective tissue that surrounds each individual muscle fiber (cell) within a fascicle.

Muscle Fiber: Also known as a myofiber or myocyte, this is a single muscle cell, multinucleated and elongated in shape.

Fascicle: A bundle of muscle fibers wrapped by perimysium.

Compartment Syndrome: A condition where increased pressure within a muscle compartment reduces blood flow, potentially leading to muscle and nerve damage.

Skeletal Muscle Fiber (Myofiber)

Cellular Structure

• Size: 10 to 100 μm in diameter, up to 30 cm long.

• Nuclei: Multiple peripheral nuclei per cell.

• Mitochondria: Numerous, providing ATP for contraction.

• Sarcolemma: The plasma membrane of a muscle fiber.

• T-tubules (Transverse Tubules): Deep invaginations of the sarcolemma that help propagate action potentials into the cell.

• Sarcoplasm: The cytoplasm of a muscle fiber, containing glycogen granules and myoglobin for oxygen storage.

• Myofibrils: Hundreds to thousands per cell, composed of repeating sarcomeres.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions (Ca2+).

Neuromuscular Junction and Action Potential

Structure and Function

• Neuromuscular Junction (NMJ): The synapse between a motor neuron and a skeletal muscle fiber, typically located midway along the muscle fiber.

• Synaptic Vesicles: Contain the neurotransmitter acetylcholine (ACh).

• End Plate Potential (EPP): A graded depolarization of the muscle membrane caused by ACh binding to nicotinic acetylcholine receptors (nAChRs).

• Action Potential (AP): If the EPP reaches threshold, it triggers an all-or-none action potential in the sarcolemma, leading to muscle contraction.

Key Steps:

1. AP arrives at axon terminal of motor neuron.

2. Voltage-gated Ca2+ channels open, Ca2+ enters axon terminal.

3. ACh is released into synaptic cleft and binds to nAChRs on the muscle fiber.

4. Na+ influx depolarizes the sarcolemma, generating an EPP.

5. If threshold is reached, a muscle AP is generated and propagates along the sarcolemma and T-tubules.

6. ACh is broken down by acetylcholinesterase, terminating the signal.

Sarcomere Structure

Organization of Myofibrils

• Myofibrils: Rod-like structures composed of repeating units called sarcomeres.

• Sarcomere: The functional contractile unit of muscle, defined as the segment between two Z discs.

• Thick Filaments: Composed of myosin, which has heads that bind to actin and ATP.

• Thin Filaments: Composed of actin, tropomyosin, and troponin.

• Z Disc: Anchors thin filaments and connects adjacent myofibrils.

• M Line: Holds adjacent thick filaments together.

• I Band: Light band containing only thin filaments.

• A Band: Dark band containing the length of thick filaments, including regions of overlap with thin filaments.

• H Zone: Lighter region in the middle of the A band where thick and thin filaments do not overlap.

Ultrastructure of Filaments

Thick and Thin Filaments

• Myosin (Thick Filament): Each myosin molecule has a tail and two heads. The heads have binding sites for actin and ATP, and possess ATPase activity.

• Actin (Thin Filament): Composed of globular (G) actin subunits arranged in double helical strands (F-actin). Each G-actin has a myosin-binding site.

• Tropomyosin: A regulatory protein that blocks myosin-binding sites on actin in resting muscle.

• Troponin: A regulatory protein complex that binds Ca2+ and moves tropomyosin away from binding sites upon activation.

Sarcoplasmic Reticulum and T-Tubules

Calcium Storage and Release

• Sarcoplasmic Reticulum (SR): Stores Ca2+ and releases it upon stimulation by an action potential.

• Terminal Cisternae: Enlarged regions of the SR adjacent to T-tubules, forming a triad structure.

• T-Tubules: Conduct action potentials from the sarcolemma into the muscle fiber, triggering Ca2+ release from the SR.

Excitation-Contraction Coupling

Linking Electrical and Mechanical Events

• Action potentials travel along the sarcolemma and down T-tubules.

• Voltage-sensitive proteins in T-tubules trigger Ca2+ release from the SR.

• Ca2+ binds to troponin, causing a conformational change that moves tropomyosin and exposes myosin-binding sites on actin.

• Myosin heads bind to actin, initiating contraction.

Cross-Bridge Cycle

Steps of the Cross-Bridge Cycle

1. Cross-Bridge Attachment: Energized myosin head binds to exposed binding site on actin.

2. Power Stroke: Myosin head pivots, pulling the thin filament toward the M line; ADP and Pi are released.

3. Cross-Bridge Detachment: A new ATP molecule binds to myosin, causing it to detach from actin.

4. Cocking of the Myosin Head: ATP is hydrolyzed to ADP and Pi, re-energizing the myosin head for the next cycle.

The cycle repeats as long as Ca2+ and ATP are available.

Sliding Filament Model of Contraction

Mechanism of Muscle Shortening

• During contraction, thin filaments slide past thick filaments, increasing their overlap.

• Sarcomeres shorten, causing the muscle fiber and the whole muscle to shorten.

• The I bands and H zones become narrower, while the A band remains the same length.

Key Principle: The length of the thick and thin filaments does not change; only their degree of overlap changes.

Length-Tension Relationship

Optimal Sarcomere Length

• There is an optimal sarcomere length (usually around 2.0–2.2 μm) at which maximal force can be generated.

• If sarcomeres are too stretched or too compressed, force generation decreases due to less optimal overlap between actin and myosin.

Application: This relationship explains why muscle strength varies with joint angle and muscle length.

Table: Connective Tissue Layers of Skeletal Muscle

Key Terms and Definitions

• Myofibril: A long, filamentous organelle found within muscle cells that has a banded appearance.

• Sarcomere: The basic contractile unit of muscle fiber.

• Myosin: The motor protein that forms thick filaments and interacts with actin to produce contraction.

• Actin: The protein that forms thin filaments and provides binding sites for myosin.

• Tropomyosin: A regulatory protein that blocks myosin-binding sites on actin in resting muscle.

• Troponin: A regulatory protein that binds calcium and moves tropomyosin away from binding sites on actin.

• Sarcolemma: The plasma membrane of a muscle fiber.

• Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores and releases calcium ions.

• Neuromuscular Junction: The synapse between a motor neuron and a muscle fiber.

Key Equations

• Force Generation (Length-Tension Relationship):

• ATP Hydrolysis (for Myosin Head Cocking):

Additional info: Some details, such as the optimal sarcomere length and the precise sequence of excitation-contraction coupling, have been expanded for academic completeness.