Skeletal Muscle Structure and Physiology: ANP College Study Notes

MUSCULAR SYSTEM

Muscles & Muscle Tissue

The muscular system is essential for movement, posture, and various bodily functions. Skeletal muscle tissue is the most abundant muscle type in the human body and is responsible for voluntary movements.

• Skeletal muscle consists of long, cylindrical cells called fibers.

• Fibers are 10 to 100 μm in diameter and can be up to 30 cm long.

• Each fiber contains multiple nuclei located at the periphery.

• Sarcolemma: The plasma membrane of muscle cells.

• Sarcoplasm: The cytoplasm of muscle cells, containing glycosomes (for glycogen storage), myoglobin (for O2 storage), and many mitochondria.

• Unique features include myofibrils, sarcoplasmic reticulum, and T tubules.

Skeletal Muscle – Microscopic Anatomy

Cellular Structure

Skeletal muscle fibers are specialized for contraction and contain several modified structures that facilitate this function.

• Myofibrils: Rodlike contractile organelles composed of bundles of actin and myosin.

• Sarcoplasmic Reticulum (SR): Modified endoplasmic reticulum surrounding each myofibril; stores calcium for contraction.

• Transverse Tubules (T tubules): Invaginations of the sarcolemma that carry electrical signals deep into the cell.

Myofibrils and Sarcomeres

Organization and Striations

Myofibrils are densely packed organelles that run the length of the muscle fiber and make up about 80% of cell volume. They are composed of repeating units called sarcomeres, which are the basic contractile units of muscle.

• Sarcomeres: Repeating contractile units composed of bundles of myofilaments.

• Striations are due to the repeating series of dark A-bands and light I-bands.

Regions of the Myofibrils

Each sarcomere contains specific regions defined by the arrangement of thick and thin filaments.

• Thick filaments: Composed of myosin.

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

• A band: Darker region at the center of the sarcomere; contains both myosin and actin.

• I band: Lighter region at the ends of the sarcomere; contains only actin.

• Z disc: Dark line in the center of the I band; anchors thin filaments and separates sarcomeres.

• H zone: Lighter region of the A band where actin does not overlap with myosin.

• M line: Dark line in the center of the H zone; anchors thick filaments.

Structure of Filaments

Thick Filament Structure

Thick filaments are composed of the protein myosin, which consists of four polypeptide chains.

• Head: Contains two light polypeptide chains; binding sites for actin and ATP.

• Tail: Contains two heavy polypeptide chains; forms the backbone of the filament.

• Many myosin molecules bind together to form the thick filament.

Thin Filament Structure

Thin filaments are composed of actin, troponin, and tropomyosin.

• Actin: Twisting double strands of fibrous protein; each subunit has a binding site for myosin.

• Tropomyosin: Thread-like regulatory protein that covers myosin binding sites on actin.

• Troponin: Ball-like regulatory protein with binding sites for calcium.

• Troponin-tropomyosin complex regulates muscle contraction by controlling access to myosin binding sites.

Other Proteins

Additional proteins contribute to the structural integrity and function of the sarcomere.

• Titin: Elastic protein filament that anchors thick filaments to the Z disc and helps with recoil.

• Myosin-binding Protein C: Ensures thin and thick filaments stay parallel.

Sarcoplasmic Reticulum and T-Tubules

Sarcoplasmic Reticulum (SR)

The SR is a network of modified endoplasmic reticulum that surrounds each myofibril and is crucial for muscle contraction.

• Regulates intracellular Ca2+ levels.

• Stores and releases Ca2+ as needed.

• Terminal cisterns are channels located on either side of T-tubules; together they form a triad.

T-Tubules

T-tubules are tunnel-like continuations of the sarcolemma that invaginate the cell at every A band-I band junction.

• Conduct electrical signals deep into muscle fiber.

• Stimulate release of calcium from SR.

• Associated with terminal cisterns to form the triad.

Neuromuscular Junction

Structure and Function

The neuromuscular junction is the site where a motor neuron communicates with a skeletal muscle fiber, initiating contraction.

• Located mid-way along a muscle fiber.

• Consists of three parts: axon terminal (synaptic knob), synaptic cleft, and motor end plate.

• Axon terminal contains synaptic vesicles with acetylcholine (ACh).

• Motor end plate has receptors for ACh and serves as chemically gated ion channels.

• Synaptic cleft is a fluid-filled space separating axon terminal from motor end plate; contains acetylcholinesterase (AChE).

Muscle Contraction Physiology

Sliding Filament Model

The sliding filament model explains how muscles contract by the interaction of actin and myosin filaments.

• In relaxed state, thin and thick filaments overlap only slightly.

• Upon stimulation, myosin heads bind to actin and sliding begins.

• Thin filaments slide past thick filaments, increasing overlap.

Sarcomere Contraction

Contraction involves the shortening of sarcomeres, myofibrils, muscle fibers, and ultimately the whole muscle.

• Myosin heads bind to actin, forming cross bridges.

• Cross bridges form and break several times, pulling actin toward the center of the sarcomere.

• Z discs are pulled toward the M line; H zone and I band shorten, A band remains unchanged.

• Thick and thin filaments do not change length, only their degree of overlap increases.

Requirements for Skeletal Muscle Contraction

Several steps are required for skeletal muscle contraction, including activation, action potential generation, and excitation-contraction coupling.

• Activation: Stimulation by nerve ending at the neuromuscular junction.

• Action potential: Electrical impulse propagates along the sarcolemma.

• Excitation-contraction coupling: Action potential triggers release of Ca2+ from SR, initiating contraction.

Four Steps of Skeletal Muscle Contraction

Muscle contraction occurs in four main steps:

1. Events at neuromuscular junction

2. Generation and propagation of an action potential

3. Excitation-contraction (E-C) coupling

4. Cross bridge cycling

Resting Membrane Potential and Membrane Transport

Resting Membrane Potential (RMP)

RMP is the electrical charge difference across the sarcolemma while at rest, typically about –90 mV in skeletal muscle cells.

• Inside of sarcolemma is negative compared to outside.

• Established and maintained by ion channels and Na+/K+ pumps.

• ACh receptors and voltage-gated ion channels are closed; Ca2+ is stored in SR.

• Myofilaments are in relaxed position.

Membrane Transport Review

Diffusion is the passive movement of ions down their electrochemical gradients, driven by electrical and chemical forces.

• Electrical: Ions move toward areas of opposite charge.

• Chemical: Ions move from high to low concentration.

Nerve Stimulus and Neuromuscular Junction

Stimulation by Motor Neurons

Skeletal muscle is stimulated by somatic motor neurons under voluntary control.

• Axons travel from the central nervous system to skeletal muscle.

• Each axon forms several branches and each terminal forms a neuromuscular junction with a single muscle fiber.

Neuromuscular Junction Details

The axon terminal and muscle fiber are separated by the synaptic cleft. The axon terminal contains synaptic vesicles with ACh, Ca2+ pumps, and voltage-gated Ca2+ channels.

• Ca2+ pumps actively pump calcium out of the cell.

• Voltage-gated Ca2+ channels open or close due to voltage changes.

Muscle Fiber Features

The muscle fiber at the junction has a motor end plate with ACh receptors, junctional folds, and Na+/K+ pumps.

• Na+/K+ pumps create a negative charge inside the cell (RMP).

• Pumps out 3 sodium for every 2 potassium in.

Events at the Neuromuscular Junction

Six Steps of Transmission

Transmission at the neuromuscular junction involves six steps:

1. Action potential arrives at axon terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ enters terminal.

3. Ca2+ causes exocytosis of synaptic vesicles; ACh released into synaptic cleft.

4. ACh diffuses and binds to receptors on motor end plate.

5. ACh binding opens chemically-gated ion channels; Na+ enters, K+ exits, causing end plate potential.

6. ACh degraded by acetylcholinesterase; ion channels close, effects terminated.

Action Potential

Definition and Phases

An action potential is a rapid electrical signal caused by reversal of membrane potential, traveling down the plasma membrane of excitable cells.

• Occurs in neurons and muscle cells.

• Involves Na+/K+ pumps, voltage-gated sodium and potassium channels.

• Three phases: depolarization, repolarization, hyperpolarization.

Resting Membrane Potential

Excitable cells maintain a negative charge inside compared to outside, created by Na+/K+ pumps.

• 3 sodium out for every 2 potassium in; requires ATP.

• Voltage-gated channels are closed at rest.

• Skeletal muscle RMP: –80 mV to –90 mV; neurons: –70 mV.

Depolarization

Depolarization occurs when a stimulus causes the cell to reach threshold, opening voltage-gated sodium channels.

• Sodium moves into cell, making inside more positive.

• Depolarization is due to sodium channels opening and Na+ moving into cell.

Repolarization

Repolarization follows depolarization, closing sodium channels and opening potassium channels.

• Potassium moves out of cell, making inside more negative.

• Repolarization is due to K+ moving out of cell.

Hyperpolarization

After repolarization, the cell briefly overshoots its resting membrane potential before returning to normal via Na+/K+ pumps.

• Voltage-gated potassium channels close; cell drifts back to RMP.

Action Potential Propagation

Depolarization causes voltage-gated sodium channels in adjacent areas to open, resulting in a wave of action potential traveling down the cell.

• This process is called action potential propagation or a depolarization wave.

Summary Table: Sarcomere Regions and Filament Types

This table summarizes the main regions of the sarcomere and the types of filaments present.

Region	Filament Type	Description
A band	Myosin & Actin	Darker region, center of sarcomere, thick filament with some overlap
I band	Actin	Lighter region, ends of sarcomere, only thin filament
Z disc	Actin (anchored)	Anchors thin filaments, separates sarcomeres
H zone	Myosin	Lighter region of A band, no actin overlap
M line	Myosin (anchored)	Anchors thick filaments, center of H zone

Additional info: Table entries inferred from context and standard anatomy references.