Muscle Tissue: Structure, Function, and Contraction Mechanisms

Module 10.1 Overview of Muscle Tissue

Types of Muscle Tissue

Muscle tissue is essential for movement, posture, and various physiological functions. There are three main types of muscle tissue, each with distinct characteristics and roles.

• Skeletal muscle: Composed of long, cylindrical fibers with striations. Responsible for voluntary movements and attached to bones.

• Cardiac muscle: Found only in the heart, striated, branched, and interconnected by intercalated discs. Contractions are involuntary.

• Smooth muscle: Lacks striations, spindle-shaped cells, found in walls of hollow organs (e.g., intestines, blood vessels). Contractions are involuntary.

Common functions of all muscle tissue types include generating force, maintaining posture, stabilizing joints, and regulating movement of materials through hollow organs.

Properties of Muscle Cells

Muscle cells possess unique properties that enable their function:

• Contractility: Ability to contract when proteins in the cell draw closer together.

• Excitability: Ability to respond to stimuli, usually from the nervous system.

• Conductivity: Ability to conduct electrical changes across the plasma membrane.

• Distensibility: Ability to be stretched without being ruptured.

• Elasticity: Ability to return to original length after being stretched.

Structure of Muscle Cells

Muscle cells, or myocytes, have specialized terminology:

• Sarcolemma: Plasma membrane of the myocyte.

• Sarcoplasm: Cytoplasm of the myocyte.

• Myofibrils: Cylindrical organelles made of bundles of specialized proteins for contraction.

• Sarcoplasmic reticulum (SR): Modified endoplasmic reticulum that stores and releases calcium ions.

Module 10.2 Structure and Function of Skeletal Muscle Fibers

Structure of the Skeletal Muscle Fiber

Skeletal muscle fibers are long, cylindrical cells formed by the fusion of many embryonic myoblasts. They contain several specialized structures:

• Myofibrils: Most abundant organelle, made up of mostly contractile proteins.

• Sarcoplasmic reticulum (SR): Surrounds myofibrils, stores and releases calcium ions.

• Transverse tubules (T-tubules): Deep inward extensions of sarcolemma filled with extracellular fluid.

• Terminal cisternae: Enlarged sections of SR found flanking each T-tubule, forming a triad.

Structure of the Myofibril

Myofibrils contain contractile proteins that generate tension, regulatory proteins that dictate when a fiber may contract, and structural proteins that maintain proper alignment and fiber stability.

• Thick filaments: Bundles of myosin proteins. Each myosin has a head and tail, with the head binding to actin.

• Thin filaments: Composed of actin, tropomyosin, and troponin. Actin binds with myosin heads; tropomyosin and troponin regulate contraction.

• Elastic filaments: Made of titin, a spring-like protein that stabilizes the myofibril structure and resists excessive stretching.

Myofilament Arrangement and the Sarcomere

Striations in skeletal muscle are due to the arrangement of myofilaments within sarcomeres:

• A bands: Dark bands where thick and thin filaments overlap.

• I bands: Light bands containing only thin filaments.

• H zone: Middle of the A band, only thick filaments present.

• M line: Center of the A band, contains structural proteins.

Module 10.3 Skeletal Muscle Fibers as Electrically Excitable Cells

Membrane Potentials in Muscle Cells

Muscle cells maintain a membrane potential due to unequal distribution of ions across the plasma membrane:

• Resting membrane potential: Electrical potential across the sarcolemma of a resting muscle fiber, typically $-85\ \text{mV}$.

• Na+/K+ pump: Maintains the resting membrane potential by moving Na+ out and K+ into the cell.

Ion Channels and Gradients

• Sodium and potassium ions move through the sarcolemma using protein channels and carriers, only if a gradient exists.

Action Potentials

Action potentials are brief, temporary changes in membrane potential:

• Generated by opening gated ion channels (ligand-gated or voltage-gated).

• Depolarization: Voltage-gated Na+ channels open, Na+ enters, membrane potential rises to about $+30\ \text{mV}$.

• Repolarization: Voltage-gated K+ channels open, K+ exits, membrane potential returns to resting value.

Module 10.4 The Process of Skeletal Muscle Contraction and Relaxation

The Neuromuscular Junction (NMJ)

The NMJ is where a motor neuron communicates with muscle fibers to stimulate contraction.

• Axon terminal: Contains synaptic vesicles filled with acetylcholine (ACh).

• Synaptic cleft: Space between axon terminal and muscle fiber.

• Motor end plate: Region of muscle fiber membrane with ligand-gated Na+ channels activated by ACh.

Muscle Contraction

Muscle contraction occurs in three phases: excitation, excitation-contraction coupling, and contraction.

1. Excitation phase: Action potential from motor neuron triggers release of ACh, which binds to receptors on the motor end plate, opening Na+ channels and generating an end-plate potential.

2. Excitation-contraction coupling: End-plate potential triggers action potentials along the sarcolemma and T-tubules, leading to Ca2+ release from the SR.

3. Contraction phase: Ca2+ binds to troponin, moving tropomyosin and exposing actin binding sites. Myosin heads bind to actin, forming crossbridges and initiating the crossbridge cycle.

The Crossbridge Cycle

• Myosin head binds to actin after ATP hydrolysis ($\text{ATP} \rightarrow \text{ADP} + \text{P}_i$).

• Power stroke occurs as myosin pulls actin toward the center of the sarcomere.

• ATP binds to myosin, causing it to detach from actin and reset for another cycle.

Muscle Relaxation

• Motor neuron stops signaling, ACh is degraded by acetylcholinesterase.

• Ca2+ is pumped back into the SR, tropomyosin covers actin binding sites, and muscle returns to resting state.

Module 10.5 Energy Sources for Skeletal Muscle Contraction

Immediate Sources of Energy

• ATP: Stored in muscle fiber, rapidly consumed during contraction.

• Creatine phosphate: Concentration in cytosol is 5-6 times higher than ATP, can regenerate ATP for about 10 seconds of maximum activity.

Glycolytic Energy Sources

• Glycolysis: Series of reactions breaking down glucose to pyruvate, producing ATP. Can occur with or without oxygen.

• If oxygen is present, pyruvate enters mitochondria for oxidative catabolism.

• If oxygen is absent, pyruvate is converted to lactic acid.

• Glycolysis can supply ATP for 30-40 seconds of sustained contraction.

Oxidative Energy Sources

• Oxidative catabolism: Requires oxygen, occurs in mitochondria, produces more ATP than glycolysis.

• Provides nearly 100% of ATP after several minutes of activity.

Summary Table: Types of Muscle Tissue

Type	Location	Striations	Control	Key Features
Skeletal	Attached to bones	Present	Voluntary	Long, cylindrical, multinucleated
Cardiac	Heart	Present	Involuntary	Branched, intercalated discs
Smooth	Walls of hollow organs	Absent	Involuntary	Spindle-shaped, single nucleus

Key Equations

• ATP hydrolysis: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

• Resting membrane potential: $V_{\text{rest}} = -85\ \text{mV}$

Additional info:

• Some terminology and details were inferred for completeness and clarity.

• Table structure and summary points were expanded for academic context.