Muscle Tissue: Structure, Function, and Physiology

Module 10.1 Overview of Muscle Tissue

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and various bodily functions. There are three main types of muscle tissue:

• Skeletal muscle: Voluntary, striated muscle attached to bones for movement.

• Cardiac muscle: Involuntary, striated muscle found only in the heart.

• Smooth muscle: Involuntary, non-striated muscle found in walls of hollow organs.

Common functions of all muscle tissue types include generating force (muscle tension), producing movement, maintaining posture, stabilizing joints, generating heat, and regulating flow of materials through hollow organs.

Striated Muscle Tissue

• Skeletal and cardiac muscle are both striated, meaning they have a striped appearance due to the arrangement of contractile proteins.

• Skeletal muscle cells are long fibers, multinucleated, and can be controlled voluntarily.

• Cardiac muscle cells are branched, found only in the heart, and connected by intercalated discs. They contract involuntarily.

Smooth Muscle Tissue

• Smooth muscle cells do not have striations and are spindle-shaped with a single nucleus.

• They are found in the walls of hollow organs (e.g., digestive tract, blood vessels) and contract involuntarily.

Properties of Muscle Cells

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

• Excitability: Ability to respond to stimuli.

• 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

• Myocytes (muscle cells) use 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

• Muscle tissue consists of many fibers and their surrounding endomysium.

• Skeletal muscle fibers are long, cylindrical, and multinucleated.

• Myofibrils are the most abundant organelle, made up of mostly contractile proteins.

• Sarcoplasmic reticulum (SR) surrounds myofibrils and stores calcium ions.

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

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

Structure of the Myofibril

• Myofibrils contain three types of myofilaments:

  • Thick filaments: Bundles of myosin protein with heads that bind to actin.

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

  • Elastic filaments: Made of titin, providing elasticity and structural support.

Myofilament Arrangement and the Sarcomere

• Sarcomere: The functional unit of muscle contraction, extending from one Z disc to the next.

• Striations are due to alternating light (I bands) and dark (A bands) regions.

• The A band contains the zone of overlap (thick and thin filaments), the H zone (only thick filaments), and the M line (structural proteins).

• The I band contains only thin filaments and the Z disc (anchors thin filaments).

The Sliding-Filament Mechanism of Contraction

The sliding-filament mechanism explains how muscle contraction occurs as myosin heads pull actin filaments toward the center of the sarcomere, shortening the muscle fiber.

• During contraction, the I band and H zone narrow, while the A band remains unchanged.

• Myosin heads attach to actin, forming crossbridges, and pull the thin filaments inward.

• Sarcomeres contract in unison, shortening the entire muscle fiber.

Module 10.3 Skeletal Muscle Fibers as Electrically Excitable Cells

Membrane Potentials in Muscle Cells

• Membrane potential: Unequal distribution of ions across the plasma membrane creates a voltage difference.

• Resting membrane potential: The electrical potential across the sarcolemma of a resting muscle fiber, typically $-85$ mV.

• This potential is maintained by the Na+/K+ pump:

  • Pumps 3 Na+ out and 2 K+ in, creating a high Na+ concentration outside and high K+ inside.

Ion Channels and Gradients

• Resting membrane potential changes when ion movement barriers are removed.

• Sodium and potassium ions move through channels by diffusion, driven by gradients.

Action Potentials

• Action potentials are brief, temporary changes in membrane potential from negative to positive and back.

• Generated by opening gated ion channels:

  • Ligand-gated channels: Open in response to chemicals (e.g., neurotransmitters).

  • Voltage-gated channels: Open in response to changes in membrane potential.

• Phases of action potential:

  • Depolarization: Na+ channels open, Na+ enters, membrane potential becomes positive (up to +30 mV).

  • Repolarization: Na+ channels close, K+ channels open, K+ exits, returning membrane potential to negative.

Module 10.4 The Process of Skeletal Muscle Contraction and Relaxation

The Neuromuscular Junction (NMJ)

The NMJ is where a motor neuron communicates with a muscle fiber to trigger contraction.

• Components of the NMJ:

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

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

  • Motor end plate: Region of muscle fiber membrane with ACh receptors.

• Neurotransmitter: Acetylcholine binds to ligand-gated Na+ channels, allowing Na+ influx and generating an end-plate potential.

Muscle Contraction

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

• Excitation: Action potential in neuron releases ACh, which binds to receptors on the motor end plate, generating an end-plate potential.

• Excitation-contraction coupling: Action potential travels down T-tubules, triggering Ca2+ release from SR.

• Contraction phase:

  • Ca2+ binds troponin, moving tropomyosin and exposing actin binding sites.

  • Myosin heads bind actin, perform a power stroke (using ATP), and pull actin filaments inward (crossbridge cycle).

  • ATP is required for myosin head detachment and re-cocking.

Muscle Relaxation

• Motor neuron stops releasing ACh; ACh is degraded by acetylcholinesterase.

• Ca2+ is pumped back into SR; troponin and tropomyosin return to resting positions, blocking actin binding sites.

• Muscle fiber returns to resting state.

Module 10.5 Energy Sources for Skeletal Muscle Contraction

Immediate Sources of Energy

• ATP: Immediate energy source, rapidly consumed during contraction.

• Creatine phosphate: Stores high-energy phosphate groups to regenerate ATP for short bursts (about 10 seconds).

Glycolytic Energy Sources

• Glycolysis: Anaerobic process breaking down glucose to pyruvate, producing ATP quickly.

• If oxygen is present, pyruvate enters mitochondria for oxidative metabolism; if not, it is converted to lactic acid.

• Glycolysis can provide ATP for 30–40 seconds of activity.

Oxidative Energy Sources

• Oxidative catabolism: Aerobic process using oxygen to produce large amounts of ATP for sustained activity.

• Predominant energy source after one minute of contraction and for prolonged exercise.

Summary Table: Types of Muscle Tissue

Type	Location	Striations	Control	Key Features
Skeletal	Attached to bones	Yes	Voluntary	Long, multinucleated fibers
Cardiac	Heart	Yes	Involuntary	Branched, intercalated discs
Smooth	Walls of hollow organs	No	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:

• Excitation-contraction coupling is the physiological process linking the electrical signal (action potential) to muscle contraction.

• Muscle fatigue can result from depletion of ATP, accumulation of lactic acid, or ionic imbalances.