Muscle and Muscle Tissue: Structure, Function, and Physiology

Muscle and Muscle Tissue

Overview of Muscle Tissue Types

Muscle tissue is specialized for contraction and is essential for movement, posture, and vital body functions. There are three main types of muscle tissue, each with unique structural and functional characteristics.

• Skeletal Muscle Tissue: Voluntary, striated, multinucleated; attached to bones for movement.

• Cardiac Muscle Tissue: Involuntary, striated, branched, single nucleus; found only in the heart.

• Smooth Muscle Tissue: Involuntary, non-striated, spindle-shaped; found in walls of hollow organs.

Table: Comparison of Muscle Tissue Types

Feature	Skeletal	Cardiac	Smooth
Control	Voluntary	Involuntary	Involuntary
Striations	Yes	Yes	No
Cell Shape	Long, cylindrical	Branched	Spindle-shaped
Nuclei	Multiple	Single	Single
Location	Attached to bones	Heart	Walls of organs

Special Characteristics of Muscle Tissue

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to recoil to resting length after stretching.

General Functions of Muscle

• Producing movement

• Maintaining posture

• Stabilizing joints

• Generating heat

Gross Structure of Skeletal Muscle

Organization and Connective Tissue Sheaths

Skeletal muscle is organized into bundles surrounded by connective tissue sheaths:

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds fascicles (bundles of muscle fibers).

• Endomysium: Surrounds individual muscle fibers (cells).

Single Muscle Cell: Called a muscle fiber or myocyte.

Skeletal Muscle Cell Anatomy

Key Structures and Functions

• Sarcolemma: Plasma membrane of the muscle cell; conducts action potentials.

• Sarcoplasm: Cytoplasm of the muscle cell; contains organelles and myofibrils.

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

• Transverse (T) Tubules: Invaginations of the sarcolemma; transmit action potentials into the cell.

• Triad: Structure formed by a T-tubule and two adjacent terminal cisternae of the SR.

• Cisternae of SR: Enlarged areas of the SR that store calcium.

• Myofibrils: Rod-like units within muscle fibers; composed of repeating sarcomeres.

• Sarcomere: Functional contractile unit of muscle; defined by Z discs.

Sarcomere Structure

• Proteins in the Sarcomere:

  • Actin: Thin filament; binds myosin for contraction.

  • Myosin: Thick filament; motor protein with heads that bind actin.

  • Titin: Elastic protein; stabilizes myosin and provides elasticity.

  • Nebulin: Helps align actin filaments.

  • Dystrophin: Links actin to the sarcolemma; important for structural stability.

  • Myomesin: Forms the M line; holds thick filaments together.

• A Band: Dark area; length of thick filaments.

• I Band: Light area; contains only thin filaments.

• M Line: Center of A band; holds thick filaments together.

• Z Disc: Boundary of sarcomere; anchors thin filaments.

• H Zone: Central region of A band; only thick filaments present.

Sliding Filament Theory of Muscle Contraction

The sliding filament theory explains how muscles contract by the sliding of actin (thin) filaments past myosin (thick) filaments, shortening the sarcomere without changing the length of the filaments themselves.

• Myosin heads bind to actin, forming cross-bridges.

• Using ATP, myosin heads pivot, pulling actin filaments toward the center of the sarcomere.

• Repeated cycles cause the muscle to contract.

Roles of Myofilaments, SR, and T-Tubules

• Myofilaments: Actin and myosin filaments generate force for contraction.

• Sarcoplasmic Reticulum: Releases Ca2+ to initiate contraction; reabsorbs Ca2+ for relaxation.

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

Role of Troponin and Tropomyosin

• Tropomyosin: Blocks myosin-binding sites on actin at rest.

• Troponin: Binds Ca2+; causes tropomyosin to move, exposing binding sites for myosin.

Phases of Muscle Contraction

Three Phases

1. Events at the Neuromuscular Junction:

   • Motor neuron releases acetylcholine (ACh) into synaptic cleft.

   • ACh binds to receptors on sarcolemma, triggering action potential.

2. Excitation-Contraction Coupling:

   • Action potential travels along sarcolemma and T-tubules.

   • SR releases Ca2+, which binds to troponin.

3. Cross-Bridge Cycling (Power Stroke):

   • Myosin heads bind to actin, perform power stroke, detach, and re-cock using ATP.

Effect of Drugs/Toxins: Agents that block ACh release, Ca2+ channels, or ATP production can decrease or prevent contraction.

Action Potential in Skeletal Muscle

• Definition: A rapid change in membrane potential that propagates along the sarcolemma.

• Voltage-Gated Ion Channels: Open in response to changes in membrane potential; found along sarcolemma and T-tubules.

• Ligand-Gated Ion Channels: Open in response to binding of a chemical (e.g., ACh); found at neuromuscular junction.

Phases of Action Potential:

• Depolarization: Na+ channels open; Na+ enters cell.

• Repolarization: K+ channels open; K+ leaves cell.

• Hyperpolarization: Membrane potential becomes more negative than resting.

• Threshold: Minimum membrane potential needed to trigger action potential.

Action Potential Diagram:

• Depolarization: Voltage-gated Na+ channels open; Na+ enters.

• Repolarization: Voltage-gated K+ channels open; K+ exits.

• Hyperpolarization: K+ channels remain open briefly.

Motor Units and Muscle Tension

Motor Unit

• A motor unit consists of a motor neuron and all the muscle fibers it innervates.

• Smaller motor units allow fine control; larger units generate more force.

Measuring Muscle Tension: Myogram

• A myogram records the force of muscle contraction over time.

• Used to study muscle twitch and graded responses.

Muscle Twitch

• Consists of three phases:

  1. Latent Period: Time between stimulus and contraction onset.

  2. Contraction Period: Muscle shortens as cross-bridges form.

  3. Relaxation Period: Muscle tension decreases as Ca2+ is reabsorbed.

Isotonic and Isometric Contractions

• Isotonic Contraction: Muscle changes length; tension remains constant.

  • Concentric: Muscle shortens (e.g., lifting a weight).

  • Eccentric: Muscle lengthens (e.g., lowering a weight).

• Isometric Contraction: Muscle length does not change; tension increases (e.g., holding a weight steady).

Difference: Isotonic involves movement; isometric does not.

Graded Muscle Responses

• Muscle contractions can be graded by:

  1. Changing frequency of stimulation: Higher frequency leads to summation and tetanus.

  2. Changing strength of stimulation (recruitment): More motor units activated for greater force.

• Incomplete Tetanus: Partial relaxation between stimuli.

• Complete Tetanus: No relaxation; sustained contraction.

Muscle Relaxation and Rigor Mortis

• Muscle Relaxation: Ca2+ is pumped back into SR; cross-bridges detach.

• Rigor Mortis: After death, ATP is depleted; cross-bridges cannot detach, causing stiffness. Ends as proteins degrade.

Muscle Tone

• Constant, slight contraction of muscles at rest.

• Important for posture and joint stability.

Muscle Metabolism and Fatigue

Energy Sources for Contraction

• Direct Phosphorylation: Creatine phosphate donates phosphate to ADP to form ATP.

• Anaerobic Glycolysis: Glucose broken down to lactic acid; produces ATP without oxygen.

• Aerobic Respiration: Uses oxygen to produce ATP from glucose, fatty acids, or amino acids.

ATP Pathways by Activity Level:

• Resting: Aerobic metabolism of fatty acids.

• Moderate Exercise: Aerobic metabolism of glucose and fatty acids.

• Intense Exercise: Anaerobic glycolysis predominates.

Muscle Fatigue: Inability to contract despite stimulation; due to ATP depletion, ion imbalances, or lactic acid buildup.

Excess Postexercise Oxygen Consumption (EPOC): Increased oxygen intake after exercise to restore metabolic conditions.

Types of Skeletal Muscle Fibers

Type	Contraction Speed	Fatigue Resistance	Color	Main ATP Source
Slow Oxidative (Type I)	Slow	High	Red	Aerobic
Fast Oxidative (Type IIa)	Fast	Intermediate	Red/Pink	Aerobic (some anaerobic)
Fast Glycolytic (Type IIb)	Fast	Low	White	Anaerobic glycolysis

Smooth Muscle

Innervation and Anatomy

• Innervated by autonomic nervous system via varicosities (swellings that release neurotransmitters).

• Smooth muscle cells are spindle-shaped, have a single nucleus, and lack striations.

Contraction and Relaxation in Smooth Muscle

• Contraction is slower and can be sustained longer than skeletal muscle.

• Ca2+ enters from extracellular fluid and SR; binds to calmodulin, not troponin.

• Relaxation occurs when Ca2+ is removed and myosin is dephosphorylated.

Types of Smooth Muscle

• Unitary (Visceral) Smooth Muscle: Cells connected by gap junctions; contract as a unit (e.g., intestines).

• Multi-Unit Smooth Muscle: Each cell innervated individually; allows fine control (e.g., iris of eye).

Factors Influencing Muscle Force

• Number of muscle fibers recruited (motor unit recruitment)

• Size of muscle fibers

• Frequency of stimulation

• Degree of muscle stretch (length-tension relationship)

Example: Lifting a heavy object requires recruitment of more and larger motor units, higher stimulation frequency, and optimal muscle length for maximal force.