Muscle Tissue: Structure, Function, and Physiology

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Muscle Tissue Overview

Introduction to Muscle Tissue

Muscle tissue is a primary tissue type in the human body, specialized for contraction and responsible for movement. There are three main types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.

Skeletal muscle: Moves the body by pulling on bones; voluntary control.
Cardiac muscle: Found only in the heart; controls heart contractions.
Smooth muscle: Controls movements inside organs and vessels; involuntary control.

Functions and Properties of Muscle Tissue

Common Properties of Muscle Tissue

Excitability (Responsiveness): Ability to respond to stimuli.
Contractility: Ability to shorten actively and exert a pull or tension.
Extensibility: Ability to be stretched without damage.
Elasticity: Ability to return to original length after contraction or extension.

Functions of Skeletal Muscle

Producing movement
Maintaining posture and body position
Supporting soft tissues
Guarding body entrances and exits
Maintaining body temperature
Storing nutrients

Organization of Skeletal Muscle

Structural Organization

Skeletal muscles are complex organs containing muscle tissue, connective tissues, blood vessels, and nerves.

Epimysium: Outer layer of collagen fibers; separates muscle from surrounding tissues.
Perimysium: Surrounds bundles of muscle fibers (fascicles); contains blood vessels and nerves.
Endomysium: Surrounds individual muscle fibers; contains capillaries, myosatellite cells (stem cells), and nerve fibers.

At the ends of muscles, collagen fibers from all three layers merge to form a tendon (bundle) or aponeurosis (sheet), attaching muscle to bone.

Vascular and Neural Supply

Skeletal muscles have extensive vascular networks for nutrient and waste exchange.
Muscles contract only when stimulated by the central nervous system (voluntary muscles).

Skeletal Muscle Fibers

Characteristics of Skeletal Muscle Fibers

Large, multinucleate cells formed by fusion of myoblasts.
Also called striated muscle cells due to visible striations.

Key Structures

Sarcolemma: Plasma membrane of a muscle fiber; surrounds the sarcoplasm (cytoplasm).
Transverse tubules (T tubules): Invaginations of the sarcolemma; transmit action potentials into the cell.
Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum; stores and releases calcium ions; forms terminal cisternae adjacent to T tubules (triad = 2 terminal cisternae + 1 T tubule).
Myofibrils: Cylindrical structures within muscle fibers; made of myofilaments (thin and thick filaments).

Sarcomere Structure

The sarcomere is the smallest functional unit of a muscle fiber, responsible for contraction. The arrangement of thick (myosin) and thin (actin) filaments creates the striated appearance.

A band: Dark region; contains thick filaments.
M line: Center of A band; stabilizes thick filaments.
H band: Area with only thick filaments.
Zone of overlap: Where thick and thin filaments overlap.
I band: Light region; contains only thin filaments.
Z line: Bisects I band; marks boundary between sarcomeres.
Titin: Elastic protein; helps restore resting length.

Myofilament Composition

Thin filaments: Composed of F-actin (two rows of G-actin), nebulin, tropomyosin (covers active sites), and troponin (binds tropomyosin, G-actin, and calcium).
Thick filaments: Composed of myosin molecules (each with a tail and two heads); core of titin for elasticity.

Sliding Filament Theory

Muscle contraction occurs as thin filaments slide toward the center of the sarcomere, shortening the muscle fiber. The width of the A band remains constant, while H and I bands narrow and Z lines move closer together.

The Neuromuscular Junction and Muscle Contraction

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a skeletal muscle fiber. The process of muscle contraction is initiated here.

Action potential arrives at the axon terminal.
Acetylcholine (ACh) is released into the synaptic cleft.
ACh binds to receptors on the motor end plate, opening Na+ channels and depolarizing the sarcolemma.
Action potential spreads along the sarcolemma and into T tubules.
Calcium ions are released from the SR, triggering contraction.

Excitation–Contraction Coupling

Action potential travels down T tubules to triads.
Calcium is released from terminal cisternae of SR.
Calcium binds to troponin, causing tropomyosin to move and expose active sites on actin.
Contraction cycle begins.

The Contraction Cycle

Active-site exposure (calcium binds to troponin).
Cross-bridge formation (myosin binds to actin).
Myosin head pivoting (power stroke).
Cross-bridge detachment (ATP binds to myosin).
Myosin reactivation (ATP hydrolysis).

The cycle repeats as long as calcium and ATP are available.

Muscle Relaxation

Calcium is pumped back into the SR.
Troponin returns to its original position, covering active sites.
Contraction ends.

Rigor mortis occurs after death when ATP is depleted, causing sustained contraction due to calcium buildup.

Tension Production in Muscle Fibers

Factors Affecting Tension

Number of power strokes performed.
Resting length of the fiber at stimulation.
Frequency of stimulation.

Length–Tension Relationship

Maximum tension is produced when there is optimal overlap between thick and thin filaments, allowing the greatest number of cross-bridges to form.

Frequency of Stimulation

Twitch: Single contraction from one stimulus; has latent, contraction, and relaxation phases.
Treppe: Gradual increase in tension with repeated stimulation after relaxation (<50/sec); rare in skeletal muscle.
Wave summation: Increased tension from repeated stimuli before relaxation ends (>50/sec).
Tetanus: Maximum tension; can be incomplete (rapid cycles) or complete (no relaxation phase).

Muscle Contractions

Motor Units and Recruitment

Motor unit: A motor neuron and all the muscle fibers it controls.
Recruitment: Increasing the number of active motor units for greater tension.
Muscle tone: Normal tension in resting muscle; stabilizes bones and joints.

Types of Muscle Contractions

Isotonic contractions: Muscle changes length.
- Concentric: Muscle shortens (tension > load).
- Eccentric: Muscle lengthens (tension < load).
Isometric contractions: Muscle develops tension but does not change length (tension never exceeds load).

The heavier the load, the slower and less the muscle shortens.

Muscle Relaxation and Return to Resting Length

Elastic forces (tendon recoil)
Opposing muscle contractions
Gravity

Energy for Muscle Contraction

ATP and Muscle Metabolism

ATP is the direct energy source for contraction.
Muscles store limited ATP; must generate more during activity.

ATP Generation Mechanisms

Direct phosphorylation of ADP by creatine phosphate (CP):
- ATP + Creatine ↔ ADP + Creatine Phosphate
- Enzyme: Creatine kinase (CK)
Anaerobic metabolism (glycolysis):
- Breaks down glucose to pyruvate; yields 2 ATP per glucose.
Aerobic metabolism:
- Uses fatty acids or glucose; occurs in mitochondria; yields much more ATP.

Muscle Metabolism at Different Activity Levels

At rest: Fatty acids are metabolized; glycogen and CP are stored.
Moderate activity: Aerobic breakdown of glucose.
Peak activity: Anaerobic glycolysis; pyruvate converted to lactate.

Recovery Period and Oxygen Debt

Recovery period: Time needed for muscles to return to normal after exertion.
Cori cycle: Lactate from muscles is converted to glucose in the liver.
Oxygen debt (EPOC): Extra oxygen required after exercise to restore normal metabolism.

Heat Production

Active muscles release heat, contributing up to 85% of body heat.

Hormonal Effects

Growth hormone, testosterone, thyroid hormones, and epinephrine increase muscle metabolism.

Muscle Performance and Fiber Types

Muscle Performance

Force: Maximum tension produced.
Endurance: Duration of sustained activity.
Both depend on fiber type and conditioning.

Types of Skeletal Muscle Fibers

Fiber Type	Characteristics	Example
Fast fibers	Large diameter, few mitochondria, quick to contract, fatigue rapidly, pale color	White muscles (chicken breast)
Slow fibers	Small diameter, many mitochondria, slow to contract, fatigue-resistant, high myoglobin, dark color	Red muscles (chicken legs)
Intermediate fibers	Mid-sized, little myoglobin, moderate speed and endurance	Most human muscles (pink)

Muscle Hypertrophy and Atrophy

Hypertrophy: Muscle growth from training; increases fiber diameter, myofibrils, mitochondria, and glycogen reserves.
Atrophy: Reduction in muscle size, tone, and power due to inactivity.

Aging and Muscle Tissue

Fibers become smaller and less elastic.
Increased fibrous connective tissue (fibrosis).
Decreased exercise tolerance and slower recovery.

Muscle Fatigue

Occurs when muscles cannot perform at required levels.
Causes: Depletion of reserves, damage to membranes, pH changes, and pain.

Physical Conditioning

Anaerobic endurance: Uses fast fibers; improved by brief, intense workouts (e.g., sprints, weightlifting).
Aerobic endurance: Uses slow fibers; improved by sustained, low-intensity activity (e.g., jogging).

Cardiac Muscle Tissue

Structural and Functional Differences

Found only in the heart; striated like skeletal muscle.
Small, branched cells with single nucleus.
Short, wide T tubules; no triads; SR lacks terminal cisternae.
Intercalated discs connect cells via gap junctions and desmosomes, allowing synchronized contraction.
Almost entirely dependent on aerobic metabolism; many mitochondria and myoglobin.

Functional Characteristics

Automaticity: Can contract without neural stimulation (pacemaker cells).
Nervous system can alter rate and force.
Longer contraction and refractory periods than skeletal muscle.
No wave summation or tetanus (prevents sustained contraction).

Smooth Muscle Tissue

Structural Characteristics

Long, spindle-shaped cells with single central nucleus.
No T tubules, myofibrils, or sarcomeres; nonstriated.
Thick filaments scattered; thin filaments attached to dense bodies.
Dense bodies connect adjacent cells for coordinated contraction.
No tendons or aponeuroses.

Functional Characteristics

Excitation–contraction coupling involves calcium binding to calmodulin, activating myosin light chain kinase.
Length–tension relationship: Can contract over a wide range of lengths (plasticity).
Control of contractions:
- Multiunit smooth muscle: Each cell innervated individually.
- Visceral smooth muscle: Cells connected in sheets; contract rhythmically, often without neural input.
Smooth muscle tone: Maintains a baseline level of activity, modulated by neural, hormonal, or chemical factors.

Summary Table: Muscle Tissue Types

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Location	Attached to bones	Heart	Walls of hollow organs
Control	Voluntary	Involuntary (automaticity)	Involuntary
Striations	Yes	Yes	No
Cell shape	Long, cylindrical, multinucleate	Short, branched, single nucleus	Spindle-shaped, single nucleus
Special features	T-tubules, triads, rapid contraction	Intercalated discs, pacemaker cells	Dense bodies, plasticity

Key Equations

ATP regeneration by creatine phosphate:
Glycolysis (anaerobic):
Aerobic metabolism (simplified):

Example Applications

Isotonic contraction: Lifting a dumbbell (muscle shortens).
Isometric contraction: Holding a heavy object steady (muscle length does not change).
Cardiac muscle automaticity: Heartbeat continues even if nerve supply is interrupted.
Smooth muscle plasticity: Bladder can stretch and still contract to expel urine.

Additional info: This summary integrates and expands upon the provided lecture content, ensuring a comprehensive, self-contained study guide for college-level anatomy and physiology students.