Muscle and Muscle Tissue

Overview of Muscle Tissue

Muscle tissue comprises nearly half of the body's mass and is essential for transforming chemical energy (ATP) into mechanical energy, resulting in force and movement. The study of muscle tissue includes understanding its types, characteristics, and functions.

• Types of Muscle Tissue: Skeletal, Cardiac, and Smooth

• Terminology: Myo-, Mys-, Sarco- (e.g., Sarcoplasm is muscle cell cytoplasm)

• Muscle Fiber: Only skeletal and smooth muscle cells are elongated and referred to as muscle fibers

Types of Muscle Tissue

• Skeletal Muscle: Attached to bones, longest muscle fibers, striated, voluntary, contracts rapidly but tires easily, powerful

• Cardiac Muscle: Found only in the heart, striated, involuntary, contracts at a steady rate due to pacemaker, modulated by nervous system

• Smooth Muscle: Found in walls of hollow organs (e.g., stomach, bladder, airways), non-striated, involuntary

Characteristics of Muscle Tissue

All muscle tissues share four main characteristics:

• Excitability (Responsiveness): Ability to receive and respond to stimuli

• Contractility: Ability to shorten forcibly when stimulated

• Extensibility: Ability to be stretched

• Elasticity: Ability to recoil to resting length

Functions of Muscle Tissue

• Produce Movement: Responsible for locomotion and manipulation (walking, digestion, pumping blood)

• Maintain Posture and Body Position

• Stabilize Joints

• Generate Heat: As muscles contract

Skeletal Muscle Anatomy

Structure of Skeletal Muscle

Skeletal muscle is an organ composed of multiple tissues and has three main features: nerve and blood supply, connective tissue sheaths, and attachments.

• Nerve and Blood Supply: Each muscle receives a nerve, artery, and veins; nerves supply every fiber for conscious control; muscles require large amounts of oxygen and nutrients

• Connective Tissue Sheaths: Support and reinforce muscles

  • Epimysium: Dense irregular connective tissue surrounding entire muscle

  • Perimysium: Fibrous connective tissue surrounding fascicles (groups of muscle fibers)

  • Endomysium: Fine areolar connective tissue surrounding each muscle fiber

• Attachments: Muscles span joints and attach to bones at two points

  • Insertion: Attachment to movable bone

  • Origin: Attachment to immovable or less movable bone

  • Direct Attachment: Epimysium fused to periosteum or perichondrium

  • Indirect Attachment: Connective tissue extends as tendon or aponeurosis

Muscle Fiber Microanatomy and Sliding Filament Model

Muscle Fiber Structure

• Sarcolemma: Muscle fiber plasma membrane

• Sarcoplasm: Muscle fiber cytoplasm, contains glycosomes (glycogen storage) and myoglobin (O2 storage)

• Modified Organelles: Myofibrils, Sarcoplasmic Reticulum (SR), T Tubules

Myofibrils

• Densely packed rodlike elements, account for ~80% of muscle cell volume

• Features: Striations, Sarcomeres, Myofilaments

Striations

• A Bands: Dark regions

• H Zone: Lighter region in middle of A band

• M Line: Protein line bisecting H zone

• I Bands: Light regions

• Z Disc: Sheet of proteins on midline of I band

Sarcomere

• Smallest contractile unit (functional unit) of muscle fiber

• Area between Z discs, contains A band and half of I band at each end

• Aligned end to end along myofibril

Myofilaments

• Actin (Thin Filaments): Extend across I band and part of A band, anchored to Z discs

• Myosin (Thick Filaments): Extend length of A band, connected at M line

• Hexagonal arrangement: One thick filament surrounded by six thin filaments

Molecular Composition

• Thick Filaments: Composed of myosin (2 heavy, 4 light polypeptide chains); heads form cross bridges during contraction

• Thin Filaments: Composed of actin (G actin subunits with myosin binding sites); F actin strands twist to form filament

• Regulatory Proteins: Tropomyosin and troponin bound to actin

• Elastic Filament: Titin protein holds thick filaments in place

• Dystrophin: Links thin filaments to sarcolemma proteins

• Nebulin, Myomesin, C Proteins: Maintain sarcomere alignment

Clinical Homeostatic Imbalance: Duchenne Muscular Dystrophy (DMD)

• Sex-linked recessive disease, almost exclusively in males

• Caused by defective gene for dystrophin

• Leads to muscle degeneration, weakness, and loss of muscle mass

• Progresses from extremities to head, chest, and cardiac muscles

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic Reticulum (SR): Smooth ER network surrounding myofibrils, regulates intracellular Ca2+ levels, stores and releases Ca2+

• T Tubules: Protrusions of sarcolemma into cell interior, increase surface area, allow electrical signals to reach deep muscle fibers

• Triad: Area formed by terminal cisterns of SR and T tubule

• Voltage sensors in T tubules trigger Ca2+ release from SR

Sliding Filament Model of Contraction

• Contraction: Activation of cross bridges to generate force

• Shortening occurs when tension exceeds resistance

• Sliding filament model: Thin filaments slide past thick filaments, increasing overlap

• Neither filament changes length; overlap increases

• Cross bridge attachments form and break, pulling thin filaments toward center

• Results: Z discs pulled toward M line, I bands shorten, H zones disappear, A bands move closer

Muscle Fiber Contraction

Background and Overview

• Movement initiated by brain, transmitted via motor neurons

• Neurons and muscle cells are excitable, capable of action potentials

• Action potential crosses from neuron to muscle via acetylcholine (ACh)

Ion Channels

• Chemically Gated: Opened by neurotransmitters (e.g., ACh receptors)

• Voltage-Gated: Open/close in response to voltage changes

Neuromuscular Junction (NMJ)

• Skeletal muscles stimulated by somatic motor neurons

• Axons branch and form NMJ with muscle fibers

• NMJ consists of axon terminals, synaptic cleft, and junctional folds

• Synaptic vesicles contain ACh

• Junctional folds have ACh receptors

Events at the Neuromuscular Junction

1. AP arrives at axon terminal

2. Voltage-gated Ca2+ channels open, Ca2+ enters neuron

3. ACh released into synaptic cleft

4. ACh binds to receptors, opens Na+ gates, Na+ enters, end plate potential generated

5. ACh degraded by acetylcholinesterase

Clinical Homeostatic Imbalance: Myasthenia Gravis

• Autoimmune disease; ACh receptors attacked by antibodies

• Symptoms: Drooping eyelids, difficulty swallowing/talking, muscle weakness

Generation of Action Potential Across Sarcolemma

• Resting sarcolemma is polarized (inside negative)

• AP caused by changes in electrical charges

• Three steps: End plate potential, depolarization, repolarization

End Plate Potential

• ACh binds to receptors, opens chemically gated channels

• Na+ enters, K+ leaves (less than Na+ enters)

• Interior becomes less negative (depolarization)

Depolarization

• If threshold reached, voltage-gated Na+ channels open

• Large influx of Na+ triggers AP, leading to contraction

Repolarization

• Na+ channels close, K+ channels open

• K+ efflux restores resting voltage

• Refractory period: Fiber cannot be stimulated until repolarization complete

• Na+-K+ pump restores ionic conditions

Excitation-Contraction (E-C) Coupling

• AP propagated along sarcolemma and T tubules

• Voltage-sensitive proteins stimulate Ca2+ release from SR

• Ca2+ triggers contraction

Cross Bridge Cycling

• At low Ca2+: Tropomyosin blocks actin sites, muscle relaxed

• At high Ca2+: Ca2+ binds to troponin, moves tropomyosin, myosin binds actin

• Cycle: Formation, power stroke, detachment, cocking of myosin head

Cross Bridge Cycle Steps

1. Cross bridge formation: Myosin head attaches to actin

2. Power stroke: Myosin head pivots, pulls actin toward M line

3. Detachment: ATP attaches to myosin, cross bridge detaches

4. Cocking: ATP hydrolysis cocks myosin head for next cycle

Clinical Homeostatic Imbalance: Rigor Mortis

• After death, ATP synthesis stops, Ca2+ cannot be pumped into SR

• Cross bridges form and cannot detach, causing stiffness

• Muscles remain contracted until proteins break down

Whole Muscle Contraction

Principles of Muscle Contraction

• Contraction produces muscle tension

• Isometric contraction: Tension increases, no shortening

• Isotonic contraction: Muscle shortens, tension exceeds load

Motor Unit

• Motor unit: Motor neuron and all muscle fibers it supplies

• Smaller units allow greater fine control

• Fibers spread throughout muscle; stimulation causes weak contraction

Muscle Twitch

• Simplest contraction from single AP

• Phases: Latent (E-C coupling), contraction (cross bridge formation), relaxation (Ca2+ reentry)

• Muscle contracts faster than it relaxes

Graded Muscle Responses

• Normal contraction is smooth, strength varies with needs

• Graded by frequency and strength of stimulation

Response to Frequency

• Single stimulus: Single twitch

• Wave summation: Rapid stimuli, twitches increase in force

• Unfused tetanus: Sustained, quivering contraction

• Fused tetanus: Maximum tension, smooth sustained contraction

• Prolonged contraction leads to fatigue

Response to Strength

• Recruitment: More fibers activated, more precise control

• Subthreshold: No contraction

• Threshold: First observable contraction

• Maximal: All motor units recruited

• Size principle: Small fibers recruited first, then larger

• Asynchronous contraction prevents fatigue

Muscle Tone

• Constant, slightly contracted state

• Due to spinal reflexes

• Keeps muscles firm, healthy, ready to respond

Isotonic and Isometric Contractions

• Isotonic: Muscle changes length, moves load

  • Concentric: Muscle shortens (e.g., picking up a book)

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

• Isometric: Tension increases, muscle does not change length

Energy for Contraction and ATP

ATP and Muscle Contraction

• ATP needed for cross bridge movement/detachment, Ca2+ pumping, Na+/K+ pump

• ATP stores depleted in 4-6 seconds

• ATP regenerated by:

  • Direct phosphorylation (creatine phosphate)

  • Anaerobic pathway (glycolysis, lactic acid formation)

  • Aerobic pathway

Direct Phosphorylation

• Creatine phosphate donates phosphate to ADP to form ATP

• Enzyme: Creatine kinase

• Equation:

Anaerobic Pathway

• Glycolysis: Glucose broken into 2 pyruvic acid, yields 2 ATP

• Low O2: Pyruvic acid converted to lactic acid

• Lactic acid diffuses into bloodstream, used as fuel or converted back to glucose

• Anaerobic yields 5% as much ATP as aerobic, but is faster

Aerobic Respiration

• Produces 95% of ATP during rest/light exercise

• Occurs in mitochondria, requires O2

• Breaks down glucose into CO2, H2O, and ATP (up to 32 ATP per glucose)

• Fuel: Glycogen, blood glucose, fatty acids (main fuel after 30 min exercise)

Energy Systems During Sports

• Aerobic Endurance: Duration muscle contracts using aerobic pathways

• Anaerobic Threshold: Point where metabolism shifts to anaerobic

Muscle Fatigue

• Inability to contract despite stimulation

• Causes: Ionic imbalances, increased inorganic phosphate, decreased ATP/glycogen, increased magnesium

• Lack of ATP rarely causes fatigue except in extreme cases

Excess Postexercise O2 Consumption (EPOC)

• O2 reserves replenished

• Lactic acid reconverted to pyruvic acid

• Glycogen stores replaced

• ATP and creatine phosphate resynthesized

• All require extra O2 (oxygen debt)

Factors Affecting Muscle Contraction

Force of Contraction

• Depends on number of cross bridges attached

• Four factors:

  • Number of fibers stimulated (recruitment)

  • Relative size of fibers (hypertrophy increases force)

  • Frequency of stimulation (higher frequency, greater force)

  • Degree of muscle stretch (optimal sarcomere length: 80-120% resting length)

Velocity and Duration of Contraction

• Influenced by muscle fiber type, load, and recruitment

• Muscle fiber types classified by:

  • Speed of contraction (slow/fast)

  • Metabolic pathway (oxidative/glycolytic)

• Three types: Slow oxidative, fast oxidative, fast glycolytic

• Different types suited for different activities

  • Slow oxidative: Endurance, posture

  • Fast oxidative: Sprinting, walking

  • Fast glycolytic: Short-term intense movements

• Load: Greater load, shorter duration, slower contraction

• Recruitment: More units, faster and more prolonged contraction

Adaptation to Exercise

• Aerobic (Endurance): Increases capillaries, mitochondria, myoglobin; greater endurance, strength, fatigue resistance

• Resistance Exercise: Muscle hypertrophy (fiber size), increased mitochondria, myofilaments, glycogen, connective tissue; increased strength and size

Homeostatic Imbalance: Atrophy

• Muscle must remain active to stay healthy

• Disuse atrophy: Degeneration and loss of mass due to immobilization or loss of neural stimulation

• Muscle strength declines rapidly; fibrous tissue replaces lost muscle

• Rehabilitation impossible if atrophy is severe

Smooth Muscle

Structure and Organization

• Found in walls of hollow organs (except heart and smallest blood vessels)

• Organized into sheets: Longitudinal and circular layers

• Alternating contractions mix and propel substances

Smooth vs Skeletal Muscle Fibers

• Smooth: Spindle-shaped, thin, short, one nucleus, no striations, only endomysium

• Varicosities (bulbous nerve endings) release neurotransmitters into diffuse junctions

• Innervated by autonomic nervous system

• SR less developed, no T tubules; Ca2+ mainly from extracellular space

• Sarcolemma has caveolae (pouchlike infoldings with Ca2+ channels)

• Gap junctions allow electrical coupling

• No sarcomeres, but overlapping thick/thin filaments

• Thick filaments have myosin heads along entire length

• Ratio of thick to thin filaments lower (1:13 vs 1:2 in skeletal)

• No troponin; calmodulin binds Ca2+

• Filaments arranged diagonally; contraction is corkscrew-shaped

• Intermediate filaments and dense bodies anchor filaments

Contraction of Smooth Muscle

• Slow, synchronized contractions; cells coupled by gap junctions

• Some cells self-excitatory (pacemakers)

• Actin and myosin interact via sliding filament mechanism

• Ca2+ triggers contraction; ATP required

• Ca2+ binds to calmodulin, activates myosin kinase, phosphorylates myosin head

• Relaxation requires Ca2+ detachment, active transport, dephosphorylation of myosin

• Energy efficient: Maintains contraction with little energy, slow ATPases, latch mechanism

• Regulation: Controlled by nerves, hormones, local chemicals

• Special response: Stress-relaxation (adapts to stretch), can contract over wide length range

Types of Smooth Muscle

Summary Table: Muscle Tissue Types

Example:

• Skeletal Muscle: Biceps brachii contracts to lift a book (concentric contraction)

• Cardiac Muscle: Heart muscle contracts to pump blood

• Smooth Muscle: Intestinal smooth muscle contracts to propel food

Additional info: Academic context was added to clarify mechanisms, terminology, and clinical relevance. Tables were inferred and expanded for clarity.