Muscle Tissue

Overview of Muscle Tissue

Muscle tissue comprises nearly half of the body's mass and is essential for movement, posture, and heat generation. It transforms chemical energy (ATP) into directed mechanical energy, exerting force to produce movement.

• Three types of muscle tissue:

  • Skeletal

  • Cardiac

  • Smooth

• Myo-, mys-, and sarco- are prefixes commonly used for muscle-related terms.

Types of Muscle Tissue

Skeletal Muscle

Skeletal muscle is attached to bones and skin, enabling voluntary movements. Its elongated cells are called muscle fibers, which are striated and under conscious control.

• Striated (striped) appearance

• Voluntary control (e.g., conscious movement)

• Rapid, tire easily, and are powerful

• Require nervous system stimulation

Cardiac Muscle

Cardiac muscle is found only in the heart and forms the bulk of the heart walls. It is striated and contracts involuntarily, without nervous system stimulation.

• Striated

• Involuntary control

• More details in cardiovascular chapters

Smooth Muscle

Smooth muscle is located in the walls of hollow organs such as the stomach, urinary bladder, and airways. It is not striated and contracts involuntarily, often without nervous system stimulation.

• Non-striated

• Involuntary control

• Can contract without nervous system stimulation

Special Characteristics of Muscle Tissue

• 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

• Movement of bones or fluids

• Maintaining posture and body position

• Stabilizing joints

• Heat generation (especially skeletal muscle)

• Additional functions: Protects organs, forms valves, controls pupil size, causes "goosebumps"

Skeletal Muscle Structure

General Organization

• Each muscle served by one artery, one nerve, and one or more veins

• Entrances via central part and branch through connective tissue sheaths

• Every skeletal muscle fiber supplied by nerve ending that controls its activity

• High nutrient and oxygen need; generates large amounts of waste

Connective Tissue Sheaths

Connective tissue sheaths support and reinforce muscle cells, and are organized in layers:

• Epimysium: Dense irregular connective tissue surrounding entire muscle

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

• Endomysium: Areolar connective tissue surrounding each muscle fiber

Muscle Attachments

• Muscles attach in at least two places:

  • Insertion: Movable bone

  • Origin: Immovable (less movable) bone

• Attachments can be:

  • Direct: Epimysium fused to periosteum of bone or perichondrium of cartilage

  • Indirect: Connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis

Microscopic Anatomy of Skeletal Muscle Fiber

Muscle Fiber Structure

• Long, cylindrical cell

• 10 to 100 μm in diameter; up to 30 cm long

• Multiple peripheral nuclei

• Sarcolemma: Plasma membrane

• Sarcoplasm: Cytoplasm

Myofibrils

Myofibrils are densely packed, rodlike elements that account for about 80% of cell volume. They contain contractile units called sarcomeres and exhibit striations due to the arrangement of myofilaments.

• Striations: Alternating dark A bands and light I bands

• Sarcomeres: Contractile units; perfectly aligned repeating series

Striations and Sarcomere Structure

• H zone: Lighter region in midsection of dark A band where filaments do not overlap

• M line: Line of protein myomesin bisects H zone

• Z disc (line): Coin-shaped sheet of proteins on midline of I band that anchors thin filaments and connects myofibrils to one another

• Thick filaments: Run entire length of A band

• Thin filaments: Run length of I band and partway into A band

• Sarcomere: Region between two successive Z discs

Sarcomere

• Smallest contractile unit (functional unit) of muscle fiber

• Align along myofibril like boxcars of a train

• Contains A band with ½ I band at each end

• Composed of thick and thin myofilaments made of contractile proteins

Myofilament Banding Pattern

• Actin myofilaments: Thin filaments, anchored to Z discs

• Myosin myofilaments: Thick filaments, extend length of A band, connected at M line

Ultrastructure of Thick Filament

• Composed of protein myosin

• Each myosin molecule has two heavy and four light polypeptide chains

• Myosin heads contain binding sites for actin, ATP, and ATPase enzymes

• Myosin heads act as cross bridges during contraction

Ultrastructure of Thin Filament

• Composed of protein actin

• Actin subunits bear active sites for myosin head attachment during contraction

• Tropomyosin and troponin are regulatory proteins bound to actin

Structure of Myofibril

• Titin: Elastic filament, holds thick filaments in place, helps muscle recoil after stretch

• Dystrophin: Links thin filaments to proteins of sarcolemma

• Nebulin, myomesin, C proteins bind filaments or sarcomeres together

Sarcoplasmic Reticulum (SR)

The SR is a network of smooth endoplasmic reticulum surrounding each myofibril, functioning in regulation of intracellular Ca2+ levels.

• Stores and releases Ca2+

• Pairs of terminal cisterns form perpendicular cross channels

T Tubules

• Continuations of sarcolemma

• Lumen continuous with extracellular space

• Increase muscle fiber's surface area

• Penetrate cell's interior at each A band–I band junction

• Associate with paired terminal cisterns to form triads that encircle each sarcomere

Triad Relationships

• T tubules conduct impulses deep into muscle fiber; every sarcomere

• Integral proteins protrude into intermembrane space from T tubule and SR cistern membranes and connect with each other

• T tubule integral proteins act as voltage sensors and change shape in response to voltage changes

• SR integral proteins are channels that release Ca2+ from SR cisterns when voltage sensors change shape

Sliding Filament Model of Contraction

Mechanism of Muscle Contraction

During contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more. Myosin heads bind to actin, forming cross bridges and pulling thin filaments toward the center of the sarcomere, resulting in muscle shortening.

• In relaxed state, thin and thick filaments overlap only at ends of A band

• During contraction, thin filaments slide past thick filaments → actin and myosin overlap more

• Occurs when myosin heads bind to actin → cross bridges

• Pulls Z discs toward M line

• I bands shorten; Z discs closer; H zones disappear; A bands move closer (length stays same)

Physiology of Skeletal Muscle Fibers

Activation (at Neuromuscular Junction)

• Must be nervous system stimulation

• Must generate action potential in sarcolemma

Excitation-Contraction Coupling

• Action potential propagated along sarcolemma

• Intracellular Ca2+ levels must rise briefly

The Nerve Stimulus and Events at the Neuromuscular Junction

• Skeletal muscles stimulated by somatic motor neurons

• Axons of motor neurons travel from central nervous system via nerves to skeletal muscle

• Each axon forms several branches at its entrance to muscle

• Each axon ending forms neuromuscular junction with single muscle fiber

• Usually only one per muscle fiber

Events at the Neuromuscular Junction

• Nerve impulse arrives at axon terminal → ACh released into synaptic cleft

• ACh diffuses across cleft and binds with receptors on sarcolemma

• Electrical events → generation of action potential

Destruction of Acetylcholine

• ACh effects quickly terminated by enzyme acetylcholinesterase in synaptic cleft

• Breaks down ACh to acetic acid and choline

• Prevents continued muscle fiber contraction in absence of additional stimulation

Generation of an Action Potential

• Resting sarcolemma polarized

• Action potential caused by changes in electrical charges

• Occurs in three steps:

  1. End plate potential

  2. Depolarization

  3. Repolarization

Generation of an Action Potential Across the Sarcolemma

• End plate potential (local depolarization):

  • ACh binding opens chemically (ligand) gated ion channels

  • Simultaneous diffusion of Na+ (inward) and K+ (outward)

  • More Na+ diffuses in, so interior of sarcolemma becomes less negative

  • Local depolarization = end plate potential

• Depolarization: Generation and propagation of an action potential (AP)

• Repolarization: Restoring electrical conditions of RMP

• Na+ channels close and voltage-gated K+ channels open

• K+ efflux rapidly restores resting polarity

• Fiber cannot be stimulated – in refractory period until repolarization complete

• Ionic conditions of resting state restored by Na+-K+ pump

Excitation-Contraction (E-C) Coupling

Events in E-C Coupling

• Events that transmit AP along sarcolemma lead to sliding of myofilaments

• AP brief; ends before contraction

• Causes rise in intracellular Ca2+ which leads to contraction

• Latent period: Time between E-C coupling events and contraction

Events of Excitation-Contraction (E-C) Coupling

• AP propagated along sarcolemma to T tubules

• Voltage-sensitive proteins stimulate Ca2+ release from SR

• Ca2+ necessary for contraction

Channels Involved in Initiating Muscle Contraction

• Nerve impulse reaches axon terminal → voltage-gated calcium channels open

• ACh released to synaptic cleft

• ACh binds to its receptors on sarcolemma → opens ligand-gated Na+ and K+ channels → end plate potential

• Opens voltage-gated Na+ channels → AP propagation

• Voltage-sensitive proteins in T tubules change shape → SR releases Ca2+ to cytosol

Role of Calcium (Ca2+) in Contraction

• At low intracellular Ca2+ concentration:

  • Tropomyosin blocks active sites on actin

  • Myosin heads cannot attach to actin

  • Muscle fiber relaxed

• At higher Ca2+ concentrations:

  • Ca2+ binds to troponin

  • Troponin changes shape and moves tropomyosin away from myosin-binding sites

  • Myosin heads bind to actin, causing sarcomere shortening and muscle contraction

  • When nervous stimulation ceases, Ca2+ pumped back into SR and contraction ends

Cross Bridge Cycle

• Continues as long as Ca2+ signal and adequate ATP present

• Cross bridge formation: High-energy myosin head attaches to thin filament

• Working (power) stroke: Myosin head pivots and pulls thin filament toward M line

• Cross bridge detachment: ATP attaches to myosin head and cross bridge detaches

• Cocking of myosin head: Energy from hydrolysis of ATP cocks myosin head into high-energy state

Homeostatic Imbalance

Rigor Mortis

• Cross bridge detachment requires ATP

• 3–4 hours after death muscles begin to stiffen with weak rigidity at 12 hours post mortem

• Dying cells take in calcium → cross bridge formation

• No ATP generated to break cross bridges

Summary Table: Types of Muscle Tissue

Key Equations

• ATP Hydrolysis:

• Na+-K+ Pump:

Additional info: Some explanations and terminology have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.