Muscle Tissue

Overview of Muscle Tissue

Muscle tissue is a fundamental component of the human body, responsible for movement, posture, and various physiological functions. It transforms chemical energy (ATP) into directed mechanical energy, exerting force and enabling motion.

• Muscle tissue comprises nearly half of the body's mass.

• Three main types: Skeletal, Cardiac, and Smooth.

• Skeletal and smooth muscle cells are elongated and called muscle fibers.

• Common prefixes: myo, mys, and sacro (refer to muscle).

• Sarcolemma: plasma membrane of muscle fiber.

• Sarcoplasm: cytoplasm of muscle fiber.

Functions of Muscle Tissue

• Movement of bones or fluids (e.g., blood).

• Maintaining posture and body position.

• Stabilizing joints.

• Heat generation (especially skeletal muscle).

• Additional functions: protection of organs, control of pupil size, formation of valves, and causing "goosebumps".

Types of Muscle Tissue

Skeletal Muscle

Skeletal muscle is attached to bones and is responsible for voluntary movements. It is striated and requires nervous system stimulation for contraction.

• Striated (striped appearance).

• Voluntary (conscious control).

• Responsible for overall motility and rapid, forceful contractions.

• Requires nervous system stimulation.

Cardiac Muscle

Cardiac muscle is found only in the heart and is responsible for pumping blood throughout the body.

• Striated.

• Involuntary (does not require conscious control).

• Can contract without nervous system stimulation.

Smooth Muscle

Smooth muscle is found in the walls of hollow organs such as the stomach, urinary bladder, and airways.

• Not striated.

• Involuntary.

• 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.

Microscopic Anatomy of Skeletal Muscle

Connective Tissue Sheaths

Connective tissue sheaths support and protect muscle fibers, reinforce the whole muscle, and prevent bursting during contraction.

• 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.

Muscle Attachments

• Muscles attach in at least two places:

  • Origin: Immovable or less movable bone.

  • Insertion: Movable bone.

• Attachments can be direct (epimysium fused to periosteum of bone or perichondrium of cartilage) or indirect (connective tissue wrappings extend beyond muscle as tendon or aponeurosis).

Skeletal Muscle Fiber Anatomy

• Long, cylindrical cell.

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

• Multiple peripheral nuclei.

• Sarcolemma: plasma membrane.

• Sarcoplasm: cytoplasm, contains glycosomes (for glycogen storage) and myoglobin (for O2 storage).

• Modified structures: myofibrils, sarcoplasmic reticulum, T tubules.

Myofibrils and Sarcomeres

Myofibrils

Myofibrils are densely packed, rodlike elements that make up about 80% of cell volume. They contain contractile units called sarcomeres.

• Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands.

Sarcomere Structure

The sarcomere is the smallest contractile unit of muscle fiber, composed of thick and thin filaments.

• Z disc: Coin-shaped sheet of proteins that anchors thin filaments and connects myofibrils to one another.

• Thick filaments: Run entire length of A band; composed of myosin.

• Thin filaments: Run length of I band and partway into A band; composed of actin.

• Sarcomere: Region between two successive Z discs.

Modified Structures in Muscle Fibers

• Sarcoplasmic reticulum (SR): Network of smooth endoplasmic reticulum surrounding each myofibril; stores and releases Ca2+.

• T tubules: Continuations of sarcolemma; conduct impulses deep into muscle fiber; every sarcomere has two T tubules.

Sliding Filament Model of Contraction

Mechanism of Contraction

Muscle contraction occurs when tension generated by cross bridges between thick and thin filaments overcomes opposing forces.

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

• During contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more.

• Cross bridges form and break several times, ratcheting thin filaments toward center of sarcomere.

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 is relaxed.

• At higher Ca2+ concentration: Ca2+ binds to troponin, causing tropomyosin to move away from myosin-binding sites, allowing myosin heads to bind to actin and initiate contraction.

Cross Bridge Cycle

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

• 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.

Neuromuscular Junction and Muscle Activation

Neuromuscular Junction (NMJ)

The NMJ is the site where a motor neuron stimulates a muscle fiber. It consists of axon terminals, synaptic cleft, and junctional folds.

• Each axon ending forms a neuromuscular junction with a single muscle fiber.

• Axon terminal and muscle fiber separated by gel-filled space called synaptic cleft.

• Synaptic vesicles of axon terminal contain neurotransmitter acetylcholine (ACh).

• Junctional folds of sarcolemma contain ACh receptors.

Events at the Neuromuscular Junction

• Nerve impulse arrives at axon terminal.

• ACh released into synaptic cleft.

• ACh binds to receptors on sarcolemma, opening ion channels and generating an action potential.

• ACh effects quickly terminated by enzyme acetylcholinesterase.

Generation of an Action Potential

• Resting sarcolemma polarized (voltage across membrane).

• Action potential caused by changes in electrical charges:

  • Depolarization: Na+ influx decreases membrane voltage toward critical threshold.

  • Repolarization: K+ efflux restores resting polarity.

• Once initiated, action potential is unstoppable and leads to muscle fiber contraction.

Excitation-Contraction (E-C) Coupling

Events of E-C Coupling

• Action potential propagated along sarcolemma to T tubules.

• Voltage-sensitive proteins stimulate Ca2+ release from sarcoplasmic reticulum.

• Ca2+ binds to troponin, initiating muscle contraction.

Summary Table: Types of Muscle Tissue

Key Equations and Concepts

• ATP Hydrolysis:

• Resting Membrane Potential:

• Action Potential Propagation:

Additional info:

• Some details about muscle metabolism, adaptations to exercise, and muscle fatigue are referenced but not fully explained in the notes. For exam preparation, students should review the biochemical pathways of ATP production and the physiological responses to prolonged muscle activity.

• Further study of the "Sliding Filament Model of Contraction" should include the roles of actin, myosin, troponin, and tropomyosin in detail.