Muscle Tissue Quiz

Muscle Physiology

• Muscle tissue constitutes 50% of the body weight.
• Muscle cells are the primary cells in muscle tissue.
• Specialized muscle cells convert chemical energy into mechanical energy using ATP.
• Diverse muscle types exist, specialized for functions like locomotion, blood pumping, and food movement.

Muscular System Functions

• Muscle movement enables body motion.
• Muscles maintain posture.
• Breathing relies on muscle action.
• Muscles produce body heat.
• Muscle contractions are crucial for communication and transmitting signals.
• Control over internal organs and blood vessel constriction involves muscle action.
• Muscle action is vital for heart function.

Properties of Muscle

• Contractility: Ability of a muscle to shorten with force.
• Excitability: Capacity of a muscle to respond to a stimulus.
• Extensibility: Ability of muscle to stretch.
• Elasticity: Ability of a muscle to recoil to its original resting length after stretching.

Muscle Tissue Types

• Skeletal Muscle: Attached to bones, striated, voluntary (conscious control), multinucleated.

  • Attached to bones.
  • Nuclei are multiple and peripherally located.
  • Striated.
  • Voluntary and involuntary (reflexes).

• Cardiac Muscle: Found in the heart; striated, involuntary, single nucleus, intercalated discs.

  • Found in the heart.
  • Single nucleus centrally located.
  • Striations.
  • Involuntary, intercalated disks.

• Smooth Muscle: Found in walls of hollow organs, blood vessels, eye, and glands. Not striated, involuntary, single nucleus, gap junctions.

  • Walls of hollow organs, blood vessels, eye, glands, and skin
  • Single nucleus centrally located.
  • Not striated.
  • Involuntary; gap junctions in visceral smooth muscle.

Skeletal Muscle (Striated)

• Attaches to the skeleton.
• Microscopically, has stripes called striations.
• Voluntary muscle, controlled consciously.
• Multinucleated.

Cardiac Muscle

• Occurs only in the heart.
• Striated, branching pattern; intercalated disks.
• Involuntary & automatic.
• Usually one nucleus but may have more.
• Neural controls; adjusts heart rate.

Smooth Muscle

• Found within most organs, arteries, and veins.
• Helps move substances through internal body channels via peristalsis.
• Not striated; involuntary.
• Single nucleus.

Structure of the Muscle Cell

• A single skeletal muscle cell is called a muscle fiber.
• Adult skeletal muscle fibers have diameters between 10 and 100 µm and lengths that may extend up to 25 cm.

Embryologic Origin

• Muscle fibers form during development via the fusion of undifferentiated, mononucleated cells called myoblasts.
• This fusion creates a multinucleated cell.
• Skeletal muscle differentiation is complete around birth.

Skeletal Muscle Fiber Repair

• Skeletal muscle fibers cannot be replaced by division of existing muscle fibers after birth.
• New fibers can form from undifferentiated cells, called satellite cells, located adjacent to the muscle fibers.
• Satellite cells differentiate similarly to embryonic myoblasts.

Skeletal Muscle Compensation for Loss

• The capacity for forming skeletal muscle fibers is considerable, but doesn't fully repair severe damage.
• Muscle tissue loss compensation often occurs through increase in the size of existing muscle fiber (hypertrophy).

Skeletal Muscle: Nerve and Blood Supply

• Each muscle receives a nerve, artery, and one or more veins.
• These enter near the muscle's center and branch throughout it.
• Skeletal muscle fibers (cells) receive nerve endings to control contraction.
• Continuous delivery of oxygen and nutrients through arteries is crucial for muscle contraction.
• Muscle cell waste removal occurs via veins.

Skeletal Muscle- CT Sheaths

• The muscle is surrounded by connective tissue sheaths.
• Endomysium: Delicate CT sheath surrounding each muscle fiber (cell).
• Perimysium: CT surrounding groups of muscle fibers (fascicles.)
• Epimysium: Dense regular CT surrounding the entire muscle.

Skeletal Muscle- CT Sheaths (at each end)

• Collagen fibers of the endomysium, perimysium, and epimysium merge to form tendons or aponeuroses.
• This provides support, strength, flexibility, and electrical isolation for muscles.

Microscopic Anatomy- Skeletal Muscle Fiber/Cell

• Sarcoplasm: Cytoplasm of the muscle fiber.
• Sarcolemma: Cell membrane of the muscle fiber.
• Myofibrils, sarcoplasmic reticulum, and T tubules are unique to muscle fibers.
• Sarcoplasm contains glycosomes (glycogen granules) and myoglobin (oxygen-binding protein).

Myofibrils - Striations

• Myofibrils (contractile elements) comprise ~80% of muscle volume.
• Made of myofilaments: thick (myosin) and thin (actin).

Ultrastructure of Myofilaments: Thick Filaments

• Composed of Myosin protein.
• Myosin found in striated muscle is class II myosin.
• Myosin has a head and tail.

Ultrastructure of Myofilaments: Thick Filaments (Further Detail)

• Each globular head has actin- and ATP-binding sites.
• The actin-binding site forms acto-myosin bridges.
• The ATP-binding site is an ATPase, hydrolyzing ATP.
• Myosin tail regions form the thick filament.

Thin Filaments (with regulatory proteins)

• Composed of actin, tropomyosin, and troponin proteins.
• Actin: Main component, with myosin-binding sites.
• Tropomyosin: Long filaments; blocks active sites in relaxed muscle.
• Troponin: Regulatory protein complex.

Troponin

• Attaches to thin filaments, tropomyosin, and actin.
• Binds calcium ions to trigger muscle contraction.
• Consist of three subunits:
  • TnT (attaches to tropomyosin)
  • TnI (attaches to actin)
  • TnC (binds calcium)

Structure of Actin and Myosin

• Myofilaments are structured into sarcomeres.
• Sarcomeres comprise the contractile units of a myofibril.
• Actin and myosin myofilaments are arranged in repeating patterns within the sarcomeres.
• Important proteins like titin, nebulin, and dystrophin are also present.

Myofibrils - Striations- Banding

• Dark bands (A bands) contain (primarily) myosin.
• Lighter bands (I bands) contain primarily actin.
• The Z disc is the midline of an I band.
• The H zone is a region in the center of the A band where actin and myosin filaments do not overlap.
• The M line is the middle of the sarcomere and the attachment point for myosin.

Sarcomere

• The smallest functional unit of contraction within a myofibril.
• The distance between neighboring Z discs.
• Z discs connect the thin filaments.
• Shortening of sarcomeres results in muscle contraction.

Myofilaments: Banding Pattern

• A transverse dark line (M line) appears in the center of the H zone.
• M line represents the protein connections for myosin tails.

Other Proteins

• Titin, Nebulin, Actinin, and Dystrophin are proteins involved in muscle structure and function.

Dystrophin

• Dystrophin is a cytoplasmic protein.
• Integral component of a protein complex associating muscle fibers' cytoskeleton with the extracellular matrix.
• Located between the sarcolemma and outermost myofilaments in muscle fibers.

Various Myopathies

• Myopathies occur as a result of defects in myofilament proteins.

Transverse Tubules (T-tubules) and Sarcoplasmic Reticulum (SR)

• T tubules are invaginations of the muscle cell membrane extending into the sarcoplasm.
• Important for rapid action potential transmission.
• The SR is specialized endoplasmic reticulum.
• Stores, releases calcium ions crucial for controlling contraction and relaxation.
• Two structures within each sarcomere: Longitudinal elements, and Terminal cisternae.

The SR (further detail)

• Longitudinal elements form a system of hollow sheets and tubes around myofibrils.
• Terminal cisternae (lateral sacs) contain calsequestrin, a calcium-binding protein.

Skeletal Muscle Contraction

• For contraction, skeletal muscle: gets stimulated by nerve endings, propagates electrical current along its sarcolemma, and has a rise in intracellular Ca2+ levels.

Electrical Characteristics (Skeletal Muscle)

• Electrical events in skeletal muscle resemble nerve cells, but there are timing and magnitude differences.
• RMP is about -90 mV, compared to -70 mV of nerve cells.
• Action potential lasts 2-4 ms vs 0.5-1 ms in nerve cells.
• Conduction speed in skeletal muscles is slower (≈5 m/s) than axons (≈100 m/s).

Neuromuscular Junction (NMJ)

• Nerve-muscle signal transfer occurs at the NMJ (myoneural junction).
• Presynaptic portion: Axon axon terminals.
• Postsynaptic portion: Muscle cell membrane.

Neuromuscular Junction (NMJ, Further Detail)

• The axon, at the NMJ, branches numerous times.
• These numerous branches form the presynaptic terminal portion of the NMJ.
• The postsynaptic portion of the NMJ is the muscle cell membrane.
• Nicotinic acetylcholine receptors are specifically clustered at postjunctional folds.

Neuromuscular Junction (NMJ, Still More Detail)

• Within nerve axon terminals lie membrane enclosed vesicles.
• Vesicles contain acetylcholine (ACh).
• Mitochondria are plentiful, associated with the terminal metabolic needs.

Neuromuscular Junction (NMJ, Final Detail)

• The postsynaptic membrane area presents folds.
• These folds locate many nicotinic acetylcholine receptors.

Neuromuscular Junction (NMJ, Ion Considerations).

• Chemically gated ion channels increase the permeability of the postsynaptic membrane to ACh.
• The synaptic cleft is the narrow space between the nerve and muscle.
• ACh must diffuse across the cleft to reach receptors.

Neuromuscular junction

• Acetylcholinesterase (AChE), an enzyme, is present in the synaptic cleft.
• AChE breaks down ACh, stopping the signal.

Electrical Events at the Neuromuscular Junction

• Action potential reaches axon terminal, depolarizing.
• Depolarization causes calcium channels to open, calcium enters.
• Calcium triggers vesicle fusion, releasing ACh.
• ACh diffuses across the cleft, binds to ACh receptors.
• Receptor binding triggers channel opening (sodium and potassium ions through).
• Voltage change (endplate potential) occurs.

Electrical Events at the NMJ (Continued)

• Depolarization spreads to adjacent membrane, activating sodium channels.
• An action potential subsequently occurs in the muscle membrane.
• Propagated along all muscle cell membrane.

Propagation of Action Potential at Muscle Membrane

• When an action potential encounters T-tubules, it's propagated down the T-tubule membrane.
• This propagation also results in numerous action potentials going towards the central part of the fiber.

T tubules

• T-tubules communicate with the outside of the muscle fiber cell membrane.
• Run deep into the muscle fiber.

DHP and Ryanodine Receptors

• Action potential in T tubules initiates DHP receptors.
• These receptors open ryanodine receptors, calcium channels.
• The opening of the calcium channels releases calcium from the terminal cisternae of the sarcoplasmic reticulum.
• Calcium increase within the cell triggers muscle contraction.
• Excitation-contraction coupling refers to these events.

Neuromuscular Transmission (with Toxins)

• Presynaptic blockade: Botulinum toxin blocks the release of ACh.
• This toxin is from Clostridium botulinum bacteria.
• Low doses of botulinum toxin (Botox) are therapeutically used for conditions like facial wrinkles.
• Curare-tubocurarine blocks postsynaptic ACh receptors.
• It's a South American arrow poison.

Sliding Filament Model of Muscle Contraction

• The process involving sequential interactions between myosin and actin.
• Results in the shortening of sarcomeres.
• Four Steps of a Cross Bridge cycle:
  • Attachment
  • Movement
  • Detachment
  • Energizing.

Sliding Filament Model in detail

• Contraction in muscles involves the activation of myosin crossbridges, structures generating the force.
• In a resting state, actin and myosin filaments do not fully overlap.
• Muscle stimulation causes myosin heads to bind to actin.
• Thin filaments are pulled past thick filaments, increasing their overlap.
• This is the basic mechanism underlying muscular contraction.

Sarcomere Shortening

• When muscles contract, sarcomeres shorten, and the H zones and I bands become narrower.
• The Z disks move closer together.

Calcium and the Contraction Mechanism

• Low intracellular calcium concentrations: tropomyosin blocks myosin binding sites on actin.
• High intracellular calcium concentrations:
  • Calcium binds to TnC (troponin C).
  • Troponin changes shape, shifting tropomyosin.
  • Myosin binding sites on actin are exposed.
  • Muscle contraction begins as myosin crossbridges cycle.