Muscle Tissue: Types and Characteristics

• Muscle tissue is a composite tissue of muscle cells and connective tissue, making up about half the body's mass.
• Voluntary muscle action is subject to conscious control.
• Involuntary muscle action is not under direct conscious control.
• Skeletal muscle is striated and found in skeletal muscles.
• Cardiac muscle is striated and found in the heart walls.
• Smooth muscle is non-striated and found in hollow organ walls.
• "Mys" and "myo" prefixes refer to muscle.
• "Sarco" refers to flesh or muscle.

Key Characteristics of Muscle Tissue

• Excitability refers to the ability of nerve signals or other stimuli to excite muscle cells, causing electrical impulses that initiate contraction.
• Contractility is the ability of muscle tissue to contract forcefully due to myofilaments containing actin and myosin proteins.
• Extensibility is the capacity of muscle tissue to be stretched, such as when one skeletal muscle contracts and stretches an opposing muscle.
• Elasticity refers to the capacity of muscle tissue to recoil passively and resume its resting length after being stretched.

Functions of Muscle Tissue

• Muscles produce movement by attaching to the skeleton and moving bones, or by squeezing in the walls of visceral organs.
• Sphincter muscles open and close passageways by encircling body openings and acting as valves.
• Muscles maintain posture and stabilize joints through continuous contraction when awake, with muscle tone aiding joint stability.
• Muscle contraction generates heat.
• Muscles attach to bones, skin, cartilage, and raphe (seam of fibrous tissue).
• A stimulus is anything that triggers a physical or behavioral change within muscle tissue.
• Striations are the dark and light bands that run transversely across muscle cells.

Muscle Fiber Structure

• Myofibrils are rod-like bundles of contractile myofilaments within skeletal muscle cells, composed of repeating sarcomeres.
• Muscle fibers are either smooth or skeletal muscle cells, capable of extending dozens of centimeters in length.
• The sarcolemma is the plasma membrane of a muscle cell.
• The epimysium is the outer layer of dense, irregular connective tissue surrounding the whole skeletal muscle.
• Perimysium is a fibrous connective tissue layer surrounding each fascicle (group of muscle fibers).
• Endomysium is a fine sheath of loose connective tissue, with reticular fibers, surrounding each muscle fiber within a fascicle.
• The epimysium, perimysium, and endomysium converge to form muscle attachments.
• A needle must pass through the epidermis, dermis, hypodermis, deep fascia, epimysium, perimysium, endomysium, and sarcolemma to reach a muscle fiber myofibril.
• The origin is the muscle attachment on the less movable bone.
• The insertion is the muscle attachment on the more movable bone.
• Direct muscle attachment is when muscle fascicles appear to attach directly to the bone due to short connective tissue strands.
• Indirect muscle attachment is when connective tissue extends beyond the muscle fibers, forming a cord-like tendon or a flat sheet.
• A tendon is a cord of dense regular connective tissue that attaches muscle to bone.
• An aponeurosis is a fibrous sheet of dense regular connective tissue connecting a muscle to the body part it moves.

Myofilaments

• Thick (myosin) filaments are in the sarcomere center, overlapping the inner ends of the thin filaments.
• Thick filaments consist primarily of myosin molecules, contain ATPase enzymes, and are studded with myosin heads.
• Thin (actin) filaments attach to each Z disc and extend towards the sarcomere's center.
• Tropomyosin forms a thin strand spiraling around the actin molecule and covers myosin head bonding sites.
• Troponin is a globular protein with three bonding sites: for actin, tropomyosin, and calcium, and attaches the tropomyosin strand to the actin molecule.
• During the sliding filament mechanism, troponin bonds with calcium to move tropomyosin from the myosin head bonding site.
• A sarcomere is the basic unit of contraction, consisting of repeating segments.

Sarcomere Bands and Zones

• A bands are dark bands created by the full length of the thick filaments and overlapping thin filaments.
• I bands are the regions on either side of the A band containing only thin filaments, creating the light portion of striations.
• The H zone is the central part of an A band where no thin filaments reach.
• The M line is in the center of the H zone, containing rods that hold thick filaments together.
• Z discs are the boundaries at the two ends of each sarcomere.
• A bands contain overlapping thick and thin filaments.
• I bands contain only thin filaments.
• The H zone contains only thick filaments.
• The M line contains thick filaments linked by accessory proteins.
• Z discs contain actin filaments from adjacent sarcomeres.
• Sarcomeres contain actin and myosin protein filaments.

Sliding Filament Mechanism

• In the sliding filament mechanism, I bands and H zones shorten, A bands and M lines do not change, and sarcomeres shorten as Z lines approach.
• The sliding filament mechanism is triggered by calcium release and the interaction of myosin heads with actin binding sites, pulling thin myofilaments towards the sarcomere's center.
• Titan molecules are springlike molecules in sarcomeres that resist overstretching, found in elastin filaments.
• Titan molecules run within thick filaments from the Z disc to the M line.
• A titan molecule holds the thick filaments in place, maintains A band organization, unfolds when stretched, refolds when released, and contributes to muscle elasticity.
• The sarcoplasmic reticulum is a smooth endoplasmic reticulum whose interconnecting tubules surround each myofibril, mostly running longitudinally, and stores calcium.
• Terminal cisterns are sarcoplasmic reticulum tubules forming perpendicular cross channels at A band and I band junctions, storing calcium.
• T tubules are deep invaginations of the sarcolemma that run between pairs of terminal cisterns.
• A triad is the complex of a T tubule flanked by two terminal cisterns at A-I junctions.
• The neuromuscular junction, or motor end plate, is the point where the nerve ending and muscle fiber meet.
• Motor neurons are nerve cells that innervate muscle fibers.
• Axon terminals are enlargements at the end of the axonal process that store neurotransmitters.
• The synaptic cleft separates axon terminals from the sarcolemma of muscle fibers.
• Synaptic vesicles are in the axon terminal and release neurotransmitters when a nerve impulse reaches the terminals.
• A neurotransmitter is a chemical released by neurons that induces impulses that contract fibers.
• The muscle fiber contraction sequence involves a nerve impulse stimulating ACh release into the synaptic cleft, ACh stimulating changes in the sarcolemma, and enzymes breaking down ACh to limit its action to a single twitch.
• A motor unit consists of a motor neuron and all the muscle fibers it innervates, contracting together when it fires.
• Recruitment is the addition of motor units to accomplish a movement.
• With single-unit innervation, units work together as one, spontaneously active, with excitement spreading from cell to cell.
• Multi-unit innervation means each cell is its own unit consisting of multiple single units, requiring excitation.
• Oxidative fibers are muscle fibers that predominantly produce ATP aerobically, using oxygen.
• Slow oxidative fibers obtain energy from aerobic reactions, have many mitochondria and capillaries, contract slowly, and resist fatigue when oxygen is present; they appear red due to abundant myoglobin.
• Slow oxidative fibers are found in the lower back
• Fast oxidative fibers are between slow oxidative and fast glycolytic; they contract quickly, are oxygen-dependent, and have high myoglobin content, many mitochondria, and capillaries.
• Fast oxidative fibers are found in the lower limbs
• Fast glycolytic fibers are pale due to little myoglobin, twice the size of slow oxidative fibers, and have many glycosomes.
• Fast glycolytic fibers contract rapidly, tire quickly, depend on anaerobic pathways for ATP, and have few mitochondria or capillaries.
• Circular fascicles are arranged in concentric rings, surrounding external body openings.
• Convergent muscles have a broad origin, with fascicles converging toward the tendon of insertion.
• Parallel muscles have fascicles running parallel to the muscle's long axis, extending from origin to insertion.
• Fusiform muscles are thick in the middle and tapered at each end.
• Unipennate muscles have all muscle fibers on the same side of the tendon.
• Bipennate muscles have muscle fibers on both sides of the tendon.
• Multipennate muscles have branches of the tendon within the muscle, resembling a feather.
• Parallel skeletal muscle fascicle arrangements have the largest range of motion.
• Multipennate skeletal muscle fascicle arrangements generate the most power.