BIO 202 ch 11

Three types of muscle tissue

Skeletal muscle (striated, voluntary, attached to bone); Cardiac muscle (striated, involuntary, heart); Smooth muscle (nonstriated, involuntary, hollow organs).

Functions of muscle tissue

Produce body movement, stabilize posture, move and store substances, pump blood, and generate heat (thermogenesis).

Electrical excitability

Ability of muscle cells to respond to stimuli by generating action potentials.

Contractility

Ability of muscle fibers to shorten forcefully and generate tension.

Extensibility

Ability of muscle tissue to stretch without damage.

Elasticity

Ability of muscle tissue to return to its original length after stretching.

Organization of skeletal muscle

Muscle → Fascicle → Muscle Fiber → Myofibril → Sarcomere → Myofilaments.

Muscle fiber

Long cylindrical multinucleated muscle cell containing myofibrils.

Sarcolemma

Plasma membrane of a muscle fiber.

Sarcoplasm

Cytoplasm of a muscle fiber.

Myofibril

Long contractile organelle composed of repeating sarcomeres.

Sarcomere

Functional contractile unit of skeletal muscle located between two Z discs.

Z disc

Boundary of each sarcomere where thin filaments attach.

A band

Region containing thick filaments; remains the same length during contraction.

I band

Region containing only thin filaments; shortens during contraction.

H zone

Center of the sarcomere containing only thick filaments; shortens during contraction.

M line

Center of the sarcomere where thick filaments are anchored.

Thin filament

Mainly composed of actin.

Thick filament

Mainly composed of myosin.

Actin

Contractile protein that contains binding sites for myosin heads.

Myosin

Contractile protein whose heads bind actin and use ATP to generate force.

Tropomyosin

Regulatory protein that blocks myosin-binding sites on actin when the muscle is relaxed.

Troponin

Regulatory protein that binds calcium and moves tropomyosin away from actin binding sites.

Titin

Structural protein that stabilizes thick filaments and provides elasticity.

Nebulin

Structural protein that stabilizes and aligns thin filaments.

Dystrophin

Structural protein linking the cytoskeleton to the sarcolemma; defective in muscular dystrophy.

Sliding filament theory

Muscles shorten because thin filaments slide over thick filaments without either filament shortening.

Step 1 of cross-bridge cycle

ATP hydrolysis cocks the myosin head into a high-energy position.

Step 2 of cross-bridge cycle

Myosin head binds to exposed actin binding sites to form a cross bridge.

Step 3 of cross-bridge cycle

Power stroke pulls the thin filament toward the center of the sarcomere.

Step 4 of cross-bridge cycle

New ATP binds myosin, causing detachment from actin.

Role of ATP in contraction

Energizes myosin heads, detaches myosin from actin, and powers calcium pumps during relaxation.

Rigor mortis

Occurs when ATP is unavailable and myosin cannot detach from actin.

Neuromuscular junction (NMJ)

Synapse between a somatic motor neuron and a skeletal muscle fiber.

Motor end plate

Specialized region of the sarcolemma containing acetylcholine receptors.

End plate potential (EPP)

Local depolarization produced when acetylcholine binds receptors on the motor end plate.

Steps at the NMJ

Action potential reaches axon terminal → Ca²⁺ enters neuron → ACh released → ACh binds receptors → Na⁺ enters muscle → Muscle action potential generated → ACh broken down by acetylcholinesterase.

Excitation-contraction coupling

Process linking a muscle action potential to calcium release and muscle contraction.

Steps of excitation-contraction coupling

Action potential travels along sarcolemma and T tubules → Sarcoplasmic reticulum releases Ca²⁺ → Ca²⁺ binds troponin → Tropomyosin moves → Cross-bridge cycling begins.

Role of calcium in contraction

Calcium binds troponin, exposing myosin-binding sites on actin.

Relaxation of skeletal muscle

Calcium is actively pumped back into the sarcoplasmic reticulum, troponin releases calcium, tropomyosin blocks binding sites, and contraction stops.

Sarcoplasmic reticulum (SR)

Stores and releases calcium for muscle contraction.

Transverse (T) tubules

Carry muscle action potentials deep into the muscle fiber.

First source of ATP during contraction

Creatine phosphate system.

Creatine phosphate system

Fastest ATP source; provides energy for about the first 15 seconds of intense activity.

Anaerobic glycolysis

Produces ATP without oxygen but generates lactic acid.

Aerobic respiration

Produces the largest amount of ATP using oxygen in mitochondria.

Muscle fatigue

Inability of a muscle to maintain force after prolonged activity.

Central fatigue

Fatigue caused by reduced stimulation from the central nervous system.

Peripheral fatigue

Fatigue caused by changes within the muscle, including ATP depletion, calcium decline, glycogen depletion, oxygen depletion, ACh depletion, and lactic acid buildup.

Oxygen debt

Increased oxygen consumption after exercise to restore ATP, creatine phosphate, glycogen, and repair tissue.

Motor unit

A somatic motor neuron and all the muscle fibers it innervates.

Muscle twitch

A single brief contraction consisting of latent, contraction, and relaxation phases.

Latent period

Time between stimulation and the start of contraction.

Contraction period

Time during which tension increases.

Relaxation period

Time during which tension decreases as calcium is returned to the SR.

Graded muscle contractions

Increased force produced by increasing action potential frequency and recruiting additional motor units.

Motor unit recruitment

Activation of additional motor units to increase muscle force.

Length-tension relationship

Maximum tension is produced when muscle fibers are near resting length.

Muscle tone

Continuous low-level contraction that maintains posture and stabilizes joints.

Origin

Stable attachment point of a muscle.

Insertion

Movable attachment point of a muscle.

Agonist

Primary muscle responsible for producing a movement.

Antagonist

Muscle that opposes the action of the agonist.

Lever

Rigid structure (bone) moved by muscles.

Fulcrum

Pivot point of a lever (joint).

Mechanical advantage

Lever arrangement that increases force production.

Isotonic contraction

Contraction in which the muscle changes length and movement occurs.

Concentric contraction

Type of isotonic contraction in which the muscle shortens.

Eccentric contraction

Type of isotonic contraction in which the muscle lengthens while generating force.

Isometric contraction

Contraction in which tension develops but muscle length does not change.

Slow oxidative (SO) fibers

Small red fibers with many mitochondria, high myoglobin, aerobic metabolism, slow contraction speed, and high fatigue resistance; important for posture and endurance.

Fast oxidative-glycolytic (FOG) fibers

Intermediate fibers that use both aerobic respiration and anaerobic glycolysis; moderate fatigue resistance; used for walking and sprinting.

Fast glycolytic (FG) fibers

Large pale fibers with few mitochondria, low myoglobin, anaerobic metabolism, rapid powerful contractions, and low fatigue resistance.

Order of muscle fiber recruitment

Slow oxidative → Fast oxidative-glycolytic → Fast glycolytic.

Cardiac muscle characteristics

Striated, branched, involuntary, autorhythmic, connected by intercalated discs, functions as a functional syncytium.

Intercalated discs

Specialized junctions containing gap junctions and desmosomes that connect cardiac muscle cells.

Functional syncytium

Cardiac muscle cells contract together as one coordinated unit.

Autorhythmicity

Ability of cardiac and some smooth muscle cells to generate spontaneous action potentials.

Source of calcium for cardiac muscle

Both the sarcoplasmic reticulum and extracellular fluid.

Smooth muscle characteristics

Nonstriated, spindle-shaped, involuntary, found in hollow organs, contracts more slowly than skeletal muscle.

Single-unit smooth muscle

Cells connected by gap junctions that contract together; found in visceral organs.

Multi-unit smooth muscle

Cells act independently; found in the iris and ciliary muscles.

Pacemaker potentials

Spontaneous depolarizations that always reach threshold in some smooth muscle cells.

Slow-wave potentials

Rhythmic depolarizations and repolarizations that may or may not reach threshold.

Regulation of smooth muscle

Controlled by the autonomic nervous system, hormones, local chemicals, and stretch.

Regulatory proteins in smooth muscle

Calmodulin and myosin light-chain kinase (MLCK).

Hypertrophy

Increase in muscle cell size; occurs in skeletal, cardiac, and smooth muscle.

Hyperplasia

Increase in the number of muscle fibers; occurs mainly in smooth muscle.

Regenerative capacity of skeletal muscle

Limited; satellite cells assist repair.

Regenerative capacity of cardiac muscle

Very limited.

Regenerative capacity of smooth muscle

Greatest regenerative ability; uses pericytes.

Homeostasis and muscle function

Muscles maintain posture, generate heat, produce movement, and move substances through organs.

Structure-function relationship in muscle

Sarcomeres generate force, while motor units determine precision and strength of movement.

Flow down calcium gradients

Action potentials trigger calcium release from the SR, allowing contraction.

Neural integration of muscle

Skeletal muscle is controlled by the somatic nervous system; cardiac and smooth muscle are regulated by the autonomic nervous system.

Clinical relevance of dystrophin

Loss of dystrophin causes muscular dystrophy and progressive muscle weakness.

Tetany

Sustained muscle contraction caused by continuous stimulation and elevated intracellular calcium.