Muscle Tissue

Three types: skeletal, smooth, and cardiac; cells are specialized for contraction

Excitability (responsiveness), contractility (ability of cells to shorten), extensibility (stretching), elasticity (recoil); only muscle tissue has all four

Movement (intentional and unintentional), stabilization, and respiration

Maintaining body temperature and storing nutrients

Contains skeletal muscle tissue (primarily), connective tissues, blood vessels, and nerves

Layer of collagen fibers that surrounds the muscle; separates muscle from surrounding tissues

Surrounds muscle fiber bundles (fasicles); innervated and has blood

Surrounds individual muscle cells (muscle fibers); contains myosatellite cells (stem cells) that repair damage

Collagen fibers of epimysium, perimysium, and endomysium come together at ends of muscles to form a tendon (bundle) or aponeurosis (sheet)

Skeletal muscles contract only when stimulated by central nervous system; often called voluntary muscles; the diaphragm usually works subconsciously; extensive vascular networks deliver oxygen, nutrients, and remove metabolic wastes

Enormous compared to other cells; contain hundreds of nuclei (multinucleate); develop by fusion of embryonic cells (myoblasts); also known as striated muscle cells due to striations

Plasma membrane of a muscle fiber; surrounds the sarcoplasm (cytoplasm of a muscle fiber); a sudden change in membrane potential initiates a contraction

Tubes that extend from surface of muscle fiber deep into sarcoplasm to transmit electrical signals

A tubular network surrounding each myofibril; similar to smooth endoplasmic reticulum; specialized for storage and release of calcium ions

Lengthwise subdivisions within a muscle fiber; responsible for muscle contraction; made of bundles of protein filaments (myofilaments)

Composed primarily of actin

Composed primarily of myosin

Smallest functional units of a muscle fiber; interactions between filaments produce contraction; arrangement of filaments accounts for striated pattern of myofibrils

Covers active sites on G-actin; prevents actin-myosin interaction (prevents muscle contraction)

A globular protein; binds tropomyosin, G-actin, and Ca2+

Found in skeletal muscle fibers and neurons; depolarization and repolarization events produce action potentials (electrical impulses); skeletal muscle fibers contract due to stimulation by motoneurons

Synapse between a neuron and a skeletal muscle fiber; axon terminal of the motor neuron releases a neurotransmitter into the synaptic cleft

The neurotransmitter released at the neuromuscular junction; binds to and opens a chemically gated Na+ channel on the muscle fiber; Na+ enters cell and depolarizes motor end plate, so an action potential is generated

Action potential travels down T tubules, Ca2+ is released from SR, Ca2+ binds to troponin and changes its shape, troponin-tropomyosin complex changes position, exposes active sites on thin filaments, contraction cycle is initiated

Duration of neural stimulus, presence of free calcium ions in cytosol, and availability of ATP

Ca2+ is pumped back into SR, Ca2+ detaches from troponin, troponin returns to original position, active sites are re-covered by tropomyosin, contraction ends; without cross-bridge formation, contraction ends; muscle relaxation occurs and the muscle returns passively to its resting length

Breaks down ACh, ending action potential generation

Fixed muscular contraction after death; results when ATP runs out and ion pumps cease to function; calcium ions build up in cytosol

A motor neuron and all of the muscle fibers it controls; may contain a few muscle fibers or thousands; all fibers in a motor unit contract at the same time

Involuntary "muscle twitch"

The normal tension and firmness of a muscle at rest; without causing movement, motor units actively stabilize positions of bones and joints and maintain balance and posture; elevated muscle tone increases resting energy consumption

Skeletal muscle changes length resulting in motion

Muscle tension > load (resistance) = muscle shortens; ex. lifting a dumbbell

Muscle tension < load = muscle elongates; ex. lowering the dumbbell

Skeletal muscle develops tension that never exceeds the load = muscle does not change length; ex. wall squat

Adenosine triphosphate; the only energy source used directly for muscle contraction; contracting muscles use a lot of ATP; muscles store enough ATP to start a contraction; more ATP must be generated to sustain a contraction

At rest, skeletal muscle fibers produce more ATP than needed; ATP transfers energy to make creatine phosphate (CP), used to store energy and convert ADP back to ATP; the enzyme creatine kinase (CK) catalyzes the conversion of ADP to ATP using the energy stored in CP; when CP is used up, other mechanisms are used to generate ATP

Direct phosphorylation of ADP by creatine phosphate (CP), anaerobic metabolism (glycolysis), aerobic metabolism (citric acid cycle and electron transport chain)

Anaerobic process; important energy source for peak muscular activity; breaks down glucose from glycogen stored in skeletal muscles; produces two ATP per molecule of glucose

Primary energy source of resting muscles; breaks down fatty acids or glucose; produces 36 ATP per molecule of glucose

Skeletal muscles at rest metabolize fatty acids and store glycogen and CP; during moderate activity, muscles generate ATP through aerobic breakdown of glucose primarily; at peak activity, pyruvate produced via glycolysis is converted to lactate

The time required after exertion for muscles to return to normal

Lactate is transferred from muscles to the liver where the liver converts lactate to pyruvate; most pyruvate molecules are converted to glucose; glucose is then used to rebuild glycogen reserves in muscle cells

Also called excess post exercise oxygen consumption; after exercise or other exertion the body needs more oxygen than usual to normalize metabolic activities; breathing rate and depth are increased

Force is the maximum amount of tension produced; endurance is the amount of time an activity can be sustained; both depend on the types of muscle fibers and physical conditioning

Majority of skeletal muscle fibers; contract very quickly; large diameter; large glycogen reserves; few mitochondria; produce strong contractions, but fatigue quickly; found in white muscles (pale, e.g., chicken breast)

Slow to contract and slow to fatigue; small diameter; numerous mitochondria; high oxygen supply from extensive capillary network; contain myoglobin (red pigment that binds oxygen); found in red muscles (dark, e.g., chicken legs)

Are mid-sized; little myoglobin; slower to fatigue than fast fibers

Contain a mixture of fiber types and are pink

Muscle growth from heavy training that causes increases in diameter of muscle fibers, number of myofibrils, number of mitochondria, glycogen reserves

Reduction of muscle size, tone, and power due to lack of activity

When muscles can no longer perform at a required level; correlated with depletion of metabolic reserves, weariness due to low blood pH and pain, and decline in pH, which affects calcium ion binding and alters enzyme activities

Uses fast fibers and stimulates hypertrophy; improved by frequent, brief, intensive workouts (e.g., 50-meter dash, weightlifting)

Supported by mitochondria; does not stimulate muscle hypertrophy; training involves sustained, low levels of activity; improvements in aerobic endurance result from alterations in the characteristics of muscle fibers and improvements in cardiovascular performance

Found only in the heart; have excitable membranes; striated like skeletal muscle cells; are small; are typically branched with a single nucleus; have short, wide T tubules; are almost totally dependent on aerobic metabolism; contain lots of myoglobin, many mitochondria; contact each other via intercalated discs

Specialized connections that join sarcolemma of adjacent cardiac muscle cells by gap junctions and desmosomes; functions include stabilizing positions of adjacent cells, maintaining three-dimensional structure of tissue, and allowing ions to move from one cell to another; so cardiac muscle cells beat in rhythm

Contraction without neural stimulation; controlled by pacemaker cells; nervous system can alter pace and tension of contractions

Integumentary system (arrector pili muscles erect hairs), cardiovascular and respiratory systems (regulates blood pressure and airflow), digestive and urinary systems (forms sphincters, moves materials along and out of the body), reproductive system (transports gametes and expels fetus)

Long, slender, spindle-shaped cells; single, central nucleus; no T tubules, myofibrils, or sarcomeres; nonstriated muscle; scattered thick filaments with many myosin heads; no tendons or aponeuroses

Normal background level of activity; can be decreased by neural, hormonal, or chemical factors