Chapter 9 Muscles and Muscle Tissue

Skeletal muscle

Voluntary, striated muscle attached to bones; produces body movement.

Fascicle

A bundle of skeletal muscle fibers surrounded by perimysium.

Muscle fiber

A single skeletal muscle cell; contains many myofibrils.

Sarcolemma

Cell membrane of a muscle fiber; conducts action potentials.

Dystrophin

Protein linking the cytoskeleton to the sarcolemma; stabilizes muscle fibers.

T tubule

Sarcolemma extension that carries action potentials deep into the muscle fiber.

Sarcoplasm

Cytoplasm of a muscle fiber; contains myofibrils, glycogen, and myoglobin.

Sarcoplasmic reticulum

Specialized smooth ER that stores and releases calcium for contraction.

Terminal cisterns

Enlarged SR regions that store calcium beside T-tubules.

Glycosomes

Organelles containing enzymes for glycogen breakdown and energy production.

Elasticity

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

Muscle functions

Produces movement
Maintains posture and body position
Stabilizes joints
Generates heat

Myoglobin

Oxygen-binding protein in muscle cells that stores and delivers oxygen.

Myofibril

Long contractile structure inside a muscle fiber; made of sarcomeres.

Sarcomere

Basic contractile unit of a myofibril; extends from one z-line to the next.

Myofilament

Protein filament within a sarcomere; includes thick and thin filaments.

A-band

Dark sarcomere region containing the entire length of thick filaments.

Myosin

A motor protein forming thick filaments; its heads bind actin and use ATP.

Thick filament

A myosin-containing filament that interacts with actin during contraction.

H-zone

Central A-band region containing only thick filaments.

M-line

A protein line at the sarcomere center that anchors thick filament.

I-band

Light sarcomere region containing only thin filaments.

Thin filaments

An actin-containing filament that interacts with myosin during contraction.

Actin

A protein that forms thin filaments; contains binding sites for myosin heads.

Z-line

A sarcomere boundary tha anchors thin filaments.

Tropomyosin

A regulatory protein that blocks myosin-binding sites on actin during rest.

Troponin

A regulatory protein that binds calcium and moves tropomyosin during contraction.

Elastic filament

A filament that helps center myosin and returns the sarcomere toward resting length.

Titin

Elastic protein connecting thick filaments to z-lines; provides recoil and stability.

Cross-bridges

Myosin heads attached to actin during muscle contraction.

Sliding Filament Model of Contraction

Actin slides past myosin, shortening sarcomeres without shortening filaments.

Epimysium

Connective tissue sheath surrounding the entire skeletal muscle.

Perimysium

Connective tissue surrounding each fascicle.

Endomysium

Connective tissue surrounding each muscle fiber.

Direct attachment

Muscle attaches to bone through the periosteum.

Indirect attachment

Muscle attaches to bone through a tendon or aponeurosis.

Tendon

Cordlike connective tissue attaching muscle to bone.

Aponeurosis

Broad, flat sheet of connective tissue attaching muscle to bone or another muscle.

Duchenne muscular dystrophy

X chromosome mutation causing progressive muscle degeneration from dystrophin deficiency.

Action potential

Rapid electrical change in membrane potential that travels along a cell membrane.

Refractory period

The brief phase after an action potential where the neuron cannot fire.

Reuptake

Return of a neurostransmitter into the presynaptic neuron after signaling.

Acetylcholine

A neurotransmitter that enables muscle action, learning, and helps with memory.

Acetylcholinesterase

An enzyme that rapidly breaks down ACh in the synaptic cleft.

Chemically-gated ion channels

Ion channels that open when a specific chemical bonds.

ACh receptor

A chemically gated channel on the motor end plate that binds ACh.

Voltage-gated ion channels

Ion channels that open or close in response to changes in membrane potential.

Somatic motor neuron

A neuron that carries motor commands from the CNS to skeletal muscle.

Neuromuscular joint

A synapse where a motor neuron communicates with a muscle fiber.

End plate potential

Local depolarizatino of the motor end plate caused by ACh binding.

Axon terminal

The end of a neuron that releases neurotransmitters into a synapse.

Synaptic vesicles

Membrane sacs in axon terminals that store neurotransmitters.

Synaptic cleft

A tiny gap between a neuron and its target cell at a synapse.

Junctional folds

Folds of the motor end plate containing many ACh receptors.

Steps for muscle contraction

ACH released
Action potential occurs
Calcium released
Cross-bridge cycling
Contraction occurs

Excitation-contraction coupling

A process linking muscle fiber action potential to calcium release.

Cross-bridge cycle

Myosin binds actin, perofms a power stroke, detaches, then resets using ATP.

Myasthenia gravis

An autoimmune disorder that reduces functional ACh receptors, causing muscle weakness.

Cardiac muscle

Involuntary, striated muscle forming the heart wall.

Smooth muscle

Involuntary, nonstriated muscle found in walls of hollow organs and vessels.

Excitability

The ability of muscle cells to respond to stimulation by producing electrical signals.

Contractility

The ability of muscle tissue to generate force and shorten when stimulated.

Extensibility

The ability of muscle tissue to be stretched without being damaged.

Muscle tension

The force produced by a muscle when its fibers contract.

Load

Force or resistance that opposes muscle contraction.

Motor unit

One motor neuron and all the muscle fibers it controls.

Myogram

A recording of muscle tension produced during a muscle contraction.

Muscle twitch

A brief contraction of a muscle fiber in response to one action potential.

Latent period

A brief delay between stimulus and muscle contraction; calcium is being released.

Period of contraction

Phase where muscle tension rises as cross-bridges generate force.

Period of relaxation

The phase when muscle tension falls as calcium returns to the SR.

Graded muscle contractions

Contractions whose strength varies according to stimulation frequency or motor-unit recruitment.

Temporal summation

Increased tension caused by repeated stimuli arriving before the muscle fully relaxes.

Unfused tetanus

Sustained contraction with partial relaxation between repeated stimuli.

Fused tetanus

Sustained maximum contraction with no relaxation between stimuli.

Recruitment

Increasing muscle tension by activating additional motor units.

Subthreshold stimuli

Stimuli too weak to produce an action potential.

Threshold stimulus

The minimum stimulus strength required to produce an action potential.

Maximal stimulus

The minimum stimulus that activates all available motor units.

Muscle tone

Continuous, low-level contraction of skeletal muscles that maintains posture.

Isotonic contraction

Muscles change length white maintaining relatively constant tension.

Concentric contraction

Isotonic contraction in which the muscle shortens while producing force.

Eccentric contraction

Isotonic contraction in which the muscle lengthens while producing force.

Isometric contraction

Muscle produces tension without changing its overall length.

Origin

Attachment site of muscle that does not move during contraction.

Insertion

Attachment site of muscle that moves during contraction.

Agonist

A main muscle that contracts to cause movement.

Antagonist

A muscle that relaxes to allow movement.

Synergists

Muscle that assist agonists in movement.

Fixators

Muscles that maintain a stable position.

Orbicularis

A circular muscle

Deltoid

A triangular muscle

Quadratus

A square muscle

Trapezius

A trapezoid-like muscle

Teres

A round muscle

Rhomboid

A rhombus-shaped muscle

Creatine phosphate

A high-energy molecule that rapidly donates a phosphate ADP to regenerate ATP during short, intense activity.

Creatine kinase

An enzyme that transfers phosphate from CP to ADP, rapidly forming ATP.

Anaerobic glycolysis

ATP production from glucose without oxygen; produces pyruvate/lactate and supports short, instense activity.

Glycolysis

The cytoplasmic breakdown of glucose into 2 pyruvate; produces 2 net ATP.