Muscle Types, Anatomy, and Contraction Mechanisms in A&P I

Muscles

Work to transform chemical energy (ATP) into mechanical energy to facilitate movement.

Major functions of muscles

Movements, stabilization of joints, maintaining posture, and producing heat.

Skeletal Muscle

Voluntary movement of the body through space, maintain posture, and stabilize joints.

Smooth Muscle

Involuntary movement of substances throughout the body.

Cardiac Muscle

Involuntary movement of blood throughout the heart and body.

Excitability

The ability of muscles to receive and respond to stimuli.

Contractability

The ability for muscles to shorten when stimulated.

Extensibility

The ability to be stretched or extended.

Elasticity

The ability to recoil to the muscles' resting length.

Epimysium

Surrounds the entire muscle and helps maintain the muscle's integrity during contraction.

Perimysium

Wraps around individual fascicles, helping to organize muscle fibers into individual bundles.

Endomysium

Surrounds each individual muscle fiber and contains extracellular fluid/nutrients to support the muscle fiber.

Insertion point

The movable bone/distal attachment of a muscle.

Origin point

The less movable bone/proximal attachment of a muscle.

Direct attachments

Epimysium fused to periosteum of bone or perichondrium of cartilage.

Indirect attachments

Connective tissue wrappings extend beyond the muscle as a ropelike tendon or a sheetlike aponeurosis.

Muscle (largest)

A group of fascicles covered externally by epimysium.

Fascicle

A group of muscle fibers (cells) surrounded by perimysium.

Myofibers (muscle cells)

Elongated multinucleated cells surrounded by endomysium.

Myofibrils

Rod-like contractile elements composed of sarcomeres arranged end to end.

Sarcomeres

Contractile unit of muscles composed of myofilaments.

Myofilaments (smallest)

Thick filaments = myosin; Thin filaments = actin.

Myofilaments

Contractile protein filaments that make up sarcomeres; the smallest unit of muscle.

Myosin

Makes up thick myofilaments; contains a head that contains both actin and ATP binding sites.

Actin

Makes up thin myofilaments.

Tropomyosin

Blocks myosin binding when the muscle is relaxed.

Troponin

Binds actin, tropomyosin, and calcium, but blocks interactions with myosin.

Z-discs

Structures that border sarcomeres.

Actin myofilaments

Thin myofilaments that are anchored to the Z-disc and move towards the center of the sarcomere.

Myosin myofilaments

Thick myofilaments that are anchored to the M-line and move towards the Z-disc.

Myofibers

Long, cylindrical, multinucleated cell.

Sarcolemma

Plasma membrane of a muscle fiber.

Sarcoplasm

Cytoplasm of a muscle fiber.

Sarcoplasmic reticulum

Specialized smooth ER which stores, releases, and retrieves Ca2+.

Glycosomes

Store glycogen used in cellular respiration.

Myoglobin

Stores oxygen used in cellular respiration.

T-tubules

microscopic invaginations of the muscle cell membrane that extend deep into the muscle fiber

Neuromuscular junction

Site where a motor neuron's terminal meets the muscle fiber; this is the site of muscle activation.

Activation

Nervous system stimulation at the NMJ must generate an action potential.

Excitation-contraction coupling

Action potential is propagated along the sarcolemma, and intracellular Ca2+ rises.

Action Potential

Electrical impulses that send signals around your body.

Depolarization

The membrane potential becomes more positive as Na+ channels open and Na+ ions rush into the cell.

Repolarization

Membrane potential starts to decrease as Na+ channels close and K+ channels open causing K+ to exit the cell.

Hyperpolarization

K+ channels close slowly, causing an excess of K+ to leave the cell and resulting in a membrane potential lower than -70mV.

Sliding Filament Model

The mechanism of muscle contraction where sarcomeres shorten through the interaction of actin and myosin filaments.

Cross-bridge Formation

The initial step in muscle contraction where calcium binds with troponin on the actin filament, shifting troponin and tropomyosin to expose myosin binding sites.

Power Stroke

The action where myosin pulls actin towards the M-line in the sarcomere, shortening it, powered by ATP hydrolysis.

Cross-bridge Detachment

The process where the attachment between the myosin head and actin binding sites is broken by ATP binding to the myosin head.

Cocking of the Myosin Head

The retraction of the myosin head to its original position after ATP hydrolysis, allowing it to bind to actin again.

Creatine Phosphate

A molecule that stores energy in phosphate bonds, providing fast energy for about 15 seconds.

Glycolysis

An anaerobic process that breaks down glucose into 2 ATP molecules, useful for short bursts of high energy output.

Aerobic Respiration

The breakdown of glucose in the presence of oxygen, producing a large amount of ATP per glucose molecule.

Concentric Contraction

A muscle contraction that shortens the muscle to move a load.

Eccentric Contraction

A muscle contraction that lengthens the muscle as the tension is diminished.

Isometric Contraction

A muscle contraction that occurs with no change in muscle length.

Motor Unit

The combination of a motor neuron and all the muscle fibers it supplies.

Sarcopenia

The degenerative loss of skeletal muscle mass and function.

Duchenne Muscular Dystrophy

A group of genetic diseases that cause progressive weakness and loss of muscle.

Dense Bodies

Sarcoplasmic structures that anchor smooth muscle fibers to the sarcolemma and facilitate muscle shortening.

Varicosities

Bulges in smooth muscle where neurotransmitters are stored to activate muscle contraction.

Calcium in Smooth Muscle

Calcium is stored in the sarcoplasmic reticulum and is essential for smooth muscle contraction.