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specialized masses of tissues that are able to contract, relax, and change their length

principal functions of muscle

movement, heat production, and posture/body support


ability to respond to neural stimulation


ability to shorten


ability to contract over a range of resting lengths


ability of a muscle to return to its original length after contraction


how well stimulus is distributed throughout cell

cardiac muscle

found only in heart; involuntary; acts constantly to pump blood through blood vessels

Cardiac muscle appearance

striated; short and branching myocytes; connected by intercalated disks.

Smooth muscle

involuntary muscle found in visceral organs and vasculature

Smooth muscle appearance

fusiform in shape; no striations, but does contain actin and myosin fibers;dense bodies that attach thin filaments to the cytoskeleton

single-unit smooth muscle

most common smooth muscle; found in viscera

multi-unit smooth muscle

direct innervation to each unit, forms motor unit; found in large arteries and iris

muscle spindles

sensory cells that control muscle tone through reflexive action; found withing the belly of the muscle running parallel along it and sense change in muscle length

golgi tendon organs

sensory cell that is found withing muscle tendons that sense muscle tension

muscle tension

force that a muscle exerts

joint receptors

sensory cells that sense a change in direction and acceleration in joint movement


innermost layer of CT that surrounds each muscle fiber/cell, connects them to other cells and contributes to then tendon of muscle


middle layer of CT that surrounds bundles of muscle fibers; vasculature and nervous supply traverse here; also contributes to the tendon


Outer layer of CT that covers the outside of the entire muscle; considered part of the deep fascia; separates muscle from surrounding tissue or bone; also contributes to the tendon of muscle


Bundle of muscle fibers


is a fibrous connective tissue in sheets, that attaches muscle to bone or to other muscle


a cordlike or bandlike mass of white fibrous connective tissue that connects a muscle to a bone


plasma membrane of muscle cells;surrounds the cytoplasm of muscle cells


the cytoplasm of a striated muscle fiber; surrounds individual myofibrils of muscle fibers

Transverse Tubules

network of tubules that extend inward at right angles to the sarcolemma; allow for nerve impulses to be transmitted rapidly to individual myofibrils

Sarcoplasmic Reticulum

longitudinal network of tubules that serves as a storage site for calcium

Mitochondrial Reticulum

network of structures for the energy producing biochemical "krebs cycle" which produces ATP through oxidative phosphorylation


contractile units of the muscle fiber; cylindrical and as long as the muscle fiber; made of up long stands of repeating subunits called sarcomeres


smallest functional unit of the myofibril; composed of bundles of myofilaments; from one Z dsic to the next


responsible for muscle contraction; 2 types are myosin and actin


thickest of the 2 myofilaments; bind parallel to one another with globular heads protruding around the circumference; centered in the sarcomere; but are connected to the Z-lines by elastic filaments


thinner myofilament; troponin molecule sits on top and blocks the binding site

concentric contraction

contraction that shortens the space between Z-lines(raising the barbell in a biceps curl)

eccentric contraction

either returns sarcomere to original (or resting) length or longer (lowering the barbell in a biceps curl)

Isometric contraction

no change in length, but a constant coupling/decoupling of cross-bridges(without shortening), such as carrying an object out in front of you

isotonic contraction

contraction with a change in length but the force generated remaining at a constant(all lifting exercise)


energy-storage polysaccharide abundant in muscle


oxygen-storing red pigment of muscle

Terminal cisternae

dilated ends of sarcoplasmic reticulum adjacent to a T-tubule

Thick filament

myofilament about 11nm in diameter composed of bundled myosin molecules

Elastic Filament

myofilament about 1nm in diameter composed of a giant protein, titin, that flanks a thick filament and anchors it to a Z-disc; centers and stabilizes the thick filament and prevents overstretching

Thin filament

myofilament about 5-6nm in diameter composed of actin, troponin, and tropomyosin

Regulatory proteins

troponin and myosin; proteins that do not directly engage in the sliding filament process of muscle contraction, but regulate myosin-actin binding


covers myosin binding sites on the actin molecules


A protein attached to tropomyosin that binds with calcium to shift the tropomyosin


springy protein that forms the elastic filaments and anchors the thick filaments to the Z discs


protein is important in maintaining the integrity of muscle fiber. absence cuases muscular dystrophy


alternate dark and light bands found on skeletal and cardiac muscle

A band

(anisotropic) thick and thin myofilaments that overlap

H band

Center band that contains only myosin

M line

center of the thick filament

I band

(isotropic) thin myofilaments only

Z disc

sheet of proteins that anchor thin filaments and connect myofibrils

neuromuscular junction

the junction between a nerve fiber and the muscle it supplies

synaptic knob

end of the axon, hold neurotransmitters

synaptic cleft

the fluid filled space at a synapse between neurons or neuron and effector

synaptic vesicles

sacs located in the cytoplasm at the tip of the presynaptic axon.


neurotransmitter released by vesciles from the axon that bind to receptors on the sarcolemma which set up an action potential and causes the muscle to contract


enzyme that catalyzes breakdown of acetylcholine, preventing sustained muscle contraction from a single nerve impulse

motor unit

a neuron and the muscle fibers it controls

intercalated discs

Rings that provide a strong connection between cardiac muscle cells, to prevent tears and leaks in the heart.

dense bodies

the structure that allows smooth muscle to generate intracellular contractile tension; analogous to z-line

smooth muscle tone

smooth-muscle tension due to low-level cross-bridge activity in absence of external stimuli


the process of wave-like muscle contractions of the alimentary tract that moves food along

cross-bridge cycle

Binding of myosin to actin, powerstroke, rigor, unbinding, cocking of the myosin heads

peak tension

all motor units contracting at maximum rate/frequency of stimulation

sustained contractions

motor units are alternated (so muscle won't fatigue as fast) to maintain force production at given level

Resting Tension

always present (muscle tone); there is random stimulation of motor units creating isometric contraction for position, stability, balance, joint stability etc

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