Muscle Tissue

Excitability

Responsiveness to chemical signals, stretch, and electrical changes across the plasma membrane.

Conductivity

Local electrical excitation that sets off a wave of excitation traveling along the muscle fiber.

Contractility

The ability of muscle fibers to shorten when stimulated.

Extensibility

The capability of muscles to be stretched between contractions.

Elasticity

The ability of a muscle to return to its original rest length after being stretched.

Skeletal muscle

Voluntary, striated muscle usually attached to bones.

Striations

Alternating light and dark transverse bands in muscle tissue.

Voluntary

Muscle movements that are usually under conscious control.

Muscle cell

A muscle fiber (myofiber) that can be as long as 30 cm.

Endomysium

Connective tissue that surrounds a muscle cell.

Perimysium

Connective tissue that surrounds a muscle fascicle.

Epimysium

Connective tissue that surrounds the entire muscle.

Tendons

Attachments between muscle and bone matrix.

Collagen

A somewhat extensible and elastic protein found in connective tissue.

Sarcolemma

The plasma membrane of a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle fiber.

Myofibrils

Long protein cords that occupy most of the sarcoplasm.

Glycogen

A stored carbohydrate that provides energy for exercise.

Myoglobin

A red pigment that provides some oxygen needed for muscle activity.

Myoblasts

Stem cells that fuse to form each muscle fiber early in development.

Satellite cells

Unspecialized myoblasts remaining between the muscle fiber and endomysium.

Sarcoplasmic reticulum (SR)

The smooth ER that forms a network around each myofibril.

Terminal cisterns

Dilated end-sacs of SR that cross the muscle fiber from one side to the other.

T tubules

Tubular infoldings of the sarcolemma that penetrate through the cell.

Triad

A T tubule and two terminal cisterns associated with it.

Thick filaments

Made of several hundred myosin molecules, with a helical array of heads.

Thin filaments

Composed primarily of two intertwined strands of fibrous actin.

Tropomyosin

Proteins that block six or seven active sites on actin.

Troponin

A small calcium-binding protein on each tropomyosin molecule.

Elastic filaments

Filaments made of titin that help stabilize thick filaments.

Contractile proteins

Myosin and actin that perform the work of contraction.

Regulatory proteins

Tropomyosin and troponin that determine when fibers can contract.

Dystrophin

A clinically important protein linking actin to membrane proteins and connective tissue.

A band

The dark part of a sarcomere where thick filaments overlap a hexagonal array of thin filaments.

Z disc

Provides anchorage for thin filaments and elastic filaments in the sarcomere.

Sarcomere

Segment from Z disc to Z disc; basic unit of muscle contraction.

Denervation atrophy

Shrinkage of paralyzed muscle when the nerve remains disconnected.

H band

____________: not as dark; middle of A band; thick filaments only.

I band

The light part of a sarcomere where thin filaments are present; 'I' stands for isotropic.

M line

____________: middle of H band.