Muscle Tissue and Muscle Fiber Structure

Muscle tension

Generation of force by muscle tissue

creates movement, maintains posture, stabilizes joints, generates heat, and regulates the flow of materials through hollow organs

Three types of muscle tissue

Skeletal muscle, cardiac muscle, and smooth muscle

Skeletal muscle tissue

Long, thin, striated, multinucleated cells arranged parallel to each other

mostly attached to the skeleton

requires nervous system stimulation and is voluntary

Cardiac muscle tissue

Shorter, wider, branching, striated cells usually containing a single nucleus

found only in the heart and is involuntary

Cells are connected by intercalated discs that help them contract as a unit

Smooth muscle tissue

Long, flattened cells with pointed ends and one centrally located oval nucleus

nonstriated and involuntary

Found in hollow organs, eyes, skin, and ducts of certain glands

Striated muscle

Muscle tissue with alternating light and dark bands

skeletal and cardiac muscle are striated, while smooth muscle is not

Contractility

Ability of a muscle cell to contract as proteins within the cell draw closer together

Excitability (responsivity)

Ability of a muscle cell to respond to a chemical, mechanical, or electrical stimulus

Conductivity

Ability to conduct electrical charges across the plasma membrane

Distensibility

Ability of a muscle cell to be stretched without rupturing

Elasticity

Ability of a muscle cell to return to its original length after being stretched

Sarcoplasm

The cytoplasm of a muscle cell

Sarcolemma

The plasma membrane of a muscle cell

Myofibrils

Bundles of specialized proteins within a muscle cell, including proteins involved in contraction

make up about 50–80% of the cell's volume

Myofibrils are the most abundant organelle in the sarcoplasm and are surrounded by the sarcoplasmic reticulum, which stores and releases calcium ions

Sarcoplasmic reticulum (SR)

Modified smooth endoplasmic reticulum that forms a network around each myofibril

stores and releases calcium ions

Skeletal muscle fiber structure

Long, cylindrical, striated cell that contains multiple nuclei and many myofibrils

Multiple nuclei—Muscle fibers arise from multiple embryonic Myoblasts, which fuse to form the muscle fibers

Why are skeletal muscle fibers multinucleated?

Multiple embryonic myoblasts fuse together to form a skeletal muscle fiber, resulting in a cell with multiple nuclei

T-tubules

tranverse tubules

Inward extensions of the sarcolemma that extend deep into the muscle fiber and surround the myofibrils

their tunnels contain extracellular fluid

Terminal cisternae

Enlarged portions of the sarcoplasmic reticulum located on each side of a T-tubule

Triad

One T-tubule plus the two terminal cisternae of the sarcoplasmic reticulum located on either side of it

Skeletal muscle fiber organization

Sarcolemma surrounds the muscle fiber →

muscle fiber contains myofibrils →

myofibrils are surrounded by sarcoplasmic reticulum →

T-tubules extend inward from the sarcolemma →

terminal cisternae of the SR flank each T-tubule