4th, 2

What is the sarcolemma?

The unique name for the muscle cell membrane.

What invaginations are found in the sarcolemma?

Transverse tubules (T-tubules).

What is the function of T-tubules?

They transmit the action potential deep into the muscle cell.

What do T-tubules wrap around?

The sarcoplasmic reticulum (SR) and myofibrils.

What forms a triad?

One T-tubule and two terminal cisternae of the sarcoplasmic reticulum (SR).

What is the structural name for the combination of a T-tubule and the SR?

A triad.

What is the functional name for the combination of a T-tubule and the SR?

Terminal cisternae.

What is the sarcoplasm?

The unique name for the cytoplasm (fluid inside a muscle cell).

What is found in the sarcoplasm?

Many mitochondria, hundreds of nuclei, myofibrils, and the sarcoplasmic reticulum.

Why are there many mitochondria in muscle fibers?

To produce ATP (energy) needed for muscle contraction.

How many nuclei does a skeletal muscle fiber usually have?

Hundreds of nuclei.

What are myofibrils?

Bundles of myofilaments that fill the sarcoplasm.

What is the sarcoplasmic reticulum (SR)?

A specialized organelle unique to muscle fibers that wraps around myofibrils.

What does the sarcoplasmic reticulum store?

Calcium ions (Ca²⁺).

Why do skeletal and cardiac muscle fibers have a striped (striated) appearance?

Because of the arrangement of the myofilaments.

What are the two types of myofilaments?

Actin (thin) and myosin (thick).

Which myofilament is thin?

Actin.

Which myofilament is thick?

Myosin.

What is a sarcomere?

The repeating structural and functional unit of a muscle fiber.

What is the structural and functional unit of a muscle fiber?

The sarcomere.

What is the Z-line (Z-disk)?

Proteins that anchor and hold two pieces of actin.

What is the M-line?

Proteins that anchor and hold two pieces of myosin.

What is the A-band?

The dark (anisotropic) band that contains myosin and overlapping actin.

Why is the A-band dark?

Because it contains thick myosin filaments and overlapping thin actin filaments.

What is the I-band?

The light (isotropic) band that contains only actin.

Why is the I-band light?

Because it contains only thin actin filaments.

What is the H-band (H-zone)?

The region within the A-band that contains only myosin.

Which band contains only myosin?

The H-band (H-zone).

Which band contains only actin?

The I-band.

Which band contains myosin and overlapping actin?

The A-band.