microscopic level of skeletal muscle Flashcards
(4 cards)
light and dark band
As mentioned previously, skeletal muscle fibers (cells) are multinucleate (Figure 6.3a). Many oval nuclei can be seen just beneath the plasma membrane, which is called the sarcolemma (sar″ko-lem′ah; “muscle husk”) in muscle fibers. The nuclei are pushed aside by long ribbonlike organelles, the myofibrils (mi″o-fi′brilz), which nearly fill the cytoplasm. Alternating light (I) bands and dark (A) bands along the length of the perfectly aligned myofibrils give the muscle fiber its striated (banded) appearance. (Think of the second letter of light, I, and the second letter of dark, A, to help you remember which band is which.) A closer look at the banding pattern reveals that the light I band has a midline interruption, a darker area called the Z disc, and the dark A band has a lighter central area called the H zone (Figure 6.3b). The M line in the center of the H zone contains tiny protein rods that hold adjacent thick filaments together.
Myofibrils are made up of repeating units called sarcomeres, which are the functional units of muscle contraction.
The light bands (called I bands) contain the Z disc and thin filaments (actin).
The dark bands (called A bands) contain thick filaments (myosin) and include the H zone and M line.
Muscles contract when myosin (thick filaments) pulls on actin (thin filaments), shortening the sarcomeres.
cross bridge
Inside each muscle cell, there are many myofibrils — long, threadlike structures responsible for muscle contraction.
These myofibrils are made up of repeating units called sarcomeres, which are the functional units of contraction.
Each sarcomere has a pattern of light and dark bands, which give muscle its striped (striated) appearance.
The light bands (I bands) contain thin filaments made of actin and have Z discs at each end. The Z discs anchor the thin filaments and mark the ends of a sarcomere.
The dark bands (A bands) contain thick filaments made of myosin, including areas where myosin overlaps with actin.
In the center of the A band is the H zone, which has only thick filaments (no actin).
In the very center of the H zone is the M line, which helps hold the thick filaments in place.
The myosin heads, found where actin and myosin overlap, use ATP to form cross-bridges with actin.
They then pull the thin filaments inward toward the M line, causing the sarcomere (and the muscle) to contract.
🚫 Common mistakes in your version (so you know what to avoid next time):
Sarcolemma is the cell membrane, not a band. You meant sarcomere or bands of the myofibril.
H band and M band are not separate bands — the H zone is a region, and the M line is a line in its center.
Myosin heads are not in the H zone — they’re at the edges of the A band where myosin overlaps actin.
thin filaments
Thin filaments are made of actin (a contractile protein) and regulatory proteins (that control if myosin can bind to actin or not).
These thin filaments are anchored to the Z disc, which is the edge of a sarcomere.
The I band (light band) contains only thin filaments — and spans across two sarcomeres because the Z disc is in the middle of it.
In a relaxed muscle, thin filaments do not reach the center of the sarcomere — that center is the H zone, which contains only thick filaments (myosin), so it looks lighter.
🔁 What happens during contraction?
Myosin heads pull the actin (thin filaments) toward the M line in the center.
As this happens, the thin filaments slide inward.
This causes the H zone to shrink or disappear, because the actin and myosin filaments now completely overlap in that region.
sarcoplasmic reticular
Another very important muscle fiber organelle—the sarcoplasmic reticulum (SR)—is a specialized smooth endoplasmic reticulum (not shown in Figure 6.3). The interconnecting tubules and sacs of the SR surround every myofibril just as the sleeve of a loosely crocheted sweater surrounds your arm. The major role of this elaborate system is to store calcium and to release it on demand when the muscle fiber is stimulated to contract. As you will see, calcium provides the final “go” signal for contraction.
