Biomedical Engineering Reference
In-Depth Information
hick filament
Sarcomere
Myosin heads
(cross bridges)
Myosin tail
(b) hick filament and myosin molecule
M line
H zone
Z disc
Z disc
Actin molecule
Tropomyosin
Troponin
Light l band
Dark A band
Light l band
Myosin-binding site
hick filament
hin filament
Elastic filament (titin)
(a) Sarcomere
(c) Portion of a thin filament and actin molecule
FIGURE A.5
Myofilament structure detail.
appear to be shaped like kidney beans. They join to form an actin filament that is
twisted into a helix. On each actin molecule is a myosin-binding site where a cross-
bridge can attach. In relaxed muscle, tropomyosin covers the myosin-binding sites
on actin and thus blocks attachment of the myosin cross-bridge to actin. The elastic
element is composed of the protein titin (connectin), which is the third most plentiful
protein in skeletal muscle after actin and myosin. Titin anchors thick filaments to
the Z-discs and thereby helps stabilize the position of the thick filaments. It may
also play a role in the recovery of the resting sarcomere length when a muscle is
stretched or relaxed. The protein was named titin because it has a huge (titanic)
molecular weight (or connectin because of its connecting function).
A fluid-filled system of cisterns called sarcoplasmic reticulum (SR) encircles
each myofibril. This elaborate tubular system is similar to smooth endoplasmic
reticulum in nonmuscle cells. In a relaxed muscle fiber, the sarcoplasmic reticulum
stores Ca
from the sarcoplasmic reticulum into the sarcoplasm
around the thick and thin filaments triggers muscle contraction. The calcium ions
pass out through pores in the sarcoplasmic reticulum that are called Ca
2+
. Release of Ca
2+
release
channels. The transverse tubules (T-tubules) are tunnel-like infoldings of sarco-
lemma. They penetrate the muscle fiber at right angles to the sarcoplasmic reticulum
and the myofilaments. T-tubules are open to the outside of the fiber and are filled
with extracellular fluid. On both sides of a transverse tubule are dilated end sacs of
the sarcoplasmic reticulum called terminal cisterns. A triad is a combined system of
a transverse tubule and the terminal cisterns on either side of it.
2+
A.1
MUSCLE CONTRACTION
In the mid-1950s, Jean Hanson and Hugh Huxley proposed the “sliding filament”
mechanism of muscle contraction (fig. A.6). They stated that skeletal muscle con-
traction was due to thick and thin filaments sliding past one another. During muscle
contraction, myosin cross-bridges pull on the thin filaments, causing them to slide
inward toward the H-zone. As the cross-bridges pull on the thin filaments, the thin
filaments meet at the center of the sarcomere. The myosin cross-bridges may even
pull the thin filaments of each sarcomere so far inward that their ends overlap. As
