Biomedical Engineering Reference
In-Depth Information
2 Sarcomeres
l band
A band
H zone
hick filament
Z disc
(a) Relaxed
Z disc
Z disc
hin filament
(b) Partially contracted
(c) Maximally contracted
FIGURE A.6
Sliding filament mechanism of muscle contraction.
the thin filaments slide inward, the Z discs come toward each other; the sarcomere
shortens but the length of thick and thin filaments does not change. The sliding of
the filaments and shortening of the sarcomere cause shortening of the whole muscle
fiber and ultimately the entire muscle.
An increase in Ca
concentration in the sarcoplasm starts filament sliding, while
a decrease turns off the sliding process. When a muscle fiber is relaxed, the concen-
tration of Ca
2+
in sarcoplasm is low. This is because the sarcoplasmic reticulum (SR)
membrane contains Ca
2+
active transport pumps that remove Ca
from the sarco-
2+
2+
plasm. Ca
is stored or sequestered inside the SR. As a muscle action potential
travels along the sarcolemma and into the transverse tubule system, however, Ca
2+
2+
from within
the SR into the sarcoplasm around the thick and thin filaments. The calcium ions
released from the SR combine with troponin, causing it to change shape. This shape
change slides the troponin-tropomyosin complex away from the myosin-binding
sites on actin.
As was described, muscle contraction requires Ca
release channels open in the SR membrane. The result is a flood of Ca
2+
. It also requires energy in
the form of ATP, which attaches to ATP-binding sites on the myosin cross-bridges
(heads). A portion of each myosin head acts as an ATPase, an enzyme that splits the
ATP into ADP +
2+
through hydrolysis reaction. This reaction transfers energy from
ATP to the myosin head even before contraction begins. The myosin cross-bridges
thus are in an activated (energized) state. Such activated myosin heads spontaneously
bind to the myosin-binding sites on actin when the Ca
P
level rises and tropomyosin
2+
slides away from its blocking position.
The shape change that occurs when myosin binds to actin produces the power
stroke of contraction. During the power stroke, the myosin cross-bridge swivels
