Biomedical Engineering Reference
In-Depth Information
toward the center of sarcomere like the oars of a boat. This action draws the thin
filaments past the thick filaments toward the H zone. As the myosin heads swivel
they release ADP. Once the power stoke is complete, ATP again combines with the
ATP-binding sites on the myosin cross-bridges. As ATP binds, the myosin head
detaches from actin. Again, ATP is split, imparting its energy to the myosin head,
which returns to its original upright position. It is then ready to combine with another
myosin-binding site further along the thin filament. The cycle repeats continually.
The myosin cross-bridges keep moving back and forth like the cogs of a ratchet,
with each power stroke moving the thin filaments toward the H-zone. At any one
instant, about half of the myosin cross-bridges are bound to actin and are swiveling.
The other half are detached and preparing to swivel again.
Contraction is analogous to running on a nonmotorized treadmill. One foot
(myosin head) strikes the belt (thin filament) and pushes it backward (toward the H-
zone). Then the other foot comes down and imparts a second push. The belt soon
moves smoothly while the runner (thin filament) remains stationary. Like the legs
of the runner, the myosin heads need a constant supply of energy to keep going.
The power stroke repeats as long as ATP is available and the Ca
2+
level near the thin
filament is high.
This continual movement applies the force draws the Z-discs toward each other
and the sarcomere shortens. The myofibrils thus contract and the whole muscle fiber
shortens. During a maximal muscle contraction, the distance between Z-discs can
decrease to half the resting length. However, the power stroke does not always result
in shortening of the muscle fibers and the whole muscle. Isometric contraction or
contraction without shortening occurs when the cross-bridges generate force but the
filaments do not slide past one another.
Two changes permit a muscle fiber to relax after it has contracted. First, ACh is
rapidly broken down by an enzyme called actylcholinesterase (AChE) present in the
synaptic cleft. When action potentials cease in the motor neuron, there is no new
release of ACh and AChE rapidly breaks down the ACh already present in the synaptic
cleft. This stops the generation of muscle action potentials and the Ca
2+
release
channels in the sarcoplasmic reticulum close. Second, Ca
2+
active transport pumps
rapidly remove Ca
from the sarcoplasm into SR. These pumps work so vigorously
that they can keep the concentration of Ca
2+
times lower in the sarcoplasm of a
relaxed muscle fiber than inside the SR. In addition, molecules of a calcium-binding
protein called calsequestrin bind to calcium ions inside the SR. This reaction takes
Ca
2+
10
4
2+
out of solution and allows even more Ca
2+
to be sequestered within the SR. As
Ca
level drops in the sarcoplasm, the tropomyosin-troponin complex slides back
over the myosin-binding sites on actin. This prevents further cross-bridge binding to
actin and the thin filaments slip back to their relaxed positions.
Sustained small contractions give firmness to a relaxed skeletal muscle known
as muscle tone. At any instant, a few muscle fibers are contracted while most are
relaxed. This small amount of contraction firms up a muscle without producing
movement and is essential for maintaining posture. Asynchronous firing of motor
units allows muscle tone to be sustained continuously.
If a skeletal muscle or group of skeletal muscles is overstimulated, the strength
of contraction becomes progressively weaker until the muscle no longer responds.
2+
