Everything you didn’t know you wanted to know.
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Microscopic Anatomy of
Skeletal Muscle
 Sarcolema: Plalsma membrane of cell (fiber)
 Myofibril: Organelle composed of bundles of
myofilaments & segmented into sarcomered
 Sarcomere: Single contractile unit (a segment of a
myofibril)
 Light (I) Band- boundary zone of sarcomeres (walls)
 Dark (A) Band- the interior of sarcomeres (pullers)
 Z-Line- marks exact boundary of sarcomere
 H-Zone- center of sarcomere and the space into which
the walls can contract
Microscopic Anatomy (continued)
 Myofilaments: Protein threads that perform the actual
contraction.
 Thick Filaments- myosin- the grabbers
 Thin Filaments- actin- the hand holds
 Sarcoplasmic Reticulum- type of endoplasmic reticulum
(smooth) that surrounds each myofibril. Stores calcium
used to stimulate contraction.
Stimulation of Muscle Cells
 Skeletal muscle is controlled by voluntary (primarily)
and involuntary stimulation by nerves. Here are the
important features involved.
 Motor Unit: a neuron and the muscle cells its axon
stimulates.
 Neuromuscular Junction: axon terminal “joins”
sarcolema of a muscle fiber.
 Synaptic Cleft: gap between axon terminals and muscle
cell.
 Neurotransmitter: a chemical released from axons that
stimulates muscle cells (Acetylcholine- ACh is the
neurotransmitter for skeletal muscle.
Stimulation of Muscle Cells
 Release of ACh makes Sarcolema permeable to Na+
and Na+ ions rush in.
 Action Potential: the electric current generated by the
in rush of Na+ ions that provides the activation energy
to cause the chemical reactions that lead to
contraction. (more on this in Ch. 7)
Sliding Filament Theory
 The sliding filament theory is the explanation for
how muscles produce force (or, usually,
shorten). It explains that the thick and thin
filaments within the sarcomere slide past one
another, shortening the entire length of the
sarcomere. In order to slide past one another, the
myosin heads will interact with the actin
filaments and, using ATP, bend to pull past the
actin.
Six Steps of Cross Bridge Cycling
1.
2.
3.
4.
5.
6.
The influx of calcium, triggering the exposure of
binding sites on actin.
The binding of myosin to actin
The power stroke of the cross bridge that causes the
sliding of the thin filaments
The binding of ATP to the cross bridge, which results
in the cross bridge disconnecting from actin
The hydrolysis of ATP, which leads to the reenergizing and repositioning of the cross bridge
The transport of calcium ions back into the
sarcoplasmic reticulum.
Step 1: A Closer Look
 The influx of calcium, triggering the exposure of
binding sites on actin. This should be obvious…
calcium has to expose actin in order for anything to
start between actin and myosin
Step 2: A Closer Look
 The binding of myosin to actin.
 Actin and myosin must make contact with one another,
or no sliding can occur.
 Important point: At the time myosin grabs onto actin, it
is already “energized”. (in its “high energy state”) This
means that in order to understand the steps you have to
keep in mind that they start with an already “energized”
myosin head.
Step 3: A Closer Look
 The Power Stroke:
 Since myosin is already energized, once it grabs onto
actin it immediately begins to pull.
 Here you have to keep in mind that there is a strong
attraction between actin and myosin. They will remain
connected unless pulled apart, even after the power
stroke is completed.
 One more thing: While the myosin head is bending in
the power stroke, this conformational change in shap of
the myosin head causes the ADP + P that remains
attached to the myosin head to fall out. This falling out
of the products of ATP hydolysis gives roomf or more
ATP to bind to myosin.
Step 4: A Closer Look
 The binding of ATP
 The myosin head has already done its work. Now it is in
its “low energy” state and needs to use more ATP in
order to get ready for another power stroke.
 The ATP binding site is free, so anytime there is more
ATP it will bind to the myosin head
 This time the conformational change prevents the
myosin head from staying attached to actin causing
them to separate.
Step 5: A Closer Look
 The hydrolysis of ATP
 Once the myosin head has ATP, it has to hydrolyze to get
its energy
 The products of ATP hydrolysis are: ADP + P
 These products remain in the ATP binding site and
myosin is back in its “high energy” state.
Step 6: A Closer Look
 The transport of Calcium ions back into the
Sarcoplasmic Reticulum (SR)
 Assuming that the signal from the nervous system for
contraction has ended, all the calcium will go back into
the SR. Each action potential on the muscle fiber
sarcolemma is extremely brief, so right after the calcium
ions spill out, they have to be sucked back up again.
(this requires an active transport mechanism)
Helpful Hints
 These steps are a part of a continuous cycle. Although
we started on Step 1, you could imagine any step as the
starting point.
 When studying this try to pick different starting points
and move through the cycle in order from you chosen
point.