CONTRACTION OF
SKELETLAL MUSCLE:
SLIDING FILAMENT
THEORY
PROPOSED BY: H.E. HUXLEY & J.
HANSEN
Introduction
• When a muscle cell contracts, the thin
filaments slide past the thick filaments
and the sarcomere shortens
Molecules Involved
1. Myosin - thick protein filament
2. Actin - thin protein filament
3. Tropomyosin - covers binding sites on
the actin to prevent cross bridges from
forming when the muscle is not
stimulated
Molecules Involved
4. Troponin - exposes binding sites on the
actin molecule during muscle
stimulation
5. ATP - provides the energy needed for
contraction
6. Calcium ions - enables actin and
myosin to bind together to form cross
bridges
Sequence of Events at a
Cross Bridge Cycle
1. The influx of calcium triggers the
exposure of binding sites on actin.
(troponin moves the tropomyosin out
of the way)
2. Myosin binds to actin to form a cross
bridge.
Sequence of Events at a
Cross Bridge Cycle
3. The cross bridge produces a power
stroke (flexion) that causes the sliding
of the thin filaments toward the center
of the sarcomere.
4. ATP binds to the myosin head, causing
actin to disconnect from the cross
bridge.
Sequence of Events at a
Cross Bridge Cycle
5. ATP is broken down and the energy
released enables the cross bridge to
be repositioned.
6. Calcium ions are transported back into
the sarcoplasmic reticulum and the
tropomyosin moves back to cover the
actin binding sites.
Sequence of Events at a
Cross Bridge Cycle
*In contraction of a typical sarcomere, step 1
occurs then steps 2 - 5 are repeated over
and over again before step 6 occurs. This
allows the thin filaments to slide all the way
inward. Steps 2 - 5 may repeat as long as
both ATP and calcium ions are available.
Multiple cross bridge cycling is coordinated
sequentially to prevent cross bridges from
being connected or disconnected at the
same time.
SUMMARY: REQUIREMENTS
FOR MUSCLE
CONTRACTION
• STIMULATION
-A nerve impulse stimulates the
neuromuscular junction and acetylcholine is
released, initiating an action potential which
triggers the release of calcium ions from the
sarcoplasmic reticulum - the calcium ions
enable actin and myosin to bind together via
cross bridges by causing troponin to move
tropomyosin out of the way.
SUMMARY: REQUIREMENTS
FOR MUSCLE
CONTRACTION
• ENERGY
-Potential energy stored in cross bridges is
converted to chemical energy (ATP)
SUMMARY: REQUIREMENTS
FOR MUSCLE
CONTRACTION
• CONTRACTION
-Energized by ATP, each cross bridge
attaches and detaches several times during a
contraction, acting much like tiny oars to
generate tension and pull the thin filaments
toward the center of the sarcomere. As this
event occurs simultaneously in sarcomeres
throughout the cell, the muscle cell shortens
to about two thirds of its normal length.
SUMMARY: REQUIREMENTS
FOR MUSCLE
CONTRACTION
• RELAXATION
-When the action potential ends, calcium ions
are immediately reabsorbed into the
sarcoplasmic reticulum storage areas, actin
and myosin filaments separate and the
muscle cell relaxes and returns to its original
length. This whole series of events takes just
a few thousandths of a second.
SUMMARY: REQUIREMENTS
FOR MUSCLE
CONTRACTION
*While the action potential is occurring, the
acetylcholine, which started the process, is
broken down by enzymes present in the
sarcolemma. In this way, a single nerve
impulse produces only one contraction,
preventing the continued contraction of a
muscle cell in the absence of additional nerve
impulses*
Animation
Sliding Filament Animation
The End