Muscles
Muscle Structure

A very high resolution E.M reveals that each myofibril
is made up of parallel filaments.
 There are 2 kinds of filament called thick & thin
filaments.
 These 2 filaments are linked at intervals called cross
bridges, which actually stick out from the thick
filaments
Thick
filament
Thin
filament
Cross
bridges
Mechanism of muscle contraction
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
The above micrographs show that the
sarcomere gets shorter when the muscle
contracts
 The light (I) bands become shorter
 The dark bands (A) bands stay the same length
The Sliding Filament Theory
 So,
when the muscle contracts,
sarcomeres become smaller
 However the filaments do not change in
length.
 Instead they slide past each other (overlap)
 So actin filaments slide between myosin
filaments
 and the zone of overlap is larger
What makes the filaments
slide past each other?

Energy for the movement comes from splitting
ATP
 ATPase that does this is located in the myosin
heads.
 The energy from the ATP causes the angle of the
myosin head to change.
 The myosin heads can attach to actin.
 Movement of the myosin heads and them
attaching and detaching from actin causes the
filaments to slide relative to one another.
 This movement reduces the sarcomere length.
Repetition of the cycle
 One ATP
molecule is split by each cross
bridge in each cycle.
 This takes only a few milliseconds
 During a contraction 1000’s of cross
bridges in each sarcomere go through
this cycle.
 However the cross bridges are all out of
synch, so there are always many cross
bridges attached at any one time to
maintain force.
The Cross Bridge Cycle
1.The cycle begins with ATP binding to
the myosin head. This causes the
myosin head to be released from actin.
The Cross Bridge Cycle
2. The ATP molecule is then hydrolysed
while the myosin head is unattached.
The ADP & Pi formed remain bound to
the myosin head.
The Cross Bridge Cycle
3. The energy released by
the hydrolysis of ATP is
absorbed by the myosin
• This causes the myosin
head to change shape
(places it in energised state
or cocked state – also
called the recovery stroke)
• It then binds to the actin
filament.
The Cross Bridge Cycle
4-5. The ADP and Pi are then released from
the myosin head
• Result = Power stroke occurs (the myosin
head changes shape)
•This draws the actin filament over the
myosin filament.
The Cross Bridge Cycle
1.The cycle begins
again when the next
ATP binds to the
myosin head.
Causing the myosin
head to be released
from actin.
Control of Muscle Contraction

How is the cross bridge cycle switched off in a
relaxed muscle?
 This is where the regulatory protein on the actin
filament, tropomyosin is involved.
 Actin filaments have myosin binding sites.
 These binding sites are blocked by tropomyosin
in relaxed muscle.
 When Ca2+ bind tropomyosin is displaced and the
myosin binding sites are uncovered.
 So myosin & actin can now bind together to start
the cross bridge cycle
Tropomyosin, Ca2+ & ATP
Ca2+ causes tropomyosin to be displaced.
 So it no longer blocks the myosin binding site
 So myosin and actin can bind together allowing
cross bridge cycling

Neuromuscular junction: Note Ach = Acetylcholine
Sarcoplasmic
Reticulum
Sequence of events
 1.
An action potential arrives at the end of
a motor neurone, at the neuromuscular
junction.
 2. This causes the release of the
neurotransmitter acetylcholine.
 3 This initiates an action potential in the
muscle cell membrane (Sarcolemma).
 4. This action potential is carried quickly
into the large muscle cell by invaginations
in the cell membrane called T-tubules.
Sequence of events
 5.
The action potential causes the
sarcoplasmic reticulum to release its store
of calcium into the myofibrils.
 6. Ca2+ causes tropomoysin to be
displaced uncovering myosin binding sites
on actin.
 7. Myosin cross bridges can now attach
and the cross bridge cycle can take place.
 Relaxation is the reverse of these steps