Transcript Muscle Structure

Muscle Structure
& Contraction
AIM
To understand the structure
and histology of muscle and the
sliding filament theory of
muscle contraction
What are the 3 types of muscle called?
• Skeletal
• Smooth
• Cardiac
Skeletal muscle
• Skeletal muscles attach to bones and act in
antagonistic pairs
 one muscle to move the bone in one direction and
another to move it back the other way
• These muscles usually contract voluntarily (act
together with the nervous system)
• Able to carry out a single contraction (twitch) or a
long, sustained contraction (tetanus)
Cardiac muscle
• found only in the heart,
Adapted for endurance and reliability
It can stretch in a limited way, like smooth
muscle, and contract with the force of a
skeletal muscle
It is a twitch muscle only and contracts
involuntarily.
Smooth muscle
• found in the digestive system, blood vessels,
bladder, airways and, in a female, the uterus
• Smooth muscle has the ability to stretch and
maintain tension for long periods of time
• It contracts involuntarily: you do not have to
think about contracting it because the nervous
system controls it automatically
The Gross structure of muscles
A single skeletal muscle, e.g.triceps muscle, is attached at its:
1. Origin - a fixed non movable part of the skeleton e.g. the
shoulder and humerous
2. Insertion – a movable part of the skeleton e.g. the ulna
Attachment is by inelastic collagen tendons
As the triceps contracts, the insertion is pulled toward the origin and
the arm is straightened or extended at the elbow
•
The triceps is an extensor
•
Because skeletal muscle exerts force only when it contracts, a
second muscle (a flexor) is needed to bend the joint
•
The biceps muscle is the flexor of the lower arm
•
Together, the biceps and triceps make up an antagonistic pair
of muscles
The antagonistic action of the
biceps and triceps muscles
What are muscles made of?
• Skeletal muscle is made up of thousands of
cylindrical muscle fibres running from origin to
insertion
• Each fibre is surrounded by a membrane
(sarcolemma)
• Fibres are bound together by connective tissue with
B.V. and nerves running through
• Each muscle fibre contains:
1) myofibrils that are stacked lengthwise and run the
entire length of the fibre
2) mitochondria
3) an extensive endoplasmic (sarcoplasmic) reticulum
4) many nuclei near the surface
The Structure of a Muscle Fibre
The structure of Myofibrils
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•
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The cross striations seen are Z lines
They occur every 2.5um in a relaxed muscle
The interval between is a sarcomere
Thin filaments of actin extend the length of
each sarcomere
• In the centre of the sarcomere the actin is
interspersed with thick filaments of myosin
• The actin and myosin filaments are crosslinked
• Myofibrils appear darker where the filaments
overlap – A bands
• 6 actins filaments surround each myosin filament
• The light bands in between where there is only
actin are the I bands
• There is a small band where it is only myosin
filaments and this makes the H zone
• The Z lines run through the middle of each I band
The Arrangement of filaments in a myofibril
The
Arrangement
of filaments
in a myofibril
Sliding Filament Theory of
Muscle contraction
• Huxley and Hanson (1954)
• Observations during muscle contraction (twitch)
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I band shortens
A band does not change length
Z lines come closer together (sarcomere shortens)
H zone shortens
• Muscle contracts by the actin and myosin
filaments sliding past each other shortening the
sarcomeres
Sliding Filament Theory
Actin Filament –
2 helical strands
G-actin twisted
around to make a
fibre
Troponin
molecules as part
of tropomyosin
are attached to
the actin filament
Myosin filament
- a long myosin
rod with 2 heads
appearing at
intervals along
the rod
Where the actin and
myosin filaments
overlap the myosin
heads can attach to
the actin filaments
Sliding Filament Theory
• Ca release from the sarcoplasmic reticulum into
the sarcoplasm cause a rise in Ca concentration
• Ca binds to the troponin, causing a change in
shape of the tropomyosin
• Tropomyosin moves to expose the myosin
binding sites on the actin
• Myosin heads form actomyosin cross-bridges
Sliding Filament Theory
• The myosin heads become attached to the actin at a
certain angle and a cross bridge forms
• The myosin head swivels to a different angle moving the
actin filaments towards the centre of the sarcomere
• The myosin binds an ATP which causes it to detach from
the actin
• The ATP is hydrolysed to ADP and the energy is used to
return the myosin head back to its original angle
• The process is repeated
• It is a ratchet or rowing mechanism
The ratchet
mechanism
of muscle
contraction
• Ca ions are actively taken back into the
sarcoplasmic reticulum
• Only small amounts of ATP are found in resting
muscles
• Instead there are larger amounts of creatine
phosphate (CP)
• ATP is recycled by using the P from the CP to
phoshorylate ADP
• CP is restored by oxidation of fatty acids or
glycogen to yield ATP which can then
rephosphorylate the CP