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Muscle Contraction
We
have 400+ muscles in the body
which comprise 40-50% of the total
body weight
There are three types of muscle:
Smooth
Cardiac
Skeletal
Skeletal muscles functions are:
Force generation for movement
Force generation for postural
support
 Heat production during periods of
cold stress
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Skeletal Muscle Structure
Skeletal muscles are wrapped in a fibrous layer
called epimysium
Each Fascile (bundle of fibres) is wrapped in
perimysium
Each muscle fibre is wrapped in endomysium
• These fibrous layers provide support for
nerves and vessels and maintain elasticity in
the muscle
• They also merge to form the tendons which
connect the muscle to the bone
Skeletal muscles fibres (cells) are long,
striated and multinucleated.
They are formed of myofibrils, mitochondria
and other organelles in the sarcoplasm
Each cell is enveloped in a cell membrane
called the sarcolemma
Inside a Muscle Fiber/Myofibril
(a single cell)
Cell membrane = Sarcolemma
Cytoplasm = sarcoplasm
Myofibrils= many myofilaments made up
of proteins Actin and Myosin
From outside in:
Muscle belly (bundle of fascicles) ->
Fascicle (bundle of fibers) -> Muscle
Fiber (also called myofibril - a single
cell)-> Myofilament (made of actin and
myosin) are divided into Sarcomeres
*see handout pg.44 and “The Structure of the Skeletal
Muscle”
Sliding Filament Theory
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Muscle contraction occurs because of the
interaction between actin and myosin in the
myofibrils.
Simultaneous shortening of adjacent sarcomeres
results in a contraction of the entire myofibril.
Myosin cross-bridges connect the actin to the
myosin
These cross-bridges draw the actin together, toward
the centre of the sarcomere. They are like
‘extending’ arms from myosin which attach on the
actin filaments
Z Band are plates that hold the actin filaments
During contraction, the distance between
the Z bands gets smaller
(actin and myosin filaments DO NOT
shorten)
*See handout p14
What happens when the muscle
gets stimulated?
An simplified overview:
Nerve impulse -> sarcolemma -> sarcoplasmic
reticulum which discharges Calcium ions ->
triggers myosin cross-bridges to pull actin ->
causes Z bands to move = muscle shortening
(contraction!!! :)
ATP (adenosine triphosphate) is required for
this process (provides energy)