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Ritika Ahmed

Muscles and muscle contraction
Three types of muscles; skeletal, cardiac and smooth

Smooth muscle
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Involuntary muscle
Not striped
Under the control of the involuntary nervous system
Found in the gut and blood vessels
Both contracts and fatigues slowly
Single nucleus
Divide and regenerate
Contract alone and under nervous control
Small and tapered

Cardiac muscle
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Found in the heart only
It is striated
Fibres joined by cross-connections
Contracts spontaneously
Does not fatigue
Uni/bi nucleate
Cells are called cardiac myocytes
Form branching networks connected at intercalated dish
Limited capacity for repair

Skeletal
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Most common type of muscle
Attached to bones by tendons (tendons made from collagen)
Voluntary
Striated
Multi-nucleated fibres crossed with a regular
pattern of fine red and white lines
Bound together by connective tissue
Involved in locomotion
Contracts rapidly
Fatigues and tires relatives quickly
Extremely adaptable
Satellite cells: like muscle “stem cells” —>
divide to make new muscle cells
There are two types of skeletal muscle: slow
twitch muscle fibres and fast twitch muscle fibres

The structure of striated muscle
• Large bundles of long cells, called muscle fibres
• The cell membrane of muscle fibres is called the sarcolemma
• Bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm (a
muscle cell’s cytoplasm)
• These folds are called transverse (T) tubules and the help to spread electrical impulses
throughout the sarcoplasm so they reach all parts of the muscle fibre
• A network of internal membranes called the sarcoplasmic reticulum runs through the
sarcoplasm
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Myofibrils and sarcomeres
• Contain bundles of thick and thin myofilaments that move past each other to make muscles
contract
• Thick myofilaments are made of the
protein myosin
• Thin myofilaments are made of the
protein actin
• Under an electron microscope there will
be a pattern of alternating dark and light
bands
• Dark bands contain thick myosin
filaments and some overlapping thin
actin filaments —> called A-bands
• Light bands contain thin actin filaments
ONLY —> called I-bands
• A myofibril is made up of many short
units called sarcomeres
• The ends of each sarcomeres are
marked with a Z line
• The middle is marked with a M-line —>
the middle line of the myosin filaments
• Around the M-line is the H-zone
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Muscle contraction (sliding filament theory)
• Myosin and actin filaments slide over each other to make sarcomeres contract - the
myofilaments DON’T contract
• The simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres
contract
• Sarcomeres return to their original length as the muscle relaxes
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-Action potential from motor neurone stimulates muscle cell

-Depolarises the sarcolemma
-Depolarisation spreads down the T-tubules to the sarcoplasmic reticulum
-Causes sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm
-Calcium binds to troponin, causing it to change shape
-This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament
-Exposes the binding site, allowing myosin head to bind to the actin filament
-The bond formed is called an actin-myosin cross bridge
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-Free ATP binds to the head, causing the shape to change again
-Breaks the cross bridge/myosin detaches from the actin filament after it has been moved
-Calcium ions activate ATPase
-Breaks down ATP into ADP and P
-ATP provides energy for the myosin head to return to its original position, primed with ADP
and P
-Myosin head reattached to a different binding site further along actin filament
-A new myosin-actin cross bridge is formed
-Cycle is repeated and continues as long calcium ions are present and bound to troponin
-Combined effect of this happening in each sarcomere is a shortening of the whole myofibril
-The shortening of many myofibrils brings about the contraction of a muscle
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Myoglobin is similar to haemoglobin except that it has one chain rather than four
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Ritika Ahmed
• Known as red muscle fibres or oxidative muscle fibres
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Fast twitch muscle
• Muscle fibres that contract very quickly
• Muscles used for fast movement
• Short bursts of speed and power
• Get tired very quickly
• Energy is releases quickly through anaerobic respiration using glycogen and glycolysis
• Few blood vessels, few mitochondria and few myoglobin
• Rich glycogen storage for glucose for both aerobic and anaerobic respiration
• Relatively high levels of creatine phosphate —> used to form ATP from ADP
• Known as glycolytic or white muscle fibres
• Many more myofibrils are packed into fast twitch fibres as little space is taken up by
mitochondria
• Cannot produce lots of ATP

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