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A2 Biology Unit 5

Alice Davies

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Structure of Skeletal Muscle
Muscles are effector organs that respond to nervous stimulation by contracting
and so bring about movement. There are three types of muscle in the body:

1. Cardiac Muscle ­ found exclusively in the heart, not under conscious
2. Smooth Muscle ­ found in the walls…

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Microscopic Structure of Skeletal Muscle
Each muscle fibre is made up of myofibrils which are made up of two types of
protein filament:

Actin ­ thinner and consists of two strands twisted around one another
Myosin ­ thicker and consists of long rod-shaped fibres with bulbous
heads that project to…

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Tropomyosin ­ forms a fibrous strand around the actin filament
Troponin ­ globular protein involved in muscle contraction

Types of Muscle Fibre
There are two types of muscle fibre, the proportions of which vary from muscle
to muscle and person to person.

Slow-Twitch Fast-Twitch
Contraction Contract more slowly and Contract…

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There are many such junctions along the muscle. If there were only one junction
of this type it would take time for a wave of contraction to travel across the
muscle, in which case not all the fibres would contract simultaneously and the
movement would be slow.

As rapid muscle…

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Contraction of Skeletal Muscles
The process of muscle contraction involves the actin and myosin
filaments sliding past one another and is therefore called the sliding
filament mechanism.

Evidence for the Sliding Filament Mechanism
Myofibrils appear darker in colour where the actin and myosin filaments
overlap and lighter where they do…

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A globular protein formed in two bulbous structures at one end
(the head)
Actin ­ a globular protein whose molecules are arranged into long
chains that are twisted around one another to form a helical strand
Tropomyosin ­ forms long thin threads that are wound around actin

Sliding Filament…

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1. Tropomyosin molecule prevents
myosin head from attaching to the
binding site on the actin molecule

2. Calcium ions released from the
endoplasmic reticulum cause the
tropomyosin molecule to pull away
from the binding sites on the actin

3. Myosin head now attaches to the
binding site on the…

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Muscle Relaxation
When nervous stimulation ceases, Ca2+ is actively transported back into
the endoplasmic reticulum using energy from the hydrolysis of ATP.
The reabsorption of Ca2+ allows tropomyosin to block the actin filament
Myosin heads are now unable to bind to actin filaments and contraction
ceases, i.e. muscle relaxes.…


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