AQA A2 BIOLOGY UNIT 5: Muscle Contraction

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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:
Skeletal muscle Always attached to bone
Acts under voluntary control via motor neurones of somatic nervous
system
Cardiac muscle Found exclusively in the heart
Not under voluntary control
Smooth muscle This is found in internal body organs such as the wall of the gut, the uterus,
blood arteries and arterioles, the iris, ciliary body and glandular ducts
It is under involuntary control via the autonomic
nervous system or hormones
Smooth muscle usually forms a ring, which tightens when it contracts, so
doesn't need a skeleton to pull against.
There are also many examples of non-muscle motility, such as cilia (in the trachea and oviducts) and
flagella (in sperm).
These movements use different "motor proteins" from muscle, though they work in similar ways.
Muscle Structure
A single muscle contains around 1000 muscle fibres running the whole
length of the muscle and joined together at the tendons.
Each muscle fibre is actually a single muscle cell. These giant cells have
many nuclei, as they were formed from the fusion of many smaller cells.
Their cytoplasm is packed full of myofibrils, bundles of protein filaments
that cause contraction. They also have many mitochondria to
provide ATP for contraction. This is called a sarcoplasm. A network of
internal membranes called sarcoplasm reticulum runs through the
sarcoplasm.
The electron microscope shows that each myofibril is made up of repeating
dark and light bands. In the middle of the dark band is a line called the M
line and in the middle of the light band is a line called the Z line. The
repeating unit from one Z line to the next is called a sarcomere.
A very high resolution electron micrograph shows that each myofibril is
made of parallel filaments. There are two kinds of alternating filaments,
called the thick and thin filaments. These two filaments are linked at
intervals by blobs called cross-bridges, which actually stick out from the
thick filaments.
The thick filament is made of a protein called myosin. Many of these
molecules stick together to form the thick filament, with the "handles" lying
together to form the backbone and the "heads" sticking out in all directions
to form the cross-bridges.
The thin filament is made of a protein called actin. Actin is a globular
molecule, but it polymerises to form a long double helix chain. The thin
filament also contains troponin and tropomyosin, two proteins involved in
the control of muscle contraction.

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Skeletal muscles are made up of two types of fibres ­ slow twitch and fast twitch.…read more

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The thick and thin filaments are arranged in a precise lattice to form a sarcomere.
The thick filaments are joined together at the M line, and the thin filaments are joined together at the
Z line, but the two kinds of filaments are not permanently joined to each other.…read more

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A neuromuscular junction is the point where a motor neurone meets a skeletal muscle fibre.
There are many 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 contraction is essential for survival there are many neuromuscular junctions spread
throughout the muscle.…read more

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Myosin filaments have globular heads that are hinged so that they can move back and forth.
Each myosin head has a binding site for actin and a binding site for ATP.
Actin filaments have binding sites for myosin heads, called actin-myosin binding sites.
Two other proteins, tropomyosin and troponin are found between actin filaments.
These proteins are attached to each other and they help myofilaments move past each other.…read more

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Muscle contraction requires considerable energy.
This is supplied by the hydrolysis of ATP to ADP and inorganic phosphate.
The energy released is needed for:
· The movement of myosin heads
· The reabsorption of calcium ions into the endoplasmic reticulum by active transport
The Aerobic System
Most of the time, when muscles are resting or moderately active, muscles use aerobic respiration of
glucose from the glycogen stores in the liver to make ATP.
This ATP is generated via oxidative phosphorylation in the cells mitochondria.…read more

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