Muscle Movement

Muscle Movement

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D
Muscle action
8.2 Neuromuscular junctions, types of muscle and the sliding filaments model
Muscles and joints
Muscles can work in antagonistic pairs. In order for smooth movement at a muscular junction, such as the elbow, two
muscles must be involved. A muscle can only produce a force when it contracts, so if there are two muscles, the bone at
the joint can only move smoothly if one muscle contracts, and the other relaxes. Below is a simply diagram showing the
behaviour of the biceps and triceps in one arm when the arm is relaxed and when the elbow is bent perpendicularly.
The elbow joint is an example of a synovial joint
where a lot of movement occurs. These muscles do
HUMERUS
operate as an antagonistic pair, and although they
HUMERUS
BICEPS
BICEPS
cannot actively contract, they re-extend when
TRICEPS
TRICEPS
pulled by the opposite antagonistic muscle.
ligament ­ holds bones
RADIUS
RADIUS together to prevent
dislocation cartilage ­ pads where
bones meet that reduce
friction as they move
Synovial joints such as the elbow, which is displayed in the
diagram to the right, produce synovial fluid (from the synovial
synovial membrane ­
membrane) which acts as a lubricant for the joint. This is there to produces synovial fluid
ease the movement of the bones at the joint. At the terminus of
each bone there is cartilage which has the purpose of reducing synovial fluid ­
lubricates the joint
friction due to movement of the bones. The protective ligament
casing keeps the bones together when they move.
Neuromuscular junctions
Muscle action is controlled by the nervous system. There are motor neurones connected to muscles cells over a
junction called a neuromuscular junction.
1 Nerve impulse arrives at
neuromuscular junction causing
vesicles containing acetylcholine to
fuse and release contents into cleft
2 Acetylcholine binds to 3 Depolarisation wave
receptors on muscle fibre travels down T-tubule
membrane causing a
depolarisation + - +
6 Acetylcholinesterase rapidly -
- - - + - +
breaks down acetylcholine so that -
+ - +
contraction only occurs when an
impulse arrives + - +
2+ 2+
4 T-system
Ca Ca depolarisation leads to
2+
2+
Ca
2+ a release of Ca from
Ca
stores in sarcoplasmic
2+
5 Ca binds to proteins in the muscle, leading to contraction reticulum
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A nerve impulse which arrives at the neuromuscular junction is transmitted across the gap via a method very similar to
that of the cholinergic synapse (see 1.4 Synapses for details on the nerve junction, because you are expected to be able to
compare similarities and differences between the two) and a small chain of events eventually stimulates a contraction of
the muscle:
1 An electrical impulse arrives at the terminus of the motor neurone, where there are vesicles containing the
neurotransmitter acetylcholine.…read more

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Cells of skeletal muscle are The cells of smooth muscle appear spindle- Cardiac muscle cells also appear
large cells, long and thin, and shaped, with small gaps (fenestrations) in striated (banded) but are not to
are multinucleated cells (have between them, and the cells are singly- be confused with skeletal muscle,
many nuclei), and under the nucleated, and not striated ­ they contain and the cells actually have
light microscope skeletal small bundles of actin and myosin and connecting platforms between
muscle appears banded contract…read more

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So there are two components of the sliding filament model. The thin filament consists of two thin strands of actin, a
globular protein, coiled around each other. Surrounding them is another rod-shaped protein called tropomyosin which
coils around the actin. The role of tropomyosin is to reinforce the actin strands. Attached to each molecule of
tropomyosin is a molecule of troponin ­ a complex consisting of three polypeptides: one binding to actin, one binding to
tropomyosin, and one binding to calcium ions.…read more

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