Muscle movement. Artwork of the mechanism of upper arm movement. First a nerve signal from the conscious part of the brain reaches a cell body (blue) of a motor neurone (nerve cell) in the grey matter of the spinal cord (at lower left). This impulse travels down an axon (blue fibre) through the spinal nerve root (at lower centre) to the arm's biceps muscle. The attachments of the axon to the long thin skeletal, or striated, muscle fibres are shown at upper right. Protein filaments (myofibrils) in the fibres slide over one another, causing the muscle to relax and the arm to straig- ten. Nerve impulses from the muscle travel back to the spinal cord through a sensory neurone (red).
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