These are the sensory receptors of the intrafusal muscle fibres. They are wound spirally around the belly of each such muscle fibre, and are thus admirably placed to detect the 'fatness' of that fibre at any moment in time. Their sensory information is conveyed back to the spinal segment by the sensory branch of the spinal nerve.
Found on http://www.encyclo.co.uk/local/20438
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