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intrafusal Describing a type of muscle fibre that occurs in the muscle spindles in voluntary muscles. Two to twelve intrafusal fibres enclosed in a capsule are found in each muscle spindle, lying parallel to the main (extrafusal) fibres. Each intrafusal fibre consists of a noncontractile (equatorial) region connected to a contractile (polar) region at each end of the fibre. The equatorial region is connected to stretch receptors.