Existence of a muscle spindle in the posterior cricoarytenoid muscle of the guinea pig

We examined a muscle spindle in the posterior cricoarytenoid muscle of the adult guinea pig. The spindle capsule contained three intrafusal muscle fibres. At the equatorial region, three intrafusal fibres did not form a typical aggregation of nuclei such as bags and chains observed in common mammalian muscle spindles. Two intrafusal fibres of the three were innervated by sensory endings which branched and arranged irregularly. These sensory endings lay on the surface of intrafusal fibres in shallow grooves or deeply penetrated into the fibre. The third intrafusal fibre, lacking sensory innervation, showed a similar fine structure to extrafusal fibres. These findings suggest that the muscle spindle in the posterior cricoarytenoid muscle of the guinea pig is strikingly different in the structure of intrafusal fibres and in the arrangement of sensory endings from that in common mammalian skeletal muscles. The occurrence and role of these different structures were discussed.