Encapsulated sensory receptors within intraorbital skeletal muscles of a camel

Background: The occurrence of encapsulated sensory receptors in extrinsic ocular muscles differs among and within orders of mammals, Beyond indications that neuromuscular and neurotendinous spindles are present in extraocular muscles of the family Camilidae, little is known of their distributional characteristics. In fact there appear to be no distribution maps for any animal that show both major types of encapsulated muscle receptor in a full set of intraorbital skeletal muscles. Method: Serial histoloscal sections of all skeletal muscles from one orbit of an adult, one-humped camel (Camelus dromedarius) were examined for encapsulated receptors. Results: Encapsulated receptors were apparent in all the intraorbital skeletal muscles. Muscle spindles outnumbered tendon organs in the fleshy part of each muscle, For all muscles, spindles were most abundant in the half of the muscle near the origin; levator palpebrae superioris had a more even distribution of spindles along its length than did extraocular muscles, These longitudinal patterns of distribution for muscle spindles related in a general way to the nerve entry zone, Tendon organs occurred anywhere along a muscle's length, but they tended to be more frequent on either side of the major concentration of muscle spindles. Both types of encapsulated receptors were generally located nearer the perimeter than the center of cross sections through muscles. Conclusions: Encapsulated receptors of classic appearance are plentiful and have distinctive configurations within intraorbital skeletal muscles of the adult dromedary. When analyzed in conjunction with other studies, the present data give rise to testable explanations for the variability among genera in the number of encapsulated receptors in extraocular muscles. (C) 1997 Wiley-Liss, Inc.