Muscle spindles in the jaw-closer muscles of the domestic cat.

The objectives of this study were to identify the exact location of spindles in jaw-closer muscles of the cat, to count the total number of spindles and to compare their distribution with the distribution of slow extrafusal fibres. The jaw-closer muscle group with all the skeletal attachments intact was fixed in a modified Carnoy solution, decalcified and processed through to wax. Complete series of sections were cut transverse, sagittal and perpendicular to the anterior temporalis muscle. At regular intervals, serial sections were stained by the Weigert-van Gieson method or immunostained for myosin isoforms. Spindle counts were made only from muscle areas where fibres were cut in transverse section, and the spindles were followed individually. The extrafusal fibres were identified by indirect immunoperoxidase staining with antibodies specific for the slow (type I) and fast (type IIM) isoforms of myosin found in jaw-closer muscles. The mean numbers and locations of spindles found were 13 in medial pterygoid (close to inferior border), 123 (one count only) in the deep anterior portion of the temporalis muscle (between the coronoid process of the mandible and the cranium), 50.5 in a small deep zone of superficial masseter anterior to the temporomandibular joint, and 50 in zygomaticomandibularis (the deepest portion of masseter). Most of the spindles were simple spindles. Spindle complexes (4 or more spindle units fused in parallel) were rare and were found only in zygomaticomandibularis and in masseter. Most parts of the jaw-closer muscles had no spindles and contained only fast fibres. Cosegregation of muscle spindles with slow fibres was found in most parts of this muscle group, but the distribution of spindles was more restricted than that of slow fibres.