A novel stretch receptor in the jaw of the rat

Origin of primary sensory neurons innervating the buccal stretch receptor

The primary sensory neurons innervating mechanoreceptors in oro-facial regions have their cell bodies in either the trigeminal ganglion or the mesencephalic nucleus of the trigeminal nerve. The buccal stretch receptor (BSR), a type of mechanoreceptor in the jaw of rodents, has recently been recognized as signaling the position of the mandible. The location of the primary afferent neurons innervating this receptor is unknown. To investigate the cell bodies of the BSR afferent neurons in rats, we applied wheat germ agglutinin-horseradish peroxidase (WGA-HRP) to the proximal stump of the severed nerve branch of the buccal nerve that supplied the BSR. HRP-labeled cell bodies were observed in the posterolateral portion of the ipsilateral trigeminal ganglion. None was found in the contralateral trigeminal ganglion or in the brainstem. All labeled cell bodies were oval or round and closely resembled pseudo-unipolar neurons. The mean diameter of the labeled somata ranged between 25.5 and 52.5 microm, with small (< or = 30 microm), medium (from 31 to 40 microm), and large somata (> or = 41 microm) accounting for 8.8%, 54.9%, and 36.3%, respectively. Among the myelinated nerve fibers in the branch in which WGA-HRP was applied, 78.5% terminated in the BSR and had larger fiber diameters than the rest, indicating that most of the medium and large HRP-labeled cell bodies were BSR afferents. From these results and the ontogenetic origin of this receptor, it is suggested that the BSR differentiated from the mechanoreceptors in the oral mucosa or the fascia of masticatory muscles.

Maturation of static sensitivity is related to expansion of the capsular space in rat buccal stretch receptors

The developmental relationship between static sensitivity and structure of the buccal stretch receptor (BSR) in rats was investigated. When responses to ramp-and-hold stretches were recorded from isolated BSRs, their static sensitivities suddenly increased between 2 and 4 weeks after birth. However, no apparent change was observed throughout other developmental stages examined. Electron microscopic examination revealed a conspicuous expansion of the fluid-filled capsular space caused by completion of the outer capsule during the same postnatal period. These findings suggest that an increase in the capsular space of BSR may be involved in increasing the static sensitivity of this receptor.