A novel stretch receptor in the jaw of the rat

Abstract

We report here an unusual type of stretch receptor found on each side of the rat jaw. This receptor has unique morphological featurs: it is quite long (24–28 mm), lies in connective tissue in between masticatooy muscle, and extends between the medial pterygoid muscle-tendon on the maxilla and the masseter-tendon on the mandible through a zigzag course, forming a Greek capital letter sigma when viewed from the side. The receptor is neither in parallel nor in series with any masticatory muscles and receives multiple innervation. The receptor increases its length when the jaw closes and shortens when the jaw opens. Electron microscopy revealed axial structures composed of a central cellular core surrounded by tightly packed collagen bundles which are separated from the capsule by a wide capsular space. Most of the sensory endings are found among axial collagen bundles, some in between core cells. The core cells have many finger-like processes on their surface, being coupled by desmosomes. The origin and nature of these cells are unclear. The wide capsular space is filled with Alcian blue positive substrate, probably acid glycosaminoglycans. The structures of outer and inner capsules are similar to those of muscle spindles, the former being composed of three to ten layers of thin flattened cells. The response of the receptor was examined with in vivo as well as in vitro preparations. In in vivo experiments, impulse discharges from this receptor increased with the increase in jaw closing. When the jaw was fully opened the impulse discharge from this receptor disappeared. It is clear from these experiments that this receptor is a slowly adapting stretch receptor, being sensitive to jaw position and movement. With in vitro preparations, a linear relation was obtained between the frequency of impulse discharge measured during the static phase of stretch and the amplitude of stretch. Beyond 20% of stretch the response tended to saturate. The dynamic component of discharge was less marked, and comparable to the response of Golgi tendon organs or secondary endings of muscle spindles. The conduction velocity of innervating axons was 23–31 m/s at 35°C.