A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. Normally there are four main types in glabrous skin: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini endings. There are also mechanoreceptors in hairy skin, and the hair cells in the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves into nerve signals sent to the brain. In the periodontal ligament, there are some mechanoreceptors, which allow the jaw to relax when biting down on hard objects; the mesencephalic nucleus is responsible for this reflex.Research into the mechanoreceptors of the human body began in the late 1970s, when Vallbo and Johansson took percutaneous recordings of human volunteers. The mechanoreceptors of rhesus monkeys are similar to those of humans.