in origin. Both the pulp and periodontal ligament have nociceptors (pain receptors), but the pulp lacks proprioceptors (motion or position receptors) and mechanoreceptors (mechanical pressure receptors). Consequently, pain originating from the dentin-pulp complex tends to be poorly localized, whereas pain from the periodontal ligament will typically be well localized, although not always. For instance, the periodontal ligament can detect the pressure exerted when biting on something smaller than a grain of sand (10–30 µm). When a tooth is intentionally stimulated, about 33% of people can correctly identify the tooth, and about 20% cannot narrow the stimulus location down to a group of three teeth. Another typical difference between pulpal and periodontal pain is that the latter is not usually made worse by thermal stimuli. Dental Pulpal The majority of pulpal toothache falls into one of the following types; however, other rare causes (which do not always fit neatly into these categories) include galvanic pain and barodontalgia. Pulpitis Pulpitis (inflammation of the pulp) can be triggered by various stimuli (insults), including mechanical, thermal, chemical, and bacterial irritants, or rarely barometric changes and ionizing radiation. Common causes include tooth decay, dental trauma (such as a crack or fracture), or a filling with an imperfect seal. Because the pulp is encased in a rigid outer shell, there is no space to accommodate swelling caused by inflammation. Inflammation therefore increases pressure in the pulp system, potentially compressing the blood vessels which supply the pulp. This may lead to ischemia (lack of oxygen) and necrosis (tissue death). Pulpitis is termed reversible when the inflamed pulp is capable of returning to a state of health, and irreversible when pulp necrosis is inevitable. Reversible pulpitis is characterized by short-lasting pain triggered by cold and sometimes heat. The symptoms of reversible pulpitis may disappear, either because the noxious stimulus is removed, such as when dental decay is removed and a filling placed, or because new layers of dentin (tertiary dentin) have been produced inside the pulp chamber, insulating against the stimulus. Irreversible pulpitis causes spontaneous or lingering pain in response to cold. Dentin hypersensitivity Dentin hypersensitivity is a sharp, short-lasting dental pain occurring in about 15% of the population, which is triggered by cold (such as liquids or air), sweet or spicy foods, and beverages. Teeth will normally have some sensation to these triggers, but what separates hypersensitivity from regular tooth sensation is the intensity of the pain.