Skin receptors, also called cutaneous receptors, are part of the somatosensory system. They detect pressure, temperature, and vibrations on or around the skin. Cutaneous mechanoreceptors respond to pressure sensations on the skin and include Merkel's corpuscles, Meissner's corpuscles, and bulboid corpuscles. Receptors that detect temperature are made up of free nerve endings on the skin and are called thermoreceptors. Pain receptors on the skin are also free nerve endings called nociceptors.
Cutaneous mechanoreceptors are probably the first type of skin receptors that come to mind for most people. These detectors respond to different kinds of touch stimulation. Merkel's corpuscles are the most densely populated in the finger tips and respond to pressure, but tend to respond slowly. These receptors are particularly sensitive to points and edges, and the response is directly related to the amount of pressure applied.
Meissner's corpuscles are one type of touch receptor in the skin. The two layers of the skin, epidermis and dermis, are separated by a waving, hill-like boundary, where the points that project into the epidermis are called dermal papillae. The tips of the dermal papillae are covered in Meissner's corpuscles, which are made of flattened layers of cells that encompass neurons, serving as the receptors. When pressure is applied to the corpuscles, the nerve endings are stimulated, which registers in the brain as a touch sensation. These sensors best detect fluttering or tapping sensations.
Bulboid corpuscles, also called Pacinian corpuscles, respond to vibrations. This type of receptor gets its name from its bulb or onion-like shape, which encloses a semi-fluid core containing medullated nerve fibers. These sensors are found all over the body, including the lips and tongue as well as the penis and clitoris. They are also found in the joints of the fingers.
Skin receptors that detect temperature are called thermoreceptors. These receptors respond to temperatures directly applied to the skin, such as a burn or an ice cube, as well as overall environmental changes in temperature. There is some debate about whether Ruffini's and bulboid corpuscles act as thermoreceptors rather than mechanoreceptors, but most scientists believe temperature is detected from free nerve endings. These nerves trigger physical reactions, such as shivering or goosebumps in cold and sweating in hot temperatures.
Nociceptors detect pain, and they are also believed to be made of free nerve endings in the skin. Due to the fact that pain can often mean danger, nociceptors respond very quickly and send information directly to the brain and spinal cord. These sensors tend to cluster in more sensitive areas of the skin and can detect any kind of pain, whether the source is temperature, chemical, or trauma. The response triggered by these receptors will vary with each circumstance, but a general response will be to protect the area where the pain occurred.