A Pacinian corpuscle is a type of touch receptor located in the skin. It is classed as a mechanoreceptor, meaning it is part of the group of sensory receptors that respond to touch and pressure. These corpuscles are especially suited to feeling rough surfaces and detecting vibration, and they respond to transient touches rather than sustained pressure. This is because they are able to quickly adapt to pressure so that it no longer acts as a stimulus.
Situated in the deeper layers of the skin, the Pacinian corpuscles look like tiny onions. Each corpuscle consists of the end of a sensory nerve fiber with layers of connective tissue wrapped around it. When the skin is touched with enough pressure, the connective tissue capsule is deformed and presses on the nerve ending, triggering an electrical impulse. This impulse is conducted along the sensory nerve to the brain, providing useful information about what is happening to the external surface of the body.
Inside the corpuscle is an important conducting structure called the first node of Ranvier. It is also here that the nerve sheath, made of an electrically insulating material known as myelin, begins. In nerves that have a myelin sheath, conduction of electrical impulses is relatively fast because a whole series of nodes of Ranvier are present, extending from the first one inside the sensory receptor. The nodes are able to rapidly transmit changes in electrical potential to one another.
When a stimulus is continually present, nerve impulses are fired off less and less over time. This feature of Pacinian corpuscle function is known as adaptation. Other types of sensory receptors also show adaptation to varying extents, but for this corpuscle, which responds to touch and pressure, adaptation occurs fairly quickly. Skin receptors that respond to pain and temperature do not have this ability to adapt speedily, as it could be dangerous to diminish such important warning signs.
Although there are similarities between the two, a Pacinian corpuscle is not as discriminatory as the type of fine-touch mechanoreceptor present in the fingertips, known as Meissner's corpuscle. This type of sensory receptor lies closer to the surface of the skin and adapts only moderately fast. It is useful on those occasions when unwanted insect visitors settle on the skin. Merkel cells are also in the mechanoreceptors group; these receptors adapt much slower than Pacinian corpuscles. When holding a coin in the palm of the hand, the Merkel cells will signal that it is still there after the Pacinian and Meissner's corpuscles have stopped firing.
