Lamellar corpuscles

Lamellar corpuscles, or Pacinian corpuscles or Golgi-Mazzoni corpuscles, are one of the four major types of mechanoreceptor. They are nerve endings in the skin responsible for sensitivity to vibration and pressure. Vibrational role may be used to detect surface, e.g., rough vs. smooth. Lamellar corpuscles are also found in the pancreas, where they detect vibration and possibly very low frequency sounds.

They were once thought to be thermoreceptors

Structure
Similar in physiology to the Meissner's corpuscle, Lamellar corpuscles are larger and fewer in number than both Merkel cells and Meissner's corpuscles.

The Lamellar corpuscle is oval-shaped and approximately 1 mm in length. The entire corpuscle is wrapped by a layer of connective tissue. It has 20 to 60 concentric lamellae composed of fibrous connective tissue and fibroblasts, separated by gelatinous material. The lamellae are very thin, flat, modified Schwann cells. In the center of the corpuscle is the inner bulb, a fluid-filled cavity with a single afferent unmyelinated nerve ending.

Function
Lamellar corpuscles detect gross pressure changes and vibrations and are rapidly adapting (phasic) receptors. Any deformation in the corpuscle causes action potentials to be generated by opening pressure-sensitive sodium ion channels in the axon membrane. This allows sodium ions to influx, creating a receptor potential.

These corpuscles are especially susceptible to vibrations, which they can sense even centimeters away. Their optimal sensitivity is 250 Hz, and this is the frequency range generated upon fingertips by textures made of features smaller than 200 µm. Lamellar corpuscles cause action potentials when the skin is rapidly indented but not when the pressure is steady, due to the layers of connective tissue that cover the nerve ending. It is thought that they respond to high-velocity changes in joint position.

Lamellar corpuscles have a large receptive field on the skin's surface with an especially sensitive center.

How it works
Lamellar corpuscles sense stimuli due to the deformation of their rings of lamellae, which press on the top of the sensory neuron and causes it to bend. When the lamellae are deformed, due to either pressure or release of pressure, a generator potential is created as it physically deforms the plasma membrane of the tip of the neuron, making it "leak" Na+ ions. If this potential reaches a certain threshold, nerve impulses or action potentials are formed by pressure-sensitive sodium channels at the first node of Ranvier, the first node of the myelinated sensory neuron. This impulse is now transferred along the axon with the use of sodium channels and sodium/potassium pumps in the axon membrane.

Once the top of the neuron is depolarized, it will depolarize the first node of Ranvier; however, as it is a rapidly adapting fibre, this does not carry on indefinitely, and the signal propagation ceases. This is a graded response, meaning that the greater the deformation, the greater the generator potential. This information is encoded in the frequency of impulses, since a bigger or faster deformation induces a higher impulse frequency. Action potentials are formed when the skin is rapidly distorted but not when pressure is continuous. The frequencies of the impulses decrease quickly and soon stop due to the layers of connective tissue that cover the nerve ending. This adaptation is useful, as it stops the nervous system from being overloaded with unnecessary information such as the pressure exerted by clothing.

Nomenclature
The Pacinian corpuscle was named after its discoverer, Italian anatomist Filippo Pacini.

The term Golgi-Mazzoni corpuscle (distinct from the Golgi organ) is used to describe a similar structure found only in the fingertips.