Pacini corpuscles: what are they and how do these receptors work?

Pacini's corpuscles they are one of the four types of mechanoreceptors that enable the sense of touch, both in humans and other mammalian species.

Thanks to these cells we can detect the pressure and vibrations on our skin, having a key importance to when detecting both possible physical threats and in aspects as daily as taking objects from the atmosphere.

It may seem that being so small they do not give much of themselves, however, neuroscience has addressed them very thoroughly, since they are relevant both in our behavior and in our survival, that is, from the point of view of Psychology and Biology. Let's see what these small structures that we all have in our largest organ, the skin, do.

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What are Pacinian corpuscles?

Beyond the simplistic idea that human beings have five senses, there is reality: there is a greater variety of sensory pathways that inform us about what is happening both in our environment and in our body. Normally, several of them are grouped under the "touch" label, some of which are capable of generating very different experiences from one another.

Pacinian corpuscles, also called lamellar corpuscles, are one of the four types of mechanoreceptors responsible for the sense of touch, found on human skin. They are especially sensitive to pressure and vibrations that may occur on the skin, either by touching an object or by the action of some movement of the individual. These cells are named after their discoverer, the Italian anatomist Filippo Pacini.

These corpuscles, although they are found throughout the skin, are found to a greater extent in places where there is no hair, such as the palms of the hands, fingers, and the soles of the feet. They have a very fast adaptability to physical stimuli, allowing a fast signal to be sent to the nervous system but progressively decreasing it as the stimulus continues to be in contact with the fur.

Thanks to this type of cells, human beings can detect physical aspects of objects such as their surface texture, roughness, in addition to exerting the appropriate force based on whether we want to grab or release the object in question.

What role do they play?

Lamellar or Pacinian corpuscles are cells that respond to sensory stimuli and possible rapid changes that may occur in it. That is why its main function is to detect vibrations in the skin, as well as changes in the pressure that this tissue can receive.

When a deformation or vibrating movement occurs in the skin, the corpuscles emit a potential of action in the nerve terminal, thus sending a signal to the nervous system that ends up reaching the brain.

Thanks to their great sensitivity, these corpuscles allow vibrations of a frequency close to 250 hertz (Hz) to be detected. This, to be understood, means that human skin is capable of detecting the movement of particles as small as one micron (1 μm) in size at the fingertips. However, some studies have indicated that they are capable of being activated by vibrations in ranges between 30 and 100 Hz.

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Where are they and what are they like?

Structurally, the Pacini corpuscles have an oval shape, sometimes very similar to a cylinder. Its size is around a millimeter in length more or less.

these cells They are made up of several sheets, also called lamellae., and it is for this reason that its other name is lamellar corpuscles. These layers can number between 20 and 60, and are made up of fibroblasts, a type of connective cell, and fibrous connective tissue. The lamellae do not have direct contact with each other, but are separated by very thin layers of collagen, with a gelatinous consistency and a high percentage of water.

At the bottom of the corpuscle enters a nerve fiber protected by myelin, which reaches the central part of the cell, becoming thicker and demyelinating as it is introduced into the corpuscle. In addition, several blood vessels also penetrate through this lower part, which branch into the various lamellar layers that make up the mechanoreceptor.

Pacini's corpuscles They are located in the hypodermis of the whole body.. This layer of the skin is found in the deepest part of the tissue, however it presents different concentrations of lamellar corpuscles depending on the area of ​​the body.

Although they can be found on both hairy and glabrous skin, that is, skin that does not have any hair, they are much more numerous on parts without hair, such as the palms of the hands and feet. In fact, about 350 corpuscles can be found on each finger, and about 800 in the palms.

Despite this, compared to other types of sensory cells related to the sense of touch, Pacini cells are found in a lower proportion. It should also be said that the other three types of touch cells, that is, those of Meissner, Merkel and Ruffini have a smaller size than those of Pacini.

It is interesting to mention the fact that Pacini corpuscles can not only be found in human skin, but also in other more internal structures of the organism. Lamellar cells are found in places as varied as they are the liver, sexual organs, pancreas, periosteum and mesentery. It has been hypothesized that these cells would have the function of detecting mechanical vibrations due to movement in these specific organs, detecting low-frequency sounds.

Mechanism of action

Pacinian corpuscles respond by emitting signals to the nervous system when their lamellae are deformed. This deformation causes both deformation and pressure to occur on the cell membrane of the sensory terminal. In turn, this membrane is deformed or curved, and that is when the nerve signal is sent to the central nervous structures, both the spinal cord and the brain.

This sending of signals has an electrochemical explanation. As the cytoplasmic membrane of the sensory neuron deforms, sodium channels, which are sensitive to pressure, open. In this way, sodium ions (Na+) are released into the synaptic space, causing the cell membrane to depolarize and the action potential to be generated, giving rise to the nerve impulse.

Pacini's corpuscles respond according to the degree of pressure exerted on the skin. That is, the more pressure, the greater the sending of nerve signals. It is for this reason that we are able to discern between a soft and delicate caress and a squeeze that can even hurt us.

However, another phenomenon also happens that may seem contrary to this fact, and that is that when dealing with receptors quickly adapt to stimuli, after a short time they begin to send fewer signals to the nervous system central. For this reason, and after a brief period of time, if we are touching an object, there comes a point where its touch becomes less conscious; that information is no longer so useful, after the first moment in which we know that the material reality that produces that sensation is there and affects us constantly.

Bibliographic references:

• Biswas, A. et al. (2015). Vibrotactile Sensitivity Threshold: Nonlinear Stochastic Mechanotransduction Model of the Pacinian Corpuscle. IEEE Transactions on Haptics 8(1). 102–113.
• Biswas, A. et al. (2015). Multiscale Layered Biomechanical Model of the Pacinian Corpuscle. IEEE Transactions on Haptics 8(1): pp. 31 - 42.
• Cherepnov, V.L.; Chadaeva, N.I. (1981). Some characteristics of soluble proteins of Pacinian corpuscles. Bulletin of Experimental Biology and Medicine. 91 (3): 346–348.
• Kandel, E. (2000). Principles of neural science. New York: McGraw-Hill, Health Professions Division.
• O'Johnson, K. (2001). The Roles and Functions of Cutaneous Mechanoreceptors. Current opinion in Neurobiology, 11: pp. 455 - 461.