The human skin is highly sensitive to changes in the external environment. Several different sensory receptors are present in the skin to detect an external stimulus. These include thermoreceptors, chemoreceptors, mechanoreceptors, nociceptors, and electromagnetic receptors. These receptors detect the external stimulus and signal the CNS to take necessary actions to cope with the changing external environment. It is necessary for the proper coordination of an organism with his surroundings.
Pacinian corpuscles or lamellar corpuscles are the mechanoreceptors found on the hairless skin of mammals including humans. They are highly sensitive to the changes in pressure and vibration. In this article, we will discuss the structure, mechanism, adaptations, and functions of Pacinian corpuscles.
Pacinian corpuscles are the capsulated endings of sensory neurons. They are large oval structures that are 0.5 to 1 mm in diameter. These corpuscles are found deep in the skin within the layers of reticular dermis and hypodermis.
Pacinian corpuscles have a single axonal fiber at the center surrounded by 15-20 lamella arranged in a concentric pattern. The entire structure is also surrounded by a connective tissue capsule. A fluid of certain nature is also present in between the lamella of the corpuscle.
The central fiber is unmyelinated near its tip. However, it becomes myelinated just before leaving the corpuscle. The myelinated fiber enters a peripheral sensory nerve.
They are not only found in skin but are also seen in the walls of organs like the pancreas, urinary bladder, and rectum, etc. here, these corpuscles detect the pressure created by distortion of the surrounding tissue and make the higher centers aware of it.
Upon histological examination of the skin with the H&E staining technique under a light microscope, Pacinian corpuscles can be identified at the border of dermis and hypodermis. They are identified by having a thick eosinophilic capsule. The capsule contains lamella of collagen fibers that appear as thin eosinophilic fibers running in a circular direction. The central axon is not apparent in most of the cut sections of the skin.
The development of Pacinian corpuscles begins in the 13th week of gestation and is completed 3 months after birth. At the initial stage, they contain a single sensory terminal at the center, surrounded by Schwann cells and mesenchymal cells.
The Schwann cells progressively become compact to form the inner core of the corpuscles. The mesenchymal cells later make the outer core as well as the capsule of the Pacinian corpuscles.
Pacinian corpuscles are responsible for detecting pressure and vibration stimuli. Any pressure or change in pressure is detected by the change in the position or shape of the lamella of Pacinian corpuscles.
When pressure is applied to the skin, the lamella of Pacinian corpuscles gets deformed. This causes stress on the membrane of sensory neuron and potential is generated, called the generator potential or receptor potential. If this generator potential is greater than the threshold, an action potential is generated that travels to the higher centers of the brain.
The basic reason of receptor potential is the change in the membrane permeability of different ions caused by the stress on the sensory axon. It causes the ions to diffuse into the cell at a different rate, resulting in a change in transmembrane potential.
Generation of Action Potential
We already know that Pacinian corpuscles have a central axon surrounded by lamella. If the corpuscle is compressed anywhere, it will result in elongation, bending, or deformation of the central fiber.
When a small area of the axonal fiber is compressed due to the deformation of the corpuscle, certain ion channels open causing the positively charged sodium ions to diffuse into the axon.
These positive ions cause depolarization of the fiber creating a potential called receptor potential.
This receptor potential also creates local circuits of current flow that spread throughout the length of the fiber. The first node of Ranvier is located inside the capsule. When the local current circuits reach this node, the membrane becomes depolarized. The action potential now leaves the corpuscle and travels from node to node, along the sensory nerve.
Stimulus Intensity and Receptor Potential
The action potential is only generated when the receptor potential is greater than the threshold potential of the sensory fiber. The strength of the receptor potential is related to the intensity of the stimulus.
It has been shown in different experiments that the strength of action potential increases progressively with the increasing compression force applied to the corpuscle. It is because increased compression force causes more deformation of the fiber, resulting in the opening of more ion channels. The increased influx of sodium ions causes a stronger receptor potential.
The amplitude of receptor potential increases rapidly at first but then progressively at a lower rate.
It was also seen that the frequency of repetitive potential sent by the sensory nerve also increases with an increase in receptor potential strength.
It was concluded that very intense stimulation of the Pacinian corpuscle results in a less additional increase in the number of action potentials. Thus, the receptor has an extreme range of responses, from very weak to very strong. The receptor is more sensitive to the weak sensory stimuli.
It is the pathway along which the action potential generated by the Pacinian corpuscles are carried to the higher centers of the brain for interpretation and necessary action.
The deep touch, pressure, and vibratory sensations perceived by Pacinian corpuscles are carried to the higher centers via fasciculus gracilis and cuneatus, the two ascending paths of the spinal cord. Like other sensory pathways, they also have three orders of neurons.
First Order Neurons
These are the sensory neurons having cell bodies in the posterior root ganglion. Pacinian corpuscles are located at the peripheral endings of such neurons.
The central process of these first-order neurons ascend in the spinal cord in the form of two fasciculi; fasciculus gracilis and fasciculus cuneatus. They travel ipsilaterally and terminate by synapsing with the third-order neurons in nucleus gracilis and cuneatus.
Second Order Neurons
The nuclei gracilis and cuneatus are the second-order neurons. The axons of these cells are called internal arcuate fibers. These fibers decussate with similar fibers on the opposite side and form a bundle called medial lemniscus.
The medial lemnisci ascend the brainstem on both sides until it reaches the thalamus.
Third Order Neurons
The third-order neurons are located in the ventral posterolateral nucleus of the thalamus. The axons of these neurons pass through the posterior limb of the internal capsule and corona radiate to reach the somesthetic area.
The primary somesthetic area is located in the postcentral gyrus of the cerebral cortex. The nerve impulses arising in the Pacinian corpuscles are processed in this area of the brain and the necessary action is taken.
Adaptation is an important characteristic of all sensory receptors including the Pacinian corpuscles. If a continuous stimulus is applied to a sensory receptor, it first generates impulses at a very rapid rate. The response rate progressively decreases finally reaching a level when only a few or no impulses are generated. This phenomenon is known as adaptation.
Pacinian corpuscles are known to be the most rapidly adapting mechanoreceptors found in humans. If you keep applying a pressure of a certain amount to your skin, you stop having pressure sensations after a while. It is due to the highly adaptive nature of Pacinian corpuscles.
In the case of Pacinian corpuscles, adaptation occurs in two ways; fluid redistribution and accommodation.
Recall that the Pacinian corpuscle is a viscoelastic structure. Whenever some pressure is applied to the skin, a distorting force acts on one side of the corpuscle. This force is rapidly transmitted by the fluid to the same side of the central axonal fiber and a receptor potential is initiated. Within the next few milliseconds, the fluid redistributes itself and no receptor potential is generated.
Thus, the receptor potential is generated only at the beginning of the compression. It disappears in a few milliseconds even though the compression is still present. It is a very rapid mechanism of adaptation.
It is a slow adaptation process as compared to fluid redistribution. This process occurs in the central nerve fiber. If somehow the fiber continues to be distorted by compression, it becomes accommodated to the stimulus. It is due to the activation of sodium channels.
The continuous current flow through sodium channels makes them inactivated. As a result, no action potential can be generated and the receptor is said to have adapted itself to the stimulus.
This phenomenon of closing sodium channels is not limited to Pacinian corpuscles only, as it has been identified to affect all the cell membranes.
Pacinian corpuscles are a good example of rate receptors or phasic receptors. The rapidly adapting receptors cannot be used to send continuous signals as they are stimulated only when the strength of the stimulus changes. As they react only in response to an actual change, they are termed as rate receptors, phasic receptors, or movement receptors.
When a sudden pressure is applied to a tissue, it excites the Pacinian corpuscles for a few milliseconds. The excitation is over soon, although the pressure is still there. A signal is transmitted again when the pressure is released. Thus, it keeps the nervous system aware of the changing deformation of tissues but is useless in the case of the constant deformation state of the body.
Pacinian corpuscles are very important mechanoreceptors found in the human body. The major functions of these receptors are as follows.
Detection of Pressure and Vibration Changes
It is the primary function of Pacinian corpuscles. They detect any pressure change applied to the skin. The changing pressure stimuli create a sense of vibration. They have pressure-sensitive sodium channels that are opened in response to the changing pressure on the skin. An action potential is initiated that is carried to the higher centers of the brain via the ascending pathways in the spinal cord.
Detection of Pressure in Internal Organs
Recall that the Pacinian corpuscles are also present in the walls of some viscera like the rectum and urinary bladder. Here, the detect the pressure created due to the filling of the organ. As a result, a signal is sent to the CNS that makes us aware of the filled state of the organ. The CNS also make necessary arrangements to empty the filled urinary bladder or rectum. Thus, they have an important role in processes like urination and defecation.
Predictive Function of Pacinian Corpuscles
Being phasic or rate receptors, the Pacinian corpuscles can predict the state of the body after a few seconds. This prediction is done based on the rate at which the stimulus applied to the body is changing. This helps the body to prepare itself for the possible deformity.
Pacinian corpuscles are the mechanoreceptors found in the skin and some internal organs.
They are the lamellar receptors having lamella of fibrous tissue separated by a fluid. These lamellae surround a central nerve fiber and are enclosed by a connective tissue capsule.
When pressure is applied to the skin surface, the compression force is transmitted to the central nerve fiber via fluid in the corpuscle. This deformation opens certain sodium channels in the nerve fiber and receptor potential is generated.
An action potential travels along the sensory nerve if the receptor potential is greater than the threshold potential.
The sensation is carried to the brain via ascending fibers in the posterior grey column of the spinal cord. The neurons in this pathway are;
- Sensory neuron with a cell body in the posterior root ganglion
- Nucleus cuneatus and nucleus gracilis
- The ventral posteromedial nucleus of thalamus
Pacinian corpuscles are rapidly adapting mechanoreceptors that can undergo stimulus adaptation in two ways;
- Rapid adaptation by fluid redistribution
- Slow adaptation by accommodation
They perform the following functions;
- Detect changes in pressure or vibration on the skin
- Detect pressure changes in the internal organs
- Predict the state of body due to deformation
- Biswas, Abhijit; Manivannan, M.; Srinivasan, Mandyam A. (2015). “Vibrotactile Sensitivity Threshold: Nonlinear Stochastic Mechanotransduction Model of the Pacinian Corpuscle”. IEEE Transactions on Haptics. 8 (1): 102–113. doi:10.1109/TOH.2014.2369422. PMID 25398183.
- Biswas, Abhijit; Manivannan, M.; Srinivasan, Mandyam A. (2015). “Multiscale Layered Biomechanical Model of the Pacinian Corpuscle”. IEEE Transactions on Haptics. 8 (1): 31–42. doi:10.1109/TOH.2014.2369416. PMID 25398182.
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