The Ruffini’s corpuscles they are sensory receptors that respond to mechanical stimuli and subtle variations in temperature. These consist of a single myelinated axon that branches into multiple nerve endings that are anchored within a capsule.
This capsule may be composed of collagen synthesized by fibroblasts or perineural cells. These receptors are named after the Italian physician and biologist Angelo Ruffini (1864-1929).
They are located both in the dermis and in the hypodermis of glabrous and hairy skin of mammals and marsupials, as well as in the menisci, ligaments, and articular capsules of the joints of some birds and mammals.
The Ruffini corpuscles found in each of the above locations present slight variations in structure. However, they are all mechanoreceptors that adapt slowly to stimulus and perceive stimuli in small receptive fields.
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Ruffini’s corpuscles
They are cutaneous sensory receptors, that is, located in the skin, being specialized in perceiving temperature variations above or below body temperature. Additionally, they are capable of perceiving low levels of mechanical deformation of the skin, even in its deepest layers.
Slowly adapting mechanoreceptors are capable of detecting sustained or prolonged pressure stimuli on the skin, as well as slight deformations produced by stretching the skin. In addition to detecting this type of static stimuli, they also respond to dynamic factors such as joint angle, stimulus velocity, and stretch.
Given their ability to detect signals with very small receptive fields, Ruffini endings fall within the classification of type I mechanoreceptors.
These corpuscles are quite small in size and not very numerous.
It can be assumed that the structural change in the connective tissue (injuries, malposition of the joints, scars, degenerative processes, aging) also leads to a change in the Ruffini corpuscles. All this because they adjust to the new surroundings.
Location
Ruffini’s endings or corpuscles have been found both in the hairy and glabrous skin of mammals and marsupials, as well as in the cruciate and lateral ligaments, menisci, and joint capsules.
In glabrous or hairless skin, present on the palms of the hands, soles of the feet, lips, pubis, and penis, Ruffini’s corpuscles are located at the level of the reticular layer of the epidermis.
While, in hairy or hairy skin, these receptors are also located in the reticular layer of the epidermis between the hair follicles and in the connective tissue capsule that covers the part of the hair or hair that is inserted into the skin. The set made up of the capsule and the mechanoreceptor has been called the Pilo-Ruffini Complex.
In primates, these corpuscles have also been found associated with the regions of the dermis close to the insertion of the hairs that line the nasal mucosa.
Finally, Ruffini’s corpuscles, found in the joint capsules of birds and mammals, are located only in the areas that lie within the fibrous cap and ligaments of the capsule.
Histology
They are formed by numerous free nerve endings, coming from a common myelinated axon that are encapsulated forming a cylindrical structure. In this capsule, the nerve terminals are anchored between connective tissue collagen fibers. The axon loses its myelin sheath and bifurcates in two before encapsulating to form branched nerve terminals.
The above description corresponds to the classical structure of a Ruffini corpuscle. However, there are often subtle variations in the structure of Ruffini’s corpuscles present in glabrous skin and hairy skin overlying different anatomical structures.
For example, Ruffini’s corpuscles located in the glabrous skin of the prepuce often arise from a single axon that branches several times before losing its myelin coating within the connective tissue capsule.
In the particular case of hairy skin, where the Pilo-Ruffini Complex can form, the axon forms a spiral that approaches the hair follicle just below the sebaceous gland, where it branches, losing myelin. The branches are anchored in the connective tissue capsule of the hair follicle.
functions
They sense heat-related changes in temperature and record their stretching. Additionally, they identify the continuous deformation of the skin and deep tissues.
Mechanism of action
When a mechanical force is applied to the skin, the capsule perceives said tension at its ends. The nerve endings are then compressed around the collagen fibers. This compression is maintained for a while thanks to the inelasticity of collagen, therefore the stimulus is captured in response to prolonged stimulation.
Mechanoreceptors
Mechanoreceptors are sensory receptors located in the skin that respond to mechanical changes such as pressure. They do this thanks to the fact that they work as a signal transducer system.
That is, they are capable of capturing pressure, tension and distortion stimuli, interpreting them and transmitting this information to the interior of the cell in order to generate a physiological response.
The signal that is transmitted by this type of receptors is nervous. In other words, it consists of a repetitive electrical discharge produced by a change in the membrane potential. Which occurs as a consequence of the excitation or activation of the receptor by a given stimulus.
Classification of mechanoreceptors based on their function
Mechanoreceptors have been classified based on the response that they are capable of giving during the two phases in which the stimulus is applied. The dynamic phase and the static phase.
The dynamic phase corresponds to the stage in which the intensity of the applied stimulus varies, such as when pressure is applied and released on the skin. For its part, the static phase refers to the period of time in which the application of the stimulus is constant.
Mechanoreceptors that are only stimulated during the dynamic phase, generating a response, have been called rapidly adapting receptors or phasic receptors. Whereas, those that are capable of responding in any of the two phases are known as slowly adapting receptors or tonic receptors.
These two types of main receptors can be subclassified into two more types based on the size of the area in which they can perceive stimuli, known in physiology as the receptive field.
These have been named as: type I receptors and type II receptors. Type I receptors perceive signals in limited areas or small receptive fields, while type II receptors do so in large receptive fields.
Finally, a final classification has been established in terms of function: Meissner corpuscles, Merckel discs, Paccini corpuscles and the aforementioned Ruffini corpuscles.
References
Bradley RM. 1995. Essentials of Oral Physiology. Ed. Mosby, St. Louis.
Boyd A. The histological structure of the receptors in the knee joint of the cat correlated with their physiological response. J Physiol. 1954; 124:476-488.
Grigg P, Hoffman AH. Properties of Ruffini afferents revealed by stress analysis of isolated sections of cat knee capsule. J Neurophysiol. 1982; 47:41-54.
Halata, Z. (1988). Chapter 24 Ruffini corpuscle a stretch receptor in the connective tissue of the skin and locomotion apparatus. Transduction and Cellular Mechanisms in Sensory Receptors, 221-229.
Mountcastle, V.C. (2005). The sensory hand: neural mechanisms of somatic sensation. Harvard University Press. Page 34.
Paré M, Behets C, Cornu O. Paucity of presumptive ruffini corpuscles in the index finger pad of humans. The journal of comparative neurology. 2003; 456:260-266.