Ruffini corpuscles are SA II class mechanoreceptors found in the dermis, the skin of the tooth root, and the joint capsules. The receptors register interoceptive and exteroceptive pressure or stretch and transmit these stimuli to the brain via the spinal cord. Mutations of the receptors are usually associated with insensitivity.
What is the Ruffini corpuscle?
The first instance of human perception is the so-called sensory cells. One of the most important sensory cells of the sense of touch are the mechanoreceptors, which detect stimuli such as pressure, touch and vibration and translate them into the language of the central nervous system. The skin sense has several mechanoreceptors that fall into either the SA receptor group, the RA receptors, or the PC receptors. The Ruffini corpuscles are mechanoreceptors belonging to the class of SA-II receptors. These are slowly adapting sensory cells that have a certain resting action potential frequency and respond in particular to stretch stimuli. The cells are named after the Italian anatomist Angelo Ruffini, who is considered the first describer of these receptors. As SA-II receptors, Ruffini bodies, unlike SA-I receptors, are not inactive at rest and have a resting action potential frequency greater than 0.
Anatomy and structure
Ruffini corpuscles are located in the skin as well as the root skin of the teeth and, in addition, in the joint capsules. In the skin, they are found primarily in the stratum reticulare within the dermis. All Ruffini corpuscles have an open cylindrical shape and are flattened towards the ends. Collagenous fiber bundles of connective tissue enter the corpuscles through the cylindrical openings. On the opposite side of their entrance, they exit the cells again. Like other mechanoreceptors, Ruffini corpuscles are equipped with free nerve endings and are thus freely exposed to environmental stimuli. The endings of the nerve fibers lie spirally between fiber bundles of collagen fibers. The afferents are enclosed by insulating myelin, which improves the conductivity of the nerves and counteracts potential loss. The myelinated afferents of Ruffini bodies have a thickness of about 5 µm.
Function and tasks
Like all other mechanoreceptors, Ruffini corpuscles are responsible for detecting pressure and touch and, after translation into the language of the central nervous system, transmitting them to the brain. The Ruffini corpuscles in the dermis of the skin are so-called exteroceptors. They are thus responsible for the perception of external touch stimuli and respond to both pressure and horizontal stretch. The Ruffini corpuscles in the joint capsules must be distinguished from these. They fall into the class of interoceptors and thus deal with stimulus perception from within themselves. The Ruffini corpuscles of the joint capsules play a role primarily in depth sensitivity and their sense of position and thus belong to the proprioceptors. In the joint capsule, they register the position and displacement velocity of joints by responding to pressure correlations. When stimuli are applied, Ruffini corpuscles generate an action potential that exceeds the potential of the cells in their resting state. This action potential travels through the afferent nerves of the cells via the spinal cord into the central nervous system. Only in the brain is the stimulus processed, sensory integrated, classified and interpreted. Through the Ruffini corpuscles in the dermis, humans sense touch of varying intensity. The Ruffini corpuscles in the joint capsules also give humans self-awareness, which informs them of their own body position at all times. This connection is necessary, for example, to perform precisely directed movements. Without positional information from the joints, for example, the risk of dislocation and oversteering would be significantly higher. The proprioceptive Ruffini corpuscles work closely with the muscle spindles, which are also proprioceptive, and which primarily gather information about muscle tension to dose muscle force.
Diseases
In recent years, a new class of disease has come to light: receptor-associated disease. Such receptor-associated diseases result from receptor mutations and include a wide spectrum of hereditary and somatic individual diseases.Mutations of the Ruffini corpuscles cause a defect of the same: the affected receptors can thus, for example, no longer bind ligands, no longer transmit signals or no longer translate the stimuli into the language of the central nervous system. In some cases, mutations of receptors such as the Ruffini corpuscles also result in insufficient production or inadequate incorporation of the receptors into the membrane. In some cases, the so-called ion channel diseases are also counted among the receptor-associated diseases. The same applies to autoimmune diseases, which form autoantibodies against the receptor structures and in this way cause inflammation in the receptors. Poisoning can also damage receptors such as the Ruffini corpuscles. Ultimately, however, most of the complaints in the area of the mechanoreceptors are basically not due to the receptors themselves, but to the nerves connected to them or even in the brain, where the evaluation of the touch information takes place. Many neurological diseases can thus lead, for example, to faulty or even absent touch sensation and position sensation. One of the most common of these diseases is multiple sclerosis. This disease causes autoimmunological inflammation in the nervous tissue of the central nervous system and can thus affect both the brain and the afferent pathways of the spinal cord. Although the Ruffini corpuscles are intact, they can no longer transmit the registered information to the central nervous system after their afferents have been damaged. The consequence of this is sometimes not only the insensitivity to external pressure. The inability to dose joint deflections may also be a consequence of damaged afferents of the Ruffini corpuscles.
