Mechanoreceptors are sensory cells that enable sensation by converting mechanical stimuli such as pressure, stretch, touch, and vibration into endogenous stimuli and transmitting them to the brain via neural pathways. The medical profession distinguishes mechanoreceptors roughly according to their origin, whereby they also differ in their construction and functioning depending on the sensory organ associated with each of them. The receptors themselves are rarely affected by disease, but their nerve pathway connections to the brain can be damaged by inflammation, resulting in faulty or absent perception of pressure, stretch, touch, and vibration.
What are mechanoreceptors?
Mechanoreceptors are sensory cells in the ears, skin, and arteries. Together with thermoreceptors, chemoreceptors, photoreceptors, and pain receptors, mechanoreceptors make up the general perceptual system. The construction and functioning of mechanoreceptors differ depending on the sensory organ in which they are located. What they all have in common, however, is the conversion of mechanical force into nerve excitation. The medical profession mainly classifies the receptors according to their origin, i.e. according to their evolution. While one part of the sensory cells has developed from epithelial cells, the other part is evolutionarily derived from so-called ganglion cells. Thus, the cells are mainly divided into epithelial and ganglionic mechanoreceptors. A ganglion is an accumulation of nerve cells as found in the peripheral nervous system. Epithelium, on the other hand, is a collective term for human connective and covering tissues. Depending on their localization and the sensory organ networked with them, mechanoreceptors have different structures and thus differ in their mode of operation.
Anatomy and structure
Epthelial mechanoreceptors trace back to the cells that originally made up the organism’s surface. They contain what are known as cilia. These are cell appendages that appear on the plasma membrane as cytoplasmic protrusions. In these cilia, the conversion of an external stimulus, such as pressure or strain, into an electrical signal that can be processed by the nervous system takes place in mechanoreceptors. Unlike epithal mechanoreceptors, ganglionic mechanoreceptors are located in the tissue. Their structure is branched, yielding hundreds to thousands of individual terminals. In these terminals, the transformation of the external stimulus takes place in all ganglionic receptors. All mechanoreceptors are connected to the brain by conduction pathways, which allows the perception itself to enter consciousness. Ultimately, there are roughly five sensory systems in the human body: the auditory system, the sense of touch, the sense of balance, the sense of organ activity, and the depth sensitivity to the state of activity of tendons, muscles, and joints. They are all equipped with mechanoreceptors. While the auditory system and the sense of balance are equipped with secondary sensory cells, the rest of the above systems have primary sensory cells.
Function and tasks
All mechanoreceptors are designed to respond to mechanical stimuli. These stimuli include pressure, touch, stretch, and vibration. Sensing is thus the main task, so to speak, of any mechanoreceptor. The epithal mechanoreceptors receive a mechanical stimulus that deforms their cilia. This deformation of the cilia then opens or closes certain ion channels, resulting in excitation or inhibition of the associated receptor. This process takes place, for example, in the hair cells of human ears and plays a crucial role in the sense of hearing. In fish, flow receptors also belong to this type of sensory receptor. Insects, on the other hand, are equipped with vibration-sensitive receptors of this type. In ganglionic mechanoreceptors, on the other hand, a mechanical stimulus excites one or more of the individual terminals. In the cell body, the excitations of the individual terminals add up electrically and result in activation or inhibition of the sense. Examples of this are the sensory cells of the skin, which are responsible for the sense of touch. On the skin, physicians speak of SA-I, SA-II, RA and PC receptors. SA-I receptors map long-lasting stimuli. SA-II receptors, on the other hand, are responsible for slow stimuli and are associated with the stretching of the skin.The RA form perceives changes in stimulus intensity, whereas the PC variant detects changes in stimulus speed. While primary sensory cells themselves generate an action potential by converting the received stimulus, secondary sensory cells release neurotransmitters, the amount of which depends on the potential of the receptor. Roughly, physicians also distinguish all endogenous SA receptors from RA and PC receptors. SA receptors are responsible for the sensation of pressure. Merkel cells are one example. RA receptors handle touch sensation, such as hair follicle sensors do. PC receptors such as the Golgi-Mazzoni corpuscles perceive vibration. For sensing organ and muscle activity, the cardiac system, the digestive tract, and the muscle spindle are possible examples. Their areas of responsibility include stretching.
Diseases
Although the mechanoreceptors themselves are not usually responsible for impaired or absent perception of pressure, vibration, touch, or stretch, disorders of perceptual ability related to these mechanical stimuli may well occur in some circumstances. Most frequently, damage to the nerve pathways transmitting the stimuli to the brain is responsible for such phenomena. Such damage is often preceded by inflammation, which usually manifests itself in stabbing pain. Tumors in the central nervous system may also be responsible for misperceptions. In rare cases, the receptors themselves are affected by autoimmune diseases or symptoms of poisoning. The symptoms for a disease or dysfunction of the mechanoreceptors depend strongly on which sensory cell is specifically affected. If the receptors in the stomach, in the heart or in another internal organ are affected by a disease, the entire internal system is misregulated, which can have unpleasant to life-threatening consequences. Dizziness and nausea, on the other hand, are common symptoms of a disturbance of the vestibular receptors. Ultimately, however, even asthma, blood pressure and circulatory disorders can be related to a disturbance of the respective receptors. Thus, the symptomatic picture in this case is extremely diverse.
