Classification of Receptors: From Source to Structure
In histology, sensory apparatuses are categorized based on three main parameters.
- By the source of the incoming signal. If the stimulus originates from the external environment, it is detected by exteroreceptors. If the signal arises within the body itself, interoreceptors come into play.
- By the physicochemical nature of the stimulus (modality). Depending on the stimulus, nociceptors (pain), mechanoreceptors (mechanical pressure), baroreceptors (pressure changes), chemoreceptors (chemical composition), and thermoreceptors (temperature shifts) are distinguished. Proprioreceptors are classified separately; they provide muscle-joint sense and transmit information about body position.
- By histological structure. The baseline criterion is the relationship of the axial cylinder to glial cells. Free nerve endings lack glial wrappings and consist of naked dendritic branches. Non-free endings always maintain contact with glia.
Non-free receptors are further subdivided into unencapsulated (surrounded only by glial cells) and encapsulated (where a connective tissue capsule forms outside the glia). Unlike the receptor cells of the eye or inner ear, these tissue structures do not form independent sensory organs.
Sensory Apparatus of the Epidermis
Free nerve endings are predominantly found within the epithelial layer of the skin. They can be divided into two functional subgroups.
- Endings for pain and thermal sensitivity. Their fine branches penetrate directly into the intercellular spaces between epithelial cells. Upon stimulation, ion channels in the membrane change state (primarily calcium channels, and to a lesser extent sodium channels), triggering an action potential wave.
- Tactile complexes with Merkel cells. Located in the basal layer of the epidermis. The nerve ending closely contacts the base of a specialized Merkel epithelial cell, forming a tactile disc. This structure registers light touch. Interestingly, upon mechanical stimulation, the Merkel cell releases substances that enhance epithelial regeneration and affect vascular tone (this histological phenomenon forms the basis for the therapeutic effect of light massage). The complex is classified as a free ending because there is no connective tissue capsule, and the Merkel cell itself is of epithelial rather than glial origin. A myelin sheath around the dendrite appears only after the fiber crosses the basement membrane.
Connective Tissue Receptors (Dermis and Organ Stroma)
In the connective tissue of internal organs and the dermal layer of the skin, the receptor apparatus is more diverse.
Non-free unencapsulated receptors are branching dendrites accompanied by lemmocytes (glial cells) but lacking a capsule. Like free epidermal receptors, they perceive pain and temperature stimuli.
Encapsulated endings have the most complex structure. They always consist of three components: dendritic terminals, modified glial cells, and a connective tissue capsule.
- Tactile corpuscles (Meissner corpuscles). Responsible for touch (light pressure). They are located superficially in the dermal papillae of loose connective tissue projecting into the epidermis. Dendrites branch within a very thin capsule, and "tactile cells" (modified unmyelinated lemmocytes) lie perpendicularly to the long axis of the corpuscle between them. On histological preparations, neural elements are clearly visualized using silver nitrate impregnation.
- Lamellar corpuscles (Vater-Pacini corpuscles). Located deeply (in the reticular dermis, hypodermis, mesentery, and pancreatic stroma). They are studied using standard hematoxylin and eosin staining. The center contains an unmyelinated dendritic terminal surrounded by an inner core of glial cells. A massive outer core forms externally, consisting of a thick laminated capsule of fibroblasts and collagen, with fluid-filled spaces in between. This layered structure acts as a physical filter and powerful amplifier, allowing optimal detection of vibration and static pressure.
- Ruffini endings. Located in the reticular dermis. Dendrites intertwine with collagen fibers here, responding to tissue stretch (and described in some sources as thermoreceptors).
- End bulbs (Krause end bulbs). Spherical encapsulated structures frequently found in the skin of external genitalia. Dendrites within the capsule branch profusely like a bunch of grapes and function as mechanoreceptors.