The epithelial tissue is one of the four basic types of tissue in the human body, characterized by greater cell cohesion than the rest of tissues, due to the absence of blood flow itself and because of a clear polarization between an apical face and a basal face.
The epithelial tissue is specialized in the covering of cavities and surfaces of the body , both external and internal. In the internal surfaces it is known as endothelium, in the external ones as epithelium and in some organs it forms the parenchyma .
Within the epithelial tissue we can distinguish several types according to function. For example, the epithelial tissue that lines the internal surface of the glands, known as glandular epithelium, has an eminently secretory function.
Similarly, a sensory epithelium is a type of epithelial tissue responsible for sensory function, ie, the capture of signals from the environment (physical, mechanical chemistry) and its transformation into electrical signals (nerve impulses).
This transformation is known as transduction , and is performed by highly specialized excitable cells known as receptors or sensory neurons .
In this way, the sensory epithelium connects the environment with the nervous system and allows the body to interact quickly with the environment.
As mentioned, sensory epithelia have excitable cells capable of converting external signals into electrical signals, a type of neurons known as sensory neurons .
The structure of the sensory neurons at their apical end (on the superficial surface of the epithelium) is different according to the stimulus to which they respond, which gives rise to different types of sensory neurons in each type of sensory epithelium.
Thus, in the ear, the sensory neurons are the hair cells , in the retina are the photoreceptors and in the olfactory epithelium are the olfactory sensory neurons .
The sensory neurons are arranged between the rest of non-excitable epithelial cells, which offer mechanical and metabolic support to the sensory neurons.
The apical end of the sensory neurons is oriented towards the epithelial surface, and at this end is where external stimuli are detected and become membrane potentials, that is, electrical signals or nerve impulses.
At the basal end, sensory neurons synapse with other neurons that ultimately transmit the nerve impulse to specific areas of the brain where the information received is interpreted.
Sensory epithelia in the human body
The sensory epithelia have their origin in the ectoderm , the outermost embryonic layer , next to the epidermis and the central nervous system.
In the human body, three sensory epitheliaare distinguished : the retina or visual epithelium , the olfactory epithelium and the auditory epithelium .
Each of them is located in their respective sensory organs: eye, nose and ear, very complex organs evolved to capture, filter and concentrate external signals and direct them to the corresponding sensory epithelium.
- Visual epithelium : the visual epithelium is located in the retina of the eye. Sensory neurons are known as photoreceptors(cones, rods and ipRGCs) and are excited by interacting with photons of light through different pigments (opsin, retinal).
- Olfactory epithelium : olfactory neurons express proteins that interact with chemicals that get through the air, so they are chemoreceptors .
- Auditory epithelium : the functioning of the ear is based on hair cells, also known as auditory hair cells, whose cilia detect the vibration produced by sound waves in the endolymph, a fluid that fills the inner ear. The auditory receptors are, therefore, mechanoreceptors . The position and movement of the body ( equipoise ) is also perceived in the ear .
Of the three sensory epithelia, the visual epithelium and the olfactory epithelium are considered part of the Central Nervous System, since neither the optic nerve nor the olfactory nerve synapse with a nervous ganglion before reaching the brain, that is, these nerves connect directly the sensory epithelium and the brain.