The epithelial tissue It is one of the four basic types of tissue in the human body, characterized by a greater cell cohesion than the rest of the tissues, lack of own blood supply and for presenting a clear Polarization between an apical face and a basal face.
Epithelial tissue is specialized in the lining of cavities and body surfaces, both external and internal. On the inner surfaces it is known as the endothelium, on the outer surfaces as the 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 covers the internal surface of the glands, known as glandular epithelium, has an eminently secretory function.
Similarly, a sensory epithelium It is a type of epithelial tissue responsible for sensory function, that is, for the capturing signals from the environment (chemical, physical, mechanical) and its transformation into electrical signals (nerve impulses).
This transformation is known as transductionand is performed by highly specialized excitable cells known as sensory receptors or neurons.
In this way, the sensory epithelium connects with the environment with the nervous system and allows the organism to interact quickly with the environment.
sensory neurons
As mentioned, sensory epithelia have excitable cells capable of converting external signals into electrical signals, a type of neuron known as sensory neurons.
The structure of the sensory neurons at their apical end (on the superficial side of the epithelium) is different depending on the stimulus to which they respond, giving rise to different types of sensory neurons in each type of sensory epithelium.
Thus, in the ear the sensory neurons are the hair cellsin the retina are the photoreceptors and in the olfactory epithelium are the olfactory sensory neurons.
The sensory neurons are arranged among the rest of the non-excitable epithelial cells, which offer mechanical and metabolic support to the sensory neurons.
The apical end of sensory neurons faces the epithelial surface, and it is at this end that external stimuli are detected and converted into membrane potentialsthat is, into electrical signals or nerve impulses.
At the basal end, the sensory neurons synapse with other neurons, which are the ones that finally transmit the nerve impulse to specific areas of the brain where the information received is interpreted.
Sensory epithelia in the human body
Sensory epithelia originate from the ectodermthe outermost embryonic layer, next to the epidermis and the central nervous system.
In the human body they are distinguished three sensory epithelia: the retina or visual epitheliumthe 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 light photons through different pigments (opsin, retinal).
- olfactory epithelium: olfactory neurons express proteins that interact with chemical substances that arrive 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. Auditory receptors are therefore mechanoreceptors. The ear also perceives the position and movement of the body (equilibriumception).
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 synapses with a nerve ganglion before reaching the brain, that is, these nerves connect directly the sensory epithelium and the brain.
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