1. Sight
In vertebrates, the eye is the sensory organ for detecting visible light. It is an ovoid structure made up of the cornea, anterior chamber, iris, ciliary muscles, crystalline lens, vitreous body, retina, choroid, sclerotic, or sclera, and optic nerve. These components are distributed in three concentric layers or tunics (except for the crystalline): fibrous tunic, vascular tunic, and internal nervous tunic (Figure 1).
The cornea is the most external part of the eye, so it is in contact with the air. It is transparent and lacks blood vessels. Tvhe cornea focuses the light and protects the eye surface.
The iris is the structure of the eye that separates the anterior chamber from the posterior chamber. At the central area of the iris, there is an opening known as the pupil, through which light can reach the crystalline lens. The posterior part, the deepest one, of the iris is a two-layered, highly pigmented epithelium, which gives color to the eyes. The iris works as an adjustable diaphragm thanks to the activity of two muscles: the sphincter sets the diameter of the pupil, and the dilator increases the pupil's diameter (pupil dilation).
The crystalline lens is located behind the pupil and shows a transparent biconvex shape. The cornea and crystalline lens work together to focus the light on the retina. The vitreous body fills the cavity between the crystalline lens and the retina.
The ciliary body is found behind the iris and performs two main functions: release vitreous humor and change the shape of the crystalline to focus the light onto the retina. The ciliary epithelium releases aqueous humor.
The retina is the light-sensing structure. It is composed of photoreceptors that sense the light and other neurons for processing this information that is finally sent through the optic nerve (II) to the encephalon. The retina is made up of 10 layers of neurons (Figure 2). The most external one is the photoreceptor layer, which contains two types of photoreceptors: cones and rods. Cones are specialized in perceiving colors, whereas rods respond to light intensity. The pigment layer prevents light scattering, contributing to sharper vision.
2. Hearing
The auditory system is in charge of auditory perception. It actually consists of two subsystems: auditory and vestibular (Figure 3).
Outer ear
The outer ear is composed of the auricle (pinna) and the auditory canal. The auditory canal is a long, tube-like structure that starts in the auricle and ends in the tympanic membrane. The outer part contains many ceruminous glands that release the earwax, or cerumen.
Middle ear
The middle ear is found after the auditory canal. It is a cavity, known as the tympanic cavity. The tympanic membrane separates the auditory canal from the tympanic cavity. Inside the tympanic cavity, there are three tiny bones (ossicles): the malleus, the incus, and the stapes, along with the muscles that move them. The Eustachian tube is also part of the middle ear. It connects the tympanic cavity with the pharynx. The function of the middle ear is to transform the air waves, which carry sound information, into mechanical movements of the ossicles, which convey information to the inner ear.
Inner ear
The inner ear is the labyrinth (Figure 4), which includes the bony labyrinth and the membranous labyrinth. The bony labyrinth localizes within the temporal cranial bone and comprises the semicircular canals, vestibule, and cochlea. The vestibule is found at the center of the bony labyrinth, and the semicircular canals are connected with the vestibule. There are three semicircular canals inside the bone: superior, posterior, and lateral. On the other side, the vestibule is connected with the cochlea, which is a spiral-shaped duct.
The membranous labyrinth is located within the cavity of the bony labyrinth. In the vestibule, there are two compartments: the utricle and the saccule. The utricle along with the membranous semicircular ducts form the vestibular labyrinth. The saccule and cochlear canal form the cochlear labyrinth. All these cavities are filled with the liquid substance endolymph. On the other hand, the perilymph fills the vestibular and tympanic canals of the cochlea.
Some regions of the labyrinth contain receptor cells that can sense the changes in body speed and position, and the sound via the organ of Corti. These receptors respond to changes in angular movements, gravity (standing or lying horizontally) and acceleration of speed. The inner ear is innervated by the vestibular nerve (VIII).
3. Taste
The sense of taste is responsible for discerning the different flavors by receptor structures known as taste buds.
Taste buds
Taste buds are structures that recognize molecules responsible for flavors. They are found in the tongue papillae and in other locations of the oral cavity, such as the palate and epiglottis. The cells of taste buds are organized like an onion (without leaves), with the apical part in contact with the oral cavity (Figure 5). The receptor cells of the taste buds transmit the information to primary sensory axons that connect with the encephalon.
4. Olfaction
The olfaction process begins in the olfactory epithelium, located in the deeper part of the nasal cavity, near to the cranial bone (Figure 6). Other olfactory receptor neurons are found in the vomeronasal organ. In non-human vertebrates, there are other olfactory structures, like the septal organ of Masera and the organ of Grueneberg.
Main olfactory epithelium
The olfactory epithelium consists of neurons with transmembrane receptors that are activated by odorous molecules and join their axons to form the olfactory nerve (I) (Figure 7). This nerve crosses the cranial bone and ends in the olfactory bulb. The main olfactory epithelium is pseudostratified and about 1 cm2 in humans.
5. Skin senses
The skin, the largest sensory organ of the body, has several types of receptors that gather information from the external environment: mechanical (touch, pressure, vibration), temperature, and pain receptors (mechanical and chemical damage) (Figure 8).
Free nerve endings may function as mechanoreceptors (touch), nociceptors (pain), or thermoreceptors (temperature). They are distributed in both the epidermis and dermis. Encapsulated receptors are nerve endings wrapped by other cells. They are mostly mechanoreceptors, although a few function as thermoreceptors. Encapsulated receptors are more often found in the dermis.
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Bibliography ↷
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Salazar I, Sánchez-Quinteiro P. (2019). The risk of extrapolation in neuroanatomy: the case of the mammalian vomeronasal system. Frontiers in neuroanatomy. 3: 22.
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Peripheral nervous system