In this page, we are going to deal with structures and systems responsible for fetching information coming from outside the animal body, the environment. These systems are normally known as senses. Although we will discuss them in other pages related to other animal organs, we give here a more detailed description. The more common animal senses for external environment information are sight, hearing, smell, taste and touch.
In vertebrates, the eye is the sensory organ for detecting visible light. It is an ovoid structure made up of several tissues that are able to project and focus the light onto a layer of neurons, the retina. Retina transforms the light into nerve impulses that travel through the optic nerve (cranial nerve II) to the encephalic geniculate nucleus and other structures, and geniculate nucleus sends most of the the visual information to the visual cortex, located in the posterior part of the encephalon.
Eyes are structures that detect the light reflected by objects, transduce it into electrical information that, after a local processing, is sent to other parts of the encephalon for combining with other neuronal information that altogether eventually change the animal behavior. Eyes are rounded and polarized structures. From the anterior to the posterior part, they are made up of cornea, anterior chamber, iris, ciliary muscles, crystalline, vitreous body, retina, choroid, sclerotic or sclera and optic nerve. These components are distributed in three concentric layers or tunics (excepting the crystalline): fibrous tunic, vascular tunic, and internal nervous tunic.
Cornea is the most external part of the eye so it is in contact with the air. It is a transparent structure that focuses the light and protects the eye surface. The optic properties of the cornea is a consequence of the arrangement and type of the collagen fibers it contains. There are no blood vessels in the cornea, and that is why it is relatively easy to transplant this part of the eye in surgeries.
The cornea is a sheet of tissue made up of five layers: corneal epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. Corneal epithelium is the most outer layer, it is stratified squamous epithelium, contains many nervous fibers, and can be easily auto-repaired. Corneal epithelium is laterally continuous with the epithelium of the conjunctiva. The Bowman's layer is located inmediatelly under the corneal epithelium. This layer is not found in all mammal species. Bowman's layer contains collagen fibers, but not elastic fibers. The stroma, found under the Bowman's layer, is the thicker layer of the cornea accounting for about 90 % of the thickness of the cornea. The stroma is connective tissue with collagen fibers, mainly types I and IV, arranged in layers with different spatial orientation between each other. There are also proteoglycans like chondroitin sulfate and keratan sulfate. Some cells are also present, such as fibroblasts and errant lymphocytes. Descemet's membrane lines the inner part of the stroma, and, actually, it is the basal lamina of the endothelium. The endothelium is the inner part of the cornea, and forms the anterior wall of the anterior chamber of the eye. The cornea is laterally connected with the sclera through a region known as corneal limbus.
The ciliary body is found behind the iris and performs two main functions: releasing vitreous humor and changing the shape of the crystalline to focus the light on the retina. It links the ora serrata of the choroid to the root of the iris, and is connected to the crystalline by a ligament. The ciliary body shows ring shape, and, in transverse sections, looks like a triangle. It is divided in two components: pars plicata and pars plana. Pars plicata is located close to the crystalline lens and is organized in finger like structures known as ciliary processes, whereas pars plana shows a flattened form. The smooth muscle of the ciliary body is known as ciliary muscle, which controls accommodation by changing the shape of the crystalline lens to focus the light on the retina. The internal part of the ciliary body is dense connective tissue with abundant elastic fibers and blood vessels. The ciliary epithelium is made up of two layers, the most internal one is pigmented and releases aqueous humor. The ciliary body, together with the iris, constitutes the uvea.
The iris is the structure or the eye that separates the anterior chamber from the posterior chamber, and is attached to the ciliary body through its periphery part. In the central area of the iris, there is an opening, known as pupil, through which the light can reach the crystalline lens. The pupil area of the iris is the closest part to the pupil and the periphery part is known as ciliary area. The iris is mostly highly vascularized loose connective tissue. No matter the diameter, blood vessels show the same organization. They do not contain muscle layer. The posterior part, the deepest one, of the iris is a highly pigmented two layered epithelium, which gives color to the eyes depending on the amount and arrangement of the pigment. The iris works as a adjustable diaphragm thanks to the activity of two muscles. One of them is the sphincter of the pupil that sets the diameter of the pupil. It is smooth muscle innervated by parasympathetic fibers of the ciliary ganglion and is arranged circularly. The other muscle is the dilator muscle that increase the pupil diameter (pupil dilation). It is composed of radially oriented myoepithelial muscles, which are innervated by sympathetic neurons of the superior cervical ganglion.
The crystalline lens is located behind the pupil and shows a transparent biconvex body. Covering the crystalline, there is a transparent thick external layer known as capsule, which contains a similar molecular composition than the basal lamina of other tissues. Below the capsule, in the superficial part of the crystalline lens, there is one-cell thick layer of cuboidal cells that is not present at the peripheral parts. Besides these cuboidal cells, most of the crystalline lens is made up of cells known as crystalline fibers because they are very long, up to 10 mm, but very thin, a few µm. These cells contain a high concentration of the protein crystallin, which account for almost 90 % of the total protein content of the crystalline lens and is responsible for the optical features. The cornea and crystalline lens work together to focus the light on the retina. The crystalline lens can be stretched by the muscles of the ciliary body changing the curvature and thus the focusing properties.
The vitreous body fills the cavity between the crystalline lens and the retina. It is a gelatinous substance with similar transparency to crystal glass and composed of an aqueous solution with abundant type II and XI collagen, and hyaluronan. It also contains some scattered cells known as hyalocytes.
The retina is the light-sensitive structure of the eye and the most internal tunica of the eye. It results from an evagination of the central nervous system during the embryo development. This evagination folds to get a cup-shape morphology with two layers: a pigmented external one and a nervous internal one, which is the retina. The retina is composed of photoreceptors, neurons that convert light into electro-chemical gradients, neurons that receive and process the information from photoreceptors, and neurons that send the processed information through the optic nerve to the encephalon. There are up to 10 layers of neurons in the retina, the most external one is the photoreceptor layer with two types of photoreceptors: cones and rods. Cones are specialized in perceiving colors, whereas rods respond to light intensity. Cones are more abundant in the fovea, area of the retina where the eye focuses the light. When we rotate the eyes to see an object, we actually are moving the fovea to receive the focused light coming from the object we are interested in. The axons of the optic nerve arise from neurons located in the innermost layer of the retina, the ganglionar layer. That is why these axons must cross the photoreceptor layer leaving a spot without photoreceptors, which is then a blind spot. We are not aware of this blind spot because our encephalon "hides" it from us. The pigment layer is the outermost layer that results from the evagination and does not contain neurons but pigment cells. The pigments avoid the light dispersion contributing to a more sharp vision. This layer is in close contact with the photoreceptor layer and is involved in the homeostasis of photoreceptors.
The auditory system is in charge of auditory perception, but it is actually composed of two subsystems: auditory and vestibular. The auditory component perceives sounds and transduces them into electrical signals. The vestibular component keeps the balance of the body and spatial orientation. The auditory system is spatially divided in three parts: external ear, middle ear and inner ear.
The outer ear is composed of the auricle (pinna) and the auditory canal, which communicates the external space with the tympanic membrane or eardrum. The auricle is usually oval and is mostly made up of elastic cartilage and integument (skin). It is delimited by a thin epidermis and many hair follicles, though it depends on the animal species. 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 glands known as ceruminous glands that release lipidic components. These susbtances get mixed with the secretion of the sebaceous glands forming altogether the earwax or cerumen. The internal part of the auditory canal gets inside the cranial bone, and both types of glands progressively desappear.
Middle ear is found follows the auditory canal. It is a cavity known as tympanic cavity inside the cranial temporal bone. The tympanic membrane separates the auditory canal from the tympanic cavity. Inside the tympanic cavity, there are three tiny bones (ossicles): malleus, incus, and stapes, and the muscles that move them. The Eustachian tube is also part of the middle ear, and connects the tympanic cavity with the pharynx. This connection allows to balance the air pressure of the oral cavity (atmospheric pressure) with that of the tympanic cavity. Middle ear is separated from the inner ear by the bony inner medial wall.
The function of the middle ear is to transform the air waves, which carry the sound information, into mechanical movements of the ossicles, which convey information to the inner ear. The process starts with the pressure of the air waves, coming through the auditory canal, on the tympanic membrane. Vibrations of the tympanic membrane move the ossicles of the middle ear: first the malleus ossicle, which is in contact with the tympanic membrane, second the incus, and third the stapes. The stapes transfers the information to the labyrinth of the inner ear (see below), where it creates currents of fluid. The communication between stapes and labyrinth is through the oval window (fenestra vestibuli) and round window (fenestra cochleae) of the bone. In the tympanic cavity, there are two muscles, one attached to the malleus and the other to the stapes. Malleus keeps the tympanic membrane stretched and stapes compensates the movement of the incus. Both muscles are important for alleviating the higher vibrations and protect against very loud sounds.
The inner ear is the labyrinth. There are two parts: the bony labyrinth and the membranous labyrinth. The bony labyrinth is inside the temporal cranial bone and comprises the semicircular canals, vestibule and cochlea. Vestibule is in the center of the bony labyrinth, and semicircular canals are connected with the vestibule by both of their ends. There are three semicircular canals inside de bone: superior, posterior and lateral. The inner cavity of the canals are continuous with the cavity of the vestibule. At the other side, the vestibule is connected with the cochlea, which is a spiral-shaped conduct.
The membranous labyrinth is located inside de cavity of the bony labyrinth. In the vestibule, there are two compartments: the utricule and the saccule. The utricule cavity is connected with the cavities of the membranous labyrinth, whose membrane lines the internal surface of the semicircular canals. The utricule plus membranous semicircular conducts form the vestibular labyrinth. The saccule is connected with the cochlear canal, which extends through the interior of the cochlea. Saccule plus cochlear canal form the cochlear labyrinth. All these cavities are filled with the liquid substance endolymph, and the membrane that encloses the endolymph. Perilymph fills also the vestibular and tympanic canals of the cochlea.
Some regions of the labyrinth contain receptor cells that can sense the change of speed, position, and sound (organ of Corti by means of the ossicles of the middle ear). These cells transduce the change of movement of the liquid around them (endolymph) into electrical signals. The transduction is done by cellular apical structures known as stereocilia, which actually are modified microvellosities, and by real cilia known as kinocilium. In the ampullary crests, located between the semicircular canals and the utricule, the receptor cells (hair cells) sense angular movements of the head thanks to the bending of the stereocilia and kinocilia pushed by the current of the endolymph, and convert the mechanical change into electrical information. In the saccule and utricule, there are also receptors that can sense gravity (vertical or horizontal position of the body) as well as the lineal speed changes.
The organ of corti is located in the cochlear canal, more precisely in the scala media. It is composed of an epithelial layer containing ciliated cells. The movements of the inner ear ossicles are converted in endolymph currents, which bend the cilia and kinocilia and create the electrochemical gradients that carry the sound information.
The inner ear is innervated by the vestibular nerve (VIII). This nerve is divided in two branches: vestibular and cochlear. The vestibular branch innervates the cell receptors of the labyrinth and vestibule, whereas the cochlear branch innervates the auditive receptors. Vestibular nerves have their neuronal soma in the vestibular ganglia (there are two), which are located outside the labyrinth. The ganglion of Corti or spiral ganglion forms the cochlear branch and is located in cochlea.
The gustatory perception recognizes dissolved molecules that enter the mouth, normally food. However, the taste of food depends mostly on smelling, i.e. the olfactory sense. The structures in charge of perceiving gustatory information are the gustatory buttons, which mainly localize in the tongue papillae and in other locations of the oral cavity as well. Papillae are protrusions of the tongue surface, and are named according to their morphology: filiform, fungiform, foliate and calyciform. Gustatory buttons are found in the tips of the fungiform papillae and in the lateral and deep walls of the calyciform papillae.
The gustatory receptor cells are in the gustatory buttons. There, they make synaptic contacts with primary sensory axons, which enter the encephalon through the IX and VII cranial nerves. These nerves innervate the nucleus of the solitary tract, which in turn innervates several thalamic nuclei. From the thalamus, the gustatory information is sent to the gustatory cortex.
Tongue is the organ where papillae wiht most of the gustatory buttons are locted. Briefly, the tongue is a striate skeletal muscle inside the oral cavity that moves food, and helps to shape sounds emited by some animals.
Gustatory buttons are structures that recognize molecules responsible for the taste. They are situated in the tongue papillae and in other locations of the oral cavity such as palate and epiglottis. The cells of gustatory buttons are organized like an onion (without leaves) with the apical part in contact with the external surface. There are three cellular types: support, neuroepithelial, and basal cells. Support or subtentacular cells are located at the periphery of the gustatory button. Neuroepithelial cells are more internally situated, there are about 10 to 14 per gustatory button, and they are receptors that recognized taste molecules and transduce the information into signals that are transmitted to the nerve terminals. These nerve terminals are in close apposition to the membrane of neuroepithelial cells. The basal cells, the third type, are located basal and in the periphery of the gustatory button, in contact with the basal lamina. Basal cells have been propposed to be stem cells that are able to divide, differentiate and replace the cells that die during normal renewing of the gustatory button cell population.
The information produced by receptor cells of the gustatory buttons, after they recognize taste molecules, is gathered by different nerves depending on the zone of the tongue where gustatory buttons are located. Thus, those at the anterior part (close to the tip) and medially are innervated by the facial nerve (VII), those located at the posterior part of the tongue and those at the pharynx epithelium are innervated by the glosopharyngeal nerve (IX), and those located at the larynx and epiglottis are innervated by the vagus nerve (X). These nerve enter the rhombencephalon (posterior encephalon) at different levels, but the information converges at the rhombencephalic nucleus of the solitary tract, from here it is sent to the thalamic centers and after that to the gustatory cortex. There are other lateral pathways and outputs from the gustatory cortex that associate the gustatory information with the rest of the relevant information for the organism. For instance, the taste is a mix of gustatory information and smell information, i.e. two different type of information coming from different receptors and different nervous pathways and processed separately.
Olfaction, the sense of smell, is probably the most ancient of the senses. It is involved in feeding, social communication, predation behavior, spatial orientation, offsprinq care, parental imprinting, etcetera. The high relevance of this sense is clearly shown when the olfaction system is damaged (for instance, in rats), it causes changes in sleeping patterns and sexual behavior, increase of agresivity, poor care of progeny, and anxiety. The basic components of the olfactory system have been conserved by evolution during millions of years. It is of interest that insects show a similar basic molecular mechanism during olfaction as vertebrates, and the first cellular components of the olfactory system are similar.
Olfaction starts in the olfactory epithelium, located in the deeper part of nasal cavity, close to the cranial bone. Olfactory epithelium contains neurons (olfactory receptor neurons) with transmembrane receptors that recognize olfactory molecules. These neurons have axons that form the olfactory nerve (nerve I), which crosses the cranial bone at the so-called cribriform plate and enters the olfactory bulb, the rostral most part of the encephalon. The axons of the olfactory nerve divide in fascicles and their ends form round structures known as olfactory glomerulli. In each glomerule, the primary olfactory information is transmitted to other neurons, mainly mitral cells. From the olfactory bulb, the olfactory information sent to other deep encephalic areas where it is processed and confronted with other types of information. For instance, the smell of food has not the same impact on the behavior of the animal when the it is hungry compared when the animal is satiated.
Olfactory receptor neurons of the nasal cavity are distributed in different structures. Most of them are located in the main olfactory epithelium, close to the cribriform plate plate. Olfactory receptor neurons are also found in the vomeronasal organ, which in humans is located in a bone cavity of the septum, at the base of the nasal cavity. However, fish lack of vomeronasal organ. In vertebrates, there are other olfactory structures like the septal organ of Masera, and the organ of Grueneberg. It is thought that each of these structures sense different olfactory information.
Main olfactory epithelium
The main olfactory epithelium is pseudostratified, made up of several types of cells, and about 1 cm2 in humans. The main type of cells is the olfactory receptor neurons, which recognize the olfactory molecules and then produce an electric response by depolarization. They are bipolar with an apical part bearing cilia or microvilli, where the transmembrane olfactory receptors are located. Each olfactory receptor neuron expresses only one type of transmembrane olfactory receptor and there are hundreds of different transmembrane olfactory receptors. Thus, in the main olfactory epithelium, there are hundreds of olfactory receptor neuron populations which are able to sense a particular and different olfactory signal. At their basal part, the olfactory receptor neurons have an axon that leaves the main olfactory epithelium, joins to others olfactory axons, and together cross the cribiform plate in their way to the olfactory bulb. Among the olfactory receptor neurons there are the supporting or subtentacular cells, which are involved in the support and, likely, in the electrical isolation of the olfactory receptor neurons. Other type of cells are those forming the Bowman's glands that synthesize and release mucous substances that cover the outer surface of the epithelium. Finally, at the basal part of the main olfactory epithelium, there are the basal cells. The function of the basal cells is to divide and replace the other cell types of the epithelium. This is one salient feature of the olfactory epithelium, a permanent turnover where the cells died and are replaced by new ones.
The skin, the larger sensory organ of the body, has several types of receptors that get information from the external environment: mechanical (touch, pressure, vibration), temperature and pain (mechanical and chemical damages) receptors. Unlike other senses that gather the receptors in an organ, cutaneous receptors are scattered through the body skin as either free or encapsulated nerve endings. Different parts of the body show different receptor densities. The sensory mechanism of these receptors is similar, a sensory input changes the form or affects the nerve membrane so that the electrochemical membrane potential is modified, and this is transformed in an action potential that travels via the nerve fiber to the neuronal body. From the neuronal body, the information is sent to the central nervous system.
Cutaneous receptors can be classified regarding their location, type of stimulus they are responding to, or how their nerve endings are organized.
Free nerve endings. They are the naked final part of the nerves, not wrapped with myelin (myelinzation stops before these final segments) nor with any other structure. They can be mechanoreceptors (touch), nocireceptors (pain) and termoreceptors (temperature), and are distributed in both epidermis and dermis.
Free nerve endings may be associatated to some types of cells. For instance, Merkel disks are free endings cup-shaped to partially coat the Merkel cells of the epidermis. Merkel disks are mechanoreceptors with hight-sensitivity and slow adaptation to the stimulus, so that they can inform about long-lasting stimuli. They are abundant in the fingertips and lips, althouth they are also found scattered in the skin of other parts of the body. There are other free nerve endings, known as peritrichial endings, around the hair follicles. Peritrichial endings are mechanoreceptors usually showing a fast adaptation to the stimulus, i.e. they respond to changes of the stimulus, even slight changes, but not to sustained stimulus.
Receptors involved in pain transmission are very thin and are stimulated by molecules released by damaged cells. Analgesic works avoiding the activation at different levels depending on the drug.
Encapsulated receptors. The nerve endings are wrapped by other cells, commonly by connective tissue cells, which are arranged as onion leaves. Most of them are mechanoreceptors, although some are termoreceptors, and they are more frequently found in the dermis.
Meissner's corpuscules. They are encapsulated receptors found in the dermis, usually in the dermal papillae of skin lacking hair follicles. The capsule is made up of several layers of connective tissue and the nerve endings are twisted among these layers. A mechanical input makes the layers to be separated from each other triggering a change in the nerve ending membrane. These receptors show slow adaptation, so that even if the stimulus persists the transmission of the information stops. This ability is very useful to discern the movement of objects over the skin, but forget about them if they stay quiet. Meissner's receptors are so sensitive that the brain can discriminate the spatial direction of the movement and texture of the object. They are abundant in the fingertips and lips. Those located in the skin of genitals and nipples are known as genital's corpuscles.
Pacinian corpuscles. They are encapsulated receptors found in the deep dermis, pleura, nipples, pancreas, tendons, penis and clitoris, and in more internal locations like urinary bladder and joints. They are stimulated by fast movements like vibrations and by strong pressure forces. The size and morphology of Pacinian corpuscles is variable, and it looks like that the more deep is the location the larger they are. Golgi-Mazzoni corpuscles show similar organization to the Pacinian corpuscles, but they are more simple and are mainly located at the fingertips.
Ruffini corpuscles. They are encapsulated receptors found deep in the skin. They show slow adaptation, so that they respond to long-lasting stimuli. These receptors are also located in the joint capsules, where they process information about the rotation the bones. Besides mechanical stimulus, they can also sense temperature and pain.
Krause corpuscles. They are encapsulated receptors found in the dermis and oral cavity, where they sense pressure and temperature. They show fast adaptation.