The human body is one of the most complex living things in the universe, and there are certain factors that can cause only the human body to react in a way no other organism would. The human body is made up of dozens of organs and tissues, but none are quite as interesting and unique as the four special senses: smell, taste, sight, and hearing. Special senses, which include the eye, the ear, and the senses of smell and taste, all work together to perform specific functions in order to make the human body respond and react to certain environments unlike any other organism on Earth.
One of the most vital of the special senses is the eye, which allows humans to see. Eyes are the most complex of all the special senses, and havehas been the most researched of all four. Eyes are sphere-shaped and most of it is not visible to us at first glance. Eyes are held in place by several muscles and fat structures. The eyeball is made up of three layers, or walls, which are the fibrous layer, vascular layer, and sensory layer. These layers provide protection and strength for the eye. The first and outermost wall of the eyeball is the fibrous layer.
The fibrous layer consists of the protective sclera and the transparent cornea. The sclera is the most visible layer in the eye, and is what is referred to as the “white of the eye.” At the front of the sclera, there is a wide, clear section where the cornea sits. The cornea is supplied with much nerve endings, which make it the most sensitive and vulnerable part of the eye. The cornea is very durable and strong, and it is able to repair itself on its own after it has been damaged. Beneath the fibrous layer sits the vascular layer, which is also the middle layer. The vascular layer consists of three distinguishable regions.
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Most posterior is the choroid, which contains a dark pigment which prevents too much light from entering and damaging the eyeball. The choroid forms two smooth muscle structures, the ciliary body and the ciliary zonule, which then both form the iris. The iris is formed by smooth muscle fibers, which act like the diaphragm of a camera. A small, rounded opening in the iris is the pupil, where light passes through into the eye. Pupils ensure that neither too much or too little light enters the eye by adjusting to certain environments through shrinking and growing in size. Beneath the vascular layer lies the sensory layer. The innermost of the sensory layer is the retina. The retina contains a pigmented layer, which is composed of pigmented cells that stop too much light from scattering in the eye.
The retina also contains an inner neural layer, which contains millions of the receptor cells. Receptor cells are the rods and cones of the eye. Rods and cones are also photoreceptors, because they respond to light. The photoreceptors allow electrical signals to pass through a two-neuron chain, called the bipolar cells and then ganglion cells, before they leave the retina through the optic nerve. The optic nerve impulses are then transported to the optic cortex, which all results in vision. The retina contains a lens, which is a flexible biconvex crystal-like sculpture, where light entering the eye is focused on the retina. The lens flex and function to make the eye able to see from far away and up close. The lens also functions to divide the eye into two segments, or chambers.
Anterior to the lens sits the anterior (aqueous) segment, which contains a clear watery fluid called aqueous humor. Posterior to the lens sits the posterior (vitreous) segment, which is filled with a gel-like substance called either the vitreous humor or the vitreous body. The vitreous body is very important, because the gel-like substance to it prevents the eyeball from collapsing on itself internally. Aqueous humor, which is very similar to blood plasma, helps mainly to maintain the amount of pressure in the eyeball.
The aqueous humor also provides nutrients for the avascular lens and cornea. There are numerous physiological functions of the eyeball and vision. Accomodation is one of the major physiological functions of the eye, because it gives the eyeball the ability to focus specifically on close objects, specifically those that are less than twenty feet away. Accommodation occurs through the light-bending activity of the eye, where the eye allows light to enter itself and scatter, so that the lens bulge more to make vision more possible.
Through this light-bending process, the eye is able to adjust itself to see close up and far away without an issue. However, if the lens are too strong or too weak, vision problems occur (pg. 287). The internal and external eye muscles are necessary in order to make the eye function properly. The external eye muscles control movements and make it possible to follow moving objects, like a finger, a car, or a ball. The external eye muscles are also responsible for convergence, which is the reflexive movement of the eyes medially when we view close objects. Convergence occurs when both eyes are aimed toward the near object being viewed (pg. 290).
There is one more physiological function of the eye and vision, and it occurs when the eyeballs are exposed to bright lights. When this happens, the pupils immediately constrict, this is called the photopupillary reflex. This protective reflex prevents excessively bright light from damaging the delicate receptors (pg. 290). Without the eye muscles functioning the proper way, the daily lives of humans would be much more difficult and less comfortable. Without them, it would be difficult to read, follow moving objects, and possibly too much or too little light would enter the eye and prevent vision from occurring. Hearing is one of the most complicated and complex organs in the human body. There are several regions of the ear, each home to several different parts and components.
The ear can be split into three main regions: the external ear, middle ear, and internal ear. The inner ear contains a bony maze, which contains the organs that allow hearing and equilibrium to occur. Equilibrium can be distinguished between static and dynamic equilibrium. The equilibrium responds to various head movements, and therefore it is hard to describe because a human is not able to hear, see, or feel the sense. The equilibrium receptors of the inner ear are all called the vestibular apparatus, which can be divided into two arms. One arm is responsible for monitoring static equilibrium, while the other arm is responsible for monitoring dynamic equilibrium. Maculae are receptors of the vestibule that are within the membrane sacs that are essential and vital to our sense of static equilibrium.
Maculae help to keep our heads erect by reporting on changes in the position of the head in space with no respect to the pull of gravity when the body is not moving. Each maculae is a patch of receptor cells with their “hairs” embedded in the otolithic membrane. The otolithic membrane is studded with otoliths, which are tiny stones made of calcium salts. When gravity changes, the otoliths roll in response to the changes that have occurred. When the hair cells are activated, they send impulses along the vestibular nerve to the cerebellum of the brain, informing it of the position of the head in space.
Dynamic equilibrium receptors are found in the semicircular canals of the inner ear, respond to angular or rotary movements of the head, rather than to straight-line movements like that of static equilibrium. A swollen region at the base of the membranous semicircular canal is known as the ampulla. Within the ampulla is a receptor region called a crista ampullaris, or simply known as crista. Crista consist of a tuft of hair cells which are covered with a gelatinous cap called the cupula. As you move your head in an archlike or angular motion, the endolymph in the canal lags behind. As the cupula drags against the stationary endolymph, it acts like a swinging door with the body’s motion.
Whenever a sound wave enters the ear, the wave sets the cochlear fluids into motion. These pressure waves set up vibrations in the basilar membrane, where the receptor cells, or “hairs,” are stimulated. Once the hairs are stimulated, they transmit impulses along the cochlear nerve to the auditory cortex in the temporal lobe, where the actual hearing interpretation occurs. Taste and smell are the final of the special senses, and there are several different components that cause the tasting process to occur. The human body has thousands of olfactory receptors in the roof of each nasal cavity, which are the receptors for the sense of smell. Olfactory receptors help to intensify the sense of smell.
Neurons, known as the olfactory receptor cells, are equipped with olfactory hairs, which are long cilia that protrude from the nasal epithelium and are continuously bathed by a layer of mucus secreted by underlying glands (pg. 299). Once the olfactory receptors are stimulated, they transmit impulses along the olfactory filaments, which are bundled axons of olfactory neurons that collectively make up the olfactory nerve. The olfactory nerve conducts the impulses to the olfactory cortex of the brain (pg. 299).
With all of these components working in unison with each other, the sense of smell is allowed. The tongue is the main organ that allows the special sense of taste. The tongue is the home of thousands of tiny taste buds, or specific receptors for the sense of taste. There are also many taste buds not located on the tongue, but in other parts of the oral cavity. A few of the taste buds are scattered on the soft palate, superior part of the pharynx, and inner surface of the cheeks (pg. 300). The dorsal tongue surface is covered with small peglike projections, or papillae. The taste buds are found on the sides of the large round vallate or circumvallate papillae, and on the tops of more numerous fungiform papillae. The specific cells that respond to chemicals dissolved in the saliva are epithelial cells called gustatory cells.
Their long microvilli, or the gustatory hairs, protrude through the taste pore, and when they are stimulated, they depolarize and impulses are transmitted to the brain. Serving the anterior part of the tongue is the facial nerve. There are two other cranial nerves, which are the glossopharyngeal and vagus, which serve the other taste bud-containing areas. There are also basal cells found in the deeper regions of the taste buds (pg. 300-301). There are five basic taste sensations: the sweet receptors, sour receptors, bitter receptors, salty receptors, and Umami.
The sweet receptors respond to substances such as sugars, saccharine, some amino acids and some lead salts. Sour receptors respond to hydrogen ions, or the acidity of the solution. Bitter receptors respond to alkaloids. Salty receptors respond to metal ions in solution. Umami, which is Japanese for “delicious,” is elicited by the amino acid glutamate, which is responsible for “beefy” tastes and monosodium glutamate, a food additive (pg. 301). The special senses of the human body are what make it so unique and individual. They are one of the most complex parts of the body, with their many different components and parts that allow the special senses all to occur and take place. Without them, the human body would be a bland and boring thing, and there would be nothing to really distinguish it from some other organisms.
