The sensory systems are specialized components of the nervous system that are responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. The system is stimulated by sensory receptors; these are the organs which trigger action potentials on a sensory neuron in response to a specific type of stimulus. There are three basic classifications of sensory receptors. The first is somatic receptors: receptors in the skin, muscles, and tendons. Secondly, the visceral receptors. These are receptors in the internal organs. Finally, special receptors are the receptors positioned in specific locations. Sensory receptors can also be classified by what it is they are detecting. Mechanoreceptors respond to movement. Thermoreceptors respond to heat or cold. Photoreceptors respond selectively to light. Chemoreceptors respond only to chemicals. Nociceptors respond to pain or excess stimulation. The final way to classify receptors is by either simple or complex. This is not to suggest that any receptors are “simple” but rather that they are just slightly less simple than “complex” ones. Simple receptors are fairly small and are widely distributed throughout the body. Complex receptors govern the five special senses of taste, smell, hearing, balance, and vision. 
- Main Article: Olfactory system
The olfactory system plays a very important role in a person’s life. It affects emotions, reproductive and maternal functions, aggression, and food selection. The nose contains the receptors for the olfactory system. They are found in what is called the olfactory epithelium, located in the superior part of the nasal cavity. Each individual has two olfactory bulbs, one located in each nostril. In each nostril there is a bone, which protects the olfactory bulbs. There are holes in the cribriform plates called olfactory foramina which allow the axons of the olfactory neurons to reach the olfactory bulb. The other end of the olfactory neuron, the side exposed to the air inside the nasal cavity, has olfactory vesicles which end in olfactory hairs, encased in a mucous layer. Each olfactory neuron is bipolar and has one dendrite and one axon. Olfactory neurons are extremely unique in that they are not permanent and can be replaced by basal cells, unlike any other variation of neuron. Unlike other sensory systems, signals transmitted from the olfactory hairs travel directly to the primary sensory cortical areas and limbic system without ever involving the brain stem. 
In order for a substance to be smelled, it must pass several qualifications. Firstly, for a chemical to be smelled, it has to be airborne. A volatile substance is one that can evaporate into a vapor so that it becomes airborne. This means that the only substances able to be smelled are those which are volatile. There are some volatile substances, however, that a person cannot smell. This is because the substance must not only be volatile, but it must also be able to reach the olfactory hairs. In order for this to happen, the chemical must transport through the watery, mucous layer. This means that the substance must be at least somewhat water soluble. On top of that, the chemical must also be moderately soluble in lipids so that it can penetrate through the plasma membrane. If the chemical meets all of these requirements and is indeed able to reach the olfactory hairs, it will bind to a receptor molecule on the hair. The receptor will then generate an action potential. It is believed that smell is a result of the combining of as many as fifty primary sensations. It does not require much stimulation to create an action potential. This allows people to smell things that are floating even at a very, very low concentration in the air. However, after the olfactory hairs have been stimulated continuously for even a short period of time, the action potentials are no longer sent.
- Main Article: Visual system
The visual system is responsible for a person’s sight; detecting light, darkness, and color. The primary organs involved in sight are the eyes. The eyes have many different components which allow them to perform the remarkable task of providing visual imagery. The parts of the eye are assorted and equally varied in their purposes. The cornea is tissue which covers the area of the eye over the pupil and allows light to enter; it is therefore transparent. Next is the iris. The iris is identifiable as being the colorful portion of the eye; however it serves a higher purpose than just giving the eye its beauty. The iris controls the amount of light which enters into the eye. It is composed of smooth muscle which will constrict (condense) in bright light and dilate (expand) in dim light. The pupil is the black colored portion of the eye directly below the iris. The pupil will appear to grow larger and smaller, but, as stated previously, this is an illusion produced by the iris’s constriction and dilation. Light enters the eye through the lens. The lens focuses light before it passes to the retina, covering the back of the eye. The retina is full of blood vessels and also houses photoreceptors called rods and cones. There are about 120 million rods and 7 million cones located in the retina of each eye. There is one portion of the retina called the optic disk which has no photoreceptors. This is the part of the eye where the blood vessels enter the eye. It is also where the axons of the neuron in the eye exit to join the optic nerve, sending signals to the brain. This hole in the retina results in a blind spot in the eye. The brain automatically compensates for this blind spot and fills it in. The portion of the eye that gives us the sharpest image is the fovea centralis. This is located in a small pit in the macula lutea, a small yellow spot in the center of the retina.
Now that the structures of the eye have been established, it is important to explain their functions. There are three main steps to an eye processing an image. First, the light focuses on the retina where it stimulates the photoreceptors. Second, the photoreceptors have a reaction to the stimulus. Third, action potentials are created as result of the stimulus that are sent to the visual cortex, where they are interpreted.
The rods and cones, explained above as being located in the retina, are stimulated by the light entering the eye. Rods are sensitive to very diminutive amounts of light and therefore perform the majority of the work in low-light conditions. Though sensitive to light, rod cells cannot distinguish colors. Thus, information sent to the brain by rod cells is construed only in varying shades of gray. It is for this reason that when a person is in a room with low light levels, they have a more difficult time deducing the colors of their surroundings. Unlike rods, cones require vast quantities of light to operate properly. They are responsible for enhancing the sharpness of an image. They are also able to distinguish the wavelengths of the light that hits the retina, thereby determining color.
In the process of sending their images to the brain, the rods and cones merge with bipolar neurons, which in turn, merge with ganglion cells. Ganglion cells are sorted into two primary classifications: midget cells and parasol cells. Midget cells are called such in respect to their very small dendritic, or arching, arbors and have accordingly minute receptive fields. Likewise, parasol cells have large dendritic arbors and, consequentially, large receptive fields. Midget cells are receptive to small, slow moving stimuli and are not very sensitive to low contrast, but are, however, able to identify color. Parasol cells, however, are receptive to large, fast-moving stimuli and are very sensitive to low contrast; they, however, are not sensitive to color. All colors in the eye are perceived through blue, red, and green photoreceptors. Any color that is observed besides these three primary colors come as the result of the photoreceptors working together. 
Action potentials created by the rods and cones are processed to only a certain extent by the association neurons in the retina before they travel down the ganglion cell axons into the optic nerve. The majority of the action potentials travel to the thalamus where they join with other neurons and travel on to the visual cortex. Those that do not travel to the visual cortex go directly to the midbrain. This constitutes for part of the visual reflex system that allows people to react quickly, subconsciously, to visual threats.
- Main Article: Auditory system
The auditory system aids in hunting, warfare, defense, parenting, and general survival for people even in modern times. Sound can be heard through a process which occurs in the ear. The ear has three main sections: the outer, the middle, and the inner ear. Sound waves are vibrations in the air. These vibrations journey down the auditory canal and vibrate the tympanic membrane, the end of the external ear. When the tympanic membrane begins to citrate, it converts the original vibrations of the air to vibrations of the membrane. From there, the tympanic membrane’s vibrations trigger the first of the three auditory ossicles: the malleus. The malleus is connected through a tiny joint to the incus, which is attached to the stapes, both of which vibrate in their turn as result of the vibrations of the bone preceding them. When the stapes vibrates, it vibrates the flexible oval window. The oval window acts as a wall to keep the fluid from the inner war from leaking into the air-filled middle ear. The oval window leads to the scala vestibuli, a tube filled with a fluid called perilymph. The scala vestibuli reaches to the tip of the cochlea where it connects to another tube called the scala tympani, which also contains perilymph, via a junction called the helicotrema. The scala tympani curves all the way back around to reach the round window, just underneath the oval window. The scala vestibuli and the scala tympani are separated down the center by two membranes: the vestibular membrane and the basilar membrane. These membranes encompass the cochlear duct which houses the spiral organ, or the organ of Corti, which is the actual “hearing” organ. In the cochlear duct is a liquid called endolymph. The tectorial membrane floats in this fluid and touches hair cells on the spiral organ.
The original sound vibrations, have now been converted through the tympanic membrane, vibrated the auditory ossicles, traveled through the oval window, and are being sent down the perilymph in the inner ear. If the vibrations of the perilymph have originated from a sound with a high enough pitch, it will cause a vibration in the basilar and vestibular membrane in the scala vestibuli. As the tectorial membrane vibrates, it brushes against hair cells in the spiral organ. Then activated, the hair cells trigger action potentials in their sensory neurons. Contrastingly, if the vibrations are of a low-frequency origin, they cause a vibration in the basilar membrane near its point, once again triggering a vibration in the tectorial membrane, which brushes the hair cells on the spiral organ, which results ultimately in action potentials. Essentially, the higher-frequency a sound is the closer to the oval window the basilar membrane will vibrate, or if the sound is of low-frequency origin, the farther from the oval window the basilar membrane will vibrate. 
- Main Article: Gustatory system
The gustatory system, though still intensely complicated, is considered to be one of the simpler sensory systems. Taste is not only often pleasurable, but also plays a key role in survival. Whether warning of bitter poisons or encouraging the necessary act of consuming food, taste plays an important role in a person’s life.
The tongue houses the sensory receptors for the gustatory system. The surface of the tongue is composed of tiny little bumps called papillae. There are four different varieties of papillae: fungiform, filiform, foliate, and circumvallate. Fungiform papillae are spread irregularly over the entire surface of the tongue. Filiform papillae are the most plentiful of all the types of papillae. Foliate papillae are situated in the sides of the tongue and contain the most sensitive receptors. Finally, circumvallate papillae are the largest but least numerous of the papillae and are located on the back of the tongue. The tongue is also covered with taste buds, but, contrary to popular opinion, they are located in other areas as well. They can also be found on the palate, the lips, and the pharynx (throat), especially in children. It is estimated that each person has approximately ten thousand taste buds. Taste buds are located in the circumvallate, fungiform, and foliate papillae, being most sensitive in the foliate papillae; they are not present in the filiform papillae. Taste buds are called such due to their flower bud-like appearance. Inside each of these floral-shaped bumps are two different types of cells: the gustatory cells and the supporting cells. Each taste bud has a hole in its center appropriately named the taste pore. Protruding from the taste pore are the gustatory hairs. Taste sensory cells are among the shortest-lived cells in the body. During their brief existence of a few days, they migrate from the outside of the taste bud to its center, where they then die. Nerve fibers running out of the taste buds release neurotransmitters when stimulated. These stimuli generate an action potential on an associated afferent nerve. Fibers with smaller diameters associate with only one sensory cell, while larger fibers associate with two or more. Any given sensory cell may receive contacts from up to as many as thirty nerve fibers. Equally impressive, a single nerve fiber can innervate up to nine separate taste papillae. The afferent fibers carry the impulses directly from the tongue to the brain stem via either the chorda tympani branch of the seventh cranial verve, the lingual branch of the ninth nerve, or the pharyngeal branches of the tenth nerve. The chorda tympani works in coordination with the anterior (front) 2/3 portion of the tongue and the lingual works with the posterior (back) 1/3. In addition to taste signals, these nerves also transport information regarding temperature and texture. From there, the nerve impulses are transferred between secondary cells in the nucleus tractus solitarius, which then transport them to the brain. 
Now that the anatomical parts and functions of the parts of the tongue have been discussed, it is time to digress about the actually process of tasting. Food is able to be tasted after it has been dissolved in a person’s saliva and which then enters into the taste pore. In the taste pores, the chemicals dissolved in the saliva bind to chemical receptors in the gustatory hairs, which release neurotransmitters to stimulate an action potential on the afferent nerve. There are four sensations of taste. A person’s ability to taste any given particle of food is determined upon the combination of these four sensations in coordination with their sense of smell. The four tastes detected by the human tongue are: sweet, salt, sour, and bitter. Theoretically, any taste bud can detect all of these tastes; however, some detect some tastes more predominantly than others. Therefore, these four tastes are divided specifically into four different areas of the tongue. The tip of the tongue is where the taste buds reside that respond best to the sweet sensation. However, it also contains some taste buds that respond well to salt. The predominate sour-tasters are located on the back sides of the tongue. Directly in front of them reside the salt-sensitive taste buds. Lastly, the bitter sensing taste buds are located primarily at the back of the tongue. 
The taste of something, as well as the strength of its flavor, can vary depending on a few variables. Taste receptors adapt to new tastes fairly quickly, therefore, the flavor of some food may lose its definition after a few bites. Temperature can also affect the intensity of something’s flavor. The warmer something is, the easier it is for a person to taste it. Likewise, the colder a substance is, the longer it takes for the taste bud to produce action potentials, thus restricting the concentration of the food's flavor.
- Main Article: Somatosensory system
There are two different types of somatic receptors in the somatosensory system which join together to allow a person to feel sensations such as touch, pain, temperature, and proprioception (the positioning of joints and muscles). The first type is the cutaneous receptors, which are receptors in the skin. The second type is proprioceptors, receptors in the muscles and tendons. 
There are different types of cutaneous receptors. Free nerve endings are the simplest and most common receptors. In these receptors, nerves rise to the epidermis where they branch out until they end. Such nerves respond to heat, cold, movement, itch, and pain. There are also hair follicle receptors which go deep into the dermis. Also deep in the dermis are the Pacinian corpuscles. These are pressure and vibration receptors and resemble miniscule onions. Ruffini’s organs can also be found in this part of the dermis. They respond to pressure of the surface of the skin and the stretching of the skin. Other cutaneous receptors are the Meissner’s corpuscles which are found throughout the dermal papillae. These assist in two-point discrimination, or being able to distinguish how many points of pressure there are within a specific space.
There are only two types of proprioceptors. Muscle spindles are the first type. Located within the skeletal muscle, these receptors respond to the stretching of muscle. The Golgi tendon organ is found in the tendon and responds only to tension. Together these receptors provide the brain with the information it requires to determine whether a muscle needs to be relaxed or contracted. Technically, this is called a sense of body position.
- Basic and Clinical Neuroscience Lectures-Visual System by Vincent P. Ferrera Lecture
- Wile, Jay L., Shannon, Marilyn M., The Human Body: Fearfully and Wonderfully Made! 2001. Published by Apologia Educational Ministries, Inc. Anderson, IN. Printed by CJK. Cincinnati, OH. Sixth Printing 2008. p.257-261
- Disorders of Taste and Smell by Donald Leopold Olfactory
- Wile p.264-266
- Wile, p.276-285
- Sensory system by Duke University Medical Center, Department of Pathology.
- Auditory system Joe Steinmetz and Glen Lee.
- Wiles p.273-275
- Gustatory and Olfactory Senses Michael D. Mann.
- Wiles p.267-269
- Somatosensory system by Darryl L. Daley
- Wiles p.261-264