What Are The Sense Organs?
The ancient philosopher Aristotle is credited with classifying the five sense organs into what we now know as the "five senses." These senses are: sight, hearing, taste, smell, and touch. Each of these senses involve organs who have highly specialized receptors designed to interpret different stimuli.
These cells have links to the nervous system and the brain, which then interprets the senses and tells the body how to react to the incoming stimuli. Sensing is a very primitive - yet evolved - function that's closely integrated into the nervous system.
Let's explore the five senses in greater detail, starting with sight, which is probably the most advanced of all the senses human beings have.
Sense Of Ophthalmoception - Eyes:
Sight, also known as ophthalmoception, is the way in which the eyes are able to focus and detect images of visible light. The eyes are the organ responsible for vision.
The complex structure of the eyeball involves a transparent lens that focuses light onto the retina. The retina is covered with two types of photoreceptors: rods and cones, which allow us light perception and vision, as well as color differentiation (the human eye can detect approximately 10 million colors) and depth perception.
Our eyes automatically adapt to different situations, which allow us to see in many different environments.
Rod cells (also known as "rods"): rod cells are the photoreceptor cells able to function in lower levels of light than the cone cells. Rod cells, unlike cone cells, are not sensitive to colors, but are sensitive to levels of light. The rod cells are responsible for the ability to see in the dark as well as the ability to see things that are located to each of our sides - our peripheral vision.
Cone cells (also known as "cones"): these photoreceptor cells are located in the retina. They are the types of cells that are able to determine color; cone cells are less sensitive to levels of light, but they function best in relatively bright light.
The optic nerve, located at the back of the eyeball, connects the eyes to the brain. The brain combines the input provided by both eyes and creates one single three-dimensional image. The brain even takes the image from the retina, which is upside down, and flips it right side up.
The perception of the eye is tremendous, and likely harkens back to a time when our ancestors had to rely upon their vision in order to survive in a more hostile world.
While we are in the dark, the rod cells (responsible for night vision) increase the sensitivity of the eye so we can detect objects in very dim light. Similarly, when we are in bright lights, our iris contracts, shrinking the aperture that allows light into the eye.
Problems With The Eyes:
As with all other organ systems, the eyes are prone to various problems that can develop over time or are present at birth.
Blindness: or the inability to use the eyes to interpret the world, can occur for a number of reasons: damage to the eyeball (most especially the retina), optic nerve, poisons, medications, or from strokes.
Cataracts: as a person ages, the proteins in the lens of the eye breakdown, causing the lens to be cloudy, blurring the vision.
Conjunctivitis: a condition that involves inflammation or an infection in the membranes that line the eyelids.
Daltonism: or color blindness - is a fairly common abnormality involving vision. Being colorblind makes differentiating between certain colors challenging.
Glaucoma: a group of eye conditions that can cause damage to the optic nerve, most commonly caused by increased intraocular pressure, is the second most common cause for blindness in the US.
- Open-Angle (Chronic) Glaucoma - the most common type of glaucoma in which the intraocular pressure increases over time, pressing on the optic nerve.
- Angle-Closure (Acute) Glaucoma - a medical emergency in which the aqueous humor of the eye is blocked, leading to a sudden increase of pressure within the eye.
- Congenital Glaucoma - this type of glaucoma is present at birth and is caused by abnormal eye development.
- Secondary Glaucoma - this type of glaucoma is caused by trauma, systemic diseases, eye diseases, and corticosteriods.
Retinal Detachment - this eye condition involves the separation of the retina (the light-sensitive membrane in the back of the eye) from the layers that support it.
Problems with the eye should be addressed during routine eye examinations performed by an ophthalmologist or an optometrist.
Read more about vision impairment.
Sense Of Audioception - Ears:
Hearing, also known as audition, is the sense of sound perception and vibrations through the ear.
The ears, the organ responsible for hearing, can perceive frequencies from a deep bass frequency of 16 cycles/second to a high-pitched 28,000 cycles/second.
The reason our ears have a cuplike shape is to direct sounds from the outside world toward the tympanic membrane, which, in turn, transmits vibrations toward the inner ear through a series of tiny bones located in the middle ear. These bones are called: the malleus, incus and stapes.
The inner ear, or cochlea, is a chamber that's shaped like a spiral, that is full of nerve fibers. These fibers then react to the vibrations and transmit impulses to the brain through a nerve called, originally enough, the "auditory nerve."
The brain then takes the input from both ears to determine where the sound is coming from and approximately how far away the sound is.
The ear is also responsible for the sense of balance and orientation in space. The inner ear contains a vestibular system, a series of three semi-circular canals, at approximate right angles. These chambers are filled with fluid and particles called otoliths.
The movements of these small otoliths over the hair cells within the inner ear signals the brain which interprets these movements as motion and acceleration.
Problems With Hearing:
As with other organs, the ears can develop problems that can affect hearing, acceleration, and motion.
Acoustic Neuroma: a benign and slow-growing tumor on the vestibular cochlear nerve, located behind the ear and under the brain. This nerve connects the ear to the brain. While the tumor is non-cancerous, it can cause nerve damage as it grows.
Ear Infections: ear infections can occur in the outer, inner, or middle part of the ear and are common in infants in children. Ear infections can be caused by allergies, colds, excess mucous and saliva from teething, infected adenoids, or tobacco smoke.
Hearing Impairment: is the partial or total inability to hear. Hearing loss or impairment occurs along a spectrum - some people experience mild hearing loss while others experience total hearing loss. There are three types of hearing loss:
- Conductive Hearing Loss - this type of hearing loss occurs when sounds do not reach the cochlea, or the inner ear, due to malformation of the ear canal, ear drum dysfunction, or malfunction of the bones of the middle ear.
- Sensorineural Hearing Loss - this type of hearing loss results from dysfunction of the cochlea, the nerve that transmits cochlear impulses to the brain, or from brain damage.
- Mixed Hearing Loss - is a combination of conductive and sensorineural hearing loss.
Ménière's disease: a disorder of the inner ear that can affect both hearing and balance in varying degrees. Ménière's disease is associated with hearing loss, vertigo, and low-pitched tinnitus.
Presbycusis: the slow loss of hearing that happens slowly over time as people age. Those with presbycusis have most difficulty with high frequency sounds, such as the sound of the voice. As hearing loss progresses, lower tones become more difficult to hear.
Read more about hearing loss.
Sense Of Gustation - Mouth:
Taste, or gustation, refers to the ability to detect the way certain substances stimulate our mouths when molecules from the food we eat, the drinks we drink, and the ways that food is broken down within the mouth.
Taste cells, called gustatory cells, are clustered within the taste buds located along the tongue, roof of the mouth, and the lining around the throat. When these taste cells are stimulated, they send messages through three specialized taste nerves into the brain where the brain interprets the specific taste.
Each taste cell expresses a receptor, which responds to one of the five basic taste qualities:
- Umami, or savory.
The precise number of taste buds varies considerably among people, and are scattered throughout the tongue. It's important to note that higher numbers of taste buds mean greater sensitivity. Generally speaking, women have more taste buds than men.
Taste quality is only a part of the way humans experience food - the common chemical sense, another chemosensory mechanism, uses thousands of nerve endings that give rise to sensations, such as the coolness of mint or the burning of a chili pepper. Other nerves give us sensations of heat, cold and texture.
The combinations of these mechanisms give rise toward "flavor." The sense of taste is often confused with "flavor" which is the combination of taste and smell. Flavor also depends upon temperature, smell and texture. The senses of smell and taste are very closely related.
Problems With Taste:
Taste disorders, which can be alarming to a person, are quite common, and often occur after an injury or illness. These taste problems include:
Ageusia: a condition in which a person is unable to detect any tastes whatsoever.
Dysgeusia: a taste condition in which a salty, rancid, gross, foul or metallic sensation persists; the condition is often accompanied with a burning mouth sensation and is more common among middle aged women.
Hypogeusia: a taste problem that results in a reduced ability to taste sweet, sour, bitter, salty, and umami.
Phantom Taste Perception: a lingering (often unpleasant) taste in the mouth while there is nothing actually present within the mouth.
Sense Of Olfaction - Nose:
The sense of smell, or olfaction, similar to our sense of taste, is part of our chemical senses, or "chemosensory system."
Specialized sensory cells - olfactory sensory neurons - are found in a patch of tissue located high within the nose. These small sensory receptors connected to the olfactory nerve. Each one of these olfactory sensory neurons expresses one odor receptor. Minute molecules around us, emitted by everything from coffee beans to lilac blossoms to dog poop stimulate these receptors. When these specialized neurons sense these molecules, the neurons send a message to our brain which, in turn, identifies the smell to us.
Smells reach the olfactory sensory neurons by one of two ways:
- The nostrils
- The channel that connects the roof of the throat to the nose
When we chew food, aromas from the food molecules are released that use the secondary channel - the one through the oropharynx. If/when that channel is blocked, by allergies or a cold, odors cannot reach these sensory smells and it's much harder to detect the flavor of our food.
Our sense of smell is also influenced by "common chemical sense," which involves thousands of nerve endings on places like the nose, eyes, mouth, and throat. This is why our eyes water when we smell an onion or we feel coolness when we smell mint.
There are seven different types of smell sensations:
- Musk - often used in perfumes
- Flower - sweet
- Camphor - mixture between menthol and cinnamon
- Ether - chemical smells
- Acrid - sharp, pungent, bitter smell
- Putrid - a rotting, decaying smell
Problems With Sense of Smell:
A smell disorder is a disorder in which people have lost their ability to smell or changes the way they perceive scent. A smell disorder can be temporary - caused by a head cold, or permanent if a part of the olfactory region is damaged. There are many reasons that someone could develop a smell disorder, but most are the result of an injury or illness. Smell disorders include:
Hyposomia: the reduced ability to detect odors.
Anosmia: no ability to detect odors.
Dysomnia: a distorted identification of smell.
Parosmia: alteration in perception of smell in the presence of a (usually unpleasant) odor.
Sense Of Tactation - Skin:
The sense of touch, also called tactation or mechanoreception is a perception that results from the activation of neural receptors on the skin as well as in the throat and tongue.
As the epidermis, the skin, is the largest organ in the body, our sense of touch is distributed throughout the body.
The skin is full of nerve endings that transmit sensations to the brain. Some parts of the body have a larger distribution of nerve endings, meaning that they are more sensitive than others. Fingertips and sexual organs have the greatest concentration of nerve endings.
Hairier parts of the body tend to be more sensitive as the hairs on the skin magnify the sensation as well as acting as an early warning system for the body.
Problems With Sense of Touch:
Tactile disorders, or disorders that affect the sense of touch, are rooted within the drain or CNS, and can be the the result of brain tumors, injuries or surgical procedures. Tactile disorders include the following:
Tactile Agnosia: tactile agnosia is a disorder in which a person cannot recognize an object by touch (but can identify object through other senses), often caused by lesions or tumors of the brain.
Tactile Aphasia: someone who has tactile aphasia cannot name an object after it's been touched, and can affect one or both hands. Someone who has tactile aphasia can often describe the function of the object. Someone who has Tactile Aphasia, however, is able to name an object based upon other senses.
Tactile Apraxia: a disorder in which there are disturbances of the movements of the hands during use or interaction with an object with the use of repetitive movements or gestures.
Tactile Defensiveness: While normally seen in children, this type of tactile disorder can affect adults, leading to a diminished tolerance for tactile situations: even the most minor sensory input is felt as extreme, painful, or irritating.
Tactile Hyposensitivity: Unlike those with tactile defensiveness, someone with tactile hyposensitivity have diminished tactile stimulation. Tactile Hyposensitivity often results in injury as the pain threshold a person with tactile hyposensitivity is high. Someone who has this disorder may love extremes in sensation: surfaces that provide texture and stimulation, or hot and spicy foods.
What Is Synesthesia?
Synesthesia is a hereditary phenomenon that occurs in one out of every thousand people with variations of intensity. Synesthesia is a condition in which one type of sensory stimulation evokes the sensation of another. In this way, for someone with synesthesia purple can be both a color and a taste.
Read more about synesthesia.
What Are The Other Senses?
While there is highly debated controversy over the addition of the following senses to the traditional five senses, it's worth mentioning them.
Balance: The sense of balance, also known as vestibular sense, or equilibrioception, is a sense that allows for us to feel body movement, direction and acceleration, which allows a body to hold postural balance and equilibrium. This sense is controlled by the vestibular labyrinthine system of the inner ears.
Pain: Nociception, or physical pain, provides a signal to the brain that body tissue has been damaged. Pain was once considered to be subjective (meaning, only reported by the person experiencing pain), but it has been shown that pain is registered in a part of the brain. Pain serves to warn us of dangers and potential damage to the body.
There are three types of pain receptors in the body:
- Cutaneous - pain receptors on the skin
- Somatic - pain receptors in the joints and bones
- Visceral - pain receptors in the bodily organs
Read more about pain.
Proprioception: The sense of position in space of the parts of the body is controlled by the parietal cortex of the brain. While proprioception is influenced partially by touch, the impairment of proprioception can lead to deep issues with perception and reaction.
Temperature: the sensation of thermoception (or thermoreception) is the means by which temperature is perceived. There are at least two types of temperature sensors: those which detect heat and those which detect cold.
Additional Five Senses Resources:
Illusions and Paradoxes: a group of visual tests for perception and colorblindness.
Colorblindness Test: a test that can determine whether or not you have color-blindness.
Hear The World: An online hearing test - not for diagnostic purposes but for informational purposes only.
Neuroscience for Kids - The Senses: An overview of the senses with interactive games and other fun experiments aimed at children.