Created by CK-12 Foundation/Adapted by Christine Miller
Seeing Is Believing
At first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a , which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can’t see the hidden image, it doesn’t mean that there is anything wrong with your eyes. It’s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.
Thehuman bodyhas two basic types of senses, called special senses and general senses.have specialized sense organs that gather sensory information and change it intonerve impulses. Special senses include vision (for which theeyesare the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).,in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and otherbody tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,they alldepend oncells called sensory receptors.
Ais a specialized nerve cell that responds to a stimulus in the internal or external environment by generating anerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to thecentral nervous systemfor processing and to form a response.
There are several different types of sensory receptors that respond to different kinds of stimuli:
- respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses ofhearing and balance.
- respond to variations intemperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.
- respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.Some detect damagingheator cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).
- detect and respond to light. Most photoreceptors are found in theeyesand are needed for the sense of vision.
- respond to certain chemicals. They are found mainly in taste buds on the tongue — where they are needed for the sense of taste — and in nasal passages, where they are needed for the sense of smell.
is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.
Vision(or sight) is the ability to sense light and see. Theeye is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision. Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information. A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.
How the Eye Works
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.
- Light passes first through the, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.
- Next, light enters the interior of the eye through an opening called the. The size of this opening is controlled by the coloured part of the eye (called the ), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light. Filling the space between the cornea and the iris is a semi-gelatinous fluid called and functions to maintain the shape of the eye.
- The light then passes through the, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called , which functions to maintain the shape of the eye.
- The contains two types of photoreceptors: rod and cone cells . , which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light., which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.
- The nerve impulses from the rods and cones travel to the optic nerve via the (also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.
Humans have colour vision because we have three types of cone cells: blue, green and red. Each of these types of cone cell detects a specific wavelength of light, for which they are named. The combined stimulus is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells. Do you know what else uses these same three pieces of information to communicate colour? Your computer monitor! When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:
Figure 8.7.5 RGB colours.
Role of the Brain in Vision
The optic nerves from both eyes meet and cross just below the bottom of the in the brain. The information from both eyes is sent to the visual cortex in the of the , which is part of the . The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and “tells” us what we are seeing.
Vision problems are very common. Two of the most common are and , and they often start inchildhoodor adolescence. Another common problem, called presbyopia, occurs in most people, beginningin middleadulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.
(or nearsightedness) occurs when the light that comes into the eye does not directly focusonthe retina, butin front of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.
Hyperopia(or farsightedness)happenswhen the lightcominginto the eye does not directly focusonthe retina butbehind it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.
Presbyopia is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The preciseoriginof presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing themusclesthat control the lenstolose power as people grow older. The first signs of presbyopia — eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print — are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.
is the ability to sensesound waves, and the is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The is a coiled tube filled withliquid. The liquid moves in response to the vibrations, causing tinyhaircells(which are ) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are hearing.
The ears are also responsible for the sense of balance.is the ability to sense and maintain an appropriate body position. The inside the ear (see the figureabove) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to theperipheral nervous system, which triggers contractions ofskeletal musclesas needed to maintain balance.
and are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are . Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain “tells” us what we are tasting or smelling.
Taste receptors are found in tiny bumps on the tongue called.You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called . When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty — or savory — taste is called umami.
The most common cause of blindness in the Western hemisphere isage-related macular degeneration (AMD). Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina’s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.
In 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient’s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from,which are undifferentiated cells thatcandevelop into other cell types.Withinsix monthsofthe operation, the new cells were still surviving, and the doctor was hopeful that the patient’s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.
- Thehuman bodyhas two major types of senses: and . Special senses have specialized sense organs and include (eyes), (ears), (ears), (tongue), and (nasal passages). General senses are all associated with and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include (mechanical forces), (temperature), (pain), (light), and (chemicals).
- Touchisthe ability to sense pressure, vibration,temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and “tells” us what we are seeing.
- Common vision problems include (nearsightedness), (farsightedness), and (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or — in many cases — with laser surgery.
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and “tells” us what we are hearing.
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages toskeletal musclesvia theperipheral nervous system. Themusclesrespond by contracting to maintain balance.
- Taste and smellare both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,whileolfactory receptors in the nasal passages sense chemicals in the air.Sense of smell contributes significantly to sense of taste.
- Compare and contrast special senses and general senses.
- What are sensory receptors?
- Describe therange of tactile stimuli detected in the sense of touch.
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.
- Identify two common vision problems,along withtheir causes and their effects on vision.
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.
- What role does the ear play in balance? Which structures of the ear are involved in balance?
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.
- Is sensory information sent to the central nervous system via efferent or afferent nerves?
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?
- The auditory nerve carries _______________.
- smell information
- taste information
- balance information
- sound information
What color is Tuesday? Exploring synesthesia – Richard E. Cytowic, TED-Ed, 2013.
What Is Vertigo & Why Do We Get It?, Seeker, 2016.
How do animals see in the dark? – Anna Stöckl, TED-Ed, 2016.
What are those floaty things in your eye? – Michael Mauser, TED-Ed, 2014.
RGB colours [screenshots] from Microsoft Paint.
Age-Related Macular Degeneration. (n.d.). WebMD. https://www.webmd.com/eye-health/macular-degeneration/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
da Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell–derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36, 328–337. https://doi.org/10.1038/nbt.4114
File:Eye Diagram without text.gif. (2018, February 9).Wikimedia Commons. https://commons.wikimedia.org/w/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image fromNational Eye Institute– modified byUser:Nordelch) [public domain (https://en.wikipedia.org/wiki/Public_domain)]
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual (Section III, Chapter 5 – Noise). United States Department of Labour [online]. https://www.osha.gov/dts/osta/otm/new_noise/
Seeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https://www.youtube.com/watch?v=UL8YSLhqa5U&feature=youtu.be
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia – Richard E. Cytowic. YouTube. https://www.youtube.com/watch?v=rkRbebvoYqI&feature=youtu.be
TED-Ed. (2014, December 1). What are those floaty things in your eye? – Michael Mauser. YouTube. https://www.youtube.com/watch?v=Y6e_m9iq-4Q&feature=youtu.be
TED-Ed. (2016, August 25). How do animals see in the dark? – Anna Stöckl. YouTube. https://www.youtube.com/watch?v=t3CjTU7TaNA&feature=youtu.be