The sensory system lets us see, hear, feel, and taste the things in our daily lives and acts as a warning system against harmful stimuli. It also keeps track of what is happening inside the body and sends the appropriate signals to the various organs to help maintain homeostasis.
Chemoreceptors are stimulated by changes in the chemicals in the air you sniff or the food you eat, as well as substances such as glucose and calcium. Nociceptors are stimulated by tissue damage, oxygen deprivation, or distention. Thermoreceptors are stimulated by changes in temperature. Mechanoreceptors are stimulated by changes in pressure or movements of body fluids and photoreceptors are stimulated by light.
When studying each sensation, it is important to identify the stimulus, type of receptor, sensory nerve, and specific area of the brain that interprets the sensation. Figure 13-1 illustrates the four components of sensation using the sense of sight as an example. Light is the stimulus for the sense of sight. Light waves stimulate the photoreceptors in the back of the eye to produce a nerve impulse. The nerve impulse is conducted by a sensory nerve to the occipital lobe of the brain. The occipital lobe of the brain interprets the sensory information as sight.
The reason you see with your eyes, hear with your ears, and feel pain in your injured hand is because the cortex of your brain receives the sensation and projects it back to its source. The brain receives the sensation of the painful thumb and refers that sensation back to its source. The pain of a “phantom limb” is another example of projection. Even though the limb is not there, the patient may still feel pain in the leg. The severed nerve endings of the amputated leg continue to send sensory information to the parietal lobe of the brain. The brain interprets the information as pain.
Do all receptors have the ability to adapt quickly? No, receptors vary in this regard. Olfactory receptors adapt quickly. Receptors that determine body position and detect blood chemistries adapt very slowly and are very important in maintaining homeostasis. Pain receptors do not adapt at all. Why is it beneficial to the body that pain receptors do not adapt? Pain is a signal that something is wrong; for example, it is helpful to the body that the pain for an inflamed appendix persists until diagnosis. Clinical point: Prior to diagnosis, pain is not medicated so that useful symptoms are not masked.
Where are the receptors for the general senses distributed? They are widely distributed and are found in the skin, muscles, joints, and viscera.
Pain serves a protective function. It motivates a person to remove its cause. Pain receptors also protect the body by failing to adapt. What are the three pain triggers that have been identified? They are tissue injury, oxygen deficiency, and tissue deformation and/or stretching.
Referred pain is pain that is felt as if it is coming from an area other than the site where it originates. This is caused by shared sensory nerve pathways. Where do pain impulses for most of the body go? They travel up the spinal cord in a sensory nerve tract called the spinothalamic tract. The information is transmitted to the thalamus and then to the cerebral cortex of the parietal lobe. Analgesics are drugs that can help relieve pain.
Touch is the first sensory system to develop in the growing fetus and is required by the infant to grow and thrive. Failure to thrive in children and adults can result from lack of touch and cuddling. Kangaroo care is a response to the physiological effects of lack of touch. Refer students to Table 13-1 for more information on mechanoreceptors.
There is a normal range at which thermoreceptors respond; beyond that range, pain receptors are activated. They are stimulated at both extremes of the scale, producing a freezing or burning sensation. Why is perceiving temperature extremes protective? Temperature extremes such as boiling water can cause injury. Sensory information about temperature is sent to the parietal lobe.
The sense of proprioception allows a person to locate a body part without looking at it. It plays an important role in coordinating body movement and maintaining posture. Proprioceptors in the inner ear function in equilibrium. Ask students to use Figure 13-3 to explain why the man’s arms are in the air.
Where are the receptors of the special senses located? Refer students to Table 13-2. They are located in the nose, tongue, eye, and inner ear.
Refer back to Chapter 10, Figure 10-11, and show the olfactory area of the temporal lobe. Some odors also stimulate the pain receptors of the trigeminal nerve, CN V. An example of an odor that stimulates the trigeminal nerve is ammonia fumes. This is another example of how the senses serve protective mechanisms. To detect a faint odor we often sniff deeply. Why does this occur? The olfactory receptors are located high in the nose. Olfactory input to different parts of the brain can trigger emotional and visceral responses. What are some examples of this? Putrid odors can stimulate the emetic reflex, the smell of popcorn can cause a person to remember the last movie seen, and an odor associated with childhood memories can trigger various emotions attached to the memories. Food odors can actually make you drool.
Looking at the slide, can you determine which parts of the tongue are most sensitive to specific sensations? The tip of the tongue is most sensitive to sweet and salty substances. The sides of the tongue are most sensitive to sour sensations, and bitter substances are most strongly tasted on the back part of the tongue. Taste impulses travel along the facial, glossopharyngeal, and vagus nerves. There is evidence of two additional taste sensations, metallic and umami (a meaty flavor). When you have a cold, why does food taste different? Both taste and smell are often interpreted by the same parts of the brain, so these two senses are closely related.
The eyebrows perform a protective role by keeping perspiration out of the eyes and by shading the eyes from glaring sunlight. The eyelids, also called palpebrae, prevent the entrance of foreign objects into the eye. The conjunctiva is a thin mucous membrane lining the inner surface of the eyelid. It also folds back and forms part of the outer surface of the eyeball (the white of the eye). Why does conjunctivitis cause bloodshot eyes? The inflammation causes the blood vessels of the conjunctiva to dilate. The increased blood supply results in redness. Eyelashes help trap dust, and the lacrimal apparatus helps cleanse, moisten, and lubricate the surface of the eyes through tear production. The lacrimal gland secretes tears that drain through the lacrimal puncta and into the lacrimal sac and nasolacrimal ducts. The nasolacrimal ducts empty into the nasal cavity, flow to the back of the throat, and allow fluid to be swallowed. Why does the weeping woman in the image have tears and nasal discharge flowing down her face? Excess tears, as in crying, exceed the capacity of lacrimal drainage system.
Ask students to use the illustration to review the four components of sensation. For vision, what are the stimulus, receptor, sensory nerve, and specialized part of the brain? For vision, these are light, photoreceptors, CN II (optic nerve), and the occipital lobe of the cerebrum.
The eyeball is protected by the bony orbit and a layer of fat. The sclera helps contain the contents of the eye, shapes the eye, and is the site of attachment for the extrinsic eye muscles. The cornea is a transparent forward extension of the sclera. It covers the area over the pupil and iris (the colored portion of the eye). Because light enters the eye first through the cornea, it is called the window of the eye. If the surface of the cornea is touched lightly, the eye blinks to remove the source of irritation. This response is called the corneal reflex.
The choroid layer provides the retina with a rich supply of blood, and the dark pigments located in the choroid absorb excess light to prevent glare. Why do persons with albinism typically wear sunglasses? They lack the melanin that absorb excess light. The ciliary body gives rise to the ciliary muscles. The most anterior portion of the choroid is the iris. The iris regulates the amount of light entering the eye. The opening in the middle of the iris is called the pupil.
The retina is the nervous layer containing the visual receptors. They are sensitive to light and are therefore called photoreceptors. The area of the retina with the highest concentration of cones is the fovea centralis, an area in the center of the macula lutea. The fovea centralis is considered the area of most acute vision. The neurons of the retina converge in the back of the eye to form the optic nerve. This area is called the optic disc and contains no photoreceptors. Images that focus on the optic disc are not seen, so the optic disc is called a blind spot. What is the consequence of a detached retina? The retina is no longer nourished by the blood supplied by the choroid layer. It dies and a person with a detached retina can become blind.
Why does your pupil dilate in the dark? The dilation of the pupil allows more light to enter the eyeball and scatter to the periphery of the retina, where the rods are located. Vitamin A deficiency causes night blindness because vitamin A is needed for the production of rhodopsin, a chemical necessary for the rods to function. What is the consequence of macular degeneration? Because the greatest concentration of cones is found on the macula, macular degeneration results in blindness.
The posterior cavity of the eyeball is filled with a gel-like substance called the vitreous humor. The vitreous humor gently pushes the choroid up against the retina, thereby nourishing the retina. The aqueous humor maintains the shape of the anterior portion of the eye and provides nourishment for the cornea.
The aqueous humor leaves the anterior cavity through tiny canals, called the canals of Schlemm or the venous sinuses, which are located at the junction of the sclera and the cornea. What results from inadequate drainage of the aqueous humor? Intraocular pressure increases, causing glaucoma, which can lead to blindness.
What functions do the extrinsic eye muscles perform? They move the eyeball in various directions: up, down, sideways, and in circles (rolling the eyes). Normally, the extrinsic muscles of both eyes exert equal tension. What happens if one of the extrinsic eye muscles is weaker than the other? This disorder causes strabismus, in which the eyes appear crossed. If this condition is left untreated, it can cause “lazy eye,” or suppression amblyopia and loss of vision.
Contraction of the circular muscle of the iris constricts the pupil, causing miosis. Contraction of the radial muscle causes dilation of the pupil, resulting in mydriasis. Ciliary muscles are involved in refraction. This slide shows only the iris and not the ciliary muscles. The circular and radial muscles regulate the amount of light entering the eye. The opening and closing of the pupil in response to variation in light is called the photopupillary reflex. Pupillary function is evaluated by noting the size, shape, and reactivity to light. PERRLA is an assessment term for pupillary function: pupils equal, round, reactive to light, and accommodation. In mydriasis, sympathetic nerve stimulation causes pupillary dilation. In miosis, parasympathetic nerve stimulation causes pupillary constriction.
These are the structures that control the shape of the lens that, in turn, controls refraction.
When the lens is not present, the light rays are parallel when they hit the retina. They do not converge on the focal point. The lens bends the light rays.
Improper shape of either the lens or the cornea results in errors of refraction. Why do people wear corrective eyewear? The lens of the eyewear corrects the error of refraction, bending the rays to focus on the retina.
This is the path of the nerve impulse from the retina to the brain.
Because information from both eyes travels to the occipital lobe, you see only one image. The optic chiasm is located anterior to the pituitary gland. Any enlargement of the pituitary gland will press on the optic chiasm, causing visual disturbance.
Discuss all the places where problems with vision can occur. When all of the parts of the eye, the visual pathway, and the brain are working correctly, you can see. Seeing happens when light waves enter your eye, are bent (refracted) by the lens, and are focused on the rods and cones of the retina. The photoreceptors translate the light signal to a nerve impulse. Vision is experienced when the nerve impulse is transmitted from the retina, along the optic nerve, and finally to the occipital lobe of the brain.
Where does “swimmer’s ear” occur? It occurs in the outer ear. Modified sweat glands in the external ear secrete cerumen or ear wax. Why should you not use cotton swabs to remove ear wax? This process will pack the cerumen against the tympanic membrane, and inhibit its ability to vibrate.
The other names for the ossicles are the hammer (malleus), the anvil (incus), and stirrup (stapes). The middle ear is concerned with bone conduction. The malleus lies up against the tympanic membrane, so that it can feel the membrane’s vibration. The stapes lies up against the oval window, so that it can transmit the vibrations to the fluid or endolymph in the inner ear. Normally, the ossicles are not connected to one another, but if they become fused, there is a loss of bone conduction and hearing.
Why do your ears pop when you take off and land in an airplane? The cabin pressure changes; therefore, a pressure differential across the tympanic membrane stretches it, resulting in pain. Swallowing or chewing gum opens up the eustachian tube, equalizes the pressure and relieves the discomfort. Why are children more susceptible to otitis media? The child’s shorter and more horizontal eustachian tube allows nonsterile secretions to move from the throat into the middle ear, causing infection. With growth, the eustachian tubes become more vertical so that the secretions from the throat are less apt to enter the middle ear. Why are tubes often inserted in the ears of children who are having repeated bouts of otitis media? The tubes allow for drainage in case of a middle ear infection.
This slide focuses on Figures B and C. The movement of the mechanoreceptors or hairs in the endolymph stimulates the nerve impulse. The cochlear branch of CN VIII carries the nerve impulse to the primary auditory cortex of the temporal lobe. The inner ear is the site of nerve conduction. Excessive noise damages the CN VIII. Why are the amplifiers of a rock band placed in front of the musicians rather than behind them? This position protects the hearing of the musicians, although it might not protect the hearing of the audience. If a drug is described as “ototoxic,” what side effect might it have? Ototoxicity causes damage to CN VIII, causing hearing loss and disturbance in equilibrium.
Sound waves are collected by the external ear, causing vibrations of the tympanic membrane, which causes the vibration of the ossicles that causes movement of the oval window and endolymph. The organ of Corti transforms the vibrations into nerve impulses. Conduction deafness occurs because of disorders in the pathway of vibrations (middle ear), whereas nerve deafness occurs because of damage to CN VIII in the inner ear.
Why does hearing loss occur when ear wax immobilizes the tympanic membrane? The membrane cannot vibrate; therefore, the ossicles of the middle ear do not vibrate. Why does hearing loss occur when the ossicles are fused? In this situation, bone conduction does not occur to move the oval window and the endolymph. Why is hearing impaired when CN VIII is damaged? The nerve impulses cannot be moved to the primary auditory cortex of the temporal lobe. Damage to the primary auditory cortex of the temporal lobe causes cortical deafness. What is cortical deafness? This is deafness that occurs as a result of damage to the primary auditory cortex of the temporal lobe. Every step of the hearing pathway from the external ear to the temporal lobe must operate for hearing to occur.
This slide focuses on Figures A and C. Mention Mèniére’s disease. Changes in head position cause the fluid within the balance apparatus to move. This bends the hairs and triggers nerve impulses. These travel along the vestibular branch of CN VIII to several parts of the brain, including the cerebellum, midbrain, and temporal lobe. After receiving this sensory information, the brain sends motor information to the muscles that allow the restoration of balance. Why might a person with an inner ear disorder suffer fall-related injuries? Dizziness, or vertigo, often causes a fall.
The Human Body in Health and
Illness, 4th edition