The Senses



The Senses



Learning Objectives



1. Explain the difference between general senses and special senses, and state examples of each.


2. List and describe the five groups of sense receptors.


3. Identify the sense receptors for touch, pressure, proprioception, temperature, and pain.


4. List and identify the location of the four different taste sensations.


5. Locate the sense receptors for smell, and trace the impulse pathway to the cerebral cortex.


6. Describe the structure of the eye, and explain the function of each structure.


7. Explain how light is focused on the retina.


8. Explain the function of the following photoreceptor cells in the retina: rods and cones.


9. Explain how nerve impulses for sight are initiated in response to light.


10. Describe the structure of the ear, and explain the function of each structure.


11. Explain how nerve impulses for hearing are initiated in response to sound waves.


12. Explain the difference between static equilibrium and dynamic equilibrium.


13. Explain how impulses for static equilibrium and dynamic equilibrium are initiated.


14. Describe ways in which the aging of an individual affects the senses.


15. Identify pathology related to the senses.



Key Terms











Introduction to the Senses


Sensory perception depends on receptors that respond to various stimuli. When a stimulus triggers an impulse in a receptor, the action potentials travel to the cerebral cortex, where they are processed and interpreted. Only after this occurs is a particular sensation perceived. Some senses, such as pain, touch, pressure, and proprioception, are widely distributed in the body. These are called general senses. Other senses, such as taste, smell, hearing, and sight, are called special senses because their receptors are localized in a particular area.



Receptors and Sensations


Although there are many different kinds of sense receptors, they can be grouped into five types. The basis for these receptor types is the kind of stimulus to which they are sensitive or have a low threshold. The five types of receptors are chemoreceptors, mechanoreceptors, nociceptors, thermoreceptors, and photoreceptors (Table 10-1).



Perceived sensation occurs only after impulses have been interpreted by the brain. Steps involved in sensory perception include the following:



The impulses from all the receptors are alike. The difference in perception is where they are interpreted in the brain. For example, all impulses going to one particular region are interpreted as sound, whereas those going to another region are interpreted as taste. As the brain interprets a sensation, it projects that sense back to its original source so that the “feeling” seems to come from the receptors that are stimulated. This projection allows us to locate the source of the stimulus.


Some sense receptors, when they are continually stimulated, undergo sensory adaptation. They have a decreased sensitivity to a continued stimulus and trigger impulses only if the strength of the stimulus is increased.



General Senses


General senses, or somatic senses, are those that are found throughout the body. They are associated with the visceral organs, as well as the skin, muscles, and joints, and include the following:




Touch and Pressure


As a group, the receptors for touch and pressure are mechanoreceptors that are sensitive to forces that deform or displace tissues. They are widely distributed in the skin. Three of the mechanoreceptors involved in touch and pressure are free nerve endings, Meissner corpuscles, and pacinian corpuscles.


Free nerve endings are the dendritic ends of sensory neurons that are interspersed between the cells in epithelial tissue. They do not have a connective tissue covering. They are important in sensing objects, such as clothing, that are in continuous contact with the skin. Meissner corpuscles (MYZE-ner KOAR-pus-als) consist of the ends of sensory nerve fibers surrounded by connective tissue and are specific in localizing tactile sensations. They are located in the dermal papillae, just beneath the epidermis, where they are important in sensing light-discriminative touch stimuli. Pacinian corpuscles (pah-SIN-ee-an KOAR-pus-als) are called lamellated corpuscles because several layers of connective tissue surround the nerve endings. These are common in deeper dermis and subcutaneous tissues, tendons, and ligaments. They are stimulated by heavy pressure.




Temperature


Thermoreceptors are located immediately under the skin and are widely distributed throughout the body. They are most numerous on the lips and are least numerous on some of the broad surfaces of the trunk. Thermoreceptors include at least two types of free nerve endings that are sensitive to temperature changes. In general, there are up to 10 times more cold receptors in a given area than heat receptors. Extremes in temperature stimulate pain receptors. A person determines gradations in temperatures by the degree of stimulation of each type of receptor. Extreme cold and extreme heat feel almost the same—both are painful—because the pain receptors are being stimulated. Thermoreceptors are strongly stimulated by abrupt changes in temperature and then fade after a few seconds or minutes. In other words, thermoreceptors show rapid sensory adaptation.



Highlight on the Senses




Odors: Nearly everyone is familiar with sensory adaptation in the sense of smell. A particular odor becomes unnoticed after a short time, even though the odor molecules are still present in the air, because the system quickly adapts to the continued stimulation. Odors have the quality of being interpreted as pleasant or unpleasant. Because of this, the sense of smell is as important as taste in the selection of food. For example, a person who has became sick after eating a certain type of food is often nauseated by the smell of that same food at a later occasion.


Cold adaptation: When a person first enters a cool swimming pool on a hot day, there is an abrupt change in temperature; therefore the cold receptors are strongly stimulated and there is a feeling of discomfort. After a brief time, the receptors adapt, the stimulation fades, and the cool water feels comfortable.


Headache: If there are no pain receptors in the nervous tissue of the brain, what are headaches? Headaches are a type of referred pain, pain that is referred to the surface of the head from deeper structures. The pain stimuli may originate in the meninges or blood vessels within the cranium. Other pain stimuli may originate outside the cranium from muscular spasms, the nasal sinuses, or the eyes.


Bitter taste: The taste receptors with the highest degree of sensitivity are those that are stimulated by bitter substances, and a highly intense bitter taste usually causes a person to reject that substance. This is probably an important protective mechanism because many of the deadly toxins found in poisonous plants have an intensely bitter taste.


Blinking: The eye blinks six to 30 times a minute. Blinking stimulates the lacrimal glands to secrete a sterile fluid, or “tears,” and helps move the fluid across the eyes.


Corneal transplant: The cornea was one of the first organs transplanted. Surgical removal of deteriorating corneas and replacement with donor corneas is a common medical procedure for several reasons. The cornea is readily accessible and relatively easy to remove. The tissue is avascular, so there is no bleeding problem or difficulty in establishing circulatory pathways. Corneas are less active than other tissues immunologically and are less likely to be rejected. Long-term success after corneal implant surgery is excellent.


Pupil size: In addition to regulating the amount of light that enters the eye, pupillary reflexes may also reflect interest or emotional state. For example, frequently the pupils dilate during problem solving or when the subject is appealing. If the subject is boring or repulsive, the pupils constrict.


Detached retina: Sometimes the sensory portion of the retina breaks away from the pigmented layer, resulting in a detached retina. This may be caused by trauma, such as a blow to the head, or by certain eye disorders. If allowed to progress, the result is distorted vision and eventually blindness. In many cases the retina can be reattached by laser surgery.


Color blindness: Color blindness occurs because there is an absence or deficiency of one or more of the visual pigments in the cones, and the person cannot distinguish certain colors. In the most common form, red-green color blindness, the cones lack the red pigment and the person is unable to distinguish red from green. Most color blindness is inherited and occurs more frequently in males.


Ruptured eardrum: The eardrum is sometimes ruptured, or perforated, by shock waves from an explosion, scuba diving, trauma, or acute middle ear infections. A perforated eardrum is characterized first by acute pain, then by noise in the affected ear, and then by hearing impairment.


Motion sickness: Motion sickness is nausea and vomiting resulting from repetitive and excessive stimulation of the equilibrium receptors. Some people are more susceptible than others. image




Gustatory Sense


The gustatory sense, or taste, is one of the special senses. As previously explained, the senses of taste, smell, hearing, and sight are called special senses because their receptors are localized in a particular area.


The organs of taste, the taste buds, are localized in the mouth region, primarily on the surface of the tongue, where they lie along the walls of projections called papillae (Figure 10-1).


image
Figure 10-1 Taste buds on the papillae of the tongue. The taste hairs on taste (gustatory) cells are the receptors for taste.

The receptors belong to the chemoreceptor (kee-moh-ree-SEP-tor) category because they are sensitive to chemicals in the food we eat. In order for these chemicals to be detected by a chemoreceptor, they must be dissolved in water.


Within the taste bud, specialized epithelial cells called taste cells or gustatory cells are interspersed with supporting cells and nerve fibers (see Figure 10-1). The entire taste bud opens to the surface through a taste pore. Tiny taste hairs (microvilli) project from the taste cells through the taste pore, and it is these hairs on the taste cells that function as the receptors.


Although all the taste receptors appear to be alike, there are at least four different types, each one sensitive to a particular kind of stimulus. Consequently, there are four different taste sensations:


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Apr 16, 2017 | Posted by in NURSING | Comments Off on The Senses

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