Other Special Sense Organs
The sense organs of taste and smell are designed to respond to chemical stimuli.
Sense of Taste
The sense of taste, or gustation, involves receptors in the tongue and two different nerves that carry taste impulses to the brain (Fig. 7-18). The taste receptors, known as taste buds, are located along the edges of small, depressed areas called fissures. Taste buds are stimulated only if the substance to be tasted is in solution or dissolves in the fluids of the mouth. Receptors for four basic tastes are localized in different regions, forming a “taste map” of the tongue (see Fig. 7-18 B):
* Sweet tastes are most acutely experienced at the tip of the tongue (hence the popularity of lollipops and ice cream cones).
* Salty tastes are most acute at the anterior sides of the tongue.
* Sour tastes are most effectively detected by the taste buds located laterally on the tongue.
* Bitter tastes are detected at the posterior part of the tongue.
Taste maps vary among people, but in each person certain regions of the tongue are more sensitive to a specific basic taste. Other tastes are a combination of these four with additional smell sensations. More recently, researchers have identified some other tastes besides these basic four: water, alkaline (basic), and metallic. Another is umami, a pungent or savory taste based on a response to the amino acid glutamate. Glutamate is found in MSG (monosodium glutamate), a flavor enhancer used in Asian food. Water taste receptors are mainly in the throat and may help to regulate water balance. The nerves of taste include the facial and the glossopharyngeal cranial nerves (VII and IX). The interpretation of taste impulses is probably accomplished by the lower frontal cortex of the brain, although there may be no sharply separate gustatory center.
Sense of Smell
The importance of the sense of smell, or olfaction, is often underestimated. This sense helps to detect gases and other harmful substances in the environment and helps to warn of spoiled food. Smells can trigger memories and other psychological responses. Smell is also important in sexual behavior. The receptors for smell are located in the epithelium of the superior region of the nasal cavity (see Fig. 7-18). Again, the chemicals detected must be in solution in the fluids that line the nose. Because these receptors are high in the nasal cavity, one must “sniff” to bring odors upward in the nose. The impulses from the receptors for smell are carried by the olfactory nerve (I), which leads directly to the olfactory center in the brain’s temporal cortex. The interpretation of smell is closely related to the sense of taste, but a greater variety of dissolved chemicals can be detected by smell than by taste. The smell of foods is just as important in stimulating appetite and the flow of digestive juices as is the sense of taste. When one has a cold, food often seems tasteless and unappetizing because nasal congestion reduces ability to smell the food. The olfactory receptors deteriorate with age and food may become less appealing. It is important when presenting food to elderly people that the food look inviting so as to stimulate their appetites.
Figure 7-18 Special senses that respond to chemicals. (A) Organs of taste (gustation) and smell (olfaction). (B) A taste map of the tongue.
The General Senses
Unlike the special sensory receptors, which are localized within specific sense organs, limited to a relatively small area, the general sensory receptors are scattered throughout the body. These include receptors for touch, pressure, heat, cold, position, and pain (Fig. 7-19).
Sense of Touch
The touch receptors, tactile corpuscles, are found mostly in the dermis of the skin and around hair follicles. Sensitivity to touch varies with the number of touch receptors in different areas. They are especially numerous and close together in the tips of the fingers and the toes. The lips and the tip of the tongue also contain many of these receptors and are very sensitive to touch. Other areas, such as the back of the hand and the back of the neck, have fewer receptors and are less sensitive to touch.
Figure 7-19 Sensory receptors in the skin. Synapses are in the spinal cord.
Sense of Pressure
Even when the skin is anesthetized, it can still respond to pressure stimuli. These sensory end-organs for deep pressure are located in the subcutaneous tissues beneath the skin and also near joints, muscles, and other deep tissues. They are sometimes referred to as receptors for deep touch.
Sense of Temperature
The temperature receptors are free nerve endings, receptors that are not enclosed in capsules, but are merely branchings of nerve fibers. Temperature receptors are widely distributed in the skin, and there are separate receptors for heat and cold. A warm object stimulates only the heat receptors, and a cool object affects only the cold receptors. Internally, there are temperature receptors in the hypothalamus of the brain, which help to adjust body temperature according to the temperature of the circulating blood.
Sense of Position
Receptors located in muscles, tendons, and joints relay impulses that aid in judging one’s position and changes in the locations of body parts in relation to each other. They also inform the brain of the amount of muscle contraction and tendon tension. These rather widespread receptors, known as proprioceptors, are aided in this function by the equilibrium receptors of the internal ear. Information received by these receptors is needed for the coordination of muscles and is important in such activities as walking, running, and many more complicated skills, such as playing a musical instrument. They help to provide a sense of body movement, known as kinesthesia. Proprioceptors play an important part in maintaining muscle tone and good posture. They also help to assess the weight of an object to be lifted so that the right amount of muscle force is used. The nerve fibers that carry impulses from these receptors enter the spinal cord and ascend to the brain in the posterior part of the cord. The cerebellum is a main coordinating center for these impulses.
Sense of Pain
Pain is the most important protective sense. The receptors for pain are widely distributed free nerve endings. They are found in the skin, muscles, and joints and to a lesser extent in most internal organs (including the blood vessels and viscera). Two pathways transmit pain to the CNS. One is for acute, sharp pain, and the other is for slow, chronic pain. Thus, a single strong stimulus produces the immediate sharp pain, followed in a second or so by the slow, diffuse, burning pain that increases in severity with the passage of time. Sometimes, the cause of pain cannot be remedied quickly, and occasionally it cannot be remedied at all. In the latter case, it is desirable to lessen the pain as much as possible. Some pain relief methods that have been found to be effective include:
* Analgesic drugs. An analgesic is a drug that relieves pain. There are two main categories of such agents:
* Nonnarcotic analgesics act locally to reduce inflammation and are effective for mild to moderate pain. Most of these drugs are commonly known as nonsteroidal antiinflammatory drugs (NSAIDs). Examples are ibuprofen and naproxen.
* Narcotics act on the CNS to alter the perception and response to pain. Effective for severe pain, narcotics are administered by varied methods, including orally and by intramuscular injection. They are also effectively administered into the space surrounding the spinal cord. An example of a narcotic drug is morphine.
* Anesthetics. Although most commonly used to prevent pain during surgery, anesthetic injections are also used to relieve certain types of chronic pain.
* Endorphins are released naturally from certain regions of the brain and are associated with the control of pain. Massage, acupressure, and electric stimulation are among the techniques that are thought to activate this system of natural pain relief.
* Applications of heat or cold can be a simple but effective means of pain relief, either alone or in combination with medications. Care must be taken to avoid injury
caused by excessive heat or cold.
* Relaxation or distraction techniques include several methods that reduce perception of pain in the CNS. Relaxation techniques counteract the fight-or-flight response to pain and complement other pain-control methods.
When sensory receptors are exposed to a continuous stimulus, receptors often adjust themselves so that the sensation becomes less acute. The term for this phenomenon is sensory adaptation. For example, if you immerse your hand in very warm water, it may be uncomfortable; however, if you leave your hand there, soon the water will feel less hot (even if it has not cooled appreciably). Receptors adapt at different rates. Those for warmth, cold, and light pressure adapt rapidly. In contrast, those for pain do not adapt. In fact, the sensations from the slow pain fibers tend to increase over time. This variation in receptors allows us to save energy by not responding to unimportant stimuli while always heeding the warnings of pain.