10.1: General and Special Senses

Learning Objectives

By the end of this section, you will be able to:

• Describe different types of sensory receptors
• Describe the structures responsible for the special senses of taste, smell, hearing, balance, and vision
• Distinguish how different tastes are transduced
• Describe how the sense of smell is conducted

A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.

Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. These receptors are found in the cell membrane of these sensory neurons. Certain receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other cell membrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion movement across the membrane, and can generate an action potential in the sensory neurons.

Sensory Receptors

Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Sensory receptor cells can be classified based on their location as well as their function.

There are two classifications for sensory receptors based on the location of stimuli in the body. Sensory receptors that detect stimuli originating from outside your body are classified as exteroceptors. Exteroceptors are usually found near the body’s surface. An example of exteroceptors are the sensory receptors in your skin which detect temperature and pressure. Another example are the sensory receptors in your eye which detect light that is then processed as vision in your brain. Note that the “ex” portion of the name exteroceptor refers to the outside.

Sensory receptors that detect stimuli originating from inside the body are interoceptors. Interoceptors will thus be found inside your body, further from the surface. An example of interoceptors are the sensory receptors that detect chemical changes in your blood, or those that detect the stretch of your bladder, indicating a need to urinate. Note that the “int” portion of the name refers to the internal environment.

Sensory receptors can also be classified based on the type of stimuli they detect. These types of receptors are as follows:

• Chemoreceptors respond to different chemicals in the body such as in the blood or in the air in the lungs.
• Osmoreceptors respond to solute concentrations of body fluids.
• Thermoreceptors respond to changes in temperature. There are “hot” receptors which respond to hot temperature ranges (90-118°F) and “cold” receptors which respond to cold temperature ranges (50-104°F). Temperatures outside of this ranges activate nociceptors.
• Nociceptors respond to pain stimuli. Pain is primarily a chemical sense interpreted by the presence of chemicals from tissue damage, or similar intense stimuli.
• Proprioceptors detect movement and position of the body in the surrounding space.
• Photoreceptors detect light stimuli and are only found in the eye.
• Mechanoreceptors to mechanical stimuli such as pressure, touch, and vibration.

Lastly, sensory receptors can also be classified by how quickly or slowly they adapt to stimuli. Certain sensory receptors initially send rapid signals to the brain in response to stimuli but if this stimuli continues, these receptors decrease their firing rate. These sensory receptors are classified as rapidly adapting receptors and include receptors that detect vibration, touch, smell and thermoreceptors. For example, when you start the day in the morning and get dressed, you may initially feel your shirt on your skin as you get dressed. However, as you continue about your day, you tend not to notice the sensation of the shirt fabric on your skin. Your receptors have rapidly adapted to this sensation.

In contrast, slow adapting receptors are those that continue to send signals to the brain as long as the stimuli is present. Slow adapting receptors include nociceptors for pain, chemoreceptors and proprioceptors. These receptors are critical for maintaining homeostasis so if pain is present, you brain will want to be aware of it to address the issue. Similarly, your body needs to constantly monitor the chemical composition of the fluids in your body so your chemoreceptors will be constantly firing.

Types of Senses

We can classify the senses into the general senses and special senses. The general senses include touch, temperature, pain, and proprioception. The special senses include vision, hearing (and balance), taste, and smell. The special senses of taste and smell will be covered below while the special senses of vision, hearing and equilibrium will be covered in their own separate chapters.

Special Sense - Gustation (Taste)

Only a few recognized submodalities exist within the sense of taste, or gustation. Until recently, only four tastes were recognized: sweet, salty, sour, and bitter. Research at the turn of the 20th century led to recognition of the fifth taste, umami, during the mid-1980s. Umami is a Japanese word that means “delicious taste,” and is often translated to mean savory. Very recent research has suggested that there may also be a sixth taste for fats, or lipids.

Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae (singular = papilla) contain the structures for gustatory transduction. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.

Figure \(\PageIndex{2}\): The Tongue The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Salty taste is simply the perception of sodium ions (Na⁺) in the saliva. When you eat something salty, the salt crystals dissociate into the component ions Na⁺ and Cl^–, which dissolve into the saliva in your mouth. The Na⁺ concentration becomes high outside the gustatory cells, creating a strong concentration gradient that drives the diffusion of the ion into the cells. The entry of Na⁺ into these cells results in the depolarization of the cell membrane and the generation of a receptor potential.

Sour taste is the perception of H⁺ concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For example, orange juice—which contains citric acid—will taste sour because it has a pH value of approximately 3. Of course, it is often sweetened so that the sour taste is masked.

The first two tastes (salty and sour) are triggered by the cations Na⁺ and H⁺. The other tastes result from food molecules binding to a specialized receptors in the cell membrane of taste buds. The sweet taste is the sensitivity of taste buds to the presence of glucose dissolved in the saliva. Other monosaccharides such as fructose, or artificial sweeteners such as aspartame (NutraSweet™), saccharine, or sucralose (Splenda™) also activate the sweet receptors. The affinity for each of these molecules varies, and some will taste sweeter than glucose because they bind to the G protein–coupled receptor differently.

Bitter taste is similar to sweet in that food molecules bind to specialized receptors However, there are a number of different ways in which this can happen because there are a large diversity of bitter-tasting molecules. The specific response depends on which molecule is binding to the receptor.

One major group of bitter-tasting molecules are alkaloids. Alkaloids are nitrogen containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbe infection and less attractive to herbivores.

Therefore, the function of bitter taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are normally ingested are often combined with a sweet component to make them more palatable (cream and sugar in coffee, for example). The highest concentration of bitter receptors appear to be in the posterior tongue, where a gag reflex could still spit out poisonous food.

The taste known as umami is often referred to as the savory taste. Like sweet and bitter, it is based on the activation of specialized receptors by a specific molecule. The molecule that activates this receptor is the amino acid L-glutamate. Therefore, the umami flavor is often perceived while eating protein-rich foods. Not surprisingly, dishes that contain meat are often described as savory.

Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness.

Interactive Link

Watch this video to learn about Dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, who became interested in science at an early age because of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences between people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two groups known as “tasters” and “non-tasters” based on the density of papillae on their tongue, which also indicates the number of taste buds. Non-tasters can taste food, but they are not as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is a non-taster, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to tastes? Can you see any similarities among the members of your family?

Olfaction (Smell)

Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity (Figure \(\PageIndex{3}\)). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite.

Olfactory stimuli travels to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Olfactory stimuli also reaches structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.

The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.

Figure \(\PageIndex{3}\): The Olfactory System (a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid bone and synapse with the neurons of the olfactory bulb (tissue source: simian). LM × 812. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Disorders of the...

Olfactory System: Anosmia

Blunt force trauma to the face, such as that common in many car accidents, can lead to the loss of the olfactory nerve, and subsequently, loss of the sense of smell. This condition is known as anosmia. When the frontal lobe of the brain moves relative to the ethmoid bone, the olfactory tract axons may be sheared apart. Professional fighters often experience anosmia because of repeated trauma to face and head. In addition, certain pharmaceuticals, such as antibiotics, can cause anosmia by killing all the olfactory neurons at once. If no axons are in place within the olfactory nerve, then the axons from newly formed olfactory neurons have no guide to lead them to their connections within the olfactory bulb. There are temporary causes of anosmia, as well, such as those caused by inflammatory responses related to respiratory infections or allergies.

Loss of the sense of smell can result in food tasting bland. A person with an impaired sense of smell may require additional spice and seasoning levels for food to be tasted. Anosmia may also be related to some presentations of mild depression, because the loss of enjoyment of food may lead to a general sense of despair.

The ability of olfactory neurons to replace themselves decreases with age, leading to age-related anosmia. This explains why some elderly people salt their food more than younger people do. However, this increased sodium intake can increase blood volume and blood pressure, increasing the risk of cardiovascular diseases in the elderly.