13.5: Taste and Smell
- Page ID
- 63451
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- Describe the gross and microscopic structures responsible for the special senses of taste and smell
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. There are four types of papillae, based on their appearance (Figure \(\PageIndex{2}\)): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are taste buds that are recessed into a taste pore and resemble the shape of an onion. Taste buds contain a variety of cells that support and regenerate the taste bud structure. Specialized gustatory receptor cells have microvilli called taste hairs that can absorb the chemicals contained within foods that are ingested and then release neurotransmitters based on the amount of the specific chemical in the food. These cells are not neurons, but they are in contact with the dendrites of sensory neurons.
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 (CN VII) and glossopharyngeal (CN IX) cranial nerves, as well as a component within the vagus nerve (CN X) 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. Gustatory information is then transmitted through these cranial nerves to the brain, through the thalamus, which acts as a relay station, and finally into the primary gustatory cortex of the insula. It is here that this information is processed and perception is initiated.
Novel studies have shown that gustatory cells are not only present on the tongue but also in the guts. In humans, sweet, umami and bitter taste cells are also present in the intestines and used to "taste" the ingested food and regulate its variations.
Olfaction (Smell)
Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. Inhaled air containing odorant molecules (smells) enters the nasal cavity and passes by the nasal conchae. The olfactory receptor neurons (ORNs) are located in a small region within the superior nasal cavity (Figure \(\PageIndex{3}\)). This region is referred to as the olfactory epithelium and also contains supporting cells and basal cells (not shown), with similar functions as the homonyms in the taste buds. Each olfactory receptor neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. Olfactory glands within the basal lamina at the interface between the olfactory epithelium and the superior connective tissue produce the mucus that lines the olfactory epithelium, The olfactory sensory neurons present nonmotile cilia called olfactory cilia (or hairs) that contain the chemoreceptor for a specific odorant molecule. 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, producing an electrical change in the olfactory neurons.
The axon of an olfactory neuron extends from the basal surface of the epithelium towards the cribriform plate of the ethmoid bone forming the olfactory nerve (CN I). The olfactory fibers go through an olfactory foramen in the cribriform plate and converge to form spherical structures called glomeruli in the olfactory bulb, which resides on the ventral surface of the frontal lobe. Within the glomeruli, the olfactory receptor neurons synapse with mitral cells which in turn extend their axons to form the olfactory tract. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to 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 olfactory system can recognize seven primary odors (musky, putrid, pungent, camphoraceous, ethereal, floral, pepperminty) as well as thousands of odorant molecules that produce various scents. The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are one of the few types of neurons that undergo mitosis and is able to replace old cells. 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.
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.
Concept Review
Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs. Gustation and olfaction belong to the special senses. Gustation transduction is achieved by raised bumps on the tongue called papillae that contain taste buds, which in turn contain specialized gustatory receptor cells for chemical stimuli (”tastes”). Olfactory receptor neurons (unipolar) located in olfactory epithelium respond to chemical stimuli (“odorants”) of the nasal cavity. Their dendrites extend from the apical surface of the epithelium into the mucus lining the cavity and have receptors for odorants. Axons extend into the cribriform plate of the ethmoid bone to reach the olfactory bulb, forming the olfactory nerve (CN I). Once the olfactory nerve crosses the CNS, it becomes the olfactory tract and sends axons to several brain regions. The axons do not synapse onto the thalamus (unlike the other special senses).
Review Questions
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Glossary
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Contributors and Attributions
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology