Malcolm Rivera
Humans have a remarkable ability to detect chemical compounds in their environment, and this is primarily achieved through two senses: taste and smell. These senses, also known as gustation and olfaction, rely on specialised receptors that respond to molecules in the air we breathe and the food we eat. When we inhale, odor molecules bind to olfactory receptors in the nasal cavity, while taste involves water-soluble or lipid-soluble chemical substances detected by taste receptor cells in the mouth. Together, these senses provide us with key environmental information, influencing our decisions and behaviours, such as finding food or regulating social interactions. So, when considering which pairs constitute the chemical senses, the dynamic duo of taste and smell takes the spotlight.
| Characteristics | Values |
|---|---|
| Main Chemical Senses | Smell and Taste |
| Other Names | Olfaction and Gustation |
| Receptors | Located in the mouth and nasal cavity |
| Environmental Information | Detects chemical compounds in the environment |
| Perception | Produces olfactory and gustatory perception |
| Decision-Making | Influences decision-making and behavioural selection |
| Flavours | Result from the combined perception of smells and tastes |
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What You'll Learn
Taste and smell are the primary chemical senses
The sense of smell, or olfaction, detects airborne molecules through chemical receptors in the nasal cavity. When we inhale, odor molecules in the air bind to olfactory receptors, activating them and sending signals to the olfactory bulb in the brain, allowing us to perceive different scents.
The sense of taste, or gustation, detects soluble chemical compounds present in the food and drinks we ingest. These water-soluble or lipid-soluble chemical substances, called tastants, create the sensation of taste when detected by taste receptor cells (TRC) within the mouth. The bumps on our tongue are called papillae, and within their walls and fissures are thousands of taste buds that contain the taste receptor cells.
Taste and smell work together to create the complex sensory capacity of flavour detection. Their coordinated action involves the activation of common sensory neurons within the piriform cortex, the part of the brain that first processes olfactory information.
Through these senses, chemical information in the surrounding environment is captured by chemosensory receptors, mainly in the mouth and nasal cavity. This information is then converted into the language of the nervous system through sensory transduction and transmitted to the central nervous system, ultimately influencing decision-making and behavioural selection.
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Olfaction detects airborne molecules
Olfaction, or the sense of smell, is a chemical sense that detects airborne molecules through chemical receptors in the nasal cavity. When we inhale, odor molecules in the air bind to olfactory receptors located in the nasal cavity. The olfactory receptors then send signals to the olfactory bulb in the brain, allowing us to perceive different scents.
The olfactory system in humans is considered weaker compared to other animals, but it is very precise. The olfactory system detects airborne substances, while the accessory system senses fluid-phase stimuli. The olfactory system consists of the nostrils, ethmoid bone, nasal cavity, and olfactory epithelium. The olfactory epithelium is a layer of thin tissue covered in mucus that lines the nasal cavity.
The mucus in the nasal cavity dissolves odor molecules, which are then detected by olfactory sensory neurons in the epithelium. These neurons transmit information about the odor to the brain through a process called sensory transduction. Olfactory neurons have cilia, or tiny hairs, that contain olfactory receptors. When odor molecules bind to these receptors, it triggers an electrical response that spreads through the sensory neuron to the olfactory nerve fibers at the back of the nasal cavity.
The olfactory nerve is the first cranial nerve (CN I) and is part of the autonomic nervous system, which regulates body functions. It is the shortest sensory nerve in the human body, starting in the brain and ending in the upper, inside part of the nose. The olfactory nerve enables the sense of smell by detecting and relaying information about scents to the brain.
Damage to the olfactory system can occur due to traumatic brain injury, cancer, infection, inhalation of toxic fumes, or neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. Olfactory dysfunction can also be caused by viral infections, such as COVID-19, which is associated with a loss of smell in up to half of infected individuals.
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Gustation detects soluble compounds
Gustation, or the sense of taste, is one of the two main chemical senses, the other being olfaction (the sense of smell). Gustation detects soluble chemical compounds when they dissolve in saliva and bind to receptors on taste buds, which are primarily located on the tongue. These taste buds are distributed across the tongue's surface, with different regions of the tongue historically associated with different tastes. For example, the tip of the tongue was thought to be associated with sweetness, while the sides were associated with sourness and saltiness, and the posterior region with bitterness. However, modern taste studies have shown that no single taste bud conducts only one taste quality, although they may respond better to one quality than another.
The basic tastes identified by the tongue's taste buds include sweet, sour, salty, bitter, and umami. When we eat, the food's chemical composition interacts with both taste and smell, creating a complex sensory experience. This is why food often seems tasteless when we have a cold, as our sense of smell is dulled.
Taste buds are also found on the oral mucosa of the palate and epiglottis. They are pear-shaped structures containing around 80 cells arranged around a central taste pore. Taste receptor cells are spindle-shaped, modified neuro-epithelial cells that extend from the base to the apex of the taste buds.
The neural pathways supporting gustatory perception have been studied in detail. The facial cranial nerve, the chorda tympani, innervates taste buds in the anterior two-thirds of the tongue and part of the soft palate. The glossopharyngeal nerve innervates the posterior third of the tongue, as well as the pharynx and epiglottis. Axons from these three cranial nerves terminate on second-order sensory neurons in the nucleus of the solitary tract. In lower animals, these axons project into the parabrachial nucleus, but not in humans. Instead, in humans, the fibres of the second-order neurons travel through the central tegmental tract to the third-order sensory neurons in the ventroposterior medial nucleus (VPM) of the thalamus. The VPM then projects to the gustatory cortex, located near the post-central gyrus representing the tongue, or to the insular cortex.
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Flavours are a product of chemical sensation
The chemical senses refer to the gustatory (taste) and olfactory (smell) systems. These senses rely on specialised receptors that respond to molecules in the air we breathe and the food we eat, respectively.
Flavours are indeed a product of chemical sensation. They are the result of the combined perception of smells and tastes. Our sense of taste starts with water-soluble or lipid-soluble chemical substances, known as tastants, reaching our mouth through ingestion. These chemical compounds create the sensation of taste when they are detected by taste receptor cells (TRC) within the mouth.
The olfactory sense, on the other hand, reacts to airborne molecules that reach the nasal cavity. When we inhale, odor molecules present in the air bind to olfactory receptors located in the nasal cavity, allowing us to perceive different scents.
Through the integration of smell and taste information in the brain, we are able to perceive flavours. This involves the activation of common sensory neurons within the piriform cortex, which is responsible for initially processing olfactory information. The chemical senses, although less studied than senses like vision and audition, provide key environmental information that influences decision-making and behavioural selection.
In summary, flavours are a product of chemical sensation, specifically the combination of smell and taste perceptions. The gustatory and olfactory systems work together to provide us with important information about our environment, influencing our choices and behaviours.
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The olfactory cortex is divided into several interconnected structures
Smell and taste are the two main chemical senses in humans as they use sensory receptors that respond to molecules in the air we breathe and the food we eat. The sense of smell, or olfaction, is a chemical sense as it detects airborne molecules through chemical receptors in the nasal cavity.
The olfactory cortex is a collection of interconnected structures that receive direct input from olfactory bulb mitral and/or tufted cells. These target structures include the anterior olfactory cortex, piriform cortex, olfactory tubercle, and the cortical nucleus of the amygdala. Each of these structures is trilaminar paleocortex, with a superficial layer I that receives input from the olfactory bulb, a pyramidal cell dense layer II, and a layer III composed of association fiber axons, deep pyramidal cell somata, and interneurons. The olfactory cortex does not have a columnar organisation but rather receives input as a wave of activity spreading caudally through its arrayed dendritic trees.
The piriform cortex, located below the lateral olfactory stria, has received the most attention in the literature. It can be cytoarchitecturally divided into at least four major subregions. The orbitofrontal cortex, a portion of the prefrontal cortex located on the underside of the frontal lobe, is also important for olfactory perception. Lesions in this region can result in an inability to distinguish different odours.
The olfactory system is considered the most "primitive" sensory system due to its early phylogenetic development and connections to older, subconscious portions of the brain. It has connections with limbic system structures such as the amygdala, hippocampus, and hypothalamus, which are involved in forming emotional responses and memories.
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Frequently asked questions
Smell and taste.
Olfaction.
It reacts to airborne molecules that reach the nasal cavity.
Tastants are water-soluble or lipid-soluble chemical substances present in food or drinks.
Chemical senses are also known as gustation and olfaction.
