The binding of these odorant molecules to the protein receptors on the plasma membrane of dendrites activates a G protein-coupled signaling pathway to produce an action potential on the olfactory receptor cell. Furthermore, this action potential reaches the olfactory bulb of the brain through the axon of the olfactory nerve. From the olfactory bulb, this axon divides and travel into different regions of the brain responsible for the recognition of the smell.
Gustatory receptors are the nerve cells sensitive to the taste or gustation. Our tongue consists of bumps called papillae. Within the papillae, taste buds occur, which contains the gustatory receptor cells. These cells are sensitive to the chemicals in the food. Figure 2: Gustatory Receptors.
Basically, out tongue is sensitive to four types of tastes. The large circumvallate papillae contain up to taste buds and form a V near the posterior margin of the tongue. In addition to those two types of chemically and mechanically sensitive papillae are foliate papillae—leaf-like papillae located in parallel folds along the edges and toward the back of the tongue, as seen in the Figure Foliate papillae contain about 1, taste buds within their folds.
Each of these papillae is surrounded by a groove and contains about taste buds. These are elongated cells with hair-like processes called microvilli at the tips that extend into the taste bud pore illustrate in Figure Food molecules tastants are dissolved in saliva, and they bind with and stimulate the receptors on the microvilli.
The receptors for tastants are located across the outer portion and front of the tongue, outside of the middle area where the filiform papillae are most prominent. In humans, there are five primary tastes, and each taste has only one corresponding type of receptor.
Thus, like olfaction, each receptor is specific to its stimulus tastant. Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant. Sour tastants are acids and belong to the thermoreceptor protein family. Sweet, bitter, and umami tastants require a G-protein coupled receptor.
These tastants bind to their respective receptors, thereby exciting the specialized neurons associated with them. Both tasting abilities and sense of smell change with age. In humans, the senses decline dramatically by age 50 and continue to decline. A child may find a food to be too spicy, whereas an elderly person may find the same food to be bland and unappetizing. View this animation that shows how the sense of taste works.
Olfactory neurons project from the olfactory epithelium to the olfactory bulb as thin, unmyelinated axons. The olfactory bulb is composed of neural clusters called glomeruli , and each glomerulus receives signals from one type of olfactory receptor, so each glomerulus is specific to one odorant. From glomeruli, olfactory signals travel directly to the olfactory cortex and then to the frontal cortex and the thalamus.
Recall that this is a different path from most other sensory information, which is sent directly to the thalamus before ending up in the cortex. Olfactory signals also travel directly to the amygdala, thereafter reaching the hypothalamus, thalamus, and frontal cortex. The last structure that olfactory signals directly travel to is a cortical center in the temporal lobe structure important in spatial, autobiographical, declarative, and episodic memories. Olfaction is finally processed by areas of the brain that deal with memory, emotions, reproduction, and thought.
Taste neurons project from taste cells in the tongue, esophagus, and palate to the medulla, in the brainstem. From the medulla, taste signals travel to the thalamus and then to the primary gustatory cortex. Information from different regions of the tongue is segregated in the medulla, thalamus, and cortex. There are five primary tastes in humans: sweet, sour, bitter, salty, and umami.
Each taste has its own receptor type that responds only to that taste. Olfactory receptors present in the back of the nasal cavity are responsible for the sense of smell or olfaction. In fact, olfactory receptors are found on the olfactory epithelium. Olfactory receptors or odorant receptors are smell receptors that bind and detect air-borne odour molecules that enter the nasal cavity. They are dendrites of specialized neurons. Once odour molecules bind with olfactory receptors, they send impulses directly to the olfactory bulb of the brain.
There are hundreds of different types of olfactory receptors in the human body. Moreover, olfactory receptors are present in very large numbers millions. In each receptor, there is an external process cilia extending out to the surface of the epithelium.
Gustatory receptors are taste receptor cells that detect taste stimuli. In contrast, olfactory receptors are true neurons that detect various smells. So, this is the key difference between gustatory receptors and olfactory receptors. When we chew food or sip wine, chemicals are vaporized into air passages that connect the mouth and the back of the nose, stimulating olfactory receptors and allowing us to realize the subtleties of flavor.
Other aspects of the taste experience, such as food texture and temperature, engage additional senses. Smell, in particular, typically declines, which can make food less appealing, adversely affecting appetite and sometimes contributing to poor nutrition in the elderly.
Complete loss of the sense of smell, anosmia, afflicts some six million Americans. The causes are varied and sometimes unknown. Marked decline in olfaction may also be a sign of neurological disorders. Download this page as a PDF. Hearing is a mechanical sense. It turns physical movement into the electrical signals that make up the language of the brain, translating these vibrations into what we experience as the world of sound. All of our senses give us vital information about our surroundings, but the one we rely on most is vision.
Accordingly, the physical apparatus for gathering visual information—the eye—and the brain circuits that process this information are more complex than corresponding systems for the other senses. The human brain is a network of networks: an intricate, integrated system that coordinates operations among billions of cells. Although strokes are sudden, the brain injury they inflict typically evolves over the course of hours or even days.
Prompt, effective treatment is critical. Many of us think of hormones as chemical messengers that arrive during puberty to govern our reproductive development. But sex steroids like testosterone and estrogen also play a critical role in brain development. Sign up for monthly email updates on neuroscience discoveries, Cerebrum magazine, and upcoming events.
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