4.3 Chemical senses: olfaction and gustation

Chemical senses are crucial for survival, helping organisms detect food, mates, and dangers. Olfaction and gustation work together to process environmental cues, with olfactory receptors in the nose detecting airborne molecules and taste buds on the tongue sensing dissolved chemicals.

These sensory systems involve specialized receptors, neural pathways, and brain regions for processing and interpreting chemical signals. Understanding olfaction and gustation provides insights into how animals interact with their environment and make vital decisions.

Olfactory System

Olfactory Receptors and Bulb

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• Olfactory receptors are specialized neurons located in the nasal epithelium that detect odor molecules
  • Each olfactory receptor is specific to a particular odor molecule or group of molecules
  • Humans have approximately 400 different types of olfactory receptors (dogs have over 800)
• When an odor molecule binds to an olfactory receptor, it triggers a signal transduction cascade that generates an action potential in the receptor neuron
• Olfactory receptor neurons project their axons to the olfactory bulb, a structure in the forebrain
  • Axons from receptors that detect the same odor converge on the same glomeruli in the olfactory bulb
  • This convergence enhances the sensitivity and specificity of odor detection

Olfactory Cortex and Perception

• The olfactory bulb sends projections to the olfactory cortex, which includes parts of the amygdala, hippocampus, and orbitofrontal cortex
  • These areas are involved in the perception, discrimination, and emotional associations of odors
  • The orbitofrontal cortex is particularly important for the conscious perception of odors (allows us to identify and name specific scents)
• Olfactory information also projects to the hypothalamus, which is involved in the regulation of hormones and behavior
  • This connection may explain why certain odors can trigger strong emotional or behavioral responses (the smell of freshly baked cookies evoking feelings of comfort and nostalgia)

Pheromones and the Vomeronasal Organ

• Pheromones are chemical signals released by an individual that trigger specific behaviors or physiological responses in another individual of the same species
  • In many animals, pheromones play a crucial role in sexual attraction, territorial marking, and social communication (the queen bee releases a pheromone that suppresses ovary development in worker bees)
• Some animals, such as rodents and cats, have a specialized structure called the vomeronasal organ (VNO) that detects pheromones
  • The VNO is a pair of tubular structures located in the nasal septum that send projections to the accessory olfactory bulb
  • The accessory olfactory bulb then projects to the hypothalamus and amygdala, which mediate the behavioral and physiological responses to pheromones
• The existence and function of the VNO in humans remain controversial, and it is believed to be vestigial or non-functional in adult humans

Gustatory System

Taste Buds and Receptors

• Taste buds are the primary sensory organs for gustation and are located on the tongue, soft palate, and pharynx
  • Each taste bud contains 50-100 taste receptor cells (TRCs) that detect dissolved chemicals in the mouth
  • TRCs are modified epithelial cells that have a lifespan of about 10 days and are continuously replaced
• There are five basic taste qualities: sweet, salty, sour, bitter, and umami (savory)
  • Each taste quality is detected by a specific type of TRC that expresses receptors for that taste (sweet TRCs express receptors for sugars and artificial sweeteners)
• When a tastant binds to a receptor on a TRC, it triggers a signal transduction cascade that leads to the release of neurotransmitters onto afferent nerve fibers

Gustatory Pathways and Cortex

• Afferent nerve fibers from the taste buds project to the nucleus of the solitary tract (NST) in the brainstem
  • The NST then sends projections to the thalamus, which relays gustatory information to the primary gustatory cortex in the insula and frontal operculum
  • The primary gustatory cortex is responsible for the perception and discrimination of taste qualities (allows us to identify and differentiate between different tastes)
• Gustatory information also projects to the amygdala and orbitofrontal cortex, which are involved in the emotional and hedonic aspects of taste
  • These areas mediate the pleasurable or aversive responses to different tastes (the sweetness of chocolate eliciting feelings of enjoyment, while the bitterness of spoiled food triggers disgust)
• The gustatory system interacts closely with the olfactory system to create the overall flavor experience of food
  • The aroma of food, detected by the olfactory system, contributes significantly to the perceived taste and flavor (the distinctive flavor of coffee is largely due to its aroma, rather than its taste alone)