Biochemistry of the senses (taste, smell).

Smell and taste are among the five basic senses of humans and many other animals. Sensing chemical compounds in the environment is the most archaic sensory mechanism in living organisms. Chemosensory receptors in the mouth and nasal cavity convert chemical signals into neural activity, which is processed in the brain to guide decisions and behaviours.

Sense of taste

Taste is detected by taste buds, which are located primarily on the tongue but also on the floor of the pharynx and the upper part of the esophagus. Each taste bud contains receptor cells that respond to five basic tastes:

Sweet – Indicates energy-rich foods.

Salty – Essential for maintaining electrolyte balance.

Sour – Can signal the presence of spoiled or unripe foods.

Bitter – Helps detect potentially toxic substances.

Umami – Detects amino acids, often found in protein-rich foods.

There are two different theories where the perception of tastes is located. The sweet taste is supposed to be at the tip, the sides are sour and salty, and the root of the tongue is bitter. Some authors claim that the flavors are evenly distributed.

Taste pathway

1. Taste buds on the tongue (located in papillae) contain specialized gustatory receptor cells that detect five basic tastes.

2. When a taste molecule binds to gustatory receptors, it triggers an electrical signal, which is transmitted via unipolar neurons in:

• a) Geniculate ganglion (for the facial nerve, CN VII)
• b) Inferior ganglion of CN IX (glossopharyngeal nerve)
• c) Inferior ganglion of CN X (vagus nerve)

3. Axons from these neurons project to the nucleus gustatorius (part of the nucleus of the solitary tract, NST) in the medulla oblongata where initial processing occurs.

4. The signal is then sent to the ventral posteromedial nucleus of the thalamus, which acts as a relay station.

5. The thalamus directs the information to the primary gustatory cortex in the insula and frontal operculum, where taste perception is refined.

6. Projections to Other Brain Areas

• Some fibers project to the reticular formation (RF) in the brainstem and the limbic system, especially the hypothalamus and amygdala. These areas regulate:
• Reflexes such as salivation, swallowing, glandular secretion, and digestive motility
• Emotional and motivational aspects of taste, including food-seeking behavior and aversions

Sense of smell

The sense of smell, or olfaction, is a fundamental sensory system that enables humans and many animals to detect and interpret airborne chemical compounds in their environment. This chemosensory ability plays a crucial role in various behaviors, including food selection, detection of hazards, and social interactions.

Mechanism of Olfactory Transduction

Airborne molecules, known as odorants, dissolve in the mucus of the nasal cavity and bind to specific receptors on the cilia of ORNs. The binding of an odorant activates a G-protein-coupled receptor mechanism, leading to the production of cyclic AMP (cAMP). This secondary messenger opens ion channels, allowing the influx of Na⁺ and Ca²⁺ ions, resulting in the depolarization of the neuron. If the depolarization reaches a certain threshold, an action potential is generated and propagated along the axon of the ORN. The action potential travels through the olfactory nerve fibers to the olfactory bulb. Here, ORNs synapse with mitral and tufted cells within specialized structures called glomeruli. The processed olfactory information is then transmitted to various brain regions, including the piriform cortex, amygdala, and hypothalamus, which are involved in odor perception, emotional responses, and memory formation.

Olfactory pathway

The neural pathway for olfaction is unique among the senses, as it bypasses the thalamus and projects directly to cortical areas.

1. Olfactory receptor neurons are located in the olfactory epithelium, these neurons detect odorants and send signals to the olfactory bulb.

2. The olfactory bulb processes incoming signals and relays them via the olfactory tract.

3. The primary olfactory cortex includes the piriform cortex, which interprets the chemical nature of odorants.

4. It also has connections to the limbic system and projects to the amygdala and hippocampus which links odors to emotions and memories.

Clinical Considerations:

Olfactory dysfunction can manifest as:

• Anosmia: Complete loss of smell.
• Hyposmia: Reduced ability to detect odors.
• Parosmia: Distorted perception of odors.

Used literature

• TROJAN, Stanislav. Lékařská fyziologie. 4. edition. Grada, 2003. pp. 772. ISBN 80-247-0512-5.

• MYSLIVEČEK, Jaromír. Základy neurověd. 2. edition. Triton, 2009. ISBN 978-80-7387-088-1.

• MANGUELE, Paloma – MERLO, Emiliano. Chemical senses: taste and smell [online]. ©2023. [cit. 2025]. <https://openpress.sussex.ac.uk/introductiontobiologicalpsychology/chapter/chemical-senses/>.