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 (also on the floor of the pharynx and even in 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. Once a taste molecule binds to gustatory receptors it synapses with unipolar neurons and triggers an electrical signal. These neurons are located in:

  a) Geniculate ganglion (for the facial nerve, CN VII)
  b) Inferior ganglion of CN IX (glossopharyngeal nerve)
  3) 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