Algonquin College
8.7 - Taste and Smell
Olfactory (smell) and gustatory (taste) receptors are chemoreceptors and respond to many of the same stimuli. The olfactory sense responds to specific chemicals called odorants and includes a wide variety of different chemoreceptors. The gustatory sense only responds to a few types of chemicals called tastants and thus is very inaccurate without the olfactory sense to guide it.
  • Ex. That is why if you plug your nose while eating or are congested due to illness your food will taste bland!
Olfaction (smell)

Olfactory receptor cells (olfactory neurons) are found lining the nasal cavity, embedded in the nasal mucosa and surrounded by supporting cells. The tips of the olfactory receptor cells are covered in olfactory hairs, which contain chemoreceptors for a wide variety of chemicals. The mucus secreted by the supporting cells lining the nasal cavity dissolves chemicals entering the nose, allowing them to come into contact with the olfactory hair receptors.

If an odorant chemical comes into contact with a matching olfactory receptor, the olfactory filaments (axons of the olfactory neurons) transmit action potentials via the olfactory nerve (cranial nerve I) to the olfactory cortex in the temporal lobes. The olfactory nerve travels through the olfactory tract through the ethmoid bone of the skull.

After the olfactory cortex has processed the smell, the limbic system forms strong associations between smells, memories and emotions.

Olfaction


By Chabacano [CC-BY-SA-2.5 (www.creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons

Figure 8.7.1 - Olfactory (smell) reception structures. (1) Olfactory bulb (2) Olfactory neurons (3) Ventral portion of ethmoid bone (4) Nasal mucosa (5) Olfactory glomeruli (6) Olfactory receptor cells

Gustation (taste)

The are only five different types of gustation (taste) receptors in humans, all found on the superior surface of the tongue. Gustation receptors are non-specific chemoreceptors that only detect types of chemicals, not specific molecules.

Table 8.7.1 Types of gustation receptors

Name of receptor Types of molecules detected
Sweet sugars, saccharine, some amino acids
Sour Acids
Bitter bases
Salty metallic ions
Umami ("delicious") glutamate (an amino acid)

Tongue papillae are visible bumps on the surface of the tongue, and are often mistakenly referred to as taste buds. The tongue papillae are in fact raised bumps that surround a channel that allows saliva to flow down into a protected area surrounding each papilla. The lingual papillae are found covering most of the anterior tongue, and the larger fungiform papillae cover the posterior tongue near the pharynx.

Inside the channels created by the papillae are the taste buds, clusters of taste receptor cells. The tips of the taste buds have gustatory hairs, which are covered in taste receptors. The cluster of gustatory hairs leads to an opening called the taste pore, which allows tastant chemicals dissolved in the saliva to reach the gustatory hairs. The arrangement of the cluster of taste receptor cells around a central point gives them the appearance of a flower bud.

The gustatory receptor cells lead to the sensory branches cranial nerves: the facial nerve (VII; anterior tongue) and the glossopharyngeal nerve (IX - posterior tongue). Sensory input from these two nerves leads to the gustatory area of the cortex, just above the ears in the parietal lobe. The vagus nerve is responsible for tongue movements including swallowing.

Gustatory receptors

By NEUROtiker (Deutsch: selbst erstellt English: own work) [GFDL (www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (www.creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (www.creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons

Figure 8.7.2 - Taste bud. Saliva with dissolved tastants (chemicals) enter the taste pore. Chemoreceptors on hairs of the taste receptor cells tastants of five different categories: salty, sweet, bitter, sour and umami ("delicious").