The Chemical Senses: Smell and Taste

Introduction to Chemical Senses

The chemical senses, smell (olfaction) and taste (gustation), are essential for detecting environmental chemicals and determining whether substances should be consumed or avoided. Both senses rely on chemoreceptors, which require chemicals to be dissolved in aqueous solutions for detection.

• Smell receptors are activated by chemicals dissolved in nasal fluids.

• Taste receptors respond to chemicals dissolved in saliva.

Sense of Smell (Olfaction)

Anatomy of the Olfactory System

The olfactory system is responsible for detecting airborne chemicals and transmitting this information to the brain for processing.

• The olfactory epithelium is the organ of smell, located in the roof of the nasal cavity and covering the superior nasal conchae.

• It contains olfactory sensory neurons (bipolar neurons with olfactory cilia), supporting cells, and olfactory stem cells.

• Olfactory stem cells allow for the regeneration of olfactory neurons every 30–60 days.

Olfactory Receptors and Specificity

Olfactory receptors are highly specialized for detecting a wide variety of odorants.

• Humans have approximately 400 functional olfactory receptor genes, each encoding a unique receptor protein.

• Each olfactory sensory neuron expresses only one type of receptor protein, but each receptor can bind multiple odorants, and each odorant can activate multiple receptors.

• This combinatorial coding allows humans to distinguish thousands of different odors.

Species Differences in Olfactory Capacity

• Different species have varying numbers of olfactory receptor genes and olfactory neurons, affecting their sense of smell.

• For example, dogs have about 1,000 olfactory receptor genes and 250 million olfactory neurons, compared to humans with 350 genes and 6 million neurons.

Olfactory Transduction Process

Olfactory transduction is the process by which odorant molecules are converted into electrical signals in the nervous system.

1. Odorant binds to its receptor on the olfactory cilia.

2. The receptor activates a G protein (Golf).

3. Golf activates adenylate cyclase, which converts ATP to cAMP.

4. cAMP opens cation channels, allowing Na+ and Ca2+ influx, causing depolarization and generating a receptor potential.

5. If threshold is reached, an action potential is generated and transmitted to the brain.

The Olfactory Pathway

Olfactory signals are transmitted from the nasal cavity to the brain through a series of neural pathways.

• Filaments of the olfactory nerve (cranial nerve I) synapse with mitral cells in the olfactory bulb.

• Mitral cells (second-order neurons) form the olfactory tract, which relays signals to the olfactory cortex, hypothalamus, amygdala, and limbic system.

• Emotional responses to odors are mediated by connections to the limbic system.

Clinical Considerations

• Anosmias: Loss of smell, often due to head injuries, nasal inflammation, or neurological disorders (e.g., Parkinson’s disease).

• Olfactory hallucinations (phantosmia): Perception of odors without external stimuli, often associated with temporal lobe epilepsy.

Sense of Taste (Gustation)

Anatomy of Taste Buds

Taste is detected by chemoreceptors located in taste buds, primarily on the tongue.

• Most taste buds are found in papillae: fungiform (across the tongue), foliate (side walls), and vallate (V-shaped row at the back).

• Taste buds contain gustatory epithelial cells (taste receptor cells) with microvilli (gustatory hairs) that project into taste pores and are bathed in saliva.

• Basal epithelial cells act as stem cells, regenerating taste cells every 7–10 days.

Basic Taste Sensations

There are five primary taste sensations, each associated with different chemicals:

• Sweet: Sugars, saccharin, alcohol, some amino acids, some lead salts

• Sour: Hydrogen ions (H+) in solution

• Salty: Metal ions (e.g., Na+), sodium chloride tastes saltiest

• Bitter: Alkaloids (quinine, nicotine, caffeine) and some nonalkaloids (aspirin)

• Umami: Amino acids glutamate and aspartate (e.g., beef, cheese, monosodium glutamate)

Taste preferences have homeostatic value, guiding intake of beneficial substances and avoidance of harmful ones.

Physiology of Taste

For a chemical to be tasted, it must be dissolved in saliva, diffuse into the taste pore, and contact gustatory hairs.

• Binding of a tastant depolarizes the gustatory epithelial cell, causing neurotransmitter release and initiating a generator potential in the sensory neuron.

• Different taste modalities have different mechanisms of transduction:

  • Salty: Na+ influx directly causes depolarization.

  • Sour: H+ ions open cation channels, allowing other cations to enter.

  • Sweet, bitter, umami: Bind to G protein-coupled receptors (gustducin), activating second messenger pathways that release stored Ca2+ and open cation channels.

• All taste receptors adapt rapidly (3–5 seconds, complete adaptation in 1–5 minutes).

Other Senses Involved in Taste Perception

Taste perception is influenced by additional sensory inputs:

• Olfactory input (smell) is responsible for about 80% of taste perception.

• Touch (mechanoreceptors), temperature (thermoreceptors), and pain (nociceptors) in the oral cavity also contribute.

• Spicy foods can activate pain receptors, which some people experience as pleasurable.

Gustatory Pathway

Taste signals are transmitted to the brain via cranial nerves:

• Facial nerve (VII): Anterior two-thirds of the tongue

• Glossopharyngeal nerve (IX): Posterior one-third of the tongue and pharynx

• Vagus nerve (X): Epiglottis and lower pharynx

Pathway:

1. Gustatory fibers synapse in the solitary nucleus of the medulla.

2. Signals are relayed to the thalamus.

3. From the thalamus, signals are sent to the gustatory cortex in the insula.

4. The hypothalamus and limbic system are also involved, mediating emotional responses to taste.

Salivary Glands and Taste

Saliva, produced by three main pairs of salivary glands, acts as a solvent for tastants and facilitates their clearance from the oral cavity.

Summary Table: Comparison of Olfactory and Gustatory Systems

Additional info: The Nobel Prize in Physiology or Medicine 2004 was awarded for discoveries related to odorant receptors and the organization of the olfactory system, highlighting the importance of molecular genetics in sensory biology.