Chemical Senses: Olfaction and Gustation

Olfaction (Smell)

Olfaction is considered the "oldest" sensory system and processes information relating to the identity, concentration, and quality of odorants. Much is still unknown about how the brain interprets olfactory information.

Structure of the Olfactory System

The olfactory system involves airflow through the nasal passage. Orthonasal olfaction refers to smelling through the nose, while retronasal olfaction occurs through the back of the mouth when we chew and swallow.

Brain Regions Involved

Specific brain regions are involved in olfaction.

Projections of the Olfactory Tract

The olfactory tract's projections are unique because:

  1. They start with Cranial Nerve I.

  2. The primary targets are located in the telencephalon.

  3. There is a specialized processing center for olfaction, consisting of a 3-layered archicortex, which is phylogenetically more ancient than the 6-layered neocortex. This develops from the striatum.

  4. Multiple processing regions perform multiple roles:

    • Odor identification

    • Initiation of motor reactions

    • Initiation of visceral reactions

    • Initiation of emotional reactions

Structure of the Nasal Mucosa

The nasal mucosa contains several key cell types:

  • Olfactory receptor neurons: These are bipolar and unmyelinated. The apical surface has a single dendritic process that expands into a knob-like protrusion from which olfactory cilia expand into a thick layer of mucus.

  • Bowman's glands: These glands secrete mucus.

  • Basal cells: These are the progenitor (stem) cells of the nasal mucosa, capable of differentiating into other cell types within the mucosa.

  • Sustentacular cells (supporting cells): These cells provide metabolic and physical support, remove dead neurons, offer trophic support and detoxification. They function similarly to glial cells in the nasal mucosa.

Sensory Transduction in Olfaction

What We Know

  1. Olfactory receptors are located on the cilia.

  2. When an odorant binds to its receptor, the cell is depolarized, triggering an action potential (AP) in the soma and sending APs out the olfactory nerve.

  3. This process is similar to other sensory systems.

  4. Olfactory receptor neurons constantly regenerate.

GPCR Binding and Activation

The sensory transduction process involves the following steps:

  1. A G-protein coupled receptor (GPCR) binds an odorant.

  2. The G-protein activates adenylyl cyclase, increasing cAMP levels (↑\uparrow cAMP).

  3. cAMP-gated channels open, leading to an influx of Na+Na^+ and Ca2+Ca^{2+}.

  4. Ca2+Ca^{2+} gated Cl−Cl^- channels open, resulting in an efflux of Cl−Cl^-.

Anatomy of the Olfactory Bulb

  • Olfactory receptor axons leaving the olfactory epithelium form bundles that make up the olfactory nerve (Cranial Nerve I).

  • Each olfactory nerve projects ipsilaterally to the olfactory bulb on the same side.

  • Primary olfactory axons are received by an array of glomeruli.

  • Within each glomerulus, receptor neuron axons contact the apical dendrites of mitral cells, which are the principal projection neurons of the olfactory bulb.

  • In mice, each glomerulus includes apical dendrites from approximately 25 mitral cells and receives innervation from about 25,000 olfactory receptor axons which may increase sensitivity and average out signal noise.

  • Each glomerulus also includes dendrites from tufted cells (~50) and periglomerular cells (~25), but their function remains unclear.

Enigmatic Granule Cells

  • Granule cells are located in the innermost layer of the olfactory bulb.

  • They synapse on the basal dendrites of mitral cells but lack an axon.

  • These cells make dendrodendritic synapses on mitral cells and establish local lateral inhibition.

  • Granule cells display synaptic plasticity.

  • New granule cells are continually produced in the subventricular zone by type B cells.

Sense of Smell in Humans

Importance

While the relatively small size of the human olfactory bulb compared to other animals has been cited as a reason for humans' "inferior" sense of smell, new research is starting to overturn this notion.

Olfactory Receptor Comparison

  • Dogs: 300 million olfactory receptors

  • Humans: 6 million olfactory receptors

Vomeronasal Organ (VNO)

Pheromones released by others are sensed by the vomeronasal organ, picked up by microvilli, and sent to the accessory olfactory bulb.

VNO in Humans

Great apes and humans lack a VNO, but its function may be held elsewhere.

  • Synchronized menses has been observed.

  • Androgen and estrogen compounds at subconscious concentrations elicit behavioral responses and different patterns of brain activation.

  • Females who are not pregnant favor the scent of a conspecific with the most different genetic makeup, while pregnant females prefer the scent of familial conspecifics (similar genetic makeup).

Interpretation and Identification of Odorants

Much is not known about how we interpret and identify odorants.

  • Specificity for odorants is related to the wide variation in amino acid sequence in the G-proteins.

  • The olfactory receptor gene family is the largest known, comprising 3-5% of all genes.

cAMP-gated Channels

cAMP-gated channels are blocked at resting potential by high calcium and magnesium concentrations in mucus. Overcoming this block requires several channels to open simultaneously, making the system very specific to odorant stimulation.

Changes in concentration alter the latency to response, duration, and firing frequency, which helps encode information.

Adaptation occurs through a reduced rate of action potentials in response to a continued presence of an odorant. Increased calcium binding by calmodulin decreases the sensitivity of the channel to cAMP, and calcium is extruded through the activation of Na+Na^+/Ca2+Ca^{2+} exchange proteins.

Olfactory Receptor Neurons

Individual olfactory receptor neurons are sensitive to a subset of stimuli at specific concentrations. Neurons with specific receptors are located in particular parts of the olfactory epithelium. The topographical arrangement of receptor neurons expressing distinct odorant receptor molecules is referred to as space coding.

  • There is one GPCR type per receptor neuron.

  • Olfactory receptor neurons respond to specific odors.

Gustation (Taste)

Why We Need Taste

  • Bitter and sour tastes warn us away from potentially harmful foods.

  • Taste helps identify foods rich in nutrients.

Taste Receptors

A tastant activates taste receptor cells (TRCs), leading to the depolarization of the TRC. This causes Ca2+Ca^{2+} to enter the cytosol, resulting in the release of neurotransmitters onto gustatory afferent neurons.

There are five basic tastes: sweet (sugars), umami (amino acids), salty, sour (acids), and bitter.

Taste papillae are the bumps on the tongue. Each papilla contains taste buds, with the number varying depending on the type of papilla. Each taste bud contains 30-100 taste cells.

All tastes can be perceived over the full surface of the tongue, but certain regions have different thresholds, making them more sensitive to certain tastes.

Organization of the Human Taste System

Taste information is transmitted via:

  • Cranial nerve VII (Facial nerve) for the anterior two-thirds of the tongue.

  • Cranial nerve IX (Glossopharyngeal) for the posterior one-third of the tongue.

  • Cranial nerve X (Vagus) for the epiglottis.

These nerves project to the nucleus of the solitary tract in the brainstem, then to the ventral posterior medial nucleus (VPM) of the thalamus, and finally to the insula and frontal cortex (gustatory cortex).

Taste Transduction

A sweet tastant interacts with receptors on taste receptor cells, leading to the activation of gustatory nerves.

Comparison of Olfaction and Gustation

Similarities and Differences

  • Olfaction does not have a thalamus relay, while taste does.

  • Receptors in both systems regenerate frequently.

  • Serotonin and ATP are the principal excitatory neurotransmitters for taste.

  • Granule cells of the olfactory system do not have true axons but make dendrodendritic connections.

  • Granule cells are continually produced in one of the few neurogenic niches in the adult mammalian brain.

  • Both systems primarily use metabotropic signaling as the primary mode of transduction.

  • Sodium channels are always open.