Special Senses: Chemoreception Concepts

Special Senses: Chemoreception

• Chemoreception is the ability to detect chemicals.
• Two types of chemoreceptors are gustatory (taste) and olfactory (smell).
• Gustatory chemoreceptors detect dissolved chemicals at relatively high concentrations.
• Olfactory chemoreceptors detect airborne or dissolved chemicals at relatively low concentrations.
• Chemoreceptors have general characteristics: chemical signals bind to membrane-bound receptor proteins regulating cell membrane permeability. Information is transmitted to the cerebral frontal cortex for processing.
• Distinguishing features include different cell types and locations, different transduction mechanisms, and different neural pathways. Coverage of the frontal cortex is also a distinguishing factor.

Olfaction

• Odorants must be volatile (easily vaporized) and soluble in mucus layers to enter the nose.
• Olfactory epithelium is the organ of smell in the roof of the nasal cavity, which contains olfactory sensory neurons.
• Olfactory sensory neurons are bipolar neurons with thin apical dendrites containing several olfactory cilia.
• These neurons regenerate approximately every two months.
• Olfactory cilia are on the apical dendrites and are where odorant molecules bind.

Olfactory Transduction

• Odorants activate G protein-coupled receptors on olfactory cilia.
• A second messenger (cAMP) opens cation channels, facilitating Na+ and Ca2+ influx into the cell.
• This depolarization results in a graded receptor potential.
• The graded receptor potential triggers action potentials via voltage-gated Na+ channels.

Olfactory Receptor Cells

• Olfactory receptor cells can detect a wide variety of odorants (over 10,000).
• Each receptor cell expresses only one specific type of receptor protein, but a receptor cell can have a broad sensitivity.
• Receptor proteins bind a specific component of an odorant, and a variety of receptors can detect components of the same odorant.
• Odorants stimulate a population of receptor cells forming a combinatorial code for each odorant stimulus.

Olfactory Receptors & Odorant

• Odorant 1 will activate multiple receptors.
• Odorant 2 will also activate multiple receptors.
• The response to each odorant differs, so the responses can be distinguished.

Olfactory Pathway

• The olfactory bulb contains glomeruli where mitral cells receive input from many olfactory cells with similar characteristics.
• Olfactory tracts extend to the primary olfactory cortex in the temporal lobe and the pathway continues to the frontal lobe, allowing interpretation/identification of smells.
• Olfactory pathways also project to the limbic system causing emotional responses to odors.

Olfactory Receptors

• Olfactory receptors have a low threshold and high sensitivity (requiring low odorant concentration).
• Olfactory adaptation occurs quickly with intrinsic and extrinsic mechanisms removing odorants.
• Odorants are removed by similar detoxification enzymes found in the liver.

Gustation

• Taste buds are sensory receptor organs for taste, mainly located within papillae of the tongue.
• Within taste buds are gustatory receptor cells, epithelial sensory receptors, which aren’t neurons.
• Microvilli project into taste pores (location of receptor proteins).
• Gustatory receptor cells synapse with afferent neurons which carry signals to the brain (cortex).
• Basal cells, stem cells, rapidly turnover/replace gustatory receptor cells approximately every 10 days.
• Tastants dissolve in saliva, entering taste pores, to interact with taste receptors.

Five Primary Tastes

• Sweet, salty, sour, bitter, and umami are the five localized tastes associated with different taste receptor cells.

Gustatory Transduction

• Taste receptor proteins are in microvilli on apical surfaces of taste receptor cells.
• Various protein types & intracellular signals are used (e.g., salty – Na+ channel; sour – H+ channel; sweet, bitter, umami – G protein-coupled receptors).
• Receptor activation ↑ intracellular [Ca2+] leading to neurotransmitter release and an action potential in the afferent sensory neuron.

Salty & Sour

• Ion channels allow Na+ (salty) or H+ (sour) into the cell.
• Positive ion movement causes depolarization to receptor potential.
• Voltage-gated Ca2+ channels open and ↑ intracellular [Ca2+] causing neurotransmitter release.

Sweet, Bitter, Umami

• G protein-coupled receptor transduction systems activate phospholipase C (PLC) to increase IP3 levels.
• The increase in intracellular [Ca2+] triggers neurotransmitter release.

Thresholds & Adaptation

• Thresholds of activation vary by gustatory cell (bitter receptors most sensitive, salty/sweet less sensitive).
• Adaptation occurs within 1-5 minutes using intrinsic and extrinsic mechanisms.

Gustatory Pathway

• Three cranial nerves (facial, glossopharyngeal, vagus) carry signals to the medulla.
• Signals continue to the limbic system-hypothalamus for emotional responses and to the thalamus-gustatory cortex for conscious taste perception.
• Taste is strongly influenced by smell, thermoreceptors, mechanoreceptors, and nociceptors. Blockages of the nose can reduce taste perception.

Check Your Knowledge (Questions)

• Olfactory and gustatory receptors detect chemicals dissolved in fluids (e.g., saliva—taste; air—smell).
• Olfactory receptor cells are of epithelial origin.
• One olfactory receptor type binds to one component of an odorant, but one odorant may stimulate several receptor types.
• G protein-coupled receptor systems are involved in detecting sour tastants.
• Increased intracellular calcium levels from activation of gustatory receptor proteins trigger neurotransmitter release.
• Taste perception is affected by factors beyond stimulation of gustatory receptor cells.