Olfaction and Gustation

• A key point of differentiation involves special senses relays information from receptors in complex sensory organs, such as the tongue
• Special senses include olfaction (smell) and gustation (taste), vision, hearing, and equilibrium.
• Olfaction and gustation are considered chemical senses because of the interaction of molecules with receptor cells.

Chemoreceptors

• Located in the superior nasal cavity and cribriform plate.
• Include olfactory receptors, supporting cells, and basal cells.
• First-order neurons of the olfactory pathway.
• Each olfactory receptor is a bipolar neuron with a knob-shaped dendrite and an axon projecting through the cribriform plate to the olfactory bulb.
• Olfactory hairs respond to inhaled chemicals.
• Stimulation is converted into a graded potential.
• Odorants stimulate olfactory hairs.
• Receptors respond to chemical stimulation by producing a generator potential, initiating the olfactory pathway.
• Epithelial cells of the mucous membrane lining the nose.
• Provide physical and metabolic support to olfactory receptors.
• Detoxify chemicals.
• Stem cells located between the bases of supporting cells.
• Undergo cell division to produce new olfactory receptors.
• Replace the olfactory epithelium approximately every 30-60 days.
• Innervated by CN VII (Facial nerve).
• Stimulate lacrimal glands and nasal mucous membranes.
• Can trigger a runny nose after inhaling substances such as pepper or ammonia.
• Receptors are bipolar; each nerve has olfactory hairs.
• Both epithelium and glands are innervated by cranial nerve VII (Facial nerve).

Olfaction

• The ability to recognize 10,000 different odors depends on patterns of activity in the brain due to the activation of different combinations of olfactory receptors.
• A generator potential triggers nerve impulses during reception.
• When an odorant binds to an olfactory transmembrane receptor protein (GPCR) in the plasma membrane of an olfactory hair, it activates adenylate cyclase via a G protein.
• Adenylate cyclase produces cAMP, which opens sodium ion channels, causing an inflow of sodium ions that depolarizes the membrane and generates a nerve impulse.
• Adaptation occurs quickly, and the threshold of smell is generally low.
• Methyl mercaptan is added to natural gas as a warning agent.
• Humans are sensitive to methyl mercaptan.
• Adaptation is rapid initially, about 50% in 1 second, then adapts slowly thereafter.

Olfactory Tract

• Receptors extend through the cribriform plate of the ethmoid bone.
• Approximately 40 bundles of unmyelinated axons collectively form the left and right olfactory nerves (cranial nerve I).
• Terminate in the olfactory bulb.
• Synapses with dendrites and cell bodies of olfactory bulb neurons.
• Projects to the primary olfactory area of the cerebral cortex in the temporal lobe.
• Reach the cerebral cortex without synapsing in the thalamus.
• Project to the limbic system and hypothalamus, eliciting emotional and memory-evoked responses to odors.
• Front lobe is responsible for odor identification.
• Discrimination occurs in the orbitofrontal area.
• A decreasing sensitivity.
• Adaptation is rapid; 50% in 1 second, complete in 1 minute.
• The trigeminal nerve detects noxious stimuli within the olfactory bulb.
• The glomerular layer contains approximately ten million olfactory receptor axons which synapse in the glomeruli
• Glomeruli are permeated by dendrites from mitral cells, which output to the olfactory cortex.
• Mitral cells release glutamate as an excitatory neurotransmitter, while granular cells release GABA as an inhibitory neurotransmitter.
• Granule cells exhibit auto-inhibition and inhibit neighboring mitral cells.

Olfactory Dysfunction

• Many people experience some type caused by head trauma, upper respiratory infections, and tumors
• Anosmia (Absence of smell sensation)
• Decreased sensation of smell can be caused by head injuries, neurological changes, analgesics, and the damaging effects of smoking.
• Parosmia is a distortion of smell (Perception of smell in the absence of appropriate stimuli)
• Phantosmia is the perception of smell in the absence of an odorant, and it is a smell hallucination.
• Olfactory identification deficits may occur in early Parkinson's disease, Alzheimer's disease, and Huntington's disease and can help in diagnosis.

Gustation (Taste)

• Molecules must be dissolved in saliva to be tasted
• Odors of food can pass upward from the mouth into the nasal cavity, called retronasal olfaction.
• Plugging the nose diminishes the ability to taste because what you're tasting is what you are smelling.
• The body is sensitive to the five primary tastes: sweet, sour, salty, bitter, and umami
• Sweet tastes are activated by carbohydrates, and aldehydes, and fats
• Sour tastes are activated by metal, most commonly by Na
• Salty tastes are mostly activated by free hydrogen ions; the stronger the acid, the more "sour"
• Bitter tastes are activated by alkaloids (e.g., caffeine, nicotine).
• Umami tastes are activated by L-glutamate.
• 10,000 taste buds are found on the tongue, soft palate, epiglottis, and pharynx

Taste Papillae

• Circumvallate Papillae contain taste buds and form an inverted V-shaped row at the back of the tongue. They have about 100 taste buds in their trenches.
• Fungiform Papillae are mushroom-shaped and are scattered over the surface of the tongue, each containing about 5 taste buds.
• Foliate Papillae are found on the lateral margins of the tongue and mostly degenerate in early childhood.
• Filiform Papillae cover the entire surface of the tongue, increasing friction between the tongue and food, but don't contain taste buds.
• Gustatory receptor cells have specialized sensory receptors the hairs (cilia) that project to the surface through taste pores.
• Basal cells differentiate into supporting and receptor cells; they divide in about 10 days.
• Gustatory receptor cells that are stimulated release ATP, which stimulates the 1st order neurons
• Different tastes arise from activation of different combinations of groups of taste neurons. Bitter substances trigger the lowest thresholds while salty and sweet have higher threshold.

Gustatory Pathway

• Taste is also influenced by taste, temperature, texture, pain, sight, color, sound, and, especially, smell.
• Primary taste areas go to cranial nerves.
• VII Facial-serves anterior 2/3 of tongue
• IX Glossopharyngeal serves posterior 1/3 of tongue.
• X Vagus serves the palate, epiglottis, and esophagus.
• Signals are sent to the medulla, thalamus, limbic system & hypothalamus.
• Taste fibers extend from the thalamus to the primary gustatory area on the insular lobe of the cerebral cortex, providing conscious perception of taste.
• Info goes to thalamus (VPN), cranial nerve nuclei for salivation reflexes, then to the limbic system for emotions, and hypothalamus for satiety.
• Primary gustatory cortex is located located in the insula, in the temporal lobe
• Threshold is low for sweet, high for bitter.