Olfactory receptors and smell

• Special senses include smell, taste, vision, hearing, and equilibrium.
• Special sense receptors are arranged in complex sensory organs like the tongue, eyes, and ears.
• Olfaction (smell) and gustation (taste) are considered chemical senses
• Smell and taste have a strong connection to the limbic system, which influences memories and emotions
• The olfactory epithelium contains ten million receptors located in the superior nasal cavity and cribriform plate.

Olfactory Cell Types

• Olfactory receptors are first-order neurons of the olfactory pathway.
• Supporting cells are columnar epithelial cells that provide support and insulation to olfactory receptors
• Basal cells are stem cells that produce new olfactory receptors and supporting cells, replacing the epithelium monthly.

Olfactory Details

• Each olfactory receptor is a bipolar neuron and connects to the olfactory bulb
• Olfactory hairs on the dendrites respond to inhaled chemicals called odorants
• Olfactory (Bowman's) glands produce mucus that moistens the surface and dissolves odorants
• Most epithelium and glands are innervated by cranial nerve VII (Facial nerve).
• Ability to recognize 1,000 different odors depends on a pattern of activity in the olfactory transmembrane receptor protein (GPCR) triggers one or more nerve impulses
• Methyl mercaptan is added to natural gas to provide warning of gas leaks.
• Adaptation to smells occurs quickly, by about 50% in the first second, and then more slowly

Olfactory Transduction

• Odorant molecule binds to GPCR, activates adenylate cyclase, which produces CAMP.
• CAMP opens sodium ion channels, causing an inflow of sodium and a depolarizing generator potential.
• Depolarization then generates a nerve impulse that propagates along the olfactory receptor axon

Olfactory Pathway

• Olfactory receptors go to olfactory nerves, then to the olfactory bulbs and olfactory tracts
• The olfactory tracts then go to the cerebral cortex and limbic system
• Axons in the cribriform plate of the ethmoid bone form the olfactory nerve
• bundles of axons terminate in the olfactory bulb.
• Olfactory sensations reach the cerebral cortex without first synapsing in the thalamus
• Collateral axons from the olfactory tract project to the limbic system and hypothalamus, influencing emotional and memory-evoked responses
• Odour identification and discrimination activity is in the orbitofrontal area

More About Olfactory

• Adaptation in olfactory is rapid, with a low threshold
• The trigeminal nerve innervates the posterior nasal cavity to detect noxious stimuli
• Olfactory bulb interneuron types synapse on mitral cells within structures called glomeruli

Neural Interactions

• Mitral cells in the olfactory bulb receive input from olfactory receptor neurons
• The basal dendrites of mitral cells are connected to interneurons like granule cells
• These release GABA and produce lateral inhibition between mitral cells
• Dendro-dendritic synapses use glutamate and GABA
• Olfactory receptors form the olfactory nerve (cranial nerve I).

Tumors and Sensation

• A reduced ability to smell (hyposmia) and distortion of smell sensation (dysosmia) increases with age and neurological changes.
• Perception of smell in the absence of stimuli is phantosmia (smell hallucination
• Cacosmia is the perception of a bad smell; seen to be associated with temporal focus partial complex epilepsy
• Olfactory dysfunction can help diagnose early Parkinson's, Alzheimer's, and Huntington's diseases.

Gustation

• Molecules must be dissolved in saliva to be detected.
• What you are tasting is actually what you are smelling

Taste Sensors

• Activated by sugars, alcohol, ketones, aldehydes, metal ions (Na+), free hydrogen ions (acids).
• Also, caffeine, nicotine, activated by L-glutamate and umami receptors, activated by alkaloids.

Taste Buds and Papillae

• Taste buds are found on the tongue, soft palate, epiglottis, and pharynx
• The number of taste buds declines with age
• Papillae are elevations on the tongue.

Papillae Types

• Circumvallate papillae contain taste buds and are located in a V-shaped row at the back of the tongue
• Fungiform papillae contain taste buds and are located on the margins of the tongue
• Filiform papillae don't contain taste buds and increase friction between the and tongue

Taste Bud Details

• Taste buds are oval bodies consisting of three kinds of epithelial cells: supporting, receptor, and basal cells
• Gustatory hairs (cilia) project from receptor cells
• Basal cells differentiate into supporting and receptor cells
• Receptor cells are replaced every 10 days
• chemicals stimulate gustatory receptor cells

Sensory Pathway

• Chemicals that stimulate gustatory receptor cells are called tastants
• Tastants dissolve in saliva and bind, resulting in receptor potentials that release neurotransmitters like ATP
• Most sensitive to bitter and least sensitive to salty and sweet taste
• Na+ channels in plasma membrane cause depolarization, H+ enters the cell via the channels (sour tastants)
• Sweet, bitter and umami tastants bind to G-protein receptors (GPCR)
• Thresholds for bitter (ex. Quinine) is the lowest which is the possibly reason for having toxicity
• Adaptation to changes in taste receptors in 1-5 minutes

Sense Combinations

• First-order gustatory fibers are found in cranial nerves VII (anterior 2/3 of tongue), IX (posterior 1/3 of tongue), and X (epiglottis and esophagus).
• Signals travel through the thalamus, cortex, limbic system, and hypothalamus
• Taste fibers extend from the thalamus (VPN) to the primary gustatory area on the insular lobe of the cerebral cortex
• Signals also reach the limbic system (emotions) and hypothalamus (satiety)
• cranial nerve V Trigeminal contributes gustatory, which is no sense of taste but sends signals

Gustatory Summary

• Gustatory adaptation is fast
• The gustatory epithelial cells have a separation of approximately 10 days
• 3 papillae are in a medulla salivary reflex

Limbic and Hypothalamus Relation

• The thalamus (VPN) connects to the limbic system and hypothalamus.
• The Temporal lobe is within an Insula

Taste Preference

• Taste preferences can help determine what the body needs for building proteins
• Better and sour tastes are considered aversive, indicating food that has "gone bad."