Olfactory System: Anatomy and Function

• Sensory organs have receptors that increase environmental sensitivity.
• Special senses include smell, taste, vision, hearing, and equilibrium.
• Differentiating between special and general senses lies in the arrangement of receptors in sensory organs like the nose, tongue, and eyes.

Olfaction (Smell) and Gustation (Taste)

• Considered chemical senses because they involve the interaction of molecules with receptor cells.
• Smell and taste have a strong connection to the limbic system, so odors and tastes can trigger emotional responses and memories.

Olfaction: The Sense of Smell

• Requires a quarter inch of membrane holding 10-100 million receptors; this is within an area of 5 cm².
• The superior nasal cavity and cribriform plate are covered with olfactory receptors
• Directing inspired air toward the olfactory epithelium in the upper posterior region is the job of the turbinates, also known as nasal conchae, in the nasal cavity.
• Olfactory receptors are bipolar neurons with a knob-shaped dendrite and an axon projecting to the olfactory bulb after passing through the cribriform plate.
• Olfactory hairs, or cilia, are parts of the olfactory receptors, which respond to inhaled chemicals and project from the dendrite.
• Transduction describes how stimulus energy is converted into a graded potential in a sensory receptor.
• Chemicals having an odor that can stimulate olfactory hairs are called odorants.
• Olfactory receptors respond to chemical stimulation from odorant molecules and produce a generator potential, kickstarting the olfactory response.
• Signals are relayed in glomeruli that are well-defined micro-regions in the olfactory bulb; receptor cells of the same type converge on the same glomerulus.

Cell Types in Olfactory Epithelium

• Olfactory receptors are bipolar neurons with cilia.
• Supporting cells are columnar epithelial cells that support and provide insulation to olfactory receptors, and detoxify contacting chemicals.
• Basal cells are stem cells that replace the olfactory epithelium monthly.
• Olfactory (Bowman's) glands produce mucus to moisten the surface, and also dissolves odorants for transduction.
• All three cells and glands are innervated by cranial nerve VII.

Olfaction Transduction

• The sense of smell relies on hundreds of primary scents as suggested by genetic evidence.
• Recognizing 10,000 different odors depends on the patterns of activity in the brain.

Molecular Process of Smell

• An odorant binds to a transmembrane receptor protein (GPCR) in the plasma membrane of an olfactory hair.
• Activation of adenylate cyclase follows.
• cAMP is produced, opening sodium ion channels.
• Sodium flows inward causing depolarization.
• A generator potential occurs.
• A the impulse is generated and propagated along the axon of the olfactory receptor.

Neural Pathways

• Olfactory receptors -> olfactory nerves -> olfactory bulbs -> olfactory tracts -> cerebral cortex, limbic system
• Unmyelinated axons of olfactory receptors pass through ~20 olfactory foramina in the cribriform plate of the ethmoid bone.
• Bundles of axons form the R and L olfactory nerves, terminating in the olfactory bulb, located inferior to the frontal lobes of the cerebrum, lateral to the crista galli of the ethmoid bone.
• Axon terminals form synapses with dendrites and cell bodies of olfactory bulb neurons, extending posteriorly to form the olfactory tract, which project to the primary olfactory area of the cerebral cortex.
• The position of the olfactory cortex is debated, but it is generally accepted to be on the medial aspect of the temporal lobe and sometimes on the base of the frontal lobe.
• Olfactory sensations reach the cerebral cortex without synapsing in the thalamus first.
• Collateral axons of the olfactory tract project to the limbic system and hypothalamus to account for emotional and memory responses to odors.
• Projections go from the primary olfactory area extend to the orbitofrontal cortex for odor identification and discrimination.
• The olfactory bulb transmits smell information from the nose to the brain, so it is necessary for sensing smells.
• The glomerular layer inside the olfactory bulb receives direct input from olfactory nerves, which comprise axons from ~10 million olfactory receptor neurons in the olfactory mucosa.
• Axon ends cluster in spherical structures (glomeruli); each glomerulus receives input primarily from olfactory receptor neurons that express the same olfactory receptor.
• Dendrites from mitral cells (that output to the olfactory cortex) permeate Glomeruli, and synapse with granule cells.
• The bulb has one sensory input and one output and functions as a filter.

Cellular Interactions

• Mitral cell dendrites are connected to interneurons known as granule cells, which theoretically produce lateral inhibition between mitral cells.
• A synapse that is "dendro-dendritic" has both sides which release neurotransmitters- a rare class of synapses.
• Mitral cells release glutamate whereas granule cells release GABA.
• This synapse results in bidirectional control that leads to both auto-inhibition and lateral inhibition.
• The second-order neurons within the olfactory bulb form the olfactory tract that synapses at the primary olfactory area of the(mainly) temporal lobe
• From here, conscious awareness begins
• Other collaterals then lead to the limbic system

Clinical Issues

• Olfactory dysfunction can be associated with early Parkinson's disease and other neurodegenerative disorders to aid in diagnosis."
• Hyposmia is the medical term for reduced smell ability.
• Anosmia is the absence of smell, Hypersomnia occurs when there’s decreased sensation, Dysomnia is a distortion, Parosmia is the the perception of a smell when there isn't an appropriate stimulus.
• Caosmia smells foul when there isn't a stimulus.

Gustation: The Sense of Taste

• Taste is a chemical sense so molecules must be dissolved
• There are five categories of tastes, these are sweet, sour, bitter, salty, and umami, the last being meaty or savoury.
• All flavours have a combination of tastes
• The odour of food passing from the mouth to the nasal cavity is called retronasal olfaction.
• Taste receptors detect tastes that indicate what the body needs. -Sweet indicates energy taste -Salt indicates electrolyte-rich food -Both bitter and sour tastes are considered aversive. -Umami indicates foods high in amino acids.

Taste buds

• 10,000 exist on the tongue, soft palate, epiglottis -The number decreases with age.
• Contain elevations of papillae.
• There are three types of papillae, these are Circumvallate papillae (12 large formations with 100-300 taste buds).
• Fungiform papillae are mushroom shaped elevations scattered over the tongue.
• Taste buds are composed of basal cells, gustatory receptor cells, cilia, and supporting cells.
• The back of tongue is where taste sensitivities would peak if it were the case that each taste sensation was localized to a particular region(this is false).
• Filiform papillae act as tactile receptors and is the point on the tongue
• Gustatory cells synapse with neurons that form the first bit of the gustatory pathway.
• Adaption to taste can occur with continuous stimulation lasting 1-5 minutes.

Explore the olfactory system, detailing the roles of mucus, cranial nerves, and cellular processes in smell perception. Understand olfactory adaptation, signal transduction, and odor recognition. Examine the impact of damage to olfactory structures.