• All sensory organs contain receptors, increasing sensitivity to the environment.
• The relatively simple receptors and pathways for the general senses of touch, pressure, vibration, temperature, pain, and proprioception were considered in Chapter 16.
• The special senses include smell, taste, vision, hearing, and equilibrium.
• Special sensory afferent pathways resemble general sensory pathways.
• Special sense receptors are arranged in complex sensory organs, specifically the nose, tongue, eyes, and ears.

Chemical Senses

• Include olfaction (smell) and gustation (taste).
• These senses detect airborne and ingested chemicals.
• Play crucial roles in food preference, nutrition, and social behavior.
• These senses interact with receptor cells on a molecular level.
• Smell and taste are strongly connected to the limbic system
• Certain odors or tastes can evoke emotional responses or memories.

Olfactory Epithelium

• A one-square-inch membrane
• Contains 10-100 million receptors
• Covers the superior nasal cavity and cribriform plate
• The membrane is within a total area of 5cm squared or a little less than 1 inch squared.
• Located in the nasal cavity.
• Three cell types include olfactory receptors, supporting cells, and basal cells.
• Odorants stimulate olfactory hairs.
• Olfactory receptors respond to stimulation by producing a generator potential.
• Olfactory Bowman's glands produce mucus, moistening the surface to dissolve odorants for transduction.
• Cranial nerve VII innervates these glands and the epithelium
• Impulses in this nerve can stimulate the lacrimal glands in the eyes and nasal mucous.
• Results in tears and a runny nose after inhaling substances like pepper and ammonia.

Olfactory Receptors

• First-order neurons of the olfactory pathway
• Each receptor is a bipolar neuron with a knob-shaped dendrite.
• Each olfactory sensory neuron expresses only one type of olfactory receptor protein
• Humans have about 400 different functional olfactory receptor genes
• The axon projects through the cribriform plate and ends in the olfactory bulb.
• Olfactory hairs, or cilia, respond to inhaled chemicals.
• Chemicals that stimulate the olfactory hairs are called odorants
• Olfactory receptors respond to the chemical stimulation of an odorant molecule by producing a generator potential
• Are G-protein coupled receptors (GPCRs)

Supporting Cells

• Columnar epithelial cells of the mucous membrane provide physical and metabolic support and electrical insulation.
• These cells help detoxify chemicals.

Basal Cells

• Stem cells at the base of supporting cells, continually undergo cell division.
• Basal cells constantly differentiate, producing new olfactory epithelium about every 30 days.

Olfaction Transduction

• Odorant binds to a transmembrane receptor protein (GPCR) in the olfactory hair plasma membrane.
• When an odorant binds to an olfactory receptor protein, it activates a G protein and adenylate cyclase
• The production of cAMP results
• Cyclic AMP opens sodium ion (Na+) channels and Na+ ions enter the olfactory receptor.
• The resulting depolarization may generate an action potential
• Action potentials propagate along the axon of the olfactory receptor.

Olfaction Process

• GPCR activates adenylate cyclase via a G protein
• Adenylate cyclase produces cAMP
• cAMP opens sodium ion channels, causing an influx of sodium
• Leads to a depolarizing generator potential
• Triggers a nerve impulse to propagate along the axon of the olfactory receptor.

Smell Adaptation

• Occurs quickly.
• The threshold is low, with few molecules of certain substances needed for detection.
• Methyl mercaptan, similar to the smell of rotten eggs, is detectable at 1/25 billionth of a milligram per mL of air.
• Adaptation is rapid, about 50% in the first second but continues slowly.
• Complete insensitivity to strong odors can occur after about a minute
• Complete insensitivity involves central nervous system input.

Olfactory Nerve Impulses

• Impulses are conveyed through the olfactory nerves to the olfactory bulbs, olfactory tracts, cerebral cortex, and limbic system.
• Unmyelinated axons of olfactory receptors extend through ~20 olfactory foramina in the cribriform plate of the ethmoid bone.
• About 40 axon bundles form the right and left olfactory nerves, terminating in the olfactory bulb.

Olfactory Bulb

• Located below the frontal lobes of the cerebrum, lateral to the crista galli of the ethmoid bone
• Is the first relay station for olfactory information in the brain
• Axon terminals synapse with dendrites and cell bodies of olfactory bulb neurons
• The olfactory tract extends posteriorly and projects to the primary olfactory area of the cerebral cortex.
• Axons of olfactory sensory neurons converge onto glomeruli in the olfactory bulb
• Each glomerulus receives input from olfactory sensory neurons expressing the same type of olfactory receptor
• Within the glomeruli, the olfactory sensory neuron axons synapse with mitral cells and tufted cells, which are the main output neurons of the olfactory bulb
• Mitral and tufted cells send olfactory information to the olfactory cortex
• The primary olfactory area is where conscious awareness of smell begins.

Olfactory Sensations

• The only sensations that directly reach the cerebral cortex without synapsing in the thalamus.
• Collateral axons project to the limbic system and hypothalamus, linking to emotional and memory responses.
• Examples include sexual excitement from perfume, nausea from certain foods, and childhood memories.
• Olfactory information can trigger vivid memories and emotions
• Odor identification and discrimination occur in the frontal lobe, with the orbitofrontal area of the right hemisphere being more active.

Olfactory Cortex and Pathways

• Includes several brain regions such as the anterior olfactory nucleus, olfactory tubercle, piriform cortex, amygdala, and entorhinal cortex
• Is unique among sensory cortices because it receives direct input from the olfactory bulb without first passing through the thalamus
• Olfactory information is also sent to the thalamus and orbitofrontal cortex (OFC)
• The OFC is involved in the integration of olfactory and gustatory information and in the perception of flavor
• The amygdala mediates the emotional responses to odors, and the hippocampus is involved in olfactory memory.
• Axon terminals synapse with dendrites and cell bodies of olfactory bulb neurons
• The olfactory tract extends posteriorly and projects to the primary olfactory area of the cerebral cortex.

Olfactory Bulb Structure

• Receives direct input from olfactory nerves in the glomerular layer
• Consisting of axons from olfactory receptors
• Axons cluster into spherical structures known as glomeruli.
• Each glomerulus receives input primarily from olfactory receptor neurons that express the same olfactory receptor.
• Glomeruli are permeated by dendrites from mitral cells, which output to the olfactory cortex.

Olfactory Interneurons

• Exists within glomeruli.
• Periglomerular cells synapse within and between glomeruli.
• Granule cells synapse with mitral cells.
• Mitral cells release glutamate, while granule cells release the inhibitory neurotransmitter GABA.
• Synapses between mitral and granule cells are dendro-dendritic, releasing neurotransmitters from both sides.
• Mitral cells can inhibit themselves (auto-inhibition) and neighboring mitral cells (lateral inhibition).
• Axons from the olfactory receptors (cranial nerve I) synapse in the olfactory bulb and pass through 40 foramina in the cribriform plate.
• Second-order neurons form the olfactory tract
• Synapsing on the primary olfactory area of the temporal lobe (mainly).
• Other collaterals lead to the limbic system, for conscious awareness of smell.

Olfactory Pathologies

• Olfactory dysfunction can result from head trauma, upper respiratory infections, tumors in the anterior cranial fossa, and toxic chemicals.
• Hyposmia reduces the ability to smell, potentially caused by neurological changes, certain drugs, or smoking.
• Anosmia means the absence of smell sensation
• Anosmia is the loss of the sense of smell
• Dysosmia means the distortion of smell sensation.
• Parosmia means the perception of smell in the absence of an appropriate stimulus.
• Cacosmia means perception of a bad or foul smell.
• Phantosmia means the perception of smell in the absence of an odorant.
• Partial complex epilepsy can manifest auras of foul-smelling odors.
• Olfactory dysfunction is associated with Parkinson's disease, Alzheimer's disease, Huntington's disease, and obsessive-compulsive disorder.
• It can aid in diagnosing these conditions.

Gustation (Taste)

• Similar to olfaction
• A chemical sense.
• Ageusia is the loss of the sense of taste
• Molecules must dissolve to be detected in a medium
• Primary tastes include sour, sweet, bitter, salty, and umami (meaty or savory).
• Each taste receptor cell expresses receptors for one or more of these tastes
• Flavors are a combination of the primary tastes
• In conjunction with other somatic sensations.
• Odors of food pass upward from the mouth into the nasal cavity, known as retronasal olfaction.

Olfaction vs Gustation

• Olfaction is more sensitive
• Much of what perceived tasting is influenced by smell.
• Plugging the nose diminishes the sense of taste.
• Sweet tastes indicate energy-rich foods, salty tastes indicate electrolyte foods, and bitter and sour tastes are considered adverse tastes.
• Sweet taste is generally preferred due to its association with energy-rich foods
• Bitter taste is generally avoided due to its association with toxins
• Umami indicates food high in amino acids for building proteins.
• The classic regional localization of taste on the tongue is an oversimplification.

Taste Sensations

• Taste sensations can become localized to different regions.
• Regions include sensations like bitter, salty, sweet, sour, and umami
• The distribution of receptor cells is more uniform

Taste Buds

• Taste buds can be found on the tongue, soft palate, epiglottis, and pharynx, and larynx
• Are oval-shaped structures embedded in the epithelium of the tongue and other areas
• About 10,000 cells on average

• declines with age.

• Taste buds are located in papillae (elevations on the tongue).
• There are four types of papillae: filiform, fungiform, foliate, and circumvallate
• Filiform papillae do not contain taste buds and are involved in texture sensation
• Three types of papillae contain taste buds: circumvallate, fungiform, and foliate papillae.
• Each taste bud contains 50-100 taste receptor cells
• Taste receptor cells are arranged like segments of an orange
• Circumvallate papillae are large, inverted V-shaped structures (12 total) on the back of the tongue containing 100-300 taste buds each.
• Fungiform papillae are mushroom-shaped elevations scattered over the tongue which contain one to several taste buds
• Foliate papillae are located on the lateral margins of the posterior tongue, and degenerate in early childhood.
• Taste buds consist of gustatory receptor cells, supporting cells, and basal cells.

Gustatory Receptor Cells

• Specialized sensory receptors with hairs (cilia) that project through taste pores.
• They synapse with neurons.
• Supporting cells surround gustatory receptor cells, and stem cells (basal cells) differentiate into supporting and receptor cells every 10 days.
• Taste receptor cells are not neurons, but they form synapses with gustatory afferent neurons
• Taste receptor cells have a lifespan of about 10 days and are constantly replaced

Taste Physiology

• The tongue's surface consists of filiform papillae, for tactile purposes
• Receptors synapse with dendrites of first-order neurons
• Dendrites of each first-order neuron contact many gustatory cells.
• Tastants, chemical molecules dissolved in saliva, bind to these cells and stimulates their activity.
• Sensory neurons then cause the release of neurotransmitters to the brain.
• ATP is known as the primary neurotransmitter for these processes.
• The thresholds for taste vary.
• Most sensitive to bitter, least to salty and sweet.
• Taste receptors are activated differently by different tastants

Taste Activation Process

• Saltiness starts with Na+ entering gustatory cells via sodium ion channels via amiloride-sensitive sodium channels
• Sourness comes from H+ ions, which can block potassium channels and cause depolarization
• Sweet, bitter, and umami tastes are mediated by G-protein coupled receptors (GPCRs)
• Sweet taste receptors are T1R2 and T1R3 heterodimers
• Umami taste receptors are T1R1 and T1R3 heterodimers
• Bitter are mediated by T2R receptors
• Activation of sweet, bitter, and umami receptors leads to the activation of phospholipase C (PLC)
• PLC catalyzes the formation of IP3, which releases Ca2+ from intracellular stores, leading to depolarization and transmitter release
• Activation from different kinds of taste activation allows different sensations.
• Taste cells respond stronger to some stimuli over others.
• Stimuli are known as tastants.

Taste Thresholds

• Bitter has the lowest taste threshold
• Poisonous substances are often bitter
• The HCl in lemons elevates the acidic threshold.
• Salty and sweet have the highest taste thresholds.
• Adaptation exists for taste
• With continuous stimulation

Gustatory Pathways

• Taste receptor cells release neurotransmitters, such as ATP, onto gustatory afferent neurons
• The gustatory afferent neurons send taste information to the brainstem
• The primary gustatory cortex is located in the insula and frontal operculum
• The gustatory cortex is responsible for the conscious perception of taste
• Carried by cranial nerves.
• Temperature, texture, pain, sight, color, sound, expectations, memory, and satiety help determine taste.
• Flavor is a complex sensation that results from the integration of taste, smell, and somatosensory information
• Olfactory information plays a major role in flavor perception
• The retronasal olfaction occurs when volatile compounds from food in the mouth travel through the back of the nasal cavity and stimulate olfactory receptors
• Texture, temperature, and pain also contribute to flavor perception
• Capsaicin in chili peppers activates pain receptors, creating a sensation of heat
• Menthol in mint activates cold receptors, creating a sensation of coolness
• The facial nerve (VII) serves the anterior 2/3 of the tongue, the glossopharyngeal nerve (IX) serves the posterior 1/3, and the vagus nerve (X) serves the palate, epiglottis and esophagus.
• These nerves send information through the medulla to the thalamus and send them to the cortex, limbic system, and other cranial nerve nuclei.
• Conscious taste perception happens via the primary gustatory cortex.
• Nuclei in the medulla allow regulation of salivatory reflexes.
• Information is also routed to the limbic system and the hypothalamus.
• The gustatory nucleus lies at the rostal area of the medulla.

Sensory Integration and Flavor Perception

• The orbitofrontal cortex (OFC) is a key brain region for integrating sensory information and creating the perception of flavor

Neurological Pathways

• The neurological pathways for olfaction and gustation are distinct, but they converge in the brain
• Olfactory information travels from the olfactory bulb to the olfactory cortex, amygdala, and hippocampus
• Gustatory information travels from the taste buds to the brainstem, thalamus, and gustatory cortex
• Both olfactory and gustatory information are sent to the orbitofrontal cortex (OFC)

Impact of Olfaction and Gustation on Behavior

• The chemical senses guide feeding behavior and food preferences
• The chemical senses also play a role in social behavior, such as mate selection and kin recognition
• Pheromones are chemical signals that are used for communication between individuals of the same species