Olfactory System and Olfaction
Chemical sense
-simplest system
-includes olfaction, gustation, and visceral chemosensory
-ordorants and tastands serve as ligands
-receptors are activated by specific ligand or group of related ligands
-transduction occurs with ligand binding to receptor
-gating of ligand-gated ion channel
-activation of G-protein coupled receptor(GPCR)
Odorants
-airborne/volatile molecules
-specific for ordorant shape, size, composition
-threshold vary pM to mM
-hard to classify types
Olfactory Epithelium
Consists of olfactory receptor neurons (ORNs) and associated tissues
-nasal mucosa and cell layer
-basal lamina with blood vessels and ORN axons
-located inside nose along ethmoid bone of skull
-CRIBIFORM PLATE- porous region
-odorant TRANSDUCTION occurs within ORN CILIA that extend to mucous layer
-ORN axons project to IPSILATERAL OLFACTORY BULB via nerve bundles through
CRIBIFORM PLATE
Regeneration of Olfactory Neurons
-DO HAVE the ability to regenerate
-only other known regeneration in the hippocampal dentate gyrus
-regularly damaged to exposure to airbone pathogens and toxins
-support/protection by mucous
-traps harmful substances
-enzymes from supporting cells degrade pathogens
-OLFACTORY STEM CELLS reside near LAMINAR SURFACE of epithelium, serve as
progenitors
-other olfactory neurons also regenerate
Olfactory GPCRs
-odorant molecules bind to olfactory GPCRs on ORN membranes
-5 classes of GPCRs
1.) ordorants
2.) pheromones
3.) amino acids
4.) formyl peptides
Olfactory Transduction
1.) ordorants bind to GPCR in membrane of ORN cilia
2.) activates odorant-specific G protein
3.) activite G olf alpha subunit activates adenyl cyclase III
4.) adenyl cyclase III converts ATP  cAMP
5.) cAMP activates cyclic nucleotide-gated (CNG) cation channel
-produce influx Na+ and Ca+
-additional depol from Cl- efflux
6.) depol Na+ and Ca+ influx produce AP in ORN, transmitted via olfactory
bulb
Adaptation to ordorants
-receptor response declines with sustained ordorant exposure
-Ca+ influx through CNG cation channel activates intracellular Ca+-binding
protein calmodulin (CaM)
-CaM closes CNG cation channel
-CaM kinase II turns off adenyl cyclase II
-phosphodiesterase 1 C converts cAMP  AMP
-phosphorylation inactivates odorant GPCR
Olfactory receptor selectivity
-ORNs exhibit single type of ordorant GPCR
-GPCRs can respond to single or multiple odorants
-depend on molecular structure of odorant and region of binding
-different GPCRs have different affinities for the same odorant molecule
 explains how perception of odorant can change with odorant
concentration
Structure of Olfactory Bulb
-obligatory synapse for ascending ORN axons
-laminar structure located on ventral surface of forebrain
-MITRAL CELL dendrites receive synaptic input from ORNs in GLOMERULI
-mitral cell axons project to OLFACTORY CORTEX
-tufted cells reside between mitral cells and glomeruli
-periglomerular cells receive synaptic input in glomeruli and project to mitral
and tufted cell dendrites
-granule cells receive synaptic input from projection cell axons and feedback
to projection cell dendrites
Olfactory bulb glomeruli
-specialized synaptic structure in olfactory bulb
-each glomerulus consists of dendrites from mitral, tufted, and periglomerular cells
-receives synaptic input from thousands of ORNs expressing same odor receptor
Other olfactory bulb features
-reciprocal dendrodendritic synapses
-both sets of dendrites release NT and exhibit postsynaptic response
-may serve to sharpen odor “tuning” within and across glomeruli
-neural regeneration
-not limited to epithelial ORNs
-includes periglomerular and granule cells
Central olfactory pathways
-sensory cortical target is in uncus on ventral forebrain
-part of specialized paleocortex
-3-4 layers
-inputs in layer 1, not 4
-projection neurons layer 2, not 5
-output projects to multiple portions of the limbic system
-includes: entorhinal cortex, amygdala, thalamus
-memory and emotion
-speech and vision
Olfaction, memory, and emotion
-olfactoy pathways in brain initiate memory and emotion response
-territorial and reproductive cues in many mammals
-stimulus for synchronized menstrual cycles for female roommates
-can trigger memory association with odor
-volatile substances in food activate olfactory receptors
Olfactory pathology
Anosmia or hyposmia – loss or reduction of smell
-multiple causes: infection, mechanical blockage, excess mucus, head trauma,
aging, medication
-resolves when infection clears, when damages ORNs replaced during
regeneration
-can alter taste of food and result in appetite loss
Olfactory hallucinations-perception of noxious odors not present
-typically precedes seizures in uncus
Gustatory System and Gustation
Overview of gustatory system
-characterizes chemical and physical properties of ingested substances
-promote ingestion of nutrients, prevent ingestion of toxic compounds
-prepare digestive tract to initiate digestive process
-strong relationship between taste and appetite
-pleasant taste =enhance appetite
-unpleasant taste = diminish appetite
-hunger can enhance taste perception
-extremely unpleasant taste can initiate nausea and/or vomiting
Organization of gustatory system
-taste perception mediated by three cranial nerves
1.) facial (VII)-Chorda tympani branch
2.) glossopharengeal (IX)
3.) vagus (X)
-ipsilateral projection to nucleus of solitary tract (NTS) in brainstem
-reciprocal connections between NTS, hypothalamus, and amygdala
-responsible for affective response to taste, modulation of taste
-ipsilateral and contralateral projections to ventral posteromedial thalamus and
gustatory cortices
-connection to cortex only ipsilateral
Classification of tastants
1.) Sweet
-includes sugars, alcohols, ketones, and amino acids
-sucrose threshold around 10 mM
-everywhere
2.) Umami or savory
-monosodiumglutamate (MSG) and amino acids
-MSG threshold around 20 mM
-everywhere
3.) Salty
-inorganic salts such as NaCl, NaF, NH4Cl,MgCl2
-NaCl threshold around 10 mM
-frontal, lateral
4.) Sour
-acidic compounds that produce H+
-acid threshold around pH 4
-lateral sides of tongue
5.) Bitter
-organic alkaloids, often highly toxic
-quinine threashold around 10 mM
-back of tongue
-taste perceptions formed by combinations of these tastants and/or other
somatosensory modalities
-ex. Hot pepper, metallic, greasy
Taste bud anatomy
-PAPILLAE are structures that contain taste buds or nerve endings
-taste receptors exposed to salivary fluids on or below tongue surface
-various papillae types on tongue
1.) Circumvallate papillae (50%)
-ringed structure encircles central body on posterior tongue
-taste receptors project to glossopharyngeal nerve
2.) Fungiform papillae (25%)
-mushroom shaped, anterior tongue
-taste receptors project to facial nerve
3.) Foliate papillae (25%)
-linear ridge structure along poserior tongue
-taste receptors project to glossopharyngeal nerve
4.) Filliform –MOST NUMEROUS, NOT INVOLVED IN GUSTATION
-conical shape, cilia projections throughout tongue
-mechanosensitive nerve endings
-transmit info to CNS via trigeminal
-taste buds consist of taste and supporting cell
-basal cells support taste cell
-tastant receptors on microvilli extending into taste pore
Tastant Transduction
-2 classes of tastant transduction
1.) direct gating or permeation of ion channels
-Salty: amiloride sensitive Na+ channel
-Sour: H+ sensitive TRP channel, known as PKD2L1
2.) GPCR mediated responses in sweet, umami, and bitter
-GPCR activates G-protein that activates PLC
-PLC produces IP3
-IP3 gates TRPm Ca+ channels and Ca+ release from intracellular
stores
-Ca+ influxes leads to NT release
Trigeminal chemoreception
-consists of nocioreceptive afferents in trigeminal (V), glossopharyngeal (IX), and
vagus (X)
-provides info about ingesting irritant chemicals
irritants include:
1.) NH3
2.) Ethanol
3.) Acetic acid
4.) CO2
5.) Methanol
6.) Capsaicin
-relay to spinal trigeminal nucleus and ventral posteromedial thalamus
-initiates reflexes including:
1.) salivation
2.) perspiration
3.) tearing and nasal secretion
4.) respiratory responses
Gustatory pathology
Ageusia- loss of taste
-can result from various disorders
-LESIONS IN FACIAL NERVE or GLOSSOPHARYNGEAL NERVE
-facial nerve lesions often occur after middle ear disease or
surgery
-Chorda tympani passes through middle ear
-head trauma
-loss of taste buds during aging
-LESIONS in UNCUS region of brain can lead to abnormal taste
sensations
-gustatory hallucinations may precede seizures with temporal lobe
origins