Olfactory Flashcards
diminished sense of smell or olfactory hallucinations are often early signs of
a neurodegenerative disease
sense of smell is tied to
quick decisions and emotional states (limbic systems)
sensory neurons in olfaction
Sensory neurons span mucosal layer in superior aspect of nose through the cribiform plate
Two Odorant Pathways
Odorants reach the olfactory epithelium either through the:
Nasal cavities
Oropharynx
One Pheromone Pathway
Pheromones reach the Vomeronasal organ (VNO)
Nasal cavities carry axons through VNN to olfactory epithelium
olfactory region histological organization
Olfactory region in dome of each nasal cavity, lined with olfactory mucosa
Olfactory epithelium:
Olfactory receptor cells/Olfactory neurons (bipolar neurons)
Sustentacular cells (parallel to glia)
Basal cells (parallel to stem cells)
Brush cells (same as in respiratory epithelium)
Lamina Propria is contiguous with periosteum of underlying bone
Olfactory Glands (Bowman’s glands)
Lipofuscin + serous secretions give nasal mucous its brown-yellow color
Secretions both traps molecules and dissolves odorants
Olfactory mucosa designed to trap volatile odorants
Olfactory Neurons (ON) are bipolar neurons present throughout the epithelium
Apical pole has non-motile cilia
Large, unmyelinated axons leave epithelium and unite in lamina propria as small nerves.
Pass through small foramina in cribiform plate, come together and become the olfactory nerve (CN I)
Olfactory mucosa designed to trap volatile odorants
Sustentacular cells: columnar, broad apex, microvilli. Most numerous in epithelium.
Basal cells: small, spherical/cone shaped, replace olfactory neurons every 2-3 months
Brush cells (not seen): columnar, microvilli, basal surface comes into contact with CN V
There is laminar organization in the olfactory bulb as well
Bipolar neuron—rods/cones
But in the nose, actually have APs
Granule cells—amacrine
Periglomerular cells—horizontal cells
Sensory units are organized in groups
Pathway = bipolar cell to mitral cell, modifications made by periglomerular and granule cells
Glomerulus is functional “odor” unit where specific odors are defined and coded
Mucosal layer and cilia allow for volatile odorant capture to olfactory receptors
Sensory transduction in the olfactory epithelium involves GPCRs and cAMP
EACH ODOR RECEPTOR IS STEREOSPECIFIC FOR DISTINCT ODORS—THAT MEANS THERE ARE LIKELY THOUSANDS OF DIFFERENT ODOR RECEPTORS
After odorants bind, trigger intracellular signaling:
cAMP
Na+ channel opens
Neural stem cells in olfactory epithelium: a source of neuro-regenerative hope?
Interneurons of the olfactory bulb are rare in that they can be regenerated:
1) New neurons are born in the SVZ
- They travel through a chain of glia (a “tube”) called the Rostral Migratory Stream (RMS)
2) Olfactory ensheathing cells (OECs) help remyelinate and regenerate axons in the olfactory bulb
CNS olfactory pathways
OLFACTORY PATHWAYS PROJECT TO THE TELENCEPHALON DIRECTLY, DON’T REQUIRE THALAMUS FOR RELAY
2 major pathways: Medial and Lateral Stria
Vomeronasal system?
Many disparate parts to Primary Olfactory Cortex
Olfactory tubercle, piriform cortex, amygdala, entorhinal cortex
Olfactory information from one nostril travels to contralateral nostril to inhibit signaling via the anterior commissure. Otherwise, all pathways are ipsilateral
What 4 things constitute our primary olfactory cortex?
olfactory tubercle
piriform cortex
amygdala
entorhinal cortex
Gustatory system: Deciding what we need is based on taste (not flavor)
We can’t eat everything—need a way to encode what our body needs: Sugars Fats Proteins Ions
Taste is one of five basic properties Flavor is a mix Stimulation of taste buds Stimulation of olfactory receptors Stimulation of CN V fibers
5 Major classes of taste
Sweet Sour Salty Bitter Umami
*Each uses a different type of receptor
receptors and taste
Bitter is transduced by G protein–coupled receptors similar to Class I GPCRs (with short extracellular N termini). In contrast, sweet and umami are detected by dimers of Class III GPCRs (with long N termini that form a globular extracellular ligand-binding domain). One of the receptors for Na+ salts is a cation channel composed of three subunits, each with two transmembrane domains. Membrane receptors for sour and fat are as yet uncertain.
Taste buds are epithelial specializations on the tongue on papillae
Circumvallate
Foliate
Fungiform
TRC’s are NOT neurons. Signal through ATP
Types of papillae relate tongue distribution, cranial nerve pathways
Fungiform 25% Anterior 2/3rds of tongue Stimulated first Chorda tympani branch of CN VII
Circumvallate 50% Posterior tongue Stimulated last Lingual branch of CN IX
Foliate
25%
Mixed innervation
Taste buds of epiglottis and esophagus are innervated by
superior laryngeal branch of CN X
VII, IX, and X carry taste info to
nucleus of the solitary tract
these nerves also all project to the spinal trigeminal nucleus along with V
Taste buds contain machinery to
bind chemical compounds and code for taste sensation
Taste receptors interpret different stimuli to perceive taste
Sour—H+ ions (Type III) Salty—Na+ ions (Type I) No specialized molecular receptors involved Sweet—carbohydrates (Type II) Bitter—usually toxic molecules (Type II) Umami—amino acids (Type II)
Sweet, Bitter, and umami taste transduction involves GPCRs
Taste pathway: a simple, 2-neuron chain
Taste bud to…
Nucleus solitarius (nucleus of the solitary tract) to…
VPM Thalamus to…
Insular cortex to…
Orbitofrontal Cortex is where FLAVOR is processed
Basic touch info of the head ascends
bilaterally in two pathways
These fibers in the VPM are adjacent to the fibers for taste
- Fibers representing the oral cavity ascend ipsilaterally
