16. Olfaction

What is the Olfactory Epithelium?

- A region of epithelium found on the superior aspect of the Nasal Cavity around the Cribriform Plate which is composed of 4 different cell types and which is responsible for the transducing odorous compound presence to electrical activity in neurons

What is the takeaway clinical point of the path of olfaction?

- Inputs travel via the Olfactory Tract to the Ventromedial aspect of the Temporal Cortex

What is Sensory Localization of the Olfactory Epithelium?

- A term used to refer to the fact that various regions of the olfactory array in the Olfactory Epithelium respond to specific kinds of odorants - Odorants specifically seem to be sorted based on solubility, with questions of mucosal and tissue penetration to the given receptor being considered

What are Olfactory Binding Proteins?

- Compounds secreted by cells of Bowman's Glands into the mucous which bind to and help transport odorants from the lumenal surface of the mucous layer to the basilar surface where receptor covered neuronal cilia are found

Describe the specificity of the Olfactory System

- Each Olfactory Neuron expresses only one allele of a given G Protein Receptor gene - Each receiving Olfactory 2nd order neuron in the Olfactory Bulb only receives inputs from neurons from ONE receptor type - Thus, specificity for odor type is built into the precortical delivery system

What are Olfactory/Bowman's Glands?

- Glands in the LP of the LP of the Mucosa which deliver mucoid secretions to the surface of the Olfactory Epithelium in order to trap odorous compounds for delivery to receptors on the surface of Olfactory Neurons

How does aging affect olfaction?

- It decreases with age

What is Anosmia?

- Loss of smell

Describe the transduction mechanism for Olfactory neurons

- Olfactory Afferent Neurons have up to 1000 different kinds of G Protein Associated Receptors which all ultimately do the following: 1. Bind an Odorant specific to it 2. Activate an α Subunit in response 3. Activate Adenylyl Cyclase via α Subunit activity 4. Produce cAMP which in turn activates opens ion channels

Earlier I said that the Olfactory Bulb the Olfactory Bulb sends projections to the Amygdala & Entorhinal Center which in turn project to the Amygdala, Hippocampus, and Hypothalamus. Describe those connections and their function

- The Amygdala, Hippocampus, and Hypothalamus are all involved in the Limbic System, an array of interrelated structures in the Brain dedicated to memory and the emotional/social responses to inputs.

Earlier I mentioned that the Orbitofrontal Cortex receives projections from the Olfactory Bulb Targets; describe that connection

- The Olfactory Bulb Targets send connections either: 1. Directly to the Orbitofrontal Cortex 2. Indirectly via relay in the Medial Dorsal Nucleus of the Thalamus (MDN) first - This region, also known as the Lateroposterior Quadrant of the Orbitofrontal Cortex, is responsible for conscious perception of odor and also receives input from the Somatosensory Cortex and Gustatory Centers to integrate odor, taste, and texture into a coherent picture

Describe the Olfactory Bulb & its synapses

- The Olfactory Bulb is composed of two cell types: 1. Mitral Cells 2. Granule Cells 3. Periglomerular Cells - Mitral Cells possess multiple apical dendrites which receive synaptic connections from multiple incoming Olfactory Afferent Axons which attuned to a specific odorant - The Apical Dendrites of Mitral Cells also synapse with projections from Periglomerular Cells which connect to apical synapses of several Mitral Cells, take in information, and modulate the activity of the Apical Dendrites of the Mitral Cells in order to attune or dull their activity. These cells also receive descending input from higher centers. - Mitral Cells also possess lateral dendrites which synapse with nearby Granule Cells - Granule Cells receive descending axons from higher centers and can alter Mitral Cell activity in order to attune the olfactory system - Mitral Cells, having received the information and undergone modulation, project 2nd order axons to higher centers

What are the Commissural Connections of the Olfactory System?

- The mechanism by which neurons on one side of the nose compare their activity with neurons from the opposite side of the Nose via connections run through the Anterior Commissure to determine the location of smell sources

What are the causes of Anosmia?

- There are multiple possible causes separated broadly into a few categories: 1.Nasal - Factors which affect air flow and general health of the nose (Smoking, chronic rhinitis, etc) 2. Olfatory Epithelium - Injuries which the Bulb or Cribriform Plate, as well as toxins, metabolic deficiencies, and lesions 3. Central - Degenerative diseases and Epilepsy of the Temporal Lobe

Describe how Olfactory Afferent/Receptor Neurons respond to the presence of odorants

- There are two important ways of viewing the responses of these cells: 1. Generator Potentials - These depolarizations, which are slow, are directly proportional to the concentration of the odorant; the higher the concentration, the larger the depolarization & the more there are! 2. Action Potentials - AP frequency is in turn directly proportional to GP #

Describe the Olfactory Afferent/Receptor Neurons

- These are Bipolar Neurons with an apical bulb studded with non-motile cilia which contain receptors for the detection of odiferous compounds - The Basal Axon of these neurons is unmyelinated and passes through the Cribriform Plate to the Olfactory Bulb, where they synapse

List and describe the other major olfactory deficits

1. Agnosia - The inability to identify and name a specific sensation (in this case, olfaction) 2. Parosmia - Distortions of odor perception when and odorant is present 3. Olfactory Hallucinations - The perception of the presence of odorants when they are in fact not present 4. Aura - The presence of an overwhelming vague sense of olfactory perception after an Epileptic seizure of the Temporal Lobe

List and describe the cells which compose the Olfactory Epithelium

1. Olfactory Afferent/Receptor Neurons - These are bipolar neurons which possess odorant receptors on non motile cilia which project out from the surface of the Epithelium to bind volatile organic compounds (odorants) 2. Supporting Cells - These cells perform an analogous role to Glial Cells, supporting the Olfactory Neurons 3. Basal Cells - These are the stem cells of the region and are responsible for becoming either Olfactory Neurons or Supporting Cells 4. Brush Cells - These are microvillous epithelial cells in contact with CN V nerve endings which help mediate a sense of force

Describe the path by which smell information gets to higher centers of the Brain

1. Olfactory Receptors → Olfactory Nerve → Olfactory Bulb → Olfactory Tract → Olfactory Bulb Targets 2. The Piriform Cortex, Olfactory Tubercle, and Amygdala (Temporal Cortex Structures) all receive their own direct connections from the Olfactory Bulb and in turn direct axons to the Orbitofrontal Cortex, the Thalamus, and the Hypothalamus 3. The Entorhinal Cortex also receives innervation from the Olfactory Bulb and delivers it to the Hippocampus