Chapter 14 - Olfaction

odorants

molecules capable of eliciting responses from receptors in the olfactory mucosa

olfactory receptor neurons (ORNs)

bipolar neurons in olfactory epithelium that contain receptors for odorants

olfactory nerve

the set of bundles of unmyelinated axons originating from the olfactory receptor neurons in the olfactory epithelium of the nose that project through the cribriform plate and terminate in the olfactory bulb

cribriform plate

a bony structure of the facial portion of the skull comprising many small fenestra (tiny holes), at the level of the eyebrows, that separates the olfactory epithelium from the brain. The olfactory sensory neurons pass though the tiny holes of the cribriform plate to enter the brain

olfactory bulb

telencephalic structure that lies on the orbital surface of the frontal lobe and receives axons from cranial nerve I; contains local circuit neurons and project neurons that transmit olfactory signals to the olfactory cortex via the olfactory tracts

olfactory tract

the projection from the olfactory bulb to the various divisions of the olfactory cortex in the ventromedial forebrain. The bundle of mitral and tufted cell axons that relay olfactory information to the accessory olfactory nuclei, the olfactory tubercle, the pyriform and entorhinal cortices, and portions of the amygdala.

vomeronasal system

a specialized chemical detection system that detects pheromones (volatile chemicals released into thin air from conspecifics to regulate social interactions) or kairomones (volatile chemicals form other species indicating predator, prey, or symbiotic status); includes peripheral sensory organ, the vomeronasal organ adjacent to the olfactory epithelium in the nose, and its primary target in the forebrain, the accessory olfactory bulb.

vomeronasal organ (VNO)

a pair of chemical sensing organs in the septum (medial process) of the olfactory epithelium that are specialized for the detection and transduction of specific classes of volatile chemicals pheromones and kairomones. The sensory neurons of this organ are bipolar vomeronasal sensory receptor neurons that resemble olfactory receptor neurons in the olfactory epithelium; site of expression of a distinct family that specifically bind pheromones and kairomones

accessory olfactory bulb

the target axons from the VNO, adjacent to the main olfactory bulb relays vomeronasal information to the hypothalamus and other basal forebrain regions

conspecific interactions

social interactions between individuals of the same species. these interactions can be between adults of the same sex, often for competition for dominance of mating. they can also be between adults of opposite sexes for mating. finally, these interactions can be between adults and newborn or young offspring of the same species to provide food, protection, and behavioral guidance for newly born individuals

pheromones

species-specific odorants that play important roles in behavior in some animals, including many mammals

kairomones

volatile chemicals from other species indicating predator, prey, or symbiotic status that bind specifically to vomeronasal receptor proteins localized on subsets of vomeronasal sensory receptor cells

innate behaviors

behaviors that can be executed by an individual with minimal or learning. these behaviors include several sexual behaviors in rodents, as well as aggression, seeking of food and water, ingestion, chewing , and swallowing. Other examples include regulation of blood pressure, body temperature, and water balance for homeostasis.

olfactory cilia

actin based protrusions from the apical domain of an olfactory sensory receptor neurons; site of concentration of odorant receptor molecules and the cytoplasmic signaling intermediates necessary for odor transduction and the initiation of odor processing in the olfactory pathway

Bowman’s glands

mucous producing specializations composed of secretory cells surrounding a lumen that is continuous with the surface of the olfactory epithelium

basal cells

found in the region of the olfactory epithelium adjacent to the lamina propria, where blood vessels and connective tissue that support the olfactory epithelium are found. they retain neural stem cell identity and can generate new olfactory receptor neurons throughout life.

sustentacular cells

the primary support cells of the olfactory epithelium; help to maintain appropriate ionic milieu and epithelial integrity for the olfactory sensory neurons and their basal cell precursors throughout life

lamina propia

includes loose connective tissue as well as small blood vessels that underlie a mucosal epithelium like that of the gut, the lungs, or the lining of the nasal cavity (the olfactory epithelium. the apposition of small blood vessels to the mucosa via the connective tissue scaffold provided by the lamina propria allows for the passage of molecules between the mucosa and circulatory

nasal mucosa

general term for the entire epithelial lining of the nasal cavities. it includes both the neural respiratory epithelium and the neural olfactory sensory epithelium and their constituent cells. it is named based upon the layer of mucous that coats the entire outer surface of the respiratory and olfactory epithelia in the nose.

vomeronasal receptor neurons (VRNs)

a class of bipolar chemosensory neurons found in the vomeronasal organ that uniquely express vomeronasal receptors, and whose axons project to accessory olfactory bulb

anosmia

loss of the sense of smell; can be total or restricted to a single odorant

olfactory ensheathing cells

the glial cells that surround the unmyelinated axons of the peripheral prition of the olfactory nerve. these cells have several properties of Schwann cells that fulfill a similar function for peripheral somatosensory, motor and autonomic axons in peripheral nerves throughout the body

cyclic nucleotide-gated ion channels

a class of ion channels that are activated and inactivated by second messenger cascades. these second messenger cascades usually involve the activation of a G-protein that is coupled to a G-protein-coupled receptor leading to increased phosphorylation capacity of adenyl or guanyl cyclases: the enzymes that can phosphorylate the channels and modify their permeability to ions

adenyl cyclase III (ACIII)

membrane-bound enzyme that can be activated by G-proteins to catalyze the synthesis of cyclic AMP to ATP

adaptation

in the context of evolution, moving animal phenotypes closer to the demands of their environments. roughly synonymous with evolving “fitness”

vomeronasal receptors (VRs)

a large class of 7-transmembrane GPCRs that bind and transduce pheromonal and kairomonal signals. vomeronasal receptor proteins are localized only to vomeronasal receptor neurons in the vomeronasal organ

V1Rs

a sub-class of vomeronasal receptors that interact with the G-prtoein G alpa i2 to transduce vomeronasal sensory stimuli

V2Rs

a sub-class of vomeronasal receptors that interact with the G-protein G alpha O to transduce vomeronasal

transient receptor potential (TRP)

this family of ion channels constitute approximately 28 individual genes and the proteins that they encode. all are transmembrane cation selective channels that mediate depolarization in response primarily to various sensory stimuli. these include taste stimuli that interact with the T1R/T2R GPCR taste receptors to activate the TRPM5 channel, There also TRP channels involved in transducing mechanical displacement/ stretch across cellular membranes