the chemical senses Flashcards
anosmia
Loss of the sense of smell; can be total or restricted to a single odorant.
G-olf
A G-protein found uniquely in olfactory receptor neurons.
glomeruli
Characteristic collections of neuropil in the olfactory bulb; formed by dendrites of mitral cells and terminals of olfactory receptor cells, as well as processes from local interneurons.
insula
The portion of the cerebral cortex that is buried within the depths of the lateral fissure. Also called insular cortex.
lateral olfactory tract
The projection from the olfactory bulb to higher olfactory centers.
mitral cells
The major output neurons of the olfactory bulb.
odorants
Molecules capable of eliciting responses from receptors in the olfactory mucosa.
olfactory bulb
Olfactory relay station that receives axons from cranial nerve I and transmits this information via the olfactory tract to higher centers.
olfactory epithelium
Pseudostratified epithelium that contains olfactory receptor cells, supporting cells, and mucus-secreting glands.
olfactory receptor neurons (ORNs)
Bipolar neurons in olfactory epithelium that contain receptors for odorants.
pheromones
Species-specific odorants that play important roles in behavior in some animals, including many mammals.
pyriform cortex
Component of cerebral cortex in the temporal lobe pertinent to olfaction; so named because of its pearlike shape.
taste buds
Onion-shaped structures in the mouth and pharynx that contain taste cells.
ventral posterior medial nucleus
Component of the ventral posterior complex of thalamic nuclei that receives brainstem projections carrying somatic sensory information from the face.
What roles do the trigeminal sensory system, the olfactory system and the gustatory system play in taste?
The experience of taste is actually a combination and integration of trigeminal, gustatory and olfactory stimuli. The texture and temperature of food, along with the presence of chemicals (such as capsaicin in spicy foods) are trigeminal. Taste buds identify one of five specific characteristics of foods dissolved in the mouth (sweet, salty, sour, bitter and umami). Olfaction actually provides much of the sensory experience of food (try eating while holding your nose). All of these sensations are integrated in the cortex.
What roles does the trigeminal sensory system and TRP receptors play in the mouth and nose?
The trigeminal sensory system in the mouth and nose plays two roles. It is mostly protective of the oral and nasal passages, potentially triggering reflexes of gagging, coughing and sneezing. However, in the mouth, it also participates in taste. The peripheral end of nerves contain TRP receptors which are activated by pain, temperature and mechanical stimuli, but also by certain tastants, such as capsaicin (the ingredient in hot peppers).
What is the anatomical and functional organization of the olfactory sensory system?
Olfactory receptors exist in the upper nasal cavity (which is why sniffing enhances the presentation of odorants to receptor cells). The epithelium has sensory cells which have cilia that express GPCRs that are the receptors. Hair cells are supported by “supporting cells” and are replaced about every two months from basal cells. They are covered with mucous that traps and presents odorants. It is unmyelinated processes of these cells that form the olfactory “nerve”, which is actually a series of filaments traversing the cribriform plate to the olfactory bulb.
What are the molecular mechanisms involved in development of odorant specificity and signaling in the olfactory receptor cell?
Olfactory receptors are GPCR’s that are part of the largest family of genes (around a thousand humans, with at least half being expressed). There are two main pathways for olfactory receptors to activate the cell. These GPCRs can activate adenyl cyclase, which produces cAMP that opens cation channels directly, or it can activate phospholipase C which opens calcium channels through the action of inositol triphosphate. Note that these are both common mechanisms for GPCRs. Both of these pathways activate the olfactory receptor cells when the receptor recognizes its specific odorant.
What are the roles of glomeruli and mitral cells in the olfactory bulb and how do they participate in olfactory signaling?
All of the olfactory receptor cells terminating in a particular glomerulus in the olfactory bulb express the same odorant receptor. The receptor axons terminate on dendrites of mitral cells in the glomeruli. The axons of the mitral cells, in turn, comprise the olfactory tract that conveys olfactory stimulation to the brain.
How do glomeruli participate in identification of odors?
Individual odors generally activate more than one specific receptor, but they do so in differing proportions. It is the ability to recognize the pattern of activation of glomeruli that allows discrimination of more odors than there are individual receptor types. This also means that a single odorant may smell different based on its concentration and also if that odorant is presented in combination with another odorant.
How can sensitivity to odors be changed based on physiologic state? ?
Noradrenergic axons (from locus ceruleus of the brain stem) can enhance olfactory sensitivity by acting on transmission in the olfactory bulb when you are hungry or seeking food. Epinephrine can enhance olfactory receptor sensitivity directly and can enhance the sense of smell when you are afraid or excited.
How and where do central connections from the olfactory bulb terminate?
The olfactory tract splits into a lateral and medial olfactory stria. The medial stria terminates in the bed nucleus of the olfactory tubercle and also extends to the contralateral side of the brain. The lateral olfactory stria (the larger of the two) terminates in the anteriormost part of the parahippocampal gyrus (piriform cortex), and the amygdala. This anteromedial temporal lobe is the primary olfactory cortex.
Where are taste buds located and how are they different over the tongue?
Taste buds are located at the base of papilla on the tongue. They consist of a cluster of sensory cells with a small opening to the surface of the tongue (taste pore). They have different response properties in different regions of the tongue, with salty and sweet being anterior on the tongue and bitter and sour being more posterior. They are also connected to different cranial nerves with anteriorly-located taste receptor cells activating (via serotonin) peripheral taste sensory axons of the facial nerve (via the chorda tympani nerve). Taste receptors on the posterior third of the tongue mostly synapse with the ends of glossopharyngeal nerve fibers; and vagal nerve fibers only connect with some very posterior receptors (bitter).
What are the mechanisms involved in gustatory cells that sense sweet, salty, sour, bitter and umami?
Taste buds detecting sour and salty (which are both detecting ions: hydrogen for sour and mostly sodium for salty) are ion channels. Taste buds for bitter, sweet and umami (savory) are second messenger receptors (GPCRs).
