The chemical senses Flashcards
Broadly describe the main flow of taste information to the brain
From the taste buds to the primary gustatory axons, into the brain stem, up to the thalamus, and to the cerebral cortex.
What carries these primary gustatory axons? (3) Describe where these originate from
Three cranial nerves carry primary gustatory axons and bring taste information to the brain.
The anterior two-thirds of the tongue and the palate send axons into a branch of cranial nerve VII, the facial nerve. The posterior third of the tongue is innervated by a branch of cranial nerve IX, the glossopharyngeal nerve. The regions around the throat, including the glottis, epiglottis, and pharynx, send taste axons to a branch of cranial nerve X, the vagus nerve.
From these areas, where do these cranial nerves go?
These nerves are involved in a variety of other sensory and motor functions, but their taste axons all enter the brain stem, bundle together, and synapse within the slender gustatory nucleus, a part of the solitary nucleus in the medulla.
Describe the journey from the gustatory nucleus
From the gustatory nucleus, taste pathways diverge. The conscious experience of taste is presumably mediated by the cerebral cortex. The path to the neocortex via the thalamus is a common one for sensory information.
Neurons of the gustatory nucleus synapse on a subset of small neurons in the ventral posterior medial (VPM) nucleus, a portion of the thalamus that deals with sensory information from the head. The VPM taste neurons then send axons to the primary gustatory cortex
The taste pathways to the thalamus and cortex are primarily ipsilateral to the cranial nerves that supply them. Lesions within the VPM thalamus or the gustatory cortex—as a result of a stroke, for example— can cause ageusia, the loss of taste perception.
What role do gustatory nucleus cells play? Describe their output
Gustation is important to basic behaviors such as the control of feeding and digestion, both of which involve additional taste pathways. Gustatory nucleus cells project to a variety of brain stem regions, largely in the medulla, that are involved in swallowing, salivation, gagging, vomiting, and basic physiological functions such as digestion and respiration.
In addition, gustatory information is distributed to the hypothalamus and related parts of the basal telencephalon. These structures seem to be involved in the palatability of foods and the forces that motivate us to eat
What can localized lesions of the hypothalamus or amygdala or a nucleus of the basal telencephalon cause in regards to gustation?
Can cause an animal to either chronically overeat or ignore food, or alter its preferences for food types.
What do we use to smell?
We do not smell with our nose. Rather, we smell with a small, thin sheet of cells high up in the nasal cavity called the olfactory epithelium (see docs).
Name and describe the three main cell types of the olfactory epithelium
Olfactory receptor cells are the sites of transduction. Unlike taste receptor cells, olfactory receptors are genuine neurons, with axons of their own that penetrate into the central nervous system.
Supporting cells are similar to glia; among other things, they help produce mucus.
Basal cells are the source of new receptor cells.
What regularly happens olfactory receptors which is not commonly seen from different types of neurons?
Olfactory receptors (similar to taste receptors) continually grow, die, and regenerate, in a cycle that lasts about 4–8 weeks. In fact, olfactory receptor cells are one of the very few types of neurons in the nervous system that are regularly replaced throughout life.
Describe what happens when air is breathed through the nose?
Sniffing brings air through the convoluted nasal passages, but only a small percentage of that air passes over the olfactory epithelium. The epithelium exudes a thin coating of mucus, which flows constantly and is replaced about every 10 minutes.
Chemical stimuli in the air, called odorants, dissolve in the mucus layer before they reach receptor cells. Mucus consists of a water base with dissolved mucopolysaccharides (long chains of sugars); a variety of proteins, including antibodies, enzymes, and odorant binding proteins; and salts.
Why are the antibodies critical?
Because olfactory cells can be a direct route by which some viruses (such as the rabies virus) and bacteria enter the brain.
Where do olfactory receptor neurons send their axons?
Olfactory receptor neurons send axons into the two olfactory bulbs
Olfactory receptor neurons send axons into the two olfactory bulbs
The bulbs are a neuroscientist’s wonderland, full of neural circuits with numerous types of neurons, fascinating dendritic arrangements, unusual reciprocal synapses, and high levels of many different neurotransmitters.
What is contained in the input layer of each bulb in mice?
The input layer of each bulb in mice contains about 2000 spherical structures called glomeruli, each about 50–200 um in diameter.
Within each glomerulus, the endings of about 25,000 primary olfactory axons (axons from the receptor cells) converge and terminate on the dendrites of about 100 second-order olfactory neurons.
What have recent studies demonstrated about mapping of receptor cells onto glomeruli?
That they are astonishingly precise. Each glomerulus receives receptor axons from a large region of the olfactory epithelium. When molecular labeling methods are used to tag each receptor neuron expressing one particular receptor gene of the mouse—in this case, a gene called P2—we can see that the P2-labeled axons all converge onto only two glomeruli in each bulb. No axons seem to be out of place, but our knowledge of axonal pathfinding during development can- not yet explain the targeting accuracy of olfactory axons
