Chemical Senses Flashcards
describe the 5 basic taste qualities
- ionotropic
- sour
- salty
- metabotropic
- sweet
- bitter
- umami (monosodium glutamate)
describe the process of salty taste receptors
- salty taste is based on Na ions, which can enter amiloride sensitive sodium channels
- the resulting inward current of postively charged ions causes a depolarization of the taste receptor cell, which opens voltage-gated Ca channels
- this relases transmitter from the synaptic vesicles at the base of the receptor cell
describe the process of sour taste receptors
- sour taste is based on H+ ions (protons) which can work in 2 ways
- they can either enter amiloride sensitive Na channels
- or they can block K channels
- either of these mechanisms causes a depolarization of the taste receptor cell, which opens voltage-gated Ca channels which causes transmitter release
describe the process of sweet taste receptors
- when a molecule with sweet taste qualities (sucrose) binds to its metabotropic receptor, the enzyme adenylyl cyclase induces synthesis of cAMP
- cAMP causes closure of K channels which reduces the outward current of positive charges and leads to depolarization of the cell
- this depolarization opens voltage-gated Ca channels, which then causes release of transmitters
describe the process of bitter taste receptors
- a molecule with bitter taste qualities binds to its receptor and activates phospholipase C
- this initiates the IP3/DAG cascade, which leads to the increase of intracellular Ca (via IP3)
- this allows an inward current of sodium ions through a channel specific to taste receptor cells
describe the gustatory pathway
- CN VII, IX and X carry afferent signals (pseudounipolar neurons) to the brainstem at the ponto-medullary junction
- at the ponto-medullary junction, these afferent fibers synapse in the nucleus of the solitary tract (solitary nucleus)
- from there, fibers of the 2nd order neurons don’t cross the midline and ascend, to form synapses in the ipsilateral ventral posterior medial (VPM) nucleus of the thalamus
- fibers of the 3rd order neurons located in the thalamus synapse in the primary gustatory cortex, which is locaed in the insular lobe and the lower parts of the postcentral gyrus
describe the gustatory pathway
name the parts of the olfactory epithelium
afferent fibers in the olfactory epithelium are covered in bundles by _____ (NOT ____ and ___)
afferent fibers in the olfactory epithelium are covered in bundles by olfactory ensheathing cells (NOT oligodendrocytes or Schwann cells)
_____ are replaced through differentiation and development of _____ every 60 days
olfactory sensory neurons are replaced through differentiation and development of basal cells every 60 days
describe the signal transduction mechanism for the olfactory system
- an odorant molecule in the olfactory mucosa activates an odorant receptor embedded in the membrane of the cilia of an olfactory receptor neuron
- this stimulates adenylyl cyclase via dislocation of the alpha subunit of a G protein (GTP binding protein)
- enzymatic activity of adenylyl cyclase increases cAMP which opens cation channels
- this inward current of Ca and Na causes depolarization of the olfactory receptor neuron, a graded receptor potential
- when the graded receptor potential reaches threshold, APs are generated within the cell body of the olfactory receptor neuron
describe the olfactory pathway
- axons of the output neurons of the olfactory bulb run through the lateral olfactory tract and synapse in the olfactory cortex
- olfactory cortex areas: piriform, peri-amygdaloid and entorhinal cortices, all located in the temporal lobe, around the region of the uncus
explain why the olfactory pathway is different from other sensory systems
- the olfactory pathways reach the primary sensory cortex areas directly, not through thalamo-cortical projections
