Taste. circumvallate papilla. labelled lines. There are five recognised modalities: sweet, sour, bitter, salt & umami. All mature taste bud cells have prominent microvilli. Taste bud cells are continuously renewed from a local population of stem cells and their half-life is about ten days.
Mammals have metabotropic ligand binding receptors with second messenger signalling cascades, which indirectly activate ion channels, whereas insects use ionotropic receptors which are directly gated by the chemical stimuli.
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The vertebrate nasal cavity contains several olfactory subsystems: the main olfactory epithelium (MOE), the vomeronasal organ (VNO), the Grüneberg ganglion (GG), the septal organ (SO) and guanylate cyclase D-containing cells (GCDs) in the MOE. Sensory cells of the MOE and the SO project axons to glomeruli of the main olfactory bulb (MOB). Sensory cells of the GG and GCDs in the MOE send their axons to the necklace glomeruli (NG). Sensory cells of the VNO send their axons into the accessory olfactory bulb (AOB). Olfactory receptor neurons (ORNs) in the MOE (right) have one dendrite, which ends in a dendritic knob. From each dendritic knob, approximately 15 cilia extend into the nasal mucus. ORNs are surrounded by supporting cells and are constantly generated from basal cells.
Kaupp (2010) fig 3