9. Chemical Senses

• Olfaction


  Ability to sense and perceive volatile chemicals that are suspended in the air (smell)
• Nasal cavity


  Empty, air-filled space behind the front of your skull where your sense of smell begins. The front part of it has a patch of mucus-covered tissue called the olfactory epithelium
• Olfactory receptor neurons


  ORN
• ORNs 


  Bipolar neurons which begin processing smells. They have long dendritic protrusions that actually poke out of the epithelium into the air, exposing them to all kinds of dangers in the air. Because of this, they are one of the few known neuron populations to experience neurogenesis, with a lifespan of 30 days to a year
• Supporting cells


  Function like glial cells and help dispose of old cells, metabolize pollutants, and may help physically maintain the epithelium
• Olfactory receptors


  Estimated that there are 1000 genes that encode for 400 different ones of these, which are at the dendrites of ORNs. Each one codes for sensing a specific chemical , but we don't know which for lots of them
• Olfactory receptors are GPCRs that signal downstream effectors using Gaolf. The more neurons fire, the more intense we perceive a smell to be
• Gaolf


  Complex protein that is 90% similar to Gas (so also do AC --> cAMP--> depolarization + action potentials) that activates when the olfactory receptors get hit with their specific smell
• Glomerulus


  Highly specialized clump of tissue that is the site for synapses between ORNs and the olfactory bulb (we have like 2000) and each one only receives signals from ORNs with the same receptors
• Olfactory bulb


  Beginning of the olfactory nerve (CN I)
• Mitral cells and tufted cells


  2 cell populations that project the signals from the glomerules (where their dendrites are) directly onto the CNI
• Smell goes ORN --> Glomerule --> mitral cells --> CNI --> olfactory cortex, making it the only sense that doesn't pass through the thalamus before cortical processing
• Granule cells within the glomeruli and periglomular cells, which send projections into the glomeruli, are inhibitory and use lateral inhibition to refine synaptic processing (like the lateral inhibition in the retina)
• Piriform cortex


  Main axonal projection site for from the olfactory bulb that transmits signals on to other structures in the olfactory cotrex like the mediodorsal thalamus
• Mediodorsal thalamus


  Relay center that contributes to learning and decision making
• Amygdala


  Part of the brain that helps mediate complex emotional states that is divided into subnuclei
• Cortical amygdala


  Subdivision of the amygdala that receives strong inputs from olfactory nerve and sends projections to the hippocampus to form new memories
• Strong connections between olfactory nerve and amygdala = why we associate such strong memories with smell
• Entorhinal cortex


  Small section of the medial temporal lobe that is strongly connected with the hippocampus (like cortical amygdala) and is also involved in spatial navigation tasks
• Orbitofrontal cortex (OFC)


  Brain area found just under the eye socket hole in the skull (orbit) with circuits that function as an integration site for sensory inputs (visual, taste, smell, and somatosensory) + implicated in social behaviors and decision making (but its still under examination)
• Pheromones


  Chemicals that some species secrete to influence the behaviors of other members of the species. Debate over whether or not humans have these because we have the vomeronasal organ (specialized these detectors) and CN0 but they don't seem to have function. However, smells also influence us in choosing partners sometimes
• Hyposmia 


  A reduced ability to smell, usually caused by aging with calcification of ORNs, head injuries, or neurodegenerative diseases like AD or Parkinson's (can be an early biomarker)
• Anosmia


  Complete loss of smell
• Phantosmia


  Olfactory halluciantions caused by ORNs firing when they aren't activated. Could be caused by tumors affecting C1, seizures, strokes, or schizophrenia
• Gustatory system


  System which mediates your sense of taste and keeps you healthy by making you want to eat a balanced diet
• Lingual papillae


  Large anatomical structures that give the tongue its rough texture. Each one has up to 100 taste buds
• Taste bud

  
• Taste buds


  Onion shaped receptors that can be found on the tongue, palate and throat that each have around 100 taste receptor cells. People have around 10000 but they diminish as you age
• Taste receptor cells


  Specialized epithelial cells (not neurons) that have thin projections called taste hairs which touch the saliva
• Basal cells


  Reproduce to form supporting cells that mature into taste receptors
• Channels through which taste-related signals are sent

  Front 2/3 of tongue = facial nerve (CN VII)

  Back 1/3 of tongue = glossopharyngial nerve (CN IX)

  Back of palate + throat = vagus nerve (CN X)
• Solitary nucleus (gustatory nucleus)


  Rostral medulla area, in the medulla oblongata where the second-order taste-related nerves form synapses with the taste-sensing cranial nerves
• Gustatory cell projection is actually ipsilateral
• Ventral posteromedial (VPM) nucleus


  Thrid-order neurons of the gustatory system that get their synapses from the solitary nucleus (medulla) are here, a part of the thalamus. From here the signals get sent to areas of the gustatory cortex, where the first processing occurs
• Gustatory cortex


  Beginning of the processing of taste reception that is made up of the anterior end of the insular cortex and the frontal operculum of the frontal lobe
• Salty and sour = ionotropic

  Bitter, sweet and umami = metabotropic
• Epithelial sodium channels (ENaCs)


  NaCl dissolves in saliva and the Na+ flows freely into ion channels, which causes depolarization of the cell (like a neuron) and activates voltage-gated calcium channels that release neurotransmitters that affect the gustatory nerve fibres
• Ghrelin


  Appetite-stimulating hormone that contributes to how Na is regulated but also our appetite for it depending on its concentrations inside of us
• Salt


  Tasted by epithelial sodium channels (ENaCs)
• Sour


  Is tasted through proton-selective channels, so only acids (high H+ concentrations) get sensed. When they are present they flood in, causing the cell to depolarize