Biology - Lecture 10 (The Chemical Senses)

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  • Lecture 11 - The Chemical Senses
    • Senses
      • Sense organs: structures specialised for detecting particular types of stimuli
      • Sensory receptor neuron: converts stimulus into electrical signal
        • Receptor neurons have cilia at one end; they receive info through receptors. They output their processing at synapses
      • Sensory transduction: conversion of stimulus energy into electrical signal
        • Sensory stimulus induces ion channels to open, depolarising the cell membrane
      • Sensory (afferent) pathways to the cortex
        • Information travels from the sense organ to the brainstem, tectum, thalamus, cortex
          • Information from the left side crosses to the right side of the brainstem and vice-versa
      • A nerve is a bundle of axons. Afferent (or sensory) nerves carry information from sensory receptors towards the brain
      • The principle that sensory info remains segregated along entire pathways is often described as labelled lines
    • Gustation
      • Five types of taste:
        • Sour, bitter, salty, sweet / umami
      • Five nutritional categories:
        • Sour: acid (not good for digestion). Bitter: poisonous. Salty: NaCl (ions needed for cell function). Sweet/umami: sugar/protein (needed for energy/growth)
      • The tongue is the taste organ. Taste buds found on papillae.
        • Saliva carries tastiest through taste pore into taste bud. Each taste bud contains 50-150 taste receptor cells
      • Sensory receptors are on the apical end of the cell. Action potentials and neurotransmitter release occurs in the basal end of the cell
        • Membrane depolarisation or second messengers in cytoplasm travel from apical to basal end
      • Two tastes triggered by influx of positive tasting ions through channels. The channels remain open.
        • Depolarisation occurs when positive ions from the saliva enter through channels reside the receptor cell
      • Sweet and umami receptors use combinations of two G-protein coupled receptors.
      • Bitter tastes are triggered by a family of G-protein coupled receptors. There are 25-30 different types of T2R bitter receptor
        • A taste cell may express more than one type of bitter receptor. But a bitter receptor will never be on the same cell as a salt/sour/sweet/umami receptor
          • Bitter receptors at back of tongue can trigger reflexes to prevent food from being swallowed
    • Taste
      • Taste receptor cells are synapse (using serotonin as transmitter) with primary afferent neurons
      • These are organised in three cranial nerves that terminate in the brainstem
        • Gagging and regurgitation reflex determined by processing in brainstem. Afferent pathways continue through the thalamus to the cortex
      • Hypothalamus activates secretion of digestive fluids. Amygdala processes contextual info for future use
      • Taste receptors roles is to monitor nutritional state. Can also aid in long-term learning by providing reward signals


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