sensory

• Coordination


  Sensory receptors
• Chapter 15


  Monday 6/11/2023
• Learning outcomes


  • Candidates should be able to: describe the sequence of events that results in an action potential in a sensory neurone, using a chemoreceptor cell in a human taste bud as an example
• In normal life, action potentials are generated by a wide variety of stimuli, such as light, pressure (touch), sound, temperature or chemicals
• Receptor cell


  A cell that responds to one such stimulus by initiating an action potential
• Transducers


  Receptor cells that convert energy in one form – such as light, heat or sound – into energy in an electrical impulse in a neurone
• Sensory receptor cells


  • Located in sense organs and adapted to detect stimuli
  • Classified according to the type of stimulus they detect
• Sensory receptor cells work by receiving a stimulus. If this stimulus is greater than a certain minimum, or threshold value, then an action potential will be generated
• Chemoreceptor cells


  Gustatory cells in taste buds that detect different tastes, such as sweet, bitter and salty
• There are chemoreceptors for five tastes: sour, salty, sweet, bitter and umami
• Sour and salty taste mechanism

  Involves detection of H+ ions, or acidity, for sour tastes and the detection of Na+, from sodium chloride, for salty tastes
• Sour or salty taste mechanism

  1. Movement of ions through channels causes depolarisation of the gustatory cell membrane

  2. Depolarisation causes voltage-gated channels to open, allowing Ca+2 to enter the cell

  3. Ca+2 leads to exocytosis of neurotransmitter-containing vesicles

  4. Neurotransmitter stimulates an action potential in the sensory neurone

  5. Sensory neurone transmits impulses to the taste centre in the cerebral cortex of the brain
• Sweet or bitter taste mechanism


  • Involves specific receptors on the cell surface membranes of the gustatory cells
  • Binding of sweet or bitter substances to the receptor activates a G-protein, which triggers a signal amplification pathway via a second messenger (Cyclic AMP)
  • This pathway causes increased concentration of Ca+2 inside the cell
• Umami (savoury) taste mechanism

  • Involves receptors that bind amino acids, especially glutamate
  • These receptors are G-protein coupled receptors that, when activated, cause the release of Ca+2 within the cell
  • Increased concentration of Ca+2 inside the cells triggers the release of neurotransmitters
  • Neurotransmitters diffuse across narrow gaps to the nearby sensory neurones where an action potential can be generated
• Separate sensory neurones make connections with each type of gustatory cell. These sensory neurones carry the action potentials to the gustatory cortex in the brain where the sensation of taste is identified