SAQs

• Major signalling mechanisms cells use to communicate

  • Juxtacrine signalling
  • Paracrine signalling
  • Autocrine signalling
  • Endocrine signalling
• Juxtacrine signalling

  Signalling between two adjacent cells in physical contact, where membrane-bound proteins of one cell interact with membrane proteins or ligands of adjacent cell, and signals travel via hydrophilic membrane changes or gap junctions between cells
• Paracrine signalling

  Signalling between two adjacent cells that are not in physical contact, where neurotransmitters released by one cell diffuse to and contact adjacent cells
• Autocrine signalling

  Chemical signals released by a cell bind to receptors on that very same cell
• Endocrine signalling

  Chemical signals released by endocrine cells released into the bloodstream to communicate with distant target organs
• Graded potentials

  Small rapid changes in membrane potential
• Graded potentials

  • Can be excitatory or inhibitory
  • Spread very rapidly from the site of generation
  • Decay quickly
• Generation of graded potentials

  1. Stimulus causes an ion channel to open or close

  2. Movement of ions across the plasma membrane changing the membrane voltage
• Stimulus strength


  Proportional to the strength of the graded potential
• Summation of graded potentials

  1. Spatial summation - single neuron receives input from multiple neurons simultaneously

  2. Temporal summation - single neuron receives repetitive stimulation from a single source
• Taste receptors

  Chemoreceptors located in taste buds in the mouth that respond to chemicals dissolved in saliva
• Taste buds

  • Each taste bud contains 50 – 150 cells of 4 different cell types, 2 of them being taste receptors
• Type II taste receptors

  Responsible for tasting sweet, bitter and umami
• Type II taste receptor signalling

  Via ATP, but they don't form a clear synapse with the sensory afferents and instead the ATP diffuses out
• Type III taste receptors

  Responsible for tasting sour and salty
• Type III taste receptor signalling

  Via serotonin, and they do form a clear synapse with sensory afferents
• Salty taste transduction

  1. Na+ ions directly enter the type III cell via Na+ channels causing depolarisation

  2. This opens voltage-gated Ca2+ channels, causing the release of neurotransmitter
• Taste receptors are chemoreceptors located in taste buds in the mouth that respond to chemicals dissolved in saliva
• Each taste bud contains 50 – 150 cells of 4 different cell types, 2 of them being taste receptors
• Type II taste receptors are responsible for tasting sweet, bitter and umami
• Type II taste receptors signal via ATP, but they don't form a clear synapse with the sensory afferents and instead the ATP diffuses out
• Type III taste receptors are responsible for tasting sour and salty
• Type III taste receptors signal via serotonin, and they do form a clear synapse with sensory afferents
• Nociceptors

  Free nerve endings in skin, muscles, joints and internal organs that detect painful stimuli
• Mechanical and thermal nociceptors

  • Signal via A-delta fibres, which are thicker and myelinated
• Polymodal receptors

  • Detect mechanical, thermal and chemical stimuli and signal via thin unmyelinated C fibres
• Pain detection and transmission

  1. Nociceptors signal via the release of glutamate and substance P onto secondary afferent fibres

  2. Pain signals ascend via the spinothalamic tract to the primary somatosensory cortex and limbic system
• Fast pain and slow pain


  Two waves of pain
• Acuity (resolution)


  The precision with which the size and location of a tactile stimulus can be determined
• Factors influencing tactile acuity

  • Receptive field size of sensory units (inversely proportional to acuity)
  • Overlap of receptive fields of multiple neurons (reduces acuity)
  • Convergence of multiple sensory units onto a single secondary neuron (reduces acuity)
  • Lateral inhibition (enhances acuity)
• Two-point discrimination test


  Used to determine tactile acuity across the body
• Areas with high tactile acuity

  • Lips
  • Fingertips
• Sensory receptors in high acuity areas

  • Small receptive fields
  • High acuity
• Sensory receptors in high acuity areas

  • Meissner's corpuscles
• Areas with low tactile acuity

  • Thigh
  • Calf
• Sensory receptors in low acuity areas

  • Large receptive fields
  • Low acuity
• Sensory receptors in low acuity areas

  • Pacinian corpuscles
• Vestibular system

  System responsible for maintaining balance and spatial orientation
• Vestibular system

  • Made up of two structures:
  • Semicircular canals
  • Otolith organs
• Semicircular canals

  • Detect rotational acceleration
  • Orientated in different directions to detect acceleration in 3 directions
  • Have a crista ampullaris at the base containing hair cells for transduction