3.6.1.2

• Pacinian corpuscle at resting state

  At its resting potential, the stretch-mediated sodium channels in membrane around the neurons of a Pacinian corpuscle are too narrow to allow sodium ions to pass through them.
• Pacinian corpuscle when pressure is applied

  It changes shape, stretching the membrane around its neurone. This stretching widens the sodium channels in the membrane, enabling sodium ions to diffuse into the neurone, reversing the polarity inside the end of the sensory neurone.

  • If enough sodium channels open, enough sodium ions diffuse into the neurone, so the threshold is reached to produce a generator potential.
• A generator potential creates an action potential that passes along the neurone and then, via other neurones to the central nervous system.
• Sodium voltage gated channels cannot be opened after refractory period as it cant be depolarised again
• How a Pacinian Corpuscle produces a generator potential

  Increased pressure deforms and changes the stretch-mediated sodium ion channels

  • Allows sodium ions to diffuse into the axon
  • Depolarises the axon leading to a generator potential
• Rod cell pigment

  Rhodopsin

  • Breaks down even at low light intensities
  • Pigment breakdown triggers release of neurotransmitters
• Cone cell pigment

  Iodopsin

  • Breaks down only at high light intensities
  • Only present at fovea as light is focused there - so pigment breaks down
• High visual acuity

  Each cone connected to separate bipolar neurones

  • No retinal convergence
  • Impulses are kept separate and discrete signals are sent
  • No summation of impulses occur
• Summation

  Builds up neurotransmitters from neurones to allow action potential to be reached easier with lower stimulus.
• Temporal

  High frequency of signals

  • Builds up neurotransmitters over time
• Spatial

  Multiple neurones send signals to same post synaptic neurone

  • Neurotransmitters build up in synaptic cleft over time as all receptors are full.