Receptors

• Describe receptors

  Specialised cells that detect one specific stimulus

  Each receptor is linked to a sensory neuron

  They are transducers as they convert one form of energy to another (electrical)
• Describe how receptors produce a generator potential

  • Generator potential is the initial depolarisation of a sensory neuron, caused by Na+ diffusing in
  • If the generator potential is large enough to reach the threshold, an action potential is made
• Define the pacinian corpuscle

  A receptor that detects pressure changes

  Found in the skin, joints and genitalia
• Describe how the pacinian corpuscle works

  1. When pressure is applied, the lamellae and viscous gel are distorted, producing a ripple effect
  2. This deforms stretch-mediated Na+ channels in the sensory neuron membrane, which forces them open
  3. Na+ diffuse into the sensory neuron, causing depolarisation, which forms the generator potential
  4. If the generator potential is big enough to reach the depolarisation threshold, action potentials in the sensory neuron are produced
• Describe how photoreceptors in the eye work

  1. When light hits a rod or cone, there is a pigment that breaks down and produces a generator potential
  2. If it is big enough to reach the threshold, action potentials are produced and sent down a bipolar neuron, in the optic nerve to the brain
  3. Rhodopsin is the pigment in rods. It can't distinguish between different wavelengths of light, so images are perceived in black and white
  4. Iodopsin is the pigment in cones. There are three different types that can detect red, blue and green wavelengths, so images are perceived in colour
• Describe the connections made by rods and cones in the optic nerve

  • Rods have retinal convergence where 2 or 3 rods connect with 1 bipolar neuron, cones do not have retinal convergence so 1 cone always connects with 1 bipolar neuron
• Describe sensitivity to light in rods and cones

  • Rods have high sensitivity to light as neurotransmitters can be combined to reach the threshold of depolarisation even in low light intensities.
  • Cones have low sensitivity to light as more neurotransmitters are needed from each cone to reach the threshold, needing high light intensities
• Describe visual acuity in rods and cones

  • Rods have low visual acuity as light spots close together only produce 1 action potential.
  • Cones have high visual acuity as light spots close together produce separate action potentials
• Describe distribution of rods and cones on the retina

  • Cone cells have the highest density on the fovea
  • Rod cells have the highest density on the periphery of the retina