Receptors

• Pacinian corpuscle are stimulated by mechanical pressure and are sensitive to changes in pressure
• When stimulated, receptors change one form of energy into another, acting as transducers
• The energy change of the stimulus is converted into an electrical impulse known as a generator potential
• A generator potential is a graded response to a stimuli that could be large enough to produce an action potential
• The greater the pressure, the greater the frequency of nerve impulses along the neurone
• Pacinian corpuscles consist of a single neurone ending surrounded by a bulb of lamellae and a capsule made of connective tissue
• The Pacinian corpuscle contains stretch-mediated sodium ion channels
• The establishment of the generator potential

  1. External pressure causes lamellae in the corpuscle to deform
  2. Greater pressure causes more deformation, changing the shape
  3. Stretch-mediated sodium ion channels in the neurone membrane open and sodium ions diffuse into the neurone down their concentration gradient
  4. This depolarises the membrane creating a generator potential
  5. Greater pressure opens more sodium ion channels causing a bigger generator potential, making it more likely to reach the threshold
• With continuous pressure, the frequency of the action potentials decrease until they stop. It prevents the nervous system from being overloaded with insignificant information.
• Rod cells are sensitive to very low light intensities and therefore enable us to distinguish light from dark in very dim light
• Cone cells are sensitive to different wavelengths of light and can distinguish colour
• Multiple rod cells are connected to a single neurone which can each produce a generator potential that is added together in summation, making it more likely to reach the threshold
• Each cone cell is connected to a separate neurone which sends separate impulses to the brain, resulting in high visual acuity
• Rod cells are located more on the periphery of the retina and have poor visual acuity
• There are 3 types of cone cells that can differentiate between red, blue and green light
• Cone cells are concentrated at the fovea