3.6.1.2 Receptors

• Receptors detect changes in the internal and external environment. They are specific to a particular kind of stimuli e.g. photoreceptors detect changes in light.
• Receptors are specialised cells or proteins that detect changes in the environment (stimuli) and convert them into electrical signals (nerve impulses or generator potentials).

  • Found in sense organs (e.g., eyes, ears) or as free nerve endings in tissues.
• The Pacinian Corpuscle is a mechanoreceptor that detects pressure and vibration. It is found in the skin, particulary in areas like fingertips and soles of feet.
• Mechanism of the Pacinian corpuscle:

  • At rest, the stretch-mediated sodium ion channels in the nerve membrane are closed.
  • When pressure is applied, the layers of connective tissue are deformed.
  • This deformation stretches the nerve membrane, causing stretch-mediated sodium ion channels to open.
  • Sodium ions diffuse into the nerve ending, depolarising the membrane and generating a generator potential.
  • If the generator potential reaches the threshold, it triggers an action potential, which is transmitted along the sensory neuron to the CNS.
• Rods and Cones are photoreceptors that detect light and allow vision. They are found in the retina of the eye.
• Rod cells are sensitive to low light intensity and do not detect colour.
• Cone cells are sensitive to high light intensity and detect different wavelengths off light (colour).
• Cone cells are present at the greatest density in the fovea of the eye and contain the pigment iodopsin.
• There are three different types of cone cells each sensitive to the primary colours of light: red, green or blue.
• Cone cells provide high visual acuity because each cone cell has its own synapse via a bipolar neurone which connects to the optic sensory neurone.
• Rod cells are mainly concentrated in the highest density outside of the fovea and contain the pigment rhodopsin.

  • This makes them very sensitive to light and, therefore, stimulated in low-light conditions.
• Rod cells provide low visual acuity as more than one rod cell shares the same synapse with a bipolar cell.

  • As a result, multiple rod cells need to be stimulated to create generator potential.
• Threshold: A minimum stimulus strength is required to generate a generator potential strong enough to produce an action potential.
• Describe how a Pacinian corpuscle produces a generator potential when stimulated (3)

  • Increased pressure deforms / changes stretch-mediated sodium ion channel
  • Sodium channels open and sodium ions flow in
  • Depolarisation (leading to generator potential)
• Nocturnal mammals are active at night. Describe how the number and distribution of rods and cones across the retina would differ in a nocturnal mammal from the number and distribution in a human. Explain your answer (3)

  • more rods and fewer cones present
  • rods at the fovea / rods not mainly at periphery
  • rods have high sensitivity whereas cone cells do not
  • because rhodopsin is bleached at low light intensities
• Explain what causes vision using the fovea to be in colour (1)

  • three different wavelengths of cone cells
• Explain what causes vision using the fovea to have high visual acuity (1)

  • each receptor cell connects to a separate neurone
• Explain why it takes time for the rod cells to recover their sensitivity to light after moving into darkness (2)

  • rhodopsin bleached
  • time for resynthesis
• Explain why vision using other parts of the retina has high sensitivity to light (3)

  • many rods in other parts of retina
  • pigment in receptors work in low light
  • receptors connected in groups to ganglion cells
• Suggest and explain how the interaction between the radial muscle and circular muscle could cause the pupil to constrict (2)

  • circular muscle contracts
  • radial muscle relaxes
• The fovea of the eye of an eagle has a high density of cones. An eagle focuses the image of its prey onto the fovea. Explain how the fovea enables an eagle to see its prey in detail (3)

  • high visual acuity
  • each cone is connected to a single neurone
  • cones send separate sets of impulses to brain
• The retina of an owl has a high density of rod cells. Explain how this enables an owl to hunt its prey at night. (3)

  • high visual sensitivity
  • several rods connected to a single neurone
  • spatial summation to reach / overcome threshold