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.