Receptors

• Receptors detect stimuli and there are lots of different receptors to detect different stimuli. The stimulus always involves an energy change, e.g. temperature, light, kinetic, sound. Receptors convert these energy types into electrical energy, the impulse, which is also known as the generator potential.
• Effectors are cells that bring about a response to a stimulus by producing an effect. Effectors can be muscle cells and cells found in glands.
• The pacinian corpuscle is a skin receptor which responds to the stimulus, mechanical pressure. It is found in fingers, soles of the feet, genitals, joints, ligaments and tendons.

  The Pacinian corpuscle converts mechanical pressure, into a generator potential.
• How does the Pacinian corpuscle establish a generator potential when stimulated?

  When mechanical pressure is applied to the skin the Pacinian corpuscle layers of lamellae and sensory neuron membranes are distorted which deforms and widens specific stretch-mediated sodium ion channel proteins. This increases the sodium ion permeability so sodium ions diffuse into the neurone and the membrane becomes depolarized, producing a generator potential. If the threshold is reached an action potential is created and passed along the sensory neurone.
• When pressure is applied to the Pacinian corpuscle the higher the pressure the more sodium ion channels that open which generates a larger generator potential. If the threshold is exceeded an action potential will be established.
• The stronger a stimulus the greater the action potential frequency.
• What cells are found in the fovea of the eye?

  Only cone cells are found in the fovea.
• What cells are found at the periphery of the retina in the eye?

  Rod cells and a few cone cells.
• Why is no image perceived in the blind spot of the eye?

  There are no rod (r) or cone cells (c).

  
• Rods show retinal convergence as 3 rods are connected to one bipolar neurone.
• 3 different types of cone have pigments that absorb different wave lengths of light.
• What creates a generator potential in the eye?

  Light breaks down the pigments of the eye which alters the pigment chemical structure. Rod/Cone cells will release neurotransmitters and if these exceed the threshold a generator potential establishes in the bipolar cell.
• What pigment is broken down by light in rod cells?

  rhodopsin
• What pigment is broken down by light in cone cells?

  Iodopsin
• After a generator potential is produced in the eye how is an action potential produced?

  The bipolar cell release neurotransmitters which synapses to establish an action potential down the sensory neuron.
• What is visual acuity?

  Visual acuity is the ability to see detail.
• What is sensitivity?

  Sensitivity is the ability to detect low light levels.
• Rod cells have poor visual acuity but good sensitivity.
• Why do rod cells have poor visual acuity?

  This is because many rod cells generate one impulse to the brain which cannot distinguish between the separate light sources.
• Why do rod cells have good sensitivity?

  This is because there is enough neurotransmitter released to reach the threshold in the bipolar neurone and establish a generator potential. This is due to a type of spatial summation called retinal convergence.
• Cone cells have poor sensitivity but good visual acuity.
• Why do cone cells have good visual acuity?

  This is because each cone that is stimulated generates an individual impulse so the brain can distinguish between different light sources.
• Why do cone cells have poor sensitivity?

  This is because each individual cone cell has to produce enough neurotransmitter to reach the threshold in the bipolar neurone and this will only happen in bright light because iodopsin required high light intensity to be broken down.