The response of rods and cones to stimulation

Upper SixthBiology

Aims

• By the end of the lesson you should understand

• The response of the rod cell to light• The effect of light on rhodopsin• The response of the cone cell to light• How colour vision is facilitated• How neurotransmitters are released

Structure of the Retina

• At the back of the retina, furthest away from the vitreous humor

• Rods and cones• Rods for low

intensity light• Cones for colour

vision

Rods and cones

• Shape• Discs• Cilium• Rhodopsin• Cone pigments• RER and

mitochondria and nucleus

Effect of light on rhodopsin

• The normal structure of the retinal in rhodopsin has a kink in it at C 11 (11-cis-retinal

• The light (can be a single photon) hitting it causes it to change in shape, straighten and become all trans-retinal

• No longer fits the binding site in the opsin.

How the rod responds to light

• The rod has a resting potential of -40mV

• Maintained by the Na/K pump (NoKi)

• Outer segment has open Na channels (Na in)

• Inner segment has open K channels (K out)

• When light strikes this changes

Effect of light on rhodopsin

• Opsin changes shape K/Na channels close• Inside becomes more negative• Hyperpolarised• Impulses then sent to the brain• Rhodopsin is unstable and breaks down to

opsin and all-trans –retinal in a few minutes

• All –trans -> 11-cis retinal in the discs• Dark adaptation.

Effect of light on Rhodopsin

Pulfrich Pendulum Experiment

• Dark adapt one eye and observe the pendulum

• Describe the path of the pendulum• Effect cause by the time lag in the

dark adapted eye.

Effect of light on cone cells

• These also have pigment molecules in the membrane of the outer segment

• Needs more photons hitting to hyperpolarise

• Wavelength specific• Colour vision comes

from the brain interpreting the intensity of signals from each type

Release of neurotransmitter

• There are different transmitter substances releases by rod cells

• Cone cells release glutamate• Stimulation with light

hyperpolarise the cell• Receptors are constantly

releaseing neurotransmitter except when stimulated

Bipolar cells

• Arrival of a neurotransmitter at the bipolar cell may depolarise or hyperpolarise, which may increase or decrease the amount of neurotransmitter it releases

Ganglion cells

• Ganglion cells never total inactive• Always firing action potentials to

the brain• Frequency of APs depends on the

amount of neurotransmitter arriving from the bipolar cells

• Some fire more often when stimulated, so more slowly