Clinical Lecture: Visual defects

Question 1

What is the role of the non-receptor layer in the retina?

Answer 1

• Contains melanin to absorb light in the eye to improve resolution by stopping light bouncing around
• The pigment also provides nutrients for photoreceptor to work properly

Question 2

What state are photoreceptor in the dark?

Answer 2

Depolarised. When light comes along, they do not fire action potentials but instead became hyperpolarised. This decreases the amount of the excitatory glutamate being released.

Question 3

Describe the process of transduction.

Answer 3

• The photoreceptors have a membranous disc/invaginated membrane in the outer segment. These discs are filled with photopigments. These pigments are G protein coupled receptors such as Rhodopsin and Transducin.
• Embedded in these is Retinal - a form of Vitamin A
• When a photon of light hits Retinal, it undergoes a transformation change activating Rhodopsin.
• This causes a cascade of G protein coupled reactions
• When Rhodopsin (which contains Retinal) gets activated it activates a G protein called Transducin. This activates a phosphodiesterase which catalyses the break down of cGMP. When the levels of cGMP are low enough it causes ion channels to be closed.
  https: //www.youtube.com/watch?v=dhd2fja0LZ4

Question 4

What form do bipolar cells come in?

Answer 4

• ON Bipolar cells - depolarise in response to light onset. express metabotropic receptors - mGLuR6. When glutamate is around G-protein coupled receptor is activated mGluR6 which inhibits the ion channel TRPM1 receptors. Therefore in the dark, where Glutamate is plentiful, ion channels are closed - in the light, the channel is uninhibited causing depolarisation.
• OFF Bipolar cells - hyper polarise in response to light onset. In the dark there is lots of glutamate around, causing sustained depolarisation. In light the channels are closed causing the hyperpolarisation in response to light.

Question 5

What type of connection do we see between ganglion cells and bipolar cells in the fovea?

Answer 5

One-to-One cell connection

Question 6

What is the receptive field?

Answer 6

The area of the retina that causes any change in response to a neurone.

Receptive field centre is due to direct connection to a glutamatergic neurone and corresponds to the cell dendritic field. Receptive field surround is due to “lateral inhibition” from inhibitory neurones and its such larger than the centric field. This allows comparison of light between the centre and the surround.

Question 7

How does the Young-Helmholz theory have an effect on the retina?

Answer 7

The theory is that every colour in the rainbow can be obtained by mixing the proper ratio of red, green and blue light.

At each point in the retina there exists a cluster fo three receptor types, each type being maximally sensitive to either bleeding, green or red light.

Question 8

Why do some receptors need information about absolute luminosity?

Answer 8

• Intrinsically photosensitive retinal ganglion cells (ipRGC) in the suprachiasmic nucleus require this information as it is the master regulator of the circadian rhythm
• To prevent overexposure to light (in the OPN and Erdinger-Westphal Nucleus)

Question 9

How are Ganglion Cells classified?

Answer 9

Based on which layer in the LGN they project to:

• M-type to the M layer
• M-type to the P lLayer
• K type to the K layer