Light Detection In Man

Question 1

What do ciliary muscles do in human eye?

Answer 1

Can pull to flatten lens or relax to make lens more curved. Accommodates for objects being close or far distance.

Question 2

Distribution of cones and rods in eye?

Answer 2

Cones concentrated in fovea. Cones have highest acuity. One cone=one nerve to brain.

Question 3

What is dark-current?

Answer 3

In the dark, sodium and calcium get pumped to outer segment. This is reduced in the light. Hyper polarisation in light. Depolarisation in dark.

Opposite effect in invertebrates

Question 4

What is retinal and what does it do?

Answer 4

Light sensitive prosthetic group of the receptor protein (opsin). Pigments made from opsin and retinal are called rhodopsin.

Absorption of light conformation changes rhodopsin from cis to straight trans.

Fish and amphibians have porphyropsins (opsin and 3-dehydroretinal).

Question 5

Process of stimulation of vertebrate photoreceptors?

Answer 5

Light—> Rhodopsin—> Transducin —> Phosphodiesterase —>cGMP to GMP —> closure of dark current channel of calcium and sodium.

Question 6

What is lateral inhibition?

Answer 6

Each sensory neurones inhibits neurones adjacent to them. Lets light to be limited to distinguish edges.

Question 7

Difference between on and off center ganglion cells?

Answer 7

Central stimulation causes increases firing of AP. Whole field stimulation has less firing. Surrounding stimulation results in attenuated firing.

Opposite effect in off center ganglion.

On-center ganglion cell connected to on center bipolar cell.

Off center ganglion cell connected to off center bipolar cell.

Question 8

Order of light being detected and subsequent transduction?

Answer 8

Light enters eye and past the ganglion and bipolar cells to the photocreptors at the back of the eye where they are initially detected. This electrical stimulus is then sent to connecting bipolar cells and then to ganglion cells. These lead to the optic nerve fibres that lead to the brain.

Receptor- 1st order
Bipolar cell- 2nd order
Ganglion cell- 3rd order

Question 9

What do horizontal cells do?

Answer 9

Opposite effect of bipolar cell to adjacent bipolar cell, lateral inhibition.

Question 10

What are of brain does optic nerve take signals to?

Answer 10

From retina to lateral geniculate nucleus to primary visual cortex.

Temporal hemiretina is taken ipsilaterally to same side of the brain.

Nasal hemiretina is taken to the contra laterally to the opposite side of the brain.