Visual pathway

What cells are in the outer layer of the retina

photoreceptors, bipolar + horizontal dendrites

what cells are in the inner layer of the retina

bodies of horizontal, bipolar, and amacrines cells,

axons of bipolar, amacrines and dendrites of ganglions.

function of rods and cones in retina

generate graded potentials in response to light called photopigements.

rods - black and white vision

cones - colour vision

function of bipolar cells in retina

first neurone in the pathway to the brain, connected to photoreceptors.

what is the role of ganglion cells in the retina

A ganglion cell can receive one or more typically many bipolar inputs

• second neurone in the pathway

• axons make up optic nerve

• detect change in light themselves (effects circadian rhythm)

How do rods detect black and white

These cells generate graded potentials in response to light. The chemical responsible is called a photopigment.

In rods, this is known as rhodopsin.

It consists of two molecules: a large opsin protein long term, and retinal

During light absorption, retinal undergoes synthesis Retinol → Retinal → Retinoic acid

1.      Light triggers cis-retinal conformational change

2.      Opsin releases retinal, becomes active

3.      Active opsin activates transducin's alpha subunit → activates cGMP phosphodiesterase → converts cGMP to GMP

4.      Reduced cGMP closes Na+ channels

5.      Cell hyperpolarizes, stops glutamate release

What is the purpose of different opsin types (S.M,L)

short medium and long opsins code for different wavelengths of light that animals can see.

How is light converted to chemical then to electrical impulse?

Signal Cascade:

1.      Light triggers cis-retinal conformational change

2.      Opsin releases retinal, becomes active

3.      Active opsin activates transducin's alpha subunit → activates cGMP phosphodiesterase → converts cGMP to GMP

4.      Reduced cGMP closes Na+ channels

5.      Cell hyperpolarizes, stops glutamate release

Describe the visual (neural) pathway from the retina to the cortex.

Most neurones synapse in the LATERAL GENICULATE NUCLEUS (this is a part of the thalamus).

• optic nerve extends caudally from the retina, through the optic foramen in the presphenoid bone into the neurocranium.

• It joins the ventral aspect of the diencephalon at the optic chiasm, just rostral to the hypophysis (pituitary gland).

• In general, the majority of axons decussate, but the degree of decussation depends on the type of animal. In fishes and birds, all fibres decussate (Fig. 10.10). In mammals, there is partial decussation (ungulates about 80–90%, dogs 75%, cats 65%, primates 50%).

Define the pupillary light reflex, explaining its clinical importance

autonomic reflex that constricts the pupil in response to light adjusting the amount of light that reaches the retina

works as a CN test for the oculomotor nerve

What is amblyopia and what can cause this?

blindness with working eyes

damage to back of the head affecting visual cortex processing areas such as V2, V3, V4.

What animal sees like this?

Deer