Special Senses 1

The Eye

There are 5 special senses:

1. Olfaction (smell)
2. Gustation (taste)
3. Vision
4. Equilibrium
5. Hearing

Important structures of the eye

Sclera: the white portion of the eye composed of tough connective tissue

Cornea: the transparent portion through which light enters

Iris: the coloured portion of the eye which contracts to decrease pupil size (parasympathetic response) and dilates to increase pupil size (sympathetic response)

Lens: transparent structure that completes the focusing of light onto the retina. There are cells on the interior of the lens with no nuclei nor organelles, and are filled with crystalline which provide clarity and focusing power to the lens.

NOTE: a cataract is damage to the crystallins, where the transparency of the lens is lost.

Fovea: site of receptors for cones

Retina: inner layer of fibrous tissue surrounding the eye. The retina is comprised of three cell layers: rods and cones, bipolar cells and ganglion cells. Ganglion cells are those that transmits impulses to the brain via the optic nerve.

Common conditions of the eye

Myopia, i.e. nearsightedness, is when the eyeball is too deep and the image of a distant object is projected in front of the retina. It is corrected by a diverging, concave lens.

Hyperopia, i.e. farsightedness, is when the eyeball is too shallow or the lens is too flat. At a close range, the lens cannot provide enough refraction to focus an image on the retina and the cilia muscle must contract. It is corrected by a converging, convex lens.

Sight

Vision begins with the capture of light by photoreceptors; rods (black and white) and cones (colour). An action potential propagates along the optic nerve to the visual cortex of the occipital lobe. Visual information is used to determine both the direction and distance of an object.

Rods provide black and white vision and do no discriminate light colours. They are highly sensitive to light and do not require as much energy to be activated as cones. Cones provide colour vision. They are densely clustered in the fovea and the sharpest image is formed at the macula lutea. 

Rods and cones contain photopigments, which are unstable pigments that undergo a chemical change when they absorb light.

• Rods contain rhodopsin
• Cods contain photopsins

Sight Transduction

In the dark, rods are turned on, releasing an inhibitory neurotransmitter and preventing the bipolar cell from firing an action potential. In the light however, rods are turned off.

Rods are cells containing discs lined with proteins called rhodopsin. Without light, rhodopsin contains cis-retinal (in a curved form). However, when light interacts with rhodopsin, the retinal configuration changes from cis to trans-retinal (straight form). This also causes rhodopsin to change shape and causes transducin, a protein with an alpha segment, to bind to phosphodiesterase (PDE). PDE converts cGMP to GMP.

Now, cGMP is bound to sodium channels in the dark, allowing them to be open. When PDE decreases the cGMP concentration, the sodium channels close and the rod cell membrane hyper polarises. The rod then turns off. This ultimately prevents the release of inhibitory neurotransmitter and bipolar cells fire, sending a nerve impulse via the optic nerve to the brain.

In other words:

• In the dark, cyclic GMP levels are high and chemically gated Na+ channels open in the photoreceptor cell. The Na+ influx depolarises the membrane, causing Ca+ influx which leads to the release of an inhibitory neurotransmitter. This prevents the signalling from the bipolar cells, i.e. we don't see anything.
• In light, rhodopsin absorbs light and retinal goes from cis-retinal to trans-retinal. Rhodopsin activates a G protein which stimulates phosphodiesterase, converting cGMP to GMP. Reduced cGMP closes the Na+ channels, preventing the release of an inhibitory neurotransmitter. This allows bipolar cells to fire.

Before reaching the optic nerve, the bipolar cells synapse with ganglionic cells. These axons converge on the optic disc, penetrate the wall of the eye and proceed toward the diencephalon as the optic nerve. There are two optic nerves for each eye, and reach the diencephalon at the optic chiasm.