Rod & Cones

• Similar structure• Outer segment – part

closest to the outside of the eye

• Inner segment - part closest to the centre of the eye. Synapses with the cells in the other layers of the retina.

Rod

Outer segment

Cone Inner

segment

•Outer segments – lots of stacks of membranes – formed by invaginations of the plasma membrane:–Rod – invaginations pinch off, resulting in unconnected discs lying freely within the cytoplasm.–Cones – invaginations are still connected to the plasma membrane

Membrane discs in rods and cones are constantly discarded at the tips of the cells (where they are taken in and destroyed by phagocytic cells), and renewed by the formation of new ones – In humans three are synthesised every hour.

Rod & Cones

Invaginations provide

huge surface area

for visual pigments

Visual pigments

• When light hits molecule of pigment = changes = may result in action potentials being fired off.

• Rod – pigment consists of a protein molecule opsin, which lies within the membrane to which a small light absorbing compound – retinal, is attached to the outer surface.

• Opsin + retinal = rhodopsin

• Cones – visual pigments similar – precise opsin part not the same.

• In fact three different types of opsins in the three different types of cones cells, which gives us colour vision – sometimes known as iodopsin or cone pigments.

• Rods = more sensitive to light, so in dim light rods give us our visual information

Visual pigments

Rods and Cones

• The inner and outer segments of rods and cones are connected by microtubules arranged as a cilium

• The inner segment contains the nucleus and mitochondria, this is where the new proteins are initially formed.

Rod cells + light

• YOU SHOULD KNOW – neurones maintain a resting potential across their plasma membrane using the sodium-potassium pump.

• The pump moves 3 Na+ out and 2 K+ in by active transport – resulting in a more positive charge outside the cell than inside.

• Resting potential is usually between -60 – -70 mV inside.

Normal neurone

!!

• NO light - rod cells maintain an electrical potential difference in the same way that other neurones do, but it is around -40mV.

• It uses the sodium-potassium pump in the same manner, however the outer segments of the rod cell there are open channels that allow the Na+ ions to pass through the plasma membrane.

• In the inner segment there are potassium channels which allow the K+ ions to flow through.

• So both ions flow down their concentration gradients in the opposite direction to the sodium potassium pump.

Rod cells + light

• Light – hits the rhodopsin molecule which causes the retinal part to change shape.

• (Normal shape = kink in 11 carbon, called 11-cis-retinal – light = straight, called all-trans-retinal).

• Change in shape means that the retinal no longer fits into its binding site on the opsin molecule

• Causing the whole rhodopsin molecule to change shape into an unstable form.

• Causing the Na+ and K+ channels to close.

Rod cells + light

• The Na+/K+ channel carries on working, causing a greater potential difference to build up of -70mV, the rod cell is hyperpolarised (more polarised).

• The unstable form of rhodopsin breaks down within minutes into all-trans-retinal and opsin.

• All-trans-retinal is converted back into 11-cis-retinal and recombines with the opsin.

• This process takes time – in light most of the rhodopsin molecules break down. – therefore if you move into the dark you have no rod cells to respond to light. Wait a while and you ability to see gradually increases – process called dark adaptation

Rod cells + light

• How does a normal neurone produce an action potential?

• Presynaptic membrane only releases transmitter substance when an A.P arrives

• A.P is a ‘fleeting’ depolarisation – so that the inside is slight more positive than the outside.

• Neither rods or cones ever generate and action potential.

Rod cells + light + A.P

• Rods and cones are most depolarised when they are not receiving light (-40mV)

• Light in fact hyperpolarises them• Rods and cones release transmitter

substances into the synaptic cleft between themselves and bipolar cells when no light is falling on them and stop releasing when light does fall on them.

• Several different neurotransmitters are released from rods, the one released from cones is glutamate.

Rod cells + light + A.P

Cones + light

• Same as rod cells they have pigments on their membranes which change shape when light falls on them.

• However in rods a single photon of light can be enough to bring about the change, cone cells require much more light.

• This is why only our rods respond in dim light.

Cones + light

• There are three different cone cells each with a different light sensitive pigment.

• Each pigment responds to a particular wave length of light:– Short wavelength = blue = pigment B– Medium wavelength = green = pigment G– Long wavelength = red = pigment R

The brain interprets the colour of an image by comparing the intensity of the signal from

these three types of cones

Bipolar and ganglion cells

• Both types of neurones:– Bipolar – central body, from which two sets of

processes arise. Branch nearest rods and cones = shorter, synapse with either a single cone or many rods. Other branch longer and synapses with a ganglion cell.

– Ganglion cells – inner layer of the retina, many dendrites synapse with bipolar cells, A.P are first generated here within the retina. A.P pass along the axons of the ganglion cells, which make up the optic nerve.

Role of bipolar and ganglion cells

• Bipolar cells – the arrival of a transmitter substance from a rod or cone either depolarises or hyperpolarises the bipolar cell, which either increases or decreases the amount of transmitter substance released (different bipolar cells react differently).

• Ganglion cells – transmitters from the bipolar cells diffuse across the synaptic cleft and slot into receptors in the ganglion cell. They are never totally inactive – the frequency of A.P varies according to the amount of transmitter arriving from the bipolar cells.

• Some ganglion cells fire more often when rods and cones are illuminated others stop.

Visual acuity

• The different patterns of connections between rods and cones and bipolar cells produces the resolution of an image (sharpness).

• The resolution of an image that is perceived by the brain is known as visual acuity.

Visual acuity

• Rods – many synapse to one bipolar cells, this pools information in dim light – however the brain is not sensitive to individual rod responses – poor visual acuity.

• Cones – each bipolar cell receives information from very few cones, in the fovea each bipolar cell receives information from only one cone = high resolution of an image.

• Helped by the fact that the cones are concentrated in the fovea where the image that forms on the retina is the sharpest.

Effects of Ageing

• As we age our eyes become less acute, maybe many reasons for this:– The main one being the loss of elasticity

of the lens. This means when the ciliary muscles contact the lens doesn’t spring back into a fat shape – also it may be more difficult to stretch out when the muscles are relaxed.

Cataracts

• Lens becomes less transparent – the lens is made of living cells as the person ages the proteins may begin to denature, if the proteins coagulate then part of the lens becomes cloudy white = cataract.

Cataracts

• Most people have some degree of opacity, but is most cases it is not effect the vision enough to require treatment.

• Treatment is simple the lens is removed and it can be done under local anaesthetic.

• The lens may be replace or the person will need to wear glasses to make up for their lack of ability of the eye to bend light.