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
Rod & Cones
• Outer segments – lots of stacks of membranes
– formed by invaginations of the plasma
membrane:
Invaginations
–Rod – invaginations
pinch huge
off, resulting in
provide
unconnected discs lying freely within the cytoplasm.
surface area
–Cones – invaginations are still connected to the
plasma membrane for visual
pigments
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.
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
Visual pigments
• 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
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 sodiumpotassium pump.
Normal
• The pump moves 3 Na+ out and 2 K+ in by
neurone!!
active transport
– resulting in a more
positive charge outside the cell than
inside.
• Resting potential is usually between -60
– -70 mV inside.
Rod cells + light
• 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 11cis-retinal – light = straight, called all-transretinal).
• 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-cisretinal 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 + A.P
• 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.
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.