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Neuronal networks and behaviour summary visual system
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This document contains a summary of the lectures, and chapter 11 and 12 of the book neuroscience by purves about the visual system.
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Visual systems chapter 11 &12Vision as a creative process The sclera is the outermost tissue layer of the eye.
Gestalt psychology: what we see represents not just the The cornea is the opaque outer layer of the eye that permits
properties of objects but, more importantly, the organisation of light rays to enter the eye.
sensations by the brain. The brain makes certain assumptions
about what is to be seen in the world, expectations that seem to
derive in part from the experience and in part from the built-in
neural wiring for vision.
Vision-based on interrelationship
Just like in a melody, we recognize not the sequence of notes, but
their relationship. Played in different keys we still can recognize
the melody.
Another assumption we make is dividing objects into objects and
backgrounds. The figure-ground dichotomy illustrates one principle
of visual perception, a winner-take-all perceptual strategy: only
one part of the image can be selected as the focus of attention.
When a light ray passes through the cornea it enters the
anterior chamber and then the aqueous humour.
> The fluid that the humour is filled with, called vitreous humour
is produced in the posterior chamber.
Making sense of the patterns: occlusion The retina
There are numerous blood vessels present on the inner surface of
the retina, which enter or leave the eye through an area known
as the optic disk. This is also the region where retinal axons leave
the eye to reach targets in the thalamus and midbrain via the
optic nerve. This region of the retina contains no photoreceptors,
and because of its insensitivity to light, it produced a phenomenon
The perceived size of an object depends on other objects in the known as the “blind spot”.
visual field > When the world is viewed monocularly the visual system fills in
a missing part of the scene.
The macula is the region of the retina that supports visual acuity,
which is the ability to resolve fine details. Acuity is greatest at
the centre of the macula in the retina, which is called the fovea.
Seeing patterns There are five classes of neurons in the retina:
The visual image we see is already enhanced and adapted by the • Photoreceptors
neuron circuitry in the retina, thalamus, and cortex. Our visual • Bipolar cells
system is constantly making assumptions about the outside world • Ganglion cells
based on experience and build-in neuronal wiring to modulate the • Horizontal cells
influence of descending projections • Amacrine cells
The anatomy of the eye The cell bodies of these neurons are stacked in layers and are
The retina is the inner part of the eye that contains neurons that located in the inner, outer, and ganglion cell layers.
are sensitive to light and transmit visual signals. The layer behind
the retina is called the choroid, which is composed of the retinal The light comes through the eye and goes through all the layers
photoreceptors that absorb the light. of the retina before it reaches the photoreceptors (rods and
The ciliary body is a ring of tissue that encircles the lens and cones). The retina contains a retinotopic map of the visual field,
consists of muscles that can adjust the lens and it has a vascular which means that each point of the visual space will be processed
component that produces fluid that fills the front of the eye by a different part of the retina.
, Photoreceptors Phototransduction
There are two types of photoreceptors: rods and cones. They In most sensory systems, activation of a receptor by the
both have an outer segment that contains membranous disks with appropriate stimulus causes the cell membrane to depolarise,
light-sensitive photopigment, and an inner segment that contains stimulating an action potential and transmitter release onto the
the cell nucleus and that gives rise to synaptic terminals that neurons it contacts. In the retina, photoreceptors do not exhibit
contact bipolar or horizontal cells. The differences between rods action potentials. Light activation causes a graded change in
and cones are their shape and size. Rods are longer and membrane potential and a change in the rate of transmitter
therefore contain more optic discs. The optic discs are also part release onto postsynaptic neurons. The retina is mediated by
of the membrane. The optic discs are removed regulary and graded potentials since the distances are short.
processed by the pigment epithelium of the retina, this takes
about 12 days. The reason for this is that the photoreceptors/ In the dark, the photoreceptors are in a depolarised state and
photopigment are becoming bleached and need to be renewed. the number of open calcium channels is high, therefore the rate
> A three-neuron chain: photoreceptor cell - bipolar cell - of transmitter release is also high.
ganglion cell is the most direct path of information flow from When light falls onto a photoreceptor the calcium channels close
photoreceptors to the optic nerve. and the membrane hyperpolarises.
> The reason for this is still unknown but it probably has
something to do with the response to both increases and
decreases in luminance.
In the dark, cations Na+ and Ca2+ flow into the outer segment
through membrane channels that are gated by cGMP. This inward
current is opposed by the outward flow of K+ into the inner
segment and thus causes depolarization.
Absorption of light reduces the concentration of cGMP and leads
to the closure of the channels and a reduction in Na+ and Ca2+
The rods and cones are present in the outer layer and the cell influx, and hyperpolarizes the cell.
bodies make up the inner layer. The much larger axons of the
ganglion cells from the optic nerve carry the information to the The series of changes that leads to
rest of the CNS. The rods and cones respond to light with graded a reduction in cGMP begins when a
hyperpolarisation. photon is absorbed by the photo-
pigment in the receptor disks. The
photopigment contains the light-
absorbing chromophore retinal that
is coupled to proteins called opsins. The different opsins tune the
molecule’s absorption of light to a particular region of the light
spectrum.
When a photon gets absorbed, there is a series of alterations in
Horizontal cells enable lateral interactions between the opsin that lead to the activation of an intracellular messenger
photoreceptors and bipolar cells that maintain the visual system’s called transducin, which activates PDE that in turn hydrolyses
sensitivity to contrast over a wide range of light intensities/ cGMP and lowers its concentration. All these events take place in
luminance. the disk membrane.
Amacrine cells are in between bipolar cells and ganglion cells and
contribute to visual function. Steps of phototransduction:
> Amacrine cells can help transmit information from rods to 1) Light is absorbed and activates pigment molecules opsin or
ganglion cells, but they can also be critical for generating the rhodopsin (in rods).
direction-selective responses exhibited by specialised ganglion 2) Opsin/rhodopsin stimulates the G-protein transducin which in
cells. turn activates cGMP phosphodiesterase that catalyses the
breakdown of cGMP (to 5-GMP)
3) The cGMP-gated channels are closing and the reducing inward
current causes hyperpolarisation.
> An advantage of the cascade is that one photon can lead to the
closure of 200 channels (only in rods, since they are sensitive to
light).
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