Learning Exercise

Vision: the mismatch between external reality and the filtered and distorted perceptions that are provided by sensory input.

The goal for this assignment is for students to build an appreciation of the marvelous tailoring
of evolution that makes it possible for us to extract complex information from
sensory input. Sensory input, the information flow into the brain, is alike for all the sensory
systems including taste, touch, vision, hearing, smell, pain, equilibrium, blood
pressure, chemoreception, and proprioception. In this activity, students investigate
in detail one example showing how the sensory nervous system both dissects and
integrates information before sending that information to the brain. By dividing a
sensory field into small areas that can be monitored individually, sensory neurons
extract detailed information that is then integrated by combining features within
and between receptive fields to determine the relationships between stimuli.
Course: Human Physiology
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Exercise

Trace the signal transduction pathways and the steps from reception of the external light signal in the eye to transmission of the information about the
brightness of the light. Your job is to explain why the brightness of a wall looks different when viewed through an opaque tube compare to what you see without the tube by describing what happens to the signal from reception to
transmission through the ganglion cells and then to the optic nerve.



Write in paragraph form and use the html flags at the beginning and at the end of each paragraph. Any time you do not write in your own words, use quotation marks and include a bibliography.



Typical good student response:


The photoreceptors are the only light sensitive part of the visual system. They
consist of two types, rods and cones, and sit on the bottom of the retina, below the
bipolar and ganglion cells. Which brings up a interesting predicament that will be
explained later. Light passes through the lens of the eye and hits either the cone or
rod. In the rod, a protein called rhodopsin changes its conformation and activates a
G-protein secondary messenger, which in turn activates phosphodiesterase and
degrades cGMP to GMP. Before the light created this chain reaction. The
photoreceptor sits at a ?30 mV resting potential with Na+ leaking in through a
cGMP-mediated gate. Once the cGMP is degraded, the channel is shut, and the cell
then hyperpolarizes to ?65 mV. This does not create an action potential, but signals
the bipolar cells that are connected to them. Also, it should be pointed out that the
use of these secondary messengers allows for amplification, which makes our
photoreceptors sensitive to light, firing even if one photon hits a receptor.



The number of photoreceptors that are connected to a bipolar cell depends on the
visual acuity required. Rods are more sensitive to light and are in more concentration
on the periphery of our view.They are not sensitive to color, and thus many of them
can be connected to one bipolar cell to pick up dim light. In contrast, cones are
highly sensitive to color, and thus are usually connected to one bipolar cell to
enhance the sharpness of what we see. Binary cells aren?t limited to only the
photoreceptors that they are connected to. Through the use of horizontal cells,
bipolar cell activation can be affected by these outside receptors. However, the
affect of these other receptors seems to be an antagonizing one, when seen through
the action potential that the ganglion experience. For example, in a off center bipolar
cell (where a shadow in the center, primary receptors will exhibit a depolarization),
when a shadow in the center receptors will result in a rapid firing of the connected
ganglion, or excitatory postsynaptic potential (EPP). While when darkness passes
over the outer cells, it experiences an inhibitory postsynaptic potential (IPP), limiting
the number of action potentials of the ganglion. This phenomenon can be seen by
looking at a white wall through an opaque tube. The wall will look brighter looking
through the tube, then when not. And if you look closely through the tube, you will
notice that there is a brighter ring around the edge between light and dark, where
partial the shadow creates more of an EPP response through the ganglion cells.



The ganglion cells all converge and exit out the eye through the optic disk, which is
essentially a hole in your retina that doesn?t pick up any light activity. So, why don?t
we see a break in our vision you might ask? Then you should ask yourself, why don?t
we see the ganglion or bipolar cells in front of our photoreceptors. Or the image that
is projected to the back of our eyes is upside-down, so why isn?t the world flipped to
us? The answer is that we don?t really see with our eyes, they are just a mechanism
that picks up the information. When it is sentthrough the optic nerve, through the
optic chiasm, and finally to the temporal lobe of the brain. The temporal lobe is where
the information is filtered, filled up, and corrected to where vision becomes
comprehendible and functional for our needs.



Typical average student response:

Nerve cells take in information from cells that feed into it, and deliver it to other
cells. Nerve impulses convey that information. The presynaptic membrane contains
channels that respond to depolarization by opening and letting positively charged
calcium ions through. This leads to the release of chemicals called neurotransmitters.
Neurotransmitters act on the postsynaptic membrane, either to lower its membrane
potential or to keep its membrane potential from being lowered. If the membrane
potential is lowered, the frequency of firing increases; the synapse is termed,
excitatory. If the membrane is stabilized at a value above threshold, impulses do not
occur or occur less often; in this case, the synapse is termed inhibitory.



The wall has the same brightness with and without the tube. When viewed through
an opaque tube the wall looks more distorted. Photoreceptors respond when a photon
of light meets a photoreceptor cell and that photon strikes a rod cell after being
absorbed by a receptor protein called rhodospin, which contains Vitamin A that
absorbs light. Nerve cell depolarization leads to the release of transmitters at the
axon terminals. Hyperpolarization by light is caused by the shutting off of a flow of
ions. This flow of ions in the dark causes depolarization of the receptor at rest. As a
result the sodium pores close, the dark current decreases, and the membrane
depolarization declines and the cell hyperpolarizes. The receptor's pores are kept
open by molecules of a chemical called cGMP and a cascade of events is let loose.
The protein part of the molecule activates a large number of molecules of an enzyme
called transducin; each of these in turn activates hundreds of cGMP molecules, with
consequent closing of the pores.



The retina transfers light into nerve signals. Cells at the back of the retina contain
the light receptors, rods and cones. Rods are responsible for our vision in dim light
and are out of commission in bright light. Cones do not respond to dim light but are
responsible for our ability to see fine detail and for out color vision. Melanin mops up
the light that has passed through the retina, keeping it from being reflected back and
scattering around inside the eye. Horizontal, bipolar and amacrine cells occur in the
middle layer of the retina. The layer of cells at the front of the retina contain retinal
ganglion cells, whose axons pass across the surface of the retina, collect in a bundle
at the optic disc, and leave the eye to form the optic nerve. Light has to pass
through the ganglion-cell and bipolar-cell layers before it gets to the rods and cones.



All signals originating at the receptors and arriving at the ganglion cells must pass
through the bipolar cells. It sends a single dendrite in the direction of the receptors.
Horizontal cells are important because they are responsible for the receptive-field
surrounds of retinal-ganglion cells. Interestingly, they lack in anything that looks like
an ordinary axon. Amacrine cells? processes take place in the synaptic zone between
that layer and the ganglion cell layer. They link bipolar cells and retinal ganglion cells
and form an alternative, indirect route between them. They have several different
functions, many of them unknown. One type of amacrine seems to play a part in
specific responses to moving objects found in retinas of frogs and rabbits; another
type is interposed in the path that links ganglion cells to those bipolar cells that
receive rod input. The two main types of ganglion cells are on center, with inhibitory
surround, and off center, with excitatory surround.



Typical struggling student response:


The white wall turns a sort of gray when looked at through the tube. The edges of
the tube however are a very bright white.



The retina has several layers of neurons which perpetuate the signal to the optic
nerve and then to the cortex. From the back of the retina to the front (where the
light would be sensed) it is as follows: Melanin (pigment), Photoreceptors (rods and
cones), Bipolar Neurons, and Ganglion Neurons. In between the layers are Horizontal
cells and Amacrine Cells.



Rods and Cones are Photoreceptors that respond to light in different ways. Rods are
for night vision. They detect in shape and movement. Rods send signals to the cortex
through the visual pigment, Rhodopsin. There are millions more rods than cones in the
retina. Cones detect color in three pigments (red, green, and blue). Cones work in
bright light and are more sensitive to detail. Cones send signals through these three
pigments in a similar fashion to how Rhodopsin would send the signal to the cortex.
Rhodopsin contains two sections, opsin and retinal. When in the dark Rhodopsin is in
an inactive form. In this inactive state sodium and potassium channels are open and
the levels of cGMP are high. Sodium rushes in, depolarizing the cell (the depolarizing
of the cell is the excitatory postsynaptic potential that makes the neuron more likely
to fire) and releasing neurotransmitters to the bipolar cells. When a photon of light
activates the Rhodopsin it bleaches the cell by separating the opsin and retinal. The
opsin goes into the pigment and the retinal activates a cascade that lowers the
levels of cyclic GMP and closes the sodium channels. The decrease of the sodium
hyperpolarizes the cell (the hyperpolarization of the cell is called the inhibitory
postsynaptic potential) this causes the neuron to release less neurotransmitter to
the bipolarneuron. For the cell to recover the opsin that went into the pigment
recombines with the retinal to reform the Rhodopsin.



Axons of the Ganglion cells form the optic nerve. The optic nerve is a neuron where
the signals from the rods and cones are brought together. Ganglion cells are linked to
certain areas. These areas are known as the visual fields (the part of the field that
affects the signal of a single neuron). The visual fields have two areas a round center
and a doughnut shaped ring around. The round area is small and only has a few
receptors attached to it so it is very accurate. The second area of the visual field
has many receptors attached to it there fore it is not very accurate. This outside
field is your peripheral.

Additional Information: http://cpr.molsci.ucla.edu

Technical Notes

Students are asked to look at a white wall in bright light. They look at the wall through a dark opaque
tube with one eye, and compare that brightness to the brightness of the wall as
seen through the other eye without the tube. To understand this distortion (since
we know that the wall has the same brightness with and without the tube) it is
necessary to understand how information is processed before it is sent by the
ganglion cells through the optic nerve to the brain.

The textbook does not have enough detailed information to helpstudents understand
this process, so they fill in the details by referring to
recommended web-based source materials. Students used the following (among others) but the URL's within Brain Connection change!

Receptive fields
URL:
http://www.brainconnection.com/BC/BC_GetPage.pl?pid=Fh_Anatomy_Dir/Fh_Anatwk4
Hermann grid optical illusion
URL:
http://www.brainconnection.com/BC/BC_GetPage.l?pid=Fh_Illusions_Dir/Fh_Illusions06
Direct and indirect pathways from photoreceptors to bipolar cells
URL:
http://www.brainconnection.com/BC/BC_GetPage.pl?pid=Fh_Gallery_Dir/visimg05.htm

Requirements

Before they begin, students use their textbook and glossary to become familiar with these
basic structures and functions:

neuron,
cGMP,
rhodopsin,
photoreceptor,
ganglion cell,
bipolar cell,
horizontal cell,
retina,
neurotransmitter,
excitatory postsynaptic potential,
inhibitory postsynaptic potential,
lateral inhibition,
visual field,
off-center

Topics

Sensory perception, lateral inhibition

Learning Objectives

Students combine text and web-based resources to explain the following through an inquiry-based assignment.
1. How does a white wall look different when viewed through an opaque tube?
2. Describe the organization of the different kinds of neuron cells in the retina.
3. Describe how a photorecepter cell responds to a photon of light. List the
intracellular signals and describe how the photorecepter cell changes in
response to light.
4 .Tell what signal is sent by photorecepter cells and compare that signal sent
in response to light with the signal sent in the absence of light.
5. What is the role of the horizontal and/or amacrine cells?
6. What do ganglion cells do?