lecture 13 turning light into bioelectricity Flashcards
how does the sensory system work
1) information is conducted along neural pathways to dedicated sensory areas of brain
2) code is processed and interpreted by brain
what does the sensory system do
creates perception of world around us *but some things arent processed like uv light and high pitched sounds
what is the sensory system
how the nervous system detects and converts information from the environment to a neural code
what is the sensory system broken into
the 5 senses: touch, smell, see, taste, hear
touch, stretch, proprioception
somatosensory system
who used electrical mapping to find the motor and somatosensory systems
wilder penfield
knowing where your limbs are, pain, temperature
proprioception
which cells have sensory receptors
dorsal root ganglion cells in spinal cord that send info to brain once it’s received from sensory receptors (like 1a axon in myotatic stretch reflex)
what percent of the cortex is visual processing
40%
what does vision give us information about
shape, color, texture, direction, speed, and location
why is the visual system the most studied system
1) visual stimuli is easy to control
2) acuity- ability to distinguish between two nearby points (Snellen chart)
how is the eye like a camera lens
the image passes through both and causes an inversion (backwards and flipped upside down)
why is the image inverted in the eye
The amount of light entering the eye is controlled by the pupil, which is surrounded by the iris – the coloured part of the eye. Because the front part of the eye is curved, it bends the light, creating an upside down image on the retina. The brain eventually turns the image the right way up.
how does the eye focus
contraction of lens by ciliary muscle
how is a camera’s apeture and diaphram like an iris
limits light coming in- in dark, contracts iris for smaller pupil but in light dilates for less light in
how is film (image sensor of a camera) similar to the photoreceptors in the retina
they both cover most of the back (retina covers back of eye)
optic nerve function
It transmits sensory information for vision in the form of electrical impulses from the eye to the brain
difference between eye and camera
eye’s image sensor doesnt have uniform sensitivity
dynamic sensitivity to light (retina gets tired very quickly)
what is the visual field
entire visual space viewed by retina when eye is fixated straight ahead
label these
- Cornea. Outermost transparent layer of eye. Begins focusing process.
- Pupil. Opening to the inner eye.
- Iris. Controls size of puil.
- Lens. Focuses image of object (on retina).
- Retina. Contains cells that detect light.
- Ciliary muscle. Controls shape of the eye.
- Optic Nerve. Transmits information to the brain.
what is the degree each eye can see
about 150 degrees
what part of our vision do we see clearest in
center of vision
the part of the visual field seen by both eyes (binocular area) is located…
primarily on the temporal portion of both retinas
how is the retina ordered
laminar organization
two kinds of photoreceptors
rods and cones
name and function
Rod cells are stimulated by light over a wide range of intensities and are responsible for perceiving the size, shape, and brightness of visual images. They do not perceive colour and fine detail, tasks performed by the other major type of light-sensitive cell, the cone.
name and function
Cone cells are one of the two types of photoreceptor cells that are in the retina of the eye which are responsible for color vision as well as eye color sensitivity; they function best in relatively bright light, as opposed to rod cells that work better in dim light.
three main regions of photoreceptors
outer segment with disks (stacks of membrane with receptors)
inner segment/cell body with organelles
synaptic terminal (no action potential, cells really short so graded potential doesnt dissipate with distance)
what do photoreceptors do
absorb light and connect convert to a chemical signal
then convert again into a chemical signal at the terminal
what is rhodopsin
a GPCR in rods that is retinal gated
what does light do to rhodopsin
converts 11-cis retinal (the ligand in rhodopsin) to all-trans retinal
where is rhodopsin located
it’s a membrane protein located in the disk membrane of the outer segment
what happens once 11-cis retinal has been converted to all-trans retinal
activates rhodopsin and produces change in membrane potential
why are gpcr’s helpful in phototransduction
gpcr type receptors lead to signal amplification
resting vm of photoreceptors
-40 mv
at rest which gated cation channels (that are open) let na in and depolarize rod
cyclic gmp
what are dark currents
vgca channels continuously releasing synaptic vesicles filled with glutamate
photoreceptors ____ in response to light
hyperpolarize (and increased light intensity causes changes in hyperpolarizing magnitude and duration)
what is this showing
break this down
are rods or cones faster
cones
which is more sensitive: rods or cones
rods (cones recover from using all available substrate, photobleaching, faster
fovea
only cones, found in center of visual field
periphery
mostly cones, higher ratio of photoreceptors to post synaptic cells (more sensitive to low light)
fovea is the area of highest:
acuity
each cone expresses one
opsin
what is opsin
a family of g-protein couple receptors found in the membraneous disks of
photoreceptors. In rods opsin is coupled with retinal (a molecule derived from vitamin A) to form
rhodopsin. Retinal can have 2 different isoforms. (11-cis and all-trans retinal). The colored opsins in
cones are known by which colors (or wave length) they best detect (e.g. green/medium wavelength
opsin)
types of opsin
red blue green
name 6 cells in retina
how does color perception work
how do illusions work
caused when cones adapt to overstimulation and lose sensitivity
light on photoreceptor
hyperpolarizaion, less nt released
dark on photoreceptor
depolarization, more nt released
postsynaptic neurons receive that code and further process that information as it is sent to the brain
neurons of retina
off center bipolar cells (which photoreceptors connect to)
hyperpolarize with light
on center bipolar cells (which photoreceptors connect to)
depolarize with light
how does light move through eye
Light is first bent and focused by the cornea, and
then enters the eye through the pupil, is further focused by the lens, before hitting the retina, the layers
of neurons lining the back of the eye
Cornea
specialized transparent tissue at the
front of the eye. The curved surface of the
cornea bends the light so that the light rays
that hit the cornea at different angles are bent
such that they converge on the back of the
retina, to produce a crisp image. Thus, the
cornea provides most of the refractive power
of the eye
Pupil
the opening in the iris, that allows light
to enter the eye and eventually strike the retina
on the back.
iris
The size of the pupil is
determined by the iris, a thin circular structure
Lens
another specialized transparent tissue
that works in conjunctions with the cornea to
focus light on the retina. Although the lens provides less refractive power than the cornea, it is
adjustable so it plays a critical role in allowing the eye to bring objects at various distances into
sharp focus. The ciliary muscles are attached to the lens by zonule fibers. When the ciliary muscles
contract they reduce the tension on the zonule fibers, allowing the lens to become thicker and
rounder, increasing the refractive power and improving the focus on near objects (accomodation).
When the ciliary muscles relax, the lens becomes flatter, which is better for distance vision
Retina
innermost layer of the eye. Itself a layered structure that contains the visual sensory
neurons, circuitry for the initial processing of visual information, as well as neurons that transmit
that information to the brain
Pigment epithelium
layer of cells just behind the retina that is heavily pigmented to absorb any
scattered light that is not sensed by the sensory neurons. It also nourishes the sensory cells
Fovea
Latin for pit, the fovea is the central, thinnest part of the retina that has a high density of
cone photoreceptors, the least amount of convergence (multiple presynaptic neurons synapsing
onto a single postsynaptic target), and the highest acuity
Optic disk (blind spot)
the area on the back of the retina where the retinal blood vessels
originate and where the axons carrying visual information exit the eye and carry visual information
to the brain. There are no photoreceptors on the optic disk, which is why it creates a blind spot in
the visual field for that eye
visual field
the extent of space seen by one eye
The ability of the eye to distinguish two
points near each other is called
visual acuity. Acuity depends on several factors but especially on the
spacing of the sensory cells in the retina and the precision of the eye’s refraction. The eye test we are
familiar with at a doctor’s office is a test of acuity, specifically the acuity of the fovea
where does light first encounter neurons
The retina is where light first encounters neurons, and light must actually pass by 4 other layers of
neurons before reaching the photoreceptors, where special molecules absorb and detect light
Photoreceptor
visual sensory cell that converts light into electrical signals. Located at the
innermost layer of the retina next to the pigment epithelium. The outer segment of a photoreceptor
cell contains membranous disks with light-sensitive photopigments, the inner segment contains
the nucleus and the synaptic terminal releases glutamate onto bipolar and horizontal cells. Due to
the presence of cGMP-gated cation channels, photoreceptors are relatively depolarized (Vmem ≈
-40 mV) in the dark. Photoreceptors do not fire action potentials as their axons are very short
Phototransduction
is the biochemical process by which light is converted to an electrical signal
within photoreceptor cells. Photoreceptors are strange, in the dark (or what we think of at rest) they
are relatively depolarized, and light causes hyperpolarization. Although this seems counterintuitive, we will see that the retina is not just a photon detector, but senses the change in light, in which case
detecting lights turning OFF (or getting dimmer) can be just as informative as detecting lights turning
ON (or getting brighter).
In rods, when a photon converts the 11-cis retinal into all-trans retinal, the rhodopsin molecule also
changes shape and activates the g-protein (transducin). Transducin activates another messenger
(phosphodiesterase) which breaks down cGMP. Within the cell membrane of photoreceptor cells are
cGMP-gated cation channels which allow the flow of Na+ and Ca++ into the cell and K+ out of the cell. Since light leads to a break down of cGMP, these cGMP-gated channels close causing the cell to hyperpolarize and release less neurotransmitter.
bipolar cells
Within the retina, the photoreceptor cells pass on the visual information to bipolar cells through a
chemical synapse
Bipolar cells are also very short cells that do not have
a true axon and thus do not fire action potentials
excitatory neurons in the retina that transmit information from the photoreceptors to the
retinal ganglion cells (and amacrine cells). Bipolar cells do not fire action potentials, but have a graded
release of neurotransmitter. Bipolar cells release glutamate. There are two different types of bipolar
cells. The reason why there are these different types is unknown, but it must have provided an
evolutionary advantage at some point
glutamate
Photoreceptors release the neurotransmitter glutamate directly onto bipolar cells,
in the middle or inner nuclear layer of the retina.
List the steps in phototransduction including critical molecules and channels.
Recall that phototransduction leads to photoreceptor hyperpolarization and reduction of
neurotransmitter release upon light activation
Explain the advantages of having both rods and cones, including how the selective activation of
the three different color cones gives rise to color vision
Describe the responses of ON and OFF bipolar cells to light or dark.
Explain how different synaptic receptors on ON and OFF bipolar cells produce opposite responses
to light
on bipolar cells
ON bipolar cells depolarize in the light and hyper polarize in the dark. All photoreceptors release the same neurotransmitter (glutamate) and photoreceptors hyper polarize/release less glutamate in the light. The opposite responses of the ON and OFF bipolar cells is due to different synaptic receptors on their
dendrites. ON bipolar cell - bipolar cell that is active by light. Their dendrites contains inhibitory mGluR6 type glutamate receptors, thus they invert the signal the photoreceptors send. Since photoreceptors
release less glutamate in the light, “ON” cells invert that signal and release more NT in the light.
Mammalian photoreceptors _____ in response to light.
hyperpolarize
The fovea is _____ .
the place in the retina with the highest concentration of cones.
What would happen to the membrane potential of a rod if you blocked phosphodiesterase (PDE) and then stimulated the photoreceptor with light?
There would be no change in the membrane potential.
Rhodopsin is what kind of protein?
G-protein coupled receptor
