Psych 115: Lecture (11/22/17)

What does it mean for a synapse to be sign-conserving?

A depolarization in the pre-synaptic terminal will lead to a depolarization in the next cell. The same with a hyperpolarization.

What does it mean for a synapse to be sign-inverting?

A depolarization in the pre-synaptic terminal will lead to a hyperpolarization in the next cell and vice versa.

What is an on-center bipolar cell?

A retinal bipolar cell that is excited by light in the center of its receptive field.

What is an off-center bipolar cell?

A retinal bipolar cell that is inhibited by light in the center of its receptive field.

What is an on-center ganglion cell?

A retinal ganglion cell that is activated when light is presented to the center, rather than the periphery, of the cell's receptive field.

What is an off-center ganglion cell?

A retinal ganglion cell that is activated when light is presented to the periphery, rather than the center, of the cell's receptive field.

True or False: Any given rod is only going to absorb one photon.

True

True or False: During uniform illumination, the cones will give no response.

True

True or False: Glutamate normally inhibits on-center bipolar cells.

True

True or False: In vertebrates, shining a light on a photoreceptor causes it to HYPERpolarize, rather than depolarize.

True

True or False: Photoreceptors continually release glutamate, even when nothing is happening.

True

True or False: Photoreceptors themselves at their synaptic terminals are always OFF-center, ON-surround.

True

True or False: Rods and cones both become hyperpolarized when they are excited.

True

True or False: We have about equal numbers of on-center/off-surround cells and off-center/on-surround cells.

True

What is retinal?

Also known as vitamin A aldehyde, it is a pigmented molecule derived from vitamin A. Opsin + retinal makes rhodopsin, which is a transmembrane protein.

What is smooth pursuit?

Another type of eye movement that enables us to follow moving objects like birds or balls.

Where does the off-center/on-surround arrangement first appear?

At the synaptic terminals of the cones; the cones themselves respond to contrast

Why do we point our fovea at regions of interest on the visual scene?

Because we obtain the sharpest images from the fovea.

Why doesn't our mental image of the world jump during saccades (jumping eye movements, 4/sec, when the image DOES jump on our retinas)? A. It does, but we ignore it. B. Our brain assumes the world is stable, so it 'corrects' when the eye position changes. C. A copy of the motor signal for willed eye movements adjusts the mental image.

C

What does light do to 11-cis-retinal?

Converts it to all-trans-retinal

What is it that makes cones release more neurotransmitter: dark or light?

Dark

True or False: In invertebrates, shining a light on a photoreceptor causes it to HYPERpolarize, rather than depolarize.

False: But this is true in vertebrates!

True or False: Retinal ganglion cells are especially good at measuring diffuse illumination, whether or not there is more or less light.

False: They measure SPATIAL CONTRAST. They are most sensitive to differences in intensity of light between center and surround and are relatively unaffected by the average intensity.

What are saccades?

Fast movements of the eyes that present various parts of the visual scene to the fovea. When we fix our gaze, small saccades that we are unaware of avert photoreceptor adaptation.

Where on photoreceptors are mitochondria located?

Inner segment

What is the difference between midget and diffuse bipolar cells?

Midget bipolar cells only receive input from a single cone whereas a diffuse bipolar cell near the fovea receives input from 5-10 cones.

Hyperpolarization of a photoreceptor cell (increases/reduces) the neurotransmitter release.

reduces

The horizontal cell makes a _________-__________________ synapse using _________, back onto the synaptic terminal of the cones. This process is called ___________ _________________. If the horizontal cell is hyperpolarized, it releases (more/less) inhibitor, so the postsynaptic cones are disinhibited and depolarize.

sign-inverting, GABA; lateral inhibition; less

What is the resting potential of photoreceptors?

-35 mV

One cone drives _____ midget bipolar cells driving _____ midget ganglion cells.

2, 2

Our eyes jump (saccade) ______ times each second, so vision is constructed from a series of _____ second glimpses.

3-4, 0.25

1 degree of visual angle corresponds with _____ microns in the retina.

300

Each of the ~500 PDE molecules can convert cGMP to GMP at a rate of __________, so [cGMP] falls, the cGMP-gated Na+ channels close (shown by patch clamping), and the cell ______________________. Purpose? _____________________: One photon causes breakdown of ~______ cGMP/sec.

4000/sec, hyperpolarizes; Amplification, 106

Activated rhodopsin activates ~______ of the associated G-protein (transducin) molecules, each releasing its active (alpha) subunit. Each activated G-protein activates one ______ _________________________ (______).

500; cGMP phosphodiesterase (PDE)

How many extraocular muscles do we have to move the eye?

6 (3 pairs) that are innervated by cranial nerves (III, IV, VI)

Describe how on-center bipolar cells and ganglion cells work.

If you shine light on the photoreceptor, it becomes hyperpolarized and then releases less neurotransmitter. However, keep in mind that the glutamate neurotransmitter normally acts in an inhibitory manner. So it's like you are releasing "less inhibitor," which causes the sign-inverting on-center bipolar cell to become depolarized. This depolarization at the end of the bipolar cell to the on-center ganglion cell causes an increased firing rate. Thus, for on-center cells, the bipolar cells are sign-inverting, and the ganglion cells are sign-conserving.

Why is it useful that photoreceptors hyperpolarize upon excitation in vertebrates from a resting potential of -35 mV?

In the 'gray,' the (depolarized) photoreceptor cell releases glutamate transmitter at a high, continuous rate. It can then signal both darkening or light OFFset (depolarization = increased rate) and lightening or light ONset (hyperpolarization = reduced rate).

What are vergence movements?

Movements that allow our eyes to move inward when we view objects up close.

Photoreceptors are "_____" devices.

OFF --> meaning that light turns them off, and causes less release of neurotransmitter (remember: it closes the Na+ channels)

Where on photoreceptors is rhodopsin located?

On the discs of the outer segment

What is opsin?

One of the two components of photopigments in the retina

What does "on-center/off-surround" mean?

Referring to a concentric receptive field in which the illumination of the center excites the cell of interest while the illumination of the surround inhibits it. Note that the cell is inhibited by a dark spot in the center but excited by dark in the surround.

What does "off-center/on-surround" mean?

Referring to a concentric receptive field in which the illumination of the center inhibits the cell of interest while the illumination of the surround excites it. Note that the cell is excited by a dark spot in the center but inhibited by dark in the surround.

What is rhodopsin?

The photopigment in rods that responds to light

What happens to the Na+ channels when light hyperpolarizes the photoreceptor?

They close!

True or False: For off-center bipolar and ganglion cells, all the synapses are sign-conserving.

True: When light shines on the photoreceptor, this causes hyperpolarization and thus, decreases the release of glutamate in the synaptic terminal. The shining of light on the photoreceptors will ALWAYS lead to hyperpolarization and less release of neurotransmitter. Thus, if a bipolar cell is OFF-center (meaning that it will be inactivated by light), it must be sign-conserving. The bipolar cell must hyperpolarize as well (sign-conserving) to match the photoreceptor. This hyperpolarization then causes less neurotransmitter to be released to the ganglion cell, resulting in a decreased firing rate.

What is the best stimulus for an on-center/off-surround cell?

White spot on a black annulus

What chemical can open the Na+ channels?

cGMP --> it actually holds the channels open; thus, light causes cGMP to be destroyed

The release of more glutamate from a photoreceptor causes an off-center bipolar cell to (depolarize/hyperpolarize).

depolarize

When light is turned off, OFF-center bipolar cells ______________ (_________) OFF- center ganglion cells.

depolarize (excite)

Sign-conserving synapses are ___________________ while sign-inverting synapses are _______________.

excitatory, inhibitory

Bipolar cells respond by changes in _________ _____________ potentials; ganglion cells respond with _________ potentials.

local membrane, action