The Visual System

Question 1

Anatomy: Sclera

Answer 1

Outer Layer

Tough white covering; provide protection and is where the extraoccular muscles insert

Question 2

Anatomy: Cornea

Answer 2

Outer layer

anterior outer part, highly sensitive to touch, major refractive structure

Question 3

Anatomy: Choroid

Answer 3

Middle layer

Vascular bed; pigmented and dense; continuous anteriorly with the cilliary body

Question 4

Anatomy: Ciliarry body

Answer 4

Middle Layer

2 Parts:
Muscular - changes the thickness of the lens so that it can accomodate and focus the external image on the retina

Vascular - releases aqueous humor into the anterior compartment of the eye

Question 5

Anatomy: Iris

Answer 5

Middle Layer

thin contractile ring overlying the lens that that contains circularly arranged sphincter pupillae and radially arranged dialtor pupillae which constrict and dilate the pupil to regulate ow much light falls on the retina

Question 6

Anatomy: Retina

Optic part

Answer 6

Inner layer

2 layers:

outer pigmented layer is composed of epithelial cells and provides nutrition to the neural layer and provides protection by absorbing extra light

inner neural layer contains photoreceptors

Question 7

Anatomy - Retina

Optic Part - neural layer

Answer 7

Inner Layer

The fundus is the part of the neural layer seen through an ophthalmostcope

The optic disc is where vascular and neural elements enter and exit the eye and has no receptors therefore it is our blind spot.

The macula is the region that supports the highest visual acuity

It contains the fovea, which is the region that has the greatest density of photoreceptors, and the vascular and neural elements are shifted away from it so that nothing blocks the light getting to the receptors

Question 8

Anatomy: Lens

Answer 8

Transparent, biconvex structure held in place by the cilliary bodies. It is a refractive structure, and it separates the anterior (between the cornea and the pupil) and posterior (between the posterior to the lens) compartments of the eye.

Question 9

What are the basic steps of image formation?

Answer 9

1) Decision to examine an object
2) Extraocular muscles move the eye so that the object is at the center of the visual field. The point we focus on is our fixation point.
3) Light from the fixation point is focused on the fovea, and light from the remainder of the target is focused on the surrounding areas of the retina

Question 10

Neural Retina: Cells

What are the 5 basic classes or neurons in the neural retina? How are they subdivided?

Answer 10

They are subdivided based on the direction of information flow.

1) Direct Pathway (vertical information flow):

Photoreceptors - transduce light into a neuronal signal NOTE: the receptor is NOT part of the 1st order neuron! It is a seperate cell

Bipolar cells: interneurons that transmit the signal from the receptor cells to the ganglion cells

Ganglion cells: convey info to the CNS; form the OPTIC NERVE

2) Horizontal Pathway - Lateral information flow that modifies information form the vertical pathway (sharpen contrast)

Horizontal Cells
Amacrine Cells

Question 11

Laminar Organization of the Retina; What are the layers, and what is the direction of light flow and the direction of information flow?

Answer 11

LIGHT Travels through the layers in the following order:

1) Ganglion cells - cell bodies of ganglion cells
2) Inner plexiform layer - processes of ganglion, bipolar, and amacrine cells
3) Inner nuclear layer - cell bodies of bipolar, amacrine, and horizontal cells
4) Outer plexiform Layer - processes of photoreceptors, bipolar, and horizontal cells
5) Outer nuclear - cell bodies of photoreceptors
6) Photo receptor outer segment

INFORMATION flows through these layers in the OPPOSITE DIRECTION once the light hit the photoreceptors.

Question 12

Photoreceptors - Rods

Structural Features

Answer 12

Rods outnumber cones 20 to 1

Outer segment is long and cylindrical
Outersegment contains more membranous discs than cones (these are where phototransduction occurs)
They are absent in the fovea, and most dense along the periphery of the retina.

**Many neighboring rods converge on a single bipolar cell - amplifies the signal

Question 13

Photoreceptors - Rods

Function

Answer 13

Very sensitive to dim light

Specialized for night vision

Achromatic

Poor spatial resolution

Question 14

Photoreceptors - Cones

Structural Features

Answer 14

There are 3 types, depending on what photopigment they are sensitive to. Different photopigments are optimized for absorption of light in different parts of the visible light spectrum:

1) S - short wavelength (blue)
2) M - medium wavelength (green)
3) L = long wavelength (red)

Outer segment is short and tapering, and contains fewer membranous discs than rods.
Most dense in the fovea
Convergence is limited - one or just a few cones to one bipolar cell

Question 15

Photoreceptors - Cones

Function

Answer 15

Less sensitive to light
specialized for day vision
Trichromatic
High spatial resolution

Question 16

Steps in Phototransduction

Answer 16

1) In the dark, [cGMP] is high, which keeps open the Ca and Na ion channels on the outer segment of the photoreceptors.
2) As a result, the photoreceptors are DEPOLARIZED in the dark. They are tonically releasing the neurotransmitter glutamate.
3) Glutamate INHIBITS the receptors on the ganglion cells, so that in the dark, they cannot transmit a signal.
4) When light is absorbed, it activates phosphodiesterase, which lowers the levels of cGMP
5) Lower levels of cGMP cause Ca and Na to stop flowing into the photoreceptor. K+ then flows out of the photoreceptor and the photoreceptor is hyperpolarized
6) Hyperpolarization of the receptors stops the release of glutamate, so the ganglion cells are no longer being inhibited and they can send a signal.

Question 17

Processing visual input: Receptive fields

What is the definition of a receptive field?

Answer 17

It is the area in which the stimulus affects a single neuron

Question 18

Processing visual input: Receptive fields

What neurons in the visual system have the smallest visual fields? What is their shape?

Answer 18

The neruons with the smallest visual fields are the bipolar cells and the ganglion cells that make up the optic nerve.

These receptive fields have a concentric center-surround shape.

Question 19

Processing visual input: Receptive fields

Describe the excitation pattern of concentric center-surround receptive fields. What does this organization allow these cells to respond to?

Answer 19

Stimuli in the center respond to tone type of stimulus (like light) While the doughnut shaped ring around it responds to the opposite stimulus (like dark)

In this way, cells can respond to the differences in INTENSITY of light between the center part and the surround part of their visual fields. This allows higher systems to detect weak contrasts, and rapid changes in light intensity

Question 20

Processing visual input: Receptive fields

What are the types of center-surround cells?

Answer 20

On and off center cells

On-center - fire most rigorously when the light stimulus is in the center of the visual field

Off-center fire most vigorously when there is dark on the center of the field and light in the surrounding part

Question 21

Neuronal signaling: what kinds of signals do cells in the retina use to communicate BEFORE the level of the ganglion cells?

Answer 21

Graded, local potentials. These cells are tightly packed.

Question 22

Neuronal signaling: which are the first cells in the pathway to communicate using action potentials?

Answer 22

Ganglion Cells

Question 23

Retinal Ganglion Cells

Describe their firing patter

Answer 23

These cells are tonically active, and the firing rate either increases or decreases depending on the presence of light.

Question 24

Retinal ganglion cells - cell types

What is the function of having multiple types of retinal ganglion cells? What are the 2 types that we discussed?

Answer 24

Different types of retinal ganglion cells mediate different submodalities.

We have magnocellular and Parvocellular ganglion cells

Question 25

Retinal Ganglion cells - Magnocelular

Describe the receptive field, stimulus energy, and perception associated with these cells.

Answer 25

these cells have large receptive fields.

They detect large objects and are able to detect rapid changes in the stimulus

the appear to be responsible for the analysis of motion (location, speed, and direction)

Question 26

Retinal Ganglion cells - Magnocelular

Describe the receptive field, stimulus energy, and perception associated with these cells.

Answer 26

These cells have small receptive fields

They respond to specific wavelengths.

They appear to be responsible for the analysis of high acuity (shape and texture) and color.