Neuro 101 Exam 3 Review

What is the cornea?

transparent, dome-shaped external part of the eye that bends (refracts) light and helps focus it onto the retina

What is the pupil?

the circular opening in the center of the iris that regulates the amount of light entering the eye by constricting or dilating

What is the iris?

the colored portion of the eye surrounding and controlling the pupils size by allowing for an appropriate amount of light to enter the eye

What is the lens?

behind the pupil and iris; refracts and fine-tunes focus

What is the retina?

light-sensitive region containing photoreceptors

Optic disc

where the axons of retinal ganglion cells leave the eye

no photoreceptors → blind spot

Rod photoreceptors

• low visual acuity

• black and white vision

• peripheral retina

• functions in low-light conditions (night)

Cone photoreceptors

• high visual acuity (sharp, fine details)

• color vision

• concentrated in the fovea

• functions in bright light conditions (daytime)

  • cone = color

Phototransduction

Light causes photoreceptors to hyperpolarize

1. Light strikes photoreceptor

2. Retinal changes shape

3. Opsin activates transducin

4. cGMP decreases

5. Na+ channels close

6. Photoreceptors hyperpolarize

7. Neurotransmitter release decreases

Where is the primary visual cortex (V1)?

Occipital lobe

In darkness, photoreceptors…

depolarize (-40mV) and release lots of glutamate

In light, photoreceptors…

hyperpolarize and release less glutamate

OFF Bipolar Cells

ionotropic glutamate receptors

ON Bipolar cells release glutamate in the dark

Glutamate excites them

In the dark:

1. photoreceptor releases lots of glutamate

2. OFF bipolar cell is depolarized

3. OFF pathway is active

ON Bipolar Cells

Metabotropic glutamate receptors

ON Bipolar cells release glutamate in the light

Glutamate inhibits them

1. Less glutamate released

2. Inhibition removed

3. ON bipolar cell depolarizes

4. ON pathway ON

Visual Pathway

Retina → Optic Nerve → Optic Chiasm → Optic Tract → LGN (thalamus) → Visual Cortex (occipital lobe)

Retina Visual Pathway

Photoreceptors → Bipolar cells → Retinal Ganglion Cells (RGCs) → Optic Nerve

Lateral Geniculate Nucleus (LGN)

Located in the thalamus; Receives visual sensory information & sends it to primary visual cortex (occipital lobe)

Parvocellular Pathway

“What” pathway

• color, texture, shape, depth

• small cells with small receptive fields (detecting detail)

parvo = precise

Magnocellular Pathway

“Where” pathway

• location and movement of an object

• larger cells with large receptive fields

magno = movement