Visual Pathway

Primary visual cortex location (what surface of what lobe, surrounding what)

On the medial surface of the occipital lobe

Surrounding the calcarine sulcus

True'/False Visual info travels in crossed paths

Kind of true but visual information travels in both crossed and uncrossed paths

4 cells involved in conduction of visual impulses to visual cortex

Photoreceptor cells (rods and cones)

Bipolar neurons

Ganglion cells

Neurons of lateral geniculate body in thalamus

Bipolar neurons connect what to what

Connect rods and cones to ganglion cells

Axons of the ganglion cells pass to where

Pass to thalamus

Where do axons of the neurons of lateral geniculate body in thalamus pass to

Axons pass to visual cortex

Light rays pass through the lens to the

Retina

Outmost layer of retina

pigment epithelial layer (single layer of cells containing melanin)

Role of the pigment epithelial layer

Pigment cells absorb light that passes through retina

Photoreceptors are a layer of the retina - They are made up of what types of cells

rods and cones

Human retina has how many rods & how many cones

110-125 million rods,

6-7 million cones

Role of cones

colour vision

Role of rods

vision in light of low intensity

Bipolar cells are found in the retina. What is their role

They serve as local circuit neurons connecting rods and cones to ganglion cells.

What interneurons are present in the retina?

Horizontal cells and amacrine cells

Axons of what cells form the optic nerve

Axons of ganglion cells form optic nerve

Are ganglion cells myelinated or unmyelinated

Unmyelinated until leave eye - an optical advantage as myelin is highly refractive

What is the fovea

An area of the retina that is structurally different & allows for acute (high-resolution) vision

How is the fovea built to allow for acute (high-resolution) vision

Contains only cones

Inner retinal layers are displaced to allow light to directly reach cones

What is the visual axis

The line connecting fovea with viewed object

What is the macula lutea?

The area surrounding the fovea.

Where is the blind spot of the eye

Optic disc (papilla)

What is the optic disc (papilla)?

The area where unmyelinated optic nerve fibres exit the retina  - known as the blind spot because it lacks photoreceptors.

What happens to fibres after leaving the eye?

They become myelinated

Does the optic nerve have good regenerative capacity

No

What direction do light rays & visual impulse travel in the eye

Light rays strike retina and travel from internal to external retinal layers

Visual impulses pass from external to internal retinal layers

Describe the path of visual impulses

Rods and cones → Bipolar cells → Ganglion cells → Optic nerve → Optic chiasm → Thalamus → Visual cortex

What is binocular vision?

The overlap of right and left visual fields so both eyes project portions of each field onto both retinas, allowing depth perception

How is the right visual field projected on the retina

Nasal (medial) half of the right retina

Temporal (lateral) half of the left retina

Where is the right field of vision processed in the brain

In the left visual cortex

What fibres cross in the optic chiasm

Fibres from the nasal (medial) halves of both retinas

What fibres do not cross the optic chiasm

Fibres from the temporal (lateral) halves → continue in the optic tract of the same side

Where do most fibres terminate

In the lateral geniculate body of the thalamus

What arises from the lateral geniculate body

Optic radiation (Loop of Meyer), projecting to the primary visual cortex on the walls of the calcarine sulcus.

How are visual field quadrants arranged on the retina?

They are reversed and inverted — upper visual fields project onto lower retina and vice versa

What is the relationship of the optic chiasm to the pituitary gland?

(clinical significance?)

The optic chiasm lies just above the pituitary stalk - a pituitary tumour can damage the median portion of the chiasm → Bitemporal hemianopia

What structure lies lateral to the optic chiasm

(clinical significance?)

The internal carotid arteries — aneurysm here can damage the lateral part of the chiasm → Monocular blindness

Damage to the area of which number correlates with which visual field defect

Name the 3 Cranial Nerves responsible for controlling extraocular muscles

CN III (Oculomotor)

CN IV (Trochlear)

CN VI (Abducent)

What specific movements do each of the 3 Cranial Nerves control

CN III (Oculomotor): All other eye movements

CN IV (Trochlear): Turns the eye downward and laterally

CN VI (Abducent): Abducts the eye laterally

Where do fibres go that mediate light reflexes?

Some optic tract fibres pass from the lateral geniculate nucleus to the pretectal nucleus in the midbrain

Describe the direct and consensual light reflexes

Direct reflex: Pupil constricts in the eye directly exposed to light

Consensual reflex: Pupil of the opposite eye also constricts

What muscle is responsible for constricting the pupils

Pupillary constrictor muscle

Outline the neural pathway for the light reflex

Light stimulus → Optic nerve → Optic chiasm → Optic tract → Synapse in pretectal nucleus near the superior colliculus → Edinger-Westphal nuclei (Parasympathetic nuclei of CN III (oculomotor nerve)) → Ciliary ganglion → Short ciliary nerves → Constrictor pupillary muscles of the iris → Both pupils constrict

What triggers the accommodation reflex

Shifting gaze from a distant to a near object

What muscles are involves in the Accommodation reflex

Contraction of medial rectus muscle on both sides

What changes happen in the eye as a result of the accommodation reflex

Causes convergence of eyes

Lens thickens to increase refractive power by contraction of cillary muscle

Pupils constrict to restrict light waves to central part of lens

How does the accommodation reflex pathway work?

1. Impulses pass from retina → Visual cortex

2. Visual cortex connects to frontal eye field

3. Fibres descend to oculomotor nuclei in midbrain

4. CN III activates:

   • Medial recti → Convergence

   • Parasympathetic fibres → Pupil constriction