Visual system I: anatomy, physiology and control of the carnivore eye

Question 1

Eye: introduction

Answer 1

• Sight – vision – eyes – optic nerve (plus others for movement!)
• Eyeball (globe), ocular adnexa, musculature the move the eye and nerves that innervate the eye
• Orbit – review bones of the skull!
• In species with incomplete orbit (dogs and cats): orbital ligament

Periorbita is made of connective tissue and smooth muscle enclosing the eyeball, adjacent muscles, vessels and nerves*
* Caudal part, the apex in direct contact with the meninges so could be used to decompress intracranial pressure (more in
humans)

Question 2

Parts of the eye

Answer 2

The sclera

The Cornea

Anterior & Posterior Chambers

Iris/Pupil

Lens

Vitreous Humor

Retina

Question 3

Muscles associated with the eye

Answer 3

• Responsible for moving the eye
• Four Rectus muscles: dorsal, ventral,
medial and lateral (which insert at the
equator of the sclera)
• Position & action are as their name states
• Two oblique muscles: dorsal and ventral,
these rotate the eye
• Retractor bulbi: forms a cone around the
optic nerve and pulls the eye deeper into
the socket
• Innervated by the abducens, trochlear,
and the oculomotor nerves

Uvea: ciliary muscles and iris
• Smooth muscle – involuntary. SNS and PsNS (except in birds!!) • Lesions in the uvea can scar and have impact of functionality

Question 4

Rectus muscles insert in

Answer 4

the equator of the sclera.

Question 5

Eyeball

• Three main layers:

Answer 5

External fibrous, middle vascular, internal nervous coat

Question 6

External layers of the eye

Answer 6

1. Cornea 2. Sclera 3. Limbus

Question 7

Cornea

Answer 7

• Cranial fourth of the eyeball, transparent, avascular
• Very sensitive (nerve endings) – ophtalmic nerve, branch of trigeminal nerve (V)→corneal reflex
• Cornea is made of three layers: epithelial (and basal membrane), stroma and Descemet’s membrane
• Fluorescein stains the stroma but will not stain the Descemet membrane

Question 8

Sclera and limbus

Answer 8

• Vascular
• Attachment of muscles
(equator)
• Covers the rest of the eyeball

Question 9

Middle layer of the eyevascular tunic or uvea

Answer 9

• Deeper to the sclera and divided in three main areas: choroid, ciliary body and iris

1. Choroid (with Tapetum) – from the optic nerve to the inside of the limbus
2. Ciliary body is a thick continuation of the choroid right inside the area of the limbus
3. Iris project insides dividing the aqueous chamber in anterior and posterior

Question 10

Uvea

Answer 10

• Choroid contains vessels supplied by the posterior ciliary arteries and drained into vorticose veins
• Capillaries in the inner part is what we see when we examine the fundus of the eye
• Check the vessels here in that radiate from the optic disc (dog)
• Back of the eye: tapetum lucidum

Question 11

Lens accommodation

Answer 11

• Ciliary muscle (ciliary body) allows lens accommodation
• Lens is a solid structure suspended by the ciliary body and separates the aqueous compartment from the vitreous compartment (caudal)
• Lens is like an onion (concentric layers) allowing to focus
• Cataract = lens opacity
• Lens shape changes with species (predator vs prey)

Question 12

Iris and pupil

Answer 12

• Iris regulates size of pupil – autonomic nervous system
• Vision, function of optic nerve and optic chiasm is tested through PLR
• Iris is pigmented and in horses corpora nigrans
• Ciliary body and iris regulate IOP (aqueous humour)

Question 13

Aqueous compartment of the Eye

Answer 13

(most rostral)
• Anterior chamber – aqueous humour
• Posterior chamber – aqueous humour
Aqueous humour is produced in the ciliary body in the posterior chamber and it is drained through the limbus. Maintains IOP and provides nutrients

Question 14

• Vitreous compartment of the eye

Answer 14

(most caudal) – vitreous humour, a gelatinous fluid. Maintains retina against choroid and provides nutrients

Question 15

Deeper layer: internal tunic (neural) of the eye

Answer 15

• Retina layer: full of receptor and really sensitive to lack of vascularisation
• The caudal part is “visual”, takes up the light whereas the cranial part is “blind”
• Visual part made of rods and cones
• Blind spot at the optic disc

Question 16

Retina: five layers

Answer 16

− pigment layer
− photoreceptor layer
− bipolar layer
− ganglion layer
− nerve fibre layer

Question 17

How is light perceived

Answer 17

• Information (light!) gets in through the cornea, aqueous compartment, lens and vitreous compartment
• It reaches the retina at the back of the eye where it goes through five layers: pigment layer, captured through the photoreceptor layer and transmitted by the bipolar, ganglion and nerve fibre layers towards the optic disc to the optic nerve
• The photoreceptors are fancy neurons with modified dendrites: • Rods: sensitive to light
• Cones: sensitive to colour and detail

Question 18

Special sense organs are

Answer 18

a collection of those specialised receptor cells

• Information is received by special receptor cells that send it to the CNS
• Receptors will capture stimuli that can be:
• Mechanical
• Thermal
• Electromagnetic
• Chemical

Question 19

Vision: formation of the image

Answer 19

• Curvature of the lens is modified by the ciliary body to focus the light onto the retina
• Through the layers of the retina, light hits photoreceptor cells (rods and cones) and some light is reflected back (tapetum lucidum)
• The photoreceptors (rods and cones) send the information to the optic disc (optic nerve)
• From the optic nerve it travels to the CNS to the optic chiasm where information crosses over

Question 20

fovea

Answer 20

The fovea is the area of retina with the highest concentration of cones

Question 21

Eyelids and “friends”

Answer 21

• Medial and lateral canthus
• Conjunctiva
• Palpebral ligaments
• Cillia
• Third eyelid (nictitating membrane)
• Meibomian gland
• Harderian gland
• Lacrimal gland
• Nasolacrimal duct
• Eyelids (maxillary nerve, branch of
trigeminal)

Question 22

What are the sight receptor cells known as?

Answer 22

Photoreceptors detect light during vision.

Question 23

What is the junction between the cornea and the sclera called?

Answer 23

limbus

Question 24

4. Name the structures that make up the uvea

Answer 24

It has three parts: (1) the iris, which is the colored part of the eye

(2) the ciliary body, which is the structure in the eye that secretes the transparent liquid within the eye; and
(3) the choroid, which is the layer of blood vessels and connective tissue between the sclera and the retina.

Question 25

What is the structure called in the choroid that causes the eye to glow in the dark when light falls on it?

Answer 25

That light-reflecting surface, called the tapetum lucidum, helps animals see better in the dark.

Question 26

What are zonular fibres collectively known as?

Answer 26

These fibers are sometimes collectively referred to as the suspensory ligaments of the lens, as they act like suspensory ligaments.

Question 27

Which structure controls the amount of light that enters the eye?

Answer 27

Iris: regulates the amount of light that enters your eye. It forms the coloured, visible part of your eye in front of the lens.

Question 28

Where are bipolar ganglion cells situated?

Answer 28

As a part of the retina, bipolar cells exist between photoreceptors (rod cells and cone cells) and ganglion cells.

Question 29

Where is the posterior chamber situated?

Answer 29

The posterior chamber is a narrow space directly posterior to the iris but anterior to the lens, behind the peripheral part of the iris, and in front of the suspensory ligament of the lens and the ciliary processes, filled with aqueous humor.

Question 30

What is another term for the third eyelid?

Answer 30

The nictitating membrane

Question 31

What is the function of the Pecten Oculi?

Answer 31

It is a non-sensory, pigmented structure that projects into the vitreous humor from the point where the optic nerve enters the eyeball. The pecten is believed to both nourish the retina and control the pH of the vitreous body.