Lec 4: The Eye, Optic Nerve and the Visual Pathways

what is Lacrimation?

– Activated by Parasympathetic fibers in the Facial Nerve (CN VII)
– Reduced by Sympathetic fibers from the Superior Cervical/Sympathetic Ganglion in the neck

Excess “tears” drain into

nasal cavity

The __ refracts/focuses light through the Pupil (hole in iris) onto the lens

Cornea

Lens projects and focuses light images onto the

retina

Lens is attached, via ligaments, to the muscles of the

ciliary body 

__  in the retina are activated by photons of light

Photoreceptors 

Sensory signals, originating at the photoreceptors, are transmitted out of the eye through the

optic nerve 

iris muscles

sphincter pupillae muscle contracts:

pupil size decreases

what is two muscles of Iris

• Sphincter pupillae
• Dilator pupillae

dilator pupillae muscle contracts:

pupil size increaes

Colored part of eye function:

regulates amount of light entering eye by dilation/constriction of Pupil

Near vision and bright light

• sphincter pupillae (circular muscles) contract

• pupils constrict

• parasympathetic CN III

Distant vision and dim light

• dilator pupillae (radial muscles) contract
• pupils dilate
• sympathetic fibers from the Superior Cervical/Sympathetic Ganglion in the neck

Changes in emotional state

pupils dilate when subject matter is appealing, sexually attractive or requires problem-solving skills

Visual accommodation is the ability of the eye to change focus to maintain a clear image of an object as its distance changes. This process is achieved by the lens of the eye changing shape:

• near objects

• distant objects 

accommodation near object:

The lens becomes more round to focus on nearby objects.

accommodation distant object:

The lens flattens to focus on distant objects.

Ciliary Muscles and the Lens

The ciliary muscle and ciliary zonule are arranged sphincter-like
around the lens. As a result,

contraction loosens the ciliary zonule fibers and relaxation tightens them

what is Focusing For Distant Vision (there’s 3)?

• far point of vision

• pupils dilated

• ciliary muscles relaxed

what is Far point of vision?

• – Distance beyond which no change in lens shape needed for focusing

  • 20 feet for emmetropic (normal) eye

  • Cornea and lens focus light precisely on retina

ciliary muscles relaxed:

Lens stretched flat by tension in ciliary muscles

Lens flattens for distant vision.

Sympathetic input relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens

• Light from close objects diverges it as approaches eye

  • Requires eye to make active adjustments using three simultaneous processes

1. Accommodation of lenses – Parasym. CN III

2. Constriction of pupils – Parasym. CN III

3. Convergence of eyeballs – Med. Rectus, CN III

Accommodation of lenses –

Parasym. CN III

Constriction of pupils –

Parasym. CN III

Convergence of eyeballs –

Med. Rectus, CN III

what is accommodation?

• Changing lens shape to increase refraction

• Near point of vision

  • Closest point on which the eye can focus

• Presbyopia—loss of accommodation over age 50

what is constriction ?

Accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye

what is convergence ?

• Medial rotation of eyeballs toward object being viewed

  • Medial Rectus muscle (CN III)

Lens bulges for close vision

Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge.

what is optic disc?

the internal location, in the posterior and medial part of the eye, where the optic nerve (and retinal blood vessels) exits the eye

The __ has the highest concentration of photoreceptors

macula 

The center of the macula is called the

fovea centralis

what are Photoreceptors?

• Rods

• cones 

what are rods?

• Activated in dim/dark conditions

  • A night-vision photoreceptor

• Detect white, gray and black

• Highest concentration in the periphery of the retina

what is cones?

• Activated in bright light

• Detect color

• 3 types

  • Activated by different colors

  • Red/orange, Yellow/Green & Blue

• Highest concentration is in the macula

Signals from rods and cones (eventually) exit the eye via the

axons of ganglion cells
• The axons of ganglion cells, not photoreceptors, form the optic nerve

what is Fundoscopy of optic disc?

• Increased intraocular pressure results in edema at the optic disc

• Axons and blood vessels passing through the optic disc get compressed

  • Vision can be lost

  • Retinal arteries appear pale due to reduced blood flow

what is optic nerve?

• From the retina to the optic chiasm
• Injury to the optic nerve results in blindness in the ipsilateral eye

• From Optic Chiasm to deeper brain features

• Fibers from the nasal portion of each eye cross to the contralateral side of the brain at the optic chiasm

  • Carrying image signals from the lateral visual field

•  Fibers from the nasal side of one eye and the temporal side of the others eye (1/2 the visual field) come together at the Lateral Geniculate Nucleus of the Thalamus

optic tract

Fibers from the nasal portion of each eye cross to the contralateral side of the brain at the

optic chiasm 

Fibers from the nasal side of one eye and the temporal side of the others eye (1/2 the visual field) come together at the __ of the Thalamus

Lateral Geniculate Nucleus

Compression of the optic chiasm by pituitary adenoma will result in

• deficits in lateral vision in both eyes
• Bitemporal Hemianopia

what is Optic Radiations?

•  A collection of axons that carry visual information from the LGN to the primary visual cortex.

• Ensures visual information reaches the correct area in the primary visual cortex for interpretation

the optic radiations is divided into 2 pathways:

• Upper/Dorsal (parietal) pathway

  • carry information from the Inferior retinal quadrants

• Lower (temporal) pathway (Meyer’s loop)

  •  transmits information from the Superior quadrants.

what does Upper/Dorsal (parietal) pathway carry?

carry information from the Inferior retinal quadrants

what does Lower (temporal) pathway (Meyer’s loop) transmit?

transmits information from the Superior quadrants.

• Along the Calcarine sulcus of the Occipital lobes of the brain

• Processes basic features like edges, orientation, and movement..

• Visual information from the right visual field is processed in the left visual cortex, and vice versa

• Visual Information is sent on to other parts of the brain for higher processing and identification

primary visual cortex

what are parts of Visual Processing/Interpretation?

• parietal lobe processing of vision 

• temporal lobe processing of vision 

• frontal lobe processing of vision 

what is parietal lobe processing of vision?

• Where is it?
• How fast and in what direction is it moving?

what is temporal lobe processing of vision?

• What/who does it look like?
• What are it’s features?

what is frontal lobe processing of vision?

Combines all visual processing information to make conscious identification

Some optic tract fibers (~5%) go straight to the

superior colliculus

Afferent nerve fibers headed for the superior colliculus have three essential functions:

1. alignment of the head 

2. control of pupillary diameter 

3. adjusting to near vision accommodation 

optic tract: alignment of the head

• Turning your head towards visual/auditory stimuli
• Activates the Tectospinal Tract

optic tract: control of pupillary diameter

Each superior colliculus receives signals from both eyes – unilateral stimulation produces a bilateral response (through CN III)

optic tract: Adjusting to near vision accommodation

The lens changes shape to focus on a near object (through CN III)

• Visual information from both eyes reaches the Pretectal Nucleus in the Superior
  Colliculus

• Pretectal nucleus send information to both Oculomotor nuclei to cause constriction of both pupils

• Only injuries to the optic nerve or chiasm can produce an Afferent Pupillary Defect

Pupillary Light Reflex

what is Horoner’s Syndrome?

• Injury causing damage to the lateral medulla or superior cervical ganglion
  • Ptosis – drooping eyelid
  • Miosis – overly constricted pupil
  • Anhidrosis – lack of sweating on half of face

what is ptosis?

drooping eyelid

what is miosis?

overly constricted pupil

what is anhidrosis?

lack of sweating on half of face