This document provides information on the anatomy and physiology of the visual system. It describes the structures involved in vision, including the eye, optic nerve, optic chiasm, and visual cortex. Key points covered include the layers of the retina, blood supply to the optic nerve head, pathways of the visual system from the retina to the brain, and tests used to evaluate visual acuity and visual fields.

1. Dr Parag Moon
Senior Resident,
Dept. Of Neurology,
GMC Kota.

2.  Anatomy
 First order neurons-bipolar sensory cells in
olfactory epithelium (superior nasal concha,
upper nasal septum, roof of the nose)
 Tiny knobs on cilia-sites of chemosensory
signal transduction.
 Specific odorants stimulate specific receptor
cells.
 Central processes of olfactory neurons-
unmyelinated axons

3.  Form approximately 20 branches on each
side-olfactory nerves.
 Penetrate cribriform plate of ethmoid bone,
acquire a sheath of meninges, synapse in
olfactory bulbs.
 Basal cells in olfactory epithelium can
regenerate.
 Within olfactory bulbs, synapse on dendrites
of mitral and tufted cells in olfactory
glomeruli.

4.  Second order neurons-axons of mitral cells
 Course posteriorly through olfactory tracts in
olfactory grooves beneath frontal lobes
 Divide into medial and lateral olfactory striae.
 Medial olfactory stria terminate on medial
surface of cerebral hemisphere in
paraolfactory area, subcallosal gyrus and
inferior part of cingulate gyrus.
 Lateral olfactory stria terminate in uncus,
anterior hippocampal gyrus, piriform cortex,
entorhinal cortex, and amygdaloid nucleus

6.  History
◦ Unilateral or bilateral
◦ Altered taste
◦ Past head injury;
◦ Smoking
◦ Recent upper-respiratory infection
◦ Systemic illness
◦ Nutrition
◦ Exposure to toxins, medications or illicit drugs.

7.  Before evaluating smell- nasal passages are
open.
 Smell is tested using nonirritating stimuli.
 Avoid irritating substances-stimulate trigeminal
nerve
 Examine each nostril separately while occluding
other with eyes closed
 Perception more important than accurate
identification
 University of Pennsylvania smell identification
test (UPSIT) and Connecticut chemosensory test..

8. Term Definition
Anosmia No sense of smell
Hyposmia decrease in the sense of smell
Hyperosmia overly acute sense of smell
Dysosmia Impairment or defect in the sense of smell
Parosmia Perversion or distortion of smell
Phantosmia Perception of an odor that is not real
Presbyosmia Decrease in the sense of smell due to aging
Cacosmia Inappropriately disagreeable odors
Coprosmia fecal scent
Olfactory agnosia Inability to identify or interpret detected odors

9.  Smoking
 Pregnancy
 Chronic rhinitis
 Dental trauma
 Deviated nasal septum
 Intranasal tumors (e.g., epidermoid
carcinoma)
 Neuro-olfactory tumor
(esthesioneuroblastoma)
 Nasal polyps
 Antihistamines
 Propylthiouracil
 Olfactory dysgenesis
 Cadmium toxicity
 Chemical burns of the olfactory
epithelium
 Vitamin deficiency (B6, B12, A)
 Postviral
 Zinc or copper deficiency
 General anesthesia
 Kallmann's syndrome
 Olfactory groove
meningioma
 Craniocerebral trauma,
including surgery
 Frontal lobe tumor,
especially glioma
 Alzheimer's disease
 Parkinson's disease
 Normal aging
 Multiple sclerosis
 Sellar/parasellar tumor
 Congenital anosmia
 Meningitis
 Korsakoff's syndrome
 Familial dysautonornia
 Temporal lobectomies
 Refsums disease

10.  5 cm in length
 Extends from ganglion cell layer of retina to
optic chiasm.
 Divided into
◦ Intraocular:1mm
◦ Intraorbital:25mm
◦ Intracanalicular:9mm
◦ Intracranial:12-16mm
 Organized into 400 to 600 fascicles
separated by connective tissue septae.
 Intraorbital portion-surrounded by fat

11.  Macula-point of central fixation and greatest
visual acuity and color perception.
 Small shallow depression temporal to disc
 Fovea (L. “pit”) centralis-depression that lies
in center of macula.
 Foveola-tinier depression in center of fovea.
 Point of most acute vision as overlying retinal
layers are pushed aside
 Optical center of eye
 Macula-central 15° of vision

12.  Optic disc, or papilla-ophthalmoscopically
visible tip of intraocular portion of optic
nerve.
 1.5 mm by 1.8 mm vertical ellipse
 Pink to yellowish-white disc.
 No receptor cells
 Does not respond to visual stimuli-
physiologic blind spot
 Macula-forms center of retina
 Macular fixation point-center of clinical visual
field (VF).

13.  Blood supply to optic nerve head-circle of
Zinn-Haller,
 Composed of 2 often not connected
semicircles of short posterior ciliary arteries.
 NAION- drop in perfusion pressure in short
posterior ciliary arteries is culprit.
 Segmental disc edema corresponding to the
semicircle compromised

15.  Retinal ganglion cell axons form retinal nerve
fiber layer (NFL)
 Exit through lamina cribrosa (L. “sieve”)
 Myelinated at posterior end of optic nerve
head
 Myelin-CNS myelin
 1.2 million fibers in each optic nerve;

16.  Intracranial dura at posterior globe fuses with
Tenon's capsule
 Adherent in optic foramen to periosteum.
 Pia and arachnoid fuses with sclera where nerve
terminates
 Intracranial meninges extend forward along optic
nerves for a variable distance, forming vaginal
sheaths.
 Variations in vaginal sheath anatomy
 Intervaginal space-small subdural and a larger
sub-arachnoid space

17.  Intraorbital portion-lies within muscle cone.
 Before entering optic canal, surrounded by
annulus of Zinn formed by origins of rectus
muscles.
 Superior and medial recti partially originate from
sheath of optic nerve.
 Length of intraorbital portion greater than length
of orbit
 Blood supply-pial vascular plexus and branches
of ophthalmic artery; distally central retinal artery
also contributes intraneural branches.

18.  Intracanalicular portion-into cranium through
optic canal
 Orbital opening of canal-vertical ellipse;
intracranial end-horizontal ellipse
 1.2 cm in length, and located in lesser wing
of sphenoid bone.
 Dural sheath fused to periosteum-
immobilizing nerve.
 Medial wall of canal-thinnest part
 Blood supply-ophthalmic artery.

19.  Intracranial portion-travel 12-16 mm to optic
chiasm.
 Ophthalmic arteries above, internal carotid
arteries superiorly and medially, and anterior
cerebral arteries that cross over optic nerves
and are connected by anterior communicating
artery.
 Posterior to cavernous sinus-join together to
form optic chiasm;
 Blood supply-branches of internal carotid and
ophthalmic arteries.

21.  Optic chiasm-lies about 10 mm above
pituitary gland, separated by suprasellar
cistern.
 Fibers from temporal retina continue directly
back: nasal retina decussate to enter opposite
optic tract.
 Fibers from inferior nasal quadrant loop
forward into opposite optic nerve for a short
distance before turning back again, forming
Wilbrand's knee.

22.  Some of upper nasal fibers loop back briefly
into ipsilateral optic tract before decussation.
 In chiasm, fibers from upper retinal
quadrants lie superior: lower quadrants
inferior
 Inferior nasal fibers decussate anteriorly and
inferiorly: superior nasal fibers cross
posteriorly and superiorly
 Macular fibers-decussate as a group,
 forming a miniature chiasm within chiasm,
primarily posterior superior portion.

24.  Cavernous sinuses and carotid siphons lie just
lateral to chiasm
 Anterior cerebral and anterior communicating
arteries front and above
 Third ventricle and hypothalamus behind and
above.
 Sella tursica and sphenoid sinus lie below.
 Ophthalmic artery-same dural sheath through
canal and orbit.
 8 mm to 12 mm posterior to globe, artery enters
nerve and runs along its center to optic disc-
central retinal artery
 Divides at disc head into superior and inferior
branches.

25.  55% of axons of optic tract- contralateral
nasal retina; 45% ipsilateral temporal retina
 80% visual afferents and 20% pupillary
afferents.
 Fibers from upper retina-medial position,
inferior retina lateral.
 PMB-dorsal and lateral position,
 Majority of fibers terminate at LGB.
 Six neuronal layers in the LGB
 Ipsilateral temporal hemiretina synapse in
layers 2, 3, and 5;contralateral nasal
hemiretina synapse in layers 1, 4, and 6.

26.  Geniculocalcarine tract, or optic radiations
 Terminate in calcarine cortex of occipital lobe
 Pass through retrolenticular portion of
internal capsule and then fan out.
 Upper retinal fibers upper, and lower retinal
fibers lower in optic radiation

27.  Inferior retinal fibers arch anteriorly into
temporal lobe, sweeping forward and laterally
above inferior horn of ventricle then laterally,
down, backward.
 Meyer's loop (loop of Meyer and
Archambault).
 Fibers from superior retina run directly back
in deep parietal lobe in external sagittal
stratum, lateral to posterior horn of lateral
ventricle

28.  Primary visual cortex (calcarine area or striate
cortex)-Brodmann's area 17 on medial
surface of occipital lobe.
 Lower retinal fibers- lower lip of calcarine
fissure (lingual gyrus)
 Upper retinal fibers-upper lip of the calcarine
fissure (cuneus).
 Most peripheral parts of retina-most
anteriorly in calcarine cortex
 Macular -More posterior its calcarine
representation.

30.  Visual acuity
◦ Minimum visibility-smallest area that can be
perceived
◦ Minimum separability-ability to recognize the
separateness of two close points or lines
◦ Snellen chart for distance and near card for near
◦ In infants and children- blink to threat or bright
light, following movements, pupillary reactions
◦ Acuity- line where more than half of characters are
accurately read

31.  Distance from test chart, 20 or 6-numerator, and
distance at which smallest type read by patient should
be seen by a person with normal acuity-denominator.
 RAPD or Marcus gunn pupil
◦ Swinging light test
◦ Light shone into a pupil and then quickly switched
to other one.
◦ If one pupil dilates, even slightly, when light is
switched-RAPD present in that eye.

33.  Near vision
 Jaeger chart
 Newspaper want-ad text is approximately J-
0, regular newsprint J-6, and newspaper
headlines J-17.
 Counts fingers (CF), hand motion (HM), light
perception (LP), or no light perception (NLP).
 Count fingers at 5 ft-20/800.

34.  Non organic visual loss
1. Ask to sign
2. Schmidt-Rimpler test- look toward his hand
3. Join the forefingers
4. Menace test
5. Ask patient look into a large mirror that can
be held and moved. Tilting and moving the
mirror will elicit OKN responses

35.  Color Vision; Day and Night Vision
 Color plates or pseudoisochromatic plates
(Ishihara, Hardy-Ritter-Rand )
 In neurologic disease, red perception usually
lost first
 Compare brightness or intensity of examining
light in one eye versus other

36.  Visual field
 Normal VF- 90 degrees to 100 degrees
temporally, about 60 degrees nasally, 50
degrees to 60 degrees superiorly, and 60
degrees to 75 degrees inferiorly
 Examination most accurate in an individual
who is alert and cooperative and maintain
fixation.
 Confrontation test
 Moving pen light
 Menace reflex

37.  Formal visual field testing
 Central fields-tangent screen
 Peripheral fields-perimetry
 Notation numerator-test object size and
denominator-distance from screen

38.  Kinetic perimetry entails moving a test object
along various meridians and noting when it is
detected.
 E.g., Goldmann
 White and colored test objects varying in size
from 1 mm to 5 mm
 Smaller test object, smaller VF.
 If size of a VF defect is same with all test
objects-steep, or abrupt, margins.
 If larger with smaller test objects-gradual, or
sloping margins

39.  Automated static perimetry
 Humphrey Visual Field Analyzer
 Normal patients may appear to have
abnormal VF due to large number of
erroneous responses that can occur during
automated testing

42.  Direct opthalmoscopy
 Small aperture-examining an undilated pupil,
large aperture-dilated pupil
 Red-free filter-examining blood vessels,
looking for hemorrhages, and nerve fiber
layer
 Red reflex-assessed from distance of 12 in to
15 in.
 Areas of primary concern- disc, macula, and
arteries

43.  Disc normally round or vertically oriented
slight oval.
 Nasal margin slightly blurred
 Peripheral neuroretinal rim and central cup.
 Physiologic cup-slight depression in center of
disc that is less pinkish than rim and shows a
faint latticework due to underlying lamina
cribrosa.
 Rim is elevated slightly above cup.

44.  Myelinated axons-normal optic disc yellowish
white.
 Paler temporally where papillomacular bundle
(PMB) enters.
 When scleral opening small, disc consists
entirely of neuroretinal tissue, and
inconspicuous or nonexistent cup.
 More vulnerable to anterior ischemic optic
neuropathy-disc at risk.
 Normal cup-to-disc ratio-0.1 to 0.5.

45.  Macula-dark area that lies about 2 disc
diameters temporal to and slightly below
disc.
 Appears darker than surrounding retina
because of thinner retina
 Area of macula devoid of large blood vessels.
 Fovea centralis appears pinpoint of light
reflected from center of macula.
 Macula may be seen more easily with a red-
free filter, if patient looks directly into light

46.  Photostress Test
 In macular disease, photoreceptors require
longer to recover from bleaching of retinal
pigments after exposure to a bright light.
 Baseline visual acuity
 Then shining a bright light (e.g., a fresh penlight)
into eye for 10 seconds
 Determine time required for visual acuity to
return to baseline.
 Mainly useful with unilateral disease
 Optic nerve disease-normal photostress test.

47.  Papilledema Four stages of -early, fully
developed, chronic, and atrophic.
 Fully developed-elevation of disc surface,
humping of vessels crossing disc margin,
obliteration of disc margins, peripapillary
hemorrhages, cotton wool exudates,
engorged and tortuous retinal veins, and
marked disc hyperemia
 Early papilledema-loss of previously observed
spontaneous venous pulsations (SVPs).
 SVPs=200 mm H2O ICT

48.  Pseudopapillodema
 Common causes-optic nerve drusen,
myelinated nerve fibers, remnants of
primitive hyaloid artery (Bergmeister's
papilla), tilted discs, extreme hyperopia.
 Optic nerve drusen, or hyaloid bodies
◦ Acellular, calcified hyaline deposits within optic
nerve that may elevate and distort disc.
◦ Present 2%, bilateral in 70%
◦ Highly refractile, rock-candy appearance.

49.  In papilledema-
1. disc is usually hyperemic;
2. margin blurriness at superior and inferior poles early
in process;
3. blood vessels look normal except for fullness of veins;
4. NFL is dull with retinal blood vessels obscured because
of retinal edema.
 In pseudopapilledema
1. disc color remains normal;
2. blurriness of disc margin may be irregular,disc may
have a lumpy appearance;
3. blood vessels on the disc frequently look anomalous;
4. NFL is clear.
 If in doubt, consult an ophthalmologist.

50.  Optic neuritis
 Ischemic optic neuropathy
 Optic nerve compression
 Papillophlebitis
 Optic nerve infiltration (carcinomatous, lymphomatous)
 Sarcoidosis
 Diabetic papillopathy
 Tobacco-alcohol amblyopia
 Nutritional deficiency, especially vitamin B12
 Drugs
 Toxins
 Hereditary optic neuropathy (Leber, Kjer)
 Glaucoma

53.  Chiasmal Lesions
◦ Pituitary tumors
◦ Craniopharyngiomas
◦ Meningiomas
◦ Gliomas
◦ Carotid aneurysms
◦ Demyelination
◦ Ischemia
◦ Radionecrosis

56. Thanks

57.  Inderbir Singh's Textbook of Human
Neuroanatomy (Fundamental and Clinical)
 DeJong’s The Neurological Examination, Sixth
Edition
 Optic Nerve: Anatomy, Function, And
Common Disorders;edward A. Margolin,
Rajeshvar K . Sharda;Ophthlmology Rounds;
May/June 2014 Volume 6, Issue 3
 The optic nerve: a clinical
perspective;Pasquale Montaleone;UWOMJ |
79:2 | Fall 2012