Neuro exam #3 chapter 11: visual system part 1: Components of Visual System

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PART 1: Components of Visual System

A.) Eyes and Retina

Visual System

  • A greater portion of the brain is dedicated to vision versus any other sensory modality

  • Pathway of the visual system extends form retina lateral geniculate nuclei of thalamus primary visual cortex (PVC) of the occipital lobe

  • Lesions to the visual association cortex cause disorders of higher-order visual processing

Eyes and Retina

  • Light enters eye through lens to form image on the retina (that is inverted (A) and reversed)

    • Superior visual field projects onto lower retina and inferior visual field projects onto upper retina

    • Right visual field projects to left side of the retina of each eye, and thus the left visual field projects to right side of the retina of each eye

  • Normal Visual Fields

    • Extend ~80°-90° temporally, ~50-60° nasally and superiorly, and ~60-75° inferiorly

  • Fovea – central fixation point for each eye (not dead center, a little off to the side)

    • Is the area of retina which highest visual acuity

    • Corresponds to central 1-2° of visual space

    • It is surrounded by the macula, which also has high visual acuity and covers the central 5° of visual space

    • What is represented on the fovea is projected to the occipital pole

  • Optic Disc- formed by axons leaving the retina where they enter the optic nerve

    • No photoreceptors over the optic disc, thus a small blind spot is formed here which is located ~15° later and slightly inferior to the central fixation point of each eye

    • No functional deficit when both eyes are used

    • When 1 eye is used, our visual system seems to “fill in” the blind spot

    • Blind spot formed by axons and no receptors

  • Retina contains 3 main layers

    • Photoreceptor- outermost layer of retina

      • Respond to light and create synapses onto bipolar cells

        • Rods

          • For vision in low-level lighting conditions

          • Low resolution

          • Do not detect color

          • Outnumber cones by 20:1

        • Cones

          • High resolution

          • Highly represented in fovea where acuity is highest

          • Detect color

    • Bipolar cell Layer- Middle Layer

      • Receive input via synapses form photoreceptors

    • Ganglion cell Layer- Inner layer

      • Receive input from bipolar cells and send axons into the optic nerve

      • Fire Action Potentials

      • Ganglion cell types

      • Parasol Cells

        • Respond to gross stimulus features and movement, large cell bodies, large receptive fields

        • Their large diameter fibers project to magnocellular layers of LGN of thalamus

      • Midget Cells

        • Respond to fine visual detail and colors, small cell bodies, small receptive fields

        • Their small diameter fibers project to parvocellular layers of LGN of thalamus

  • Many neurons in the visual pathways have center-surround receptive fields, which are either “on center” or “off center”

B.) Optic Nerves/Chiasm/Tracts

Optic Nerves, Optic Chiasm, and Optic Tracts

  • Optic Nerve

    • Receives input form retinal ganglion cell axons and exits in optic canal

  • Optic Chiasm

    • Partial crossing of fibers where the optic nerve meets

      • Nasal (medial) retinal fibers cross over in chiasm

      • Thus nasal (medial) retinal fibers are responsible for temporal (lateral) hemifields of vision

  • Optic Tracts

    • Posterior to chiasm carrying visual information form ipsolateral hemiretinas of each eye

      • L Hemiretinas of both eyes end up in L optic tract and R hemiretinas of both eyes end up in R optic

      • Thus ipsolateral hemiretinas represent contralateral visual fields

      • R hemiretinas represent what visual fields

  • Lesions of the eye, retina, or optic nerves produce monocular vision field deficits

  • Lesions of optic chiasms therefore often produce bitemporal visual field deficits

  • Because of the crossover in the chiasm, lesion posterior to chiasm (optic tracts, LGN, optic radiations, or visual cortex) will produce homonymous visual field defects

    • Visual field defect occurring in the same portion of the visual field for each eye

    • Contralateral homonymous hemianopia is most common in stroke patients

  • Optic tracts wrap around the midbrain laterally to reach the LGN of thalamus

  • Axons of retinal ganglion cells traveling in optic tracts synapse on neurons in LGN, which then project to primary visual cortex

C.) Lateral Geniculate Nucleus (LGN) & Extrageniculate Pathways

Lateral Geniculate Nucleus

  • LGN has layers numbered 1-6 (ventral to dorsal)

    • Parvocellular layers- layers 6-3

      • Relay information form midget cells of retina

    • Magnocellular layers – layers 2-1

      • Relay information form parasol cells of retina (motion and spatial analysis)

  • Information from left and right eyes remains segregated even after passing through the LGN, because axons form left and right retinas synapse onto different layers of the LGN

Extrageniculate Pathways

  • A minority of fibers in the optic tract bypass the LGN to enter the superior colliculus and pretectal areas, they form the extrageniculate visual pathways

    • Pretectal areas are involved in the pupillary light reflex

    • Pretectal areas and superior colliculus are involved in directing visual attention and eye movements toward visual stimuli (via projections to brainstem and lateral parietal cortex and frontal eye fields)

D.) Optic radiations

Optic Radiations

  • Axons leaving LGN sweep over and around the lateral ventricle to project to the primary visual cortex

  • These axons fan out over a large area as they project back to PVC

  • Axons form ipsolateral and contralateral retinal layers of LGN are intermingled in optic radiations, so lesions of optic radiations cause homonymous contralateral visual field loss

  • All fibers of optic (blue and green) radiations pass through parietal and temporal lobes

    • Ipsolateral optic radiations carry info form contralateral visual fields

    • Lesions of ipsolateral radiations would cause a contralateral homonymous hemianopia visual field defect

  • Fibers of inferior optic radiations arc forward into temporal lobe forming Meyer’s loop

    • Inferior optic radiations carry information form inferior retina (or superior visual fields)

    • Temporal lobe lesions cause contralateral homonymous superior quadrantanopia “pie in the sky” visual field defect

  • Fibers of superior optic radiations pass just under the parietal lobe

    • Superior optic radiations carry information form superior retina (or inferior visual field)

    • Lower parietal lobe lesions cause contralateral homonymous inferior quadrantanopia (“pie on the floor” visual field defect)

E.) Primary Visual Cortex

Primary Visual Cortex (B1)

  • Lies on the upper and lower banks (both sides) of the calcarine fissure in the occipital lobe (medial view)

  • Brodmanns Area (BA): 18- secondary visual cortex

  • BA 19- tertiary visual cortex

  • Calcarine Fissure

    • Cuneus (wedge)- upper bank gyrus

    • Lingula (little tongue)- lower bank gyrus

  • Superior optic radiations project to upper bank of the calcarine fissure

    • Upper bank lesions thus cause contralateral homonymous inferior quadrant defects, pie on the floor

    • Lesion to right upper bank of PVS would cause what visual field defect?

      • Left inferior quadrantanopia

  • Inferior Optic Radiations project to lower bank of the calcarine fissure

    • Lower bank lesions thus cause contralateral superior quadrant defects

    • Lesion to right lower bank of PVC would cause what visual field defect

      • Left superior quadrantanopia

  • Like many other parts of the visual pathway, it is retinotopically organized

  • Fovea and macula are represented at the occipital pole

    • The fovea occupies about 50% of PVS as the fovea contains the highest density of photoreceptors and has corresponding high visual acuity

    • Thus the five and macula have a disproportionate cortical representation despite the small retinal area

  • More peripheral regions of (ipsilateral) retinas (contralateral visual fields) are represented more anteriorly along the calcarine fissure

F.) Visual Processing

Parallel Channels for Analyzing Motion, Form, and Color

  • 3 parallel channels of visual information processing analyze motion form color

  • These 3 channels project to different layers of the primary visual cortex

  • From the PVC, neurons project to other regions of visual association cortex including areas 18, 19 and other regions of the parieto-occipital and occipitotemporal cortex

  • Dorsal Pathways project to parieto-occipital association cortex

    • Pathway answers the question, Where? By analyzing motion and spatial relationships between objects as well as between body and visual stimuli

  • Ventral Pathways project to occipitotemporal association cortex

    • Pathway answers the question, what? by analyzing form with specific regions identifying colors, faces, letters, and other visual stimuli


Assessment of Visual Disturbances- KCC 11.1

  • 2 major steps needed:

    • Time course and any positive phenomena (ex bright colored lights) or negative phenomena (regions of decreased vision) present

  • Visual field

    • Description of the regions for each eye involved

  • Visual acuity often reported with Snellen notation of 20/x

  • “X” is distance at which a normal individual can see the smallest line of the eye chart seen at 20 ft.

  • Visual field defects usually do not affect visual acuity

  • Distinction between a monocular or binocular visual disturbance is essential for localization

  • Pts often describe a disturbance as being “in one eye” when in reality the left or right visual field is affected for both eyes

  • Blurred vision is hard to interpret without further description and examination

  • Often divided in positive and negative phenomenon

    • Negative phenomena

      • Region of vision the person cannot see, including a scotoma or homonymous defect

    • Positive phenomenon

      • Simple phenomena

        • Include lights colors or geometric shape caused by disturbances located anywhere form eye to cortex

      • Formed phenomena

        • Include people animals or complex scenes arising from inferior temporal-occipital visual association cortex

  • Formed visual hallucinations arise form the inferior temporal-occipital visual association cortex

  • Can be from many causes: toxic or metabolic disturbances, withdrawal from alcohol or sedatives focal seizures, complex migraine, neurodegenerative conditions, psychiatric disorders

  • Formed hallucinations can also appear as relapses phenomenon

    • Pts with visual deprivation in all or part of their visual fields cause by ocular or CNS lesions may report seeing objects, people or animals in the region of vision loss, especially in the early stages of the deficit

  • Visual Field Testing- Tests for crude deficits in the visual fields

  • Confrontation testing at bedside

  • Test each quadrant while making sure patient’s eyes stay centrally fixated

  • Test each eye using wiggling fingers and having to count the number of fingers being held up

Localization of Visual Field Defects KCC 11.2

  • Testing Visual Fields

    • Tests for crude deficits in the visual fields

    • Confrontation testing at bedside

    • Test each quadrant while making sure patient’s eyes stay centrally fixated

    • Test each eye using wiggling fingers and having pt count the number of fingers held up

    • Fields recorded as if viewing own visual field

    • Blink to threat can be used on lethargic/uncooperative patients

  • Visual field defects

  • Most important information for localization of problems in the visual pathways

    • What is the position and shape of the scotoma?

    • Does it affect one or both eyes?

  • Know the effects of lesions in the visual pathways form the retina to the PVC (figure 11.5, pg 474)

  • Monocular Scotoma:

    • Can be from lesion of retina

      • Area of retina damage would reflect damage to the associated visual filed

      • Location, size, and shape will vary depending on location and the extent of the lesion

      • Causes: retinal infarcts, hemorrhage, degeneration, and infection

    • Could also be from incomplete damage to optic nerve (ex: trauma, or small accidental cut of nerve during surgery)

  • Monocular Vision Loss

    • Can be from complete lesions of optic nerve

      • All fibers of optic nerve of the respective eye are carrying information from entire retina of that eye

      • Causes: glaucoma, optic neuritis, neuropathy, elevated ICP, tumors, trauma

    • A lesion that is severe enough to involve the entire retina would also produce a monocular vision loss

      • Entire retina= all photoreceptors for that eye!

  • Bitemporal Hemianopia:

    • Usually from damage to optic chiasm

      • Chiasm is made up of nasal (medial) retinal fibers which carry info for temporal (lateral) visual fields

      • Causes: pituitary adenoma, hypothalamus glioma, other misc tumors

      • Optic chiasm lies just in front of pituitary gland thus making it susceptible to pituitary tumors or other lesions in this area

  • Contralateral homonymous hemianopia

    • Can be from lesions of optic tracts

      • Optic tract carries info for contralateral visual field

      • For example, right optic tract carries info for left visual field

      • Optic tracts: behind the chiasm where nasal (medial) retina fibers for each eye (responsible for temporal (lateral) hemifields) have crossed over to other side

      • Causes: tumors, infarct, demyelination

    • Can be from lesions of entire optic radiations

      • Entire optic radiations-superior (parietal) and inferior (temporal/Meyer’s loop) optic radiations carry info for contralateral visual field

      • Ex: right optic radiations carry info for left visual field

      • Causes: infarcts, tumors, demyelination, trauma, hemorrhage

    • Can be form lesions of entire PVC due to termination of optic radiations on lower and upper banks of calcarine fissure

      • Causes: PCA infarcts, tumor, infection

  • Contralateral superior quadrantanopia (“pie in the sky”)

    • Can be caused by lesions in the temporal lobe

      • Inferior optic radiations (temporal/Meyer’s loop) carry info for superior contralateral visual field

      • Ex: right inferior optic radiations carry info for left superior visual field

      • Cause: MCA inferior division infarcts

    • Can be from lesions to lower bank of calcarine fissure

      • Inferior optic radiations terminate on lower bank of calcarine fissure

      • Causes: PCA infarcts, hemorrhage, tumors, infections, trauma to occipital pole

  • Contralateral inferior quadrantanopia (“pie on the floor”)

    • Can be caused by lesions in the parietal lobe

      • Superior optic radiations (parietal) carry info for inferior contralateral visual field

      • Ex: right superior optic radiations carry info for left inferior visual field

      • Causes: MCA superior division infarcts

    • Can be from lesions to upper bank of calcarine fissure

      • Superior optic radiations terminate on upper bank of calcarine fissure

      • Causes: PCA Infarcts, hemorrhage, tumors, infections, trauma to occipital pole

  • Homonymous Hemianopia

    • Is the lesion anterior or posterior to the optic chiasm?

    • Which side of the brain is the lesion in?

  • Fovea has a relatively large representation (beginning in optic nerve and continuing to the PVC) given its actual size in the retina

  • Thus, it would take a large lesion of the primary visual cortex to affect the entire fovea and macula

  • Thus, partial lesions of the visual pathways sometimes result in the central visual field being spared (because partial lesions would not usually affect entire fovea and macula representation in PVC)

  • Macular Sparing- the visual field represented by macula is intact (“macula is spared”)

  • However, a partial lesion to the PVC specifically to the occipital pole, would primarily involve the area represented by fovea and macula and could result in visual loss in the center of the visual field, which would be called a central acotoma

    • Ex: a head injury involving hitting the back of the head at the bottom of the occiput could involve the area representing the central visual filed in the PVC on both sides of the posterior occipital lobe

Blood Supply and Ischemia in the Visual Pathways- KCC 11.3

  • Retina receives blood supply mainly form branches of opthalamic artery

  • Impaired blood flow can be caused by emboli, stenosis, and vasculitits

  • Central retinal artery (branch of the opthalamic artery) supplies inner retinal layers

  • Retinal artery has 2 main branches-

    • Superior= one covering superior half

    • Inferior= covering the inferior half

  • Occlusion of one of these arteries can cause an altitudinal scotoma in one eye

  • An altitudinal scotoma in one eye can result form occlusion to one of these branches

    • Occlusion of the superior or inferior branch of the right central retinal artery (branch of opthalamic) would cause the defect below?

    • Answer: superior branch because it supplies superior retina which represents inferior visual field

  • Transient occlusion of the superior or inferior branch of the retinal artery caused by emboli results in a TIA of the retina called amaurosis fagax with “browning out” or loss of vision in 1 eye for ~10 min

  • Is sometimes described as “like a window shade” moving down or up over the eye

  • This symptom should be treated like any TIA and should e considered a warning sign or impending retinal or cerebral infarct!

  • Common cause in ipsilateral ICA stenosis causing artery-to-artery emboli

  • Optic tracts, optic chiasm and optic nerves receive blood supply form numerous small branches of anterior cerebral artery (ACA) and middle cerebral artery (MCA)

  • Clinically significant infarcts of these are therefore rarely seen

  • LGN has a variable blood supply form several vessels including anterior choroidal (branch of ICA) , thalamogeniculatem and posterior choroidal arteries (branches of PCA)

    • Could be associated contralateral hemiparesis (due to involvement of nearby posterior limb of internal capsule) and/or associated hemisensory loss (due to involvement of nearby thalamic somatosensory radiations)

  • Infarcts of the LGN would cause what type of deficit?

    • Contralateral homonymous hemianopia

  • Optic radiations passing through parietal lobe receive blood supply form superior divisions of MCA

    • Damage/infarct to parietal (superior) optic radiations cause what type of visual field deficit?

      • Contralateral inferior quadrantanopia “pie on floor”

    • Damage to left parietal optic radiations causes what?

      • Right inferior quadratanopia

  • Optic radiations passing through temporal lobe 9Meyer’s loop) receive blood supply form inferior divisions of MCA

    • Damage/infarct to the temporal (inferior/Meyer’s Loop) optic radiation cause what type of visual field deficit?

      • Contralateral superior quadratanopia “pie in sky”

    • Damage to left temporal optic radiations causes what?

      • Right superior quadratanopia

  • Primary visual cortex is supplied by PCA

  • Large infarcts involving the entire ipsilateral PVC will cause what visual field defect?

    • Contralateral homonymous hemianopia

  • R PCA infarct of entire PVC would cause what?

    • Left homonymous hemianopia

CHAPTER 12 Brainstem I: Surface Anatomy & Cranial Nerves

Anatomical & Clinical Review

  • Brainstem- compact area that carries nearly all information between brain & remainder of body

    • Contains numerous important nuclei

  • Surface features of brainstem, course & function of cranial nerves- Chapter 12

  • Cranial nerves & central pathways mediating eye movement- Chapter 13

  • Vascular supply & internal structures including ascending & descending tracts, reticular formation- Chapter 14

Surface Features of the Brainstem

  • Midbrain, pons, & medulla

  • Rostral limit= midbrain-diencephalic junction

  • Midbrain-pons= pontomesencephalic junction

  • Pons-medulla= pontomedullary junction

  • Caudal limit= cervicomedullary junction

  • Dorsal View

    • Tectum Roof

      • Superior colliculi

      • Inferior colliculi

  • Ventral View

    • Cerebral peduncles

      • Interpeduncular fossa

    • Pyramidal Decussation

      • Rostral

      • Caudal

  • Lateral View

    • Pons

      • 4th ventricle

      • Superior, middle, & inferior cerebellar peduncles

Skull Foramina & Cranial Nerve Exit Points

  • Olfactory nerve (CN1)- cribriform plate

  • Optic Nerve (CN2)- optic canal

  • Oculomotor (CN3), Trochlear (CN4), Abducens (CN6), & Trigeminal V1 (CN5) – superior orbital fissure into orbit

  • Trigeminal V2- foramen rotundum

  • Trigeminal V3- foramen ovale

  • Facial nerve (CNVII) internal auditory canal to enter auditory canal then exits skull via stylomastoid foramen

  • Vestibulocochlear nerve (CNVIII)- internal auditory canal to enter auditory canal

  • Glossopharyngeal (CNIX), Vagus (CNX), Spinal Accessory (CNXI) – jugular foramen

  • Hypoglossal (CNXII)- hypoglossal foramen

Sensory & Motor Organization of Cranial Nerves

  • Analogous to spinal nerves

    • Sensory-dorsal

    • Motor-ventral

  • More specialized due to anatomy of head & neck

  • Embryological development-adjacent to ventricular system

  • Mature nervous system

    • 3 motor columns

    • 3 sensory columns

Functions & Course of Cranial Nerves
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