LESSON 8: OLFACTORY (I), OPTIC (II) AND OCULOMOTOR (III)

normal vision pathway

1) light refracts as it goes through the lens, producing a right-left reversal image by the time it reaches the retina 2) info from left nasal visual field stimulates left temporal retinal field. it travels in the left optic nerve to the left optic tract to synapse in the left lateral geniculate body 3) info left temporal visual field stimulates left nasal retinal visual field. it travels in the left optic nerve, but crosses at the optic chiasm to the right optic tract to synapse in the right lateral geniculate body 4) optic radiations (geniculocalcarine fibers) enter the retrolenticular portion of the posterior internal capsule and divides into dorsal (superior) and ventral (inferior) bundles 5) superior bundle travels to cells in visual cortex, above the calcarine fissure and carries upper retinal quadrants 6) inferior bundle forms Meyer loop, moves rostrally and laterally around inferior horn of lateral ventricle before travelling to cells in visual cortex, below calcarine fissure and caries lower retinal quadrant info - part of optic radiation that is "pulled forward - rostral"

pupillary light reflex in the context of clinical testing

1. Light is shone in one eye for this test. [left] 2. Light signal follows the usual light pathway, BUT for the reflex (as shown), the signal does NOT go to the visual cortex. It goes directly (without synapsing) to the pretectal nucleus in the midbrain (where light reflexes converge, including those for the body responses to light) note: this reflex does not require that the person be conscious, therefore can be tested in an unconscious patient or one who is not able to answer reliably, such as a small child 3. The information (light stimulus) synapses in the pretectal nucleus bilaterally and then in the Edinger-Westphal nucleus (the parasympathetic nucleus portion of the oculomotor nucleus) bilaterally. It then travels in parasympathetic fibres along the oculomotor nerve 4. An important fact is that response (pupil constriction) occurs in BOTH eyes, even though the right eye (in this example) did not receive any light. Efferent information leaves the brainstem through both oculomotor nerves, unless one of them has a lesion reasons for crossed response: first the pathway from the nasal retina crosses and secondly, there is extensive crossing within the nuclei in the brainstem remember: you are actually testing one optic and two oculomotor nerves because the signal goes "in on CN II, out on CN III" Note it is the parasympathetic part of the oculomotor nerve. The deeper voluntary motor tract is less vulnerable and requires a deep lesion to show muscle weakness. Therefore, the shifting of the brain tends to only show pupil changes, not eye movement or eyelid changes with this type of injury

occipital lobe lesions

Alexia without agraphia (patient cannot comprehend written info but can still write) Involves infarct of corpus callosum and infarct of occipital cortex

primary visual area

Area 17 (above and below the calcarine sulcus on the medial surface of the occipital lobe) appreciates light and colour at the conscious level providing an image Cortical blindness occurs with lesions in Area 17

secondary visual cortex

Area 18 processes the information Lesions in Areas 18 or 19 don't involve vision itself (ability to see the object); they affect the quality and meaning of vision (for example association of the object with its function). Your patient may be able to see on a visual test (intact area 17) but not be able to interpret what (s)he sees (possible lesion in Areas 18 or 19).

association visual cortex

Area 19 puts it into context Lesions in Areas 18 or 19 don't involve vision itself (ability to see the object); they affect the quality and meaning of vision (for example association of the object with its function). Your patient may be able to see on a visual test (intact area 17) but not be able to interpret what (s)he sees (possible lesion in Areas 18 or 19).

Bilateral lesion at junction of parietotemporal region

Associated with balint syndrome marked by loss of voluntary control of eye while reflexive movements are preserved

Frontal eye field lesion

Causes deviation of eyes to ipsilateral side ("right way eyes")

visual reflexes

Changing of pupil size (light reflex) and lens curvature (lens accommodation) Ocular muscle fibers regulating two reflexes are controlled by the parasympathetic fibers of the CN III

monocular blindness

Complete severing of optic nerve (CN II) at any point between the eyeball and optic chiasm results in complete blindness in that eye no optic nerve fibers from the retina are spared remember that there is binocular vision alongside monocular vision! if one eye is blind or closed, other eye is still capable of covering the entire visual field (except for a small, temporal crescent-shaped peripheral field for the blind eye) therefore even after severance of the optic nerve, one is functionally blind only for this monocular portion of the field

cortical function - cortical blindness

Damage to the optic tract causes 'blindness'. Damage to the tracts AFTER synapsing in the lateral geniculate body cause 'cortical blindness' - only the cortical cells are damaged, but the remainder of the tracts intact If the lateral fibres going to the reflexes are intact while the occipital cortex is damaged, there may be some subconscious awareness of the image while no conscious awareness can occur asked about an image, they can't answer, but they might 'automatically' reach for the object even though their conscious level doesn't know it is there related to bilateral cortical lesions only.

Lesion of Trochlear nerve

Difficulty looking down toward feet. This is complicated- just remember: "difficulty getting two eyes to both look downward together properly so diplopia looking downward". Complains of difficulty going down stairs.

lesion of oculomotor nerve

Dilated pupil (unopposed action of the sympathetic dilator pupillae) eye directed outward (loss of inward turning medial rectus, unopposed strong lateral rectus directs eye outward). Diplopia (double vision)

Bitemporal Heteronymous Hemianopia

Example of heteronymous visual field deficit Loss of vision in the temporal visual fields for both eyes (left for left eye, right for right eye) Associated with optic chiasm injury (where fibers cross and form optic tract -- therefore affecting both eyes) usually induced by tumour of pituitary gland interrupts fibers from both nasal retinas and causes a visual field deficit similar to tunnel vision

sources of coordinated eye movement

Eye movement occurs through the integrated use of the 6 muscles of the eye innervated by the oculomotor (CNS III) - medial rectus muscle - inward - superior rectus muscle - upward - inferior rectus muscle - downward - inferior oblique muscle - up and out trochlear (CNS IV) - superior oblique muscle - down and out abducens nerves (CNS VI) - lateral rectus muscle - outward These muscles are coordinated through two centers: the medial longitudinal fasciculus (joining the brainstem nuclei for these nerves) the Pontine Paramedian Reticular Formation (PPRF) a coordination center within the pons and medulla

lesion in left optic nerve

For the first one that we got the same the left optic nerve has a lesion, so the nerve impulse caused by the light never makes it to the neurons in the midbrain in order to initiate a reflex response from the oculomotor nerves in either eye. no response at all

lesion in right oculomotor nerve

For the fourth the right oculomotor nerve has a lesion. As a result, the nerve impulse follows the same path as in the previous case, but this time the lesion location means the impulse never reaches the right pupil. Therefore, there is only a direct response in the left pupil. direct response but no indirect response

lesion in right optic nerve

For the second one the right optic nerve has a lesion, however this nerve is not involved in the circuit for the left eye. The light is being shone into the left eye so the nerve impulse caused by the light travels to the midbrain via the left optic nerve and synapses on the pretectal areas and then onto the edinger-westphal nuclei. The impulse then travels through both oculomotor nerves to synapse on the pupils causing the reflex in both eyes. response in both

lesion in left oculomotor nerve

For the third the left oculomotor nerve has a lesion. As a result, the light causes a nerve impulse to travel to the midbrain and synapse on the nuclei there, the impulse travels down both oculomotor nerves, but the lesion in the left oculomotor nerve means the impulse never reaches the left pupil. Therefore, there is only an indirect/consensual response of the right pupil but no response in the left pupil. no direct response, but presence of indirect response

cortical function - visual agnosia

From the primary visual cortex (area 17) information is passed to the secondary visual cortex (area 18) and then visual association area (Area 19). Lesions in areas 18 and 19 interfere with the meaning of an object producing visual agnosia, and prosopagnosia. Without input to Area 17, there is no information to process in Areas 18 and 19.

what coordinates gaze?

Gaze is coordinated by the medial longitudinal fasciculus, and PPRF.

two types of visual field defects

Homonymous: similar visual field defects for each eye, either right half of the visual fields or the left half for the visual fields for both eyes Hetereonymous: two different visual fields being impaired, e.g. left half of visual field for one eye and right half of visual field for the other eye (bitemporal hemianopia)

clinical response of pupillary light reflex

If the left eye is being tested, constriction of the left pupil is described as "presence of a direct response" and right pupil constriction is described as "presence of an indirect (or consensual) response". If one side doesn't respond that is "absence of the direct or indirect response" The pupillary light reflex is always confirmed by testing the other eye as well

pupillary light reflex pathology

If the optic nerve is damaged, the afferent signal (light) is blocked, there will be NO response in either eye With intracranial bleeding or swelling, which results in stretching of the oculomotor nerve due to shifting of the brain, THAT specific oculomotor nerve will not provide the efferent response If they are both stretched, which happens in a very severe head injury, both pupils will be 'fixed and dilated'

mydriasis

In complete darkness, constriction of radial fibers (dilator) of the iris results in pupil dilation involves both inhibition of E-W nucleus and facilitation of sympathetic activity sympathetic projections exit from T1-T3 and travel in the cervical sympathetic chain to the superior cervical ganglion, which sends postganglionic projections to the radial fibers of the iris muscle in the eyeball

tunnel vision (bitemporal hemianopia)

Information pathways from the left and right nasal retinas cross in the optic chiasm A light signal from the left side of the room goes to the right side of the brain Equally, light from the left eye goes to both sides of the brain tumour in the pituitary gland or within the cavernous sinus, this may cause compression on the optic chiasm, where this information crossing occurs This will then cause bitemporal hemianopia or tunnel vision (depending on how much of the optic chiasm is impacted

pupillary light reflex lesions

Interruption afferent projections from one eye affect light reflex in both pupils Presence of consensual w/o direct response suggests lesion involving efferent projection from the E-W nucleus to the same eye Interruption of sympathetic fibers causes paralysis of dilator fibers of iris and results in permanently constricted pupillary diameter (miosis) - Resulting condition is part of Horner syndrome, characterized by ipsilaterally constricted pupil, drooping eyelid (ptosis) and loss of facial sweating (anhidrosis)

medial longitudinal fasciculus

Is a bundle of fibres connecting the motor nuclei for CNs III, IV and VI bilaterally, with connections to gaze centres and the vestibular system

rule #3 retinal representation to the lateral geniculate body (LGB)

Left optic tract (receives projections from both eyes) contains projections from the temporal half of the left retina (nasal visual field for left eye) and nasal half of right retina (temporal visual field for right eye) Right optic tract (receives projections from both eyes) contains projections from the nasal half the left retina (temporal visual field for the right eye) and the temporal half of the right retina (nasal visual field for the left eye) optic tract projects to the lateral geniculate body of thalamus (LGB) each LGB receives point projection from homonymous (L or R) halves of visual fields of both eyes fibers from upper retinal quadrants (lower visual) terminate in MEDIAL portion of LGB fibers from lower retinal quadrants (upper visual) terminate in LATERAL portion of LGB

Primary and Association Visual Cortices Lesions

Lesion involving visual cortex in one hemisphere results in blindness in opposite field of vision (hemianopia) Large unilateral lesion in occipital lobe = posterior cerebral artery thrombosis) Central vision may be spared (collateral circulation potential from middle cerebral artery branches)

Edinger-Westphal nucleus

Midbrain nucleus containing the autonomic (parasympathetic) neurons that constitute the efferent limb of the pupillary light reflex involuntary (parasympathetic) pupil constriction

temporal visual field

Nasal retinal fibers expressed as visual field fibers

Occipital associative cortical lesion

Optic aphasia (neuro linguistic syndrome) Impaired ability to name visually presented objects although semantic knowledge ass. With objects retained Cannot name object presented visually, but can name actions ass. with object

neural mechanism of accommodation reflex

Primary visual cortex, LGB and mesencephalic reflex center 1) Image of object moving closer begins to blur → visual cortex sends projections to the superior colliculus, which mediates visual info to the pretectal area 2) pretectal nuclei send crossed and uncrossed fibers to the E-W nucleus 3) E-W nucleus projects preganglionic parasympathetic fibers in oculomotor nerve to the ciliary ganglion 4) postganglionic projections from ciliary ganglion cause constriction of the ciliary muscle 5) lens released from tension of suspensory ligaments becomes round and acquires greater refractive power

Homonymous Left Inferior Quadrantanopia

Quadrantanopia is loss of vision in the inferior left quadrant of the visual fields for both eyes Superior fascicle of the geniculocalcarine tract carrying info from the superior/upper retinal quadrants (lower visual field) travels directly through the temporoparietal substance Right sided temporoparietal lobe lesion interrupts genicolocarine tract inner (superior) fibers, affecting transmission of visual info from the right upper retinal quadrants, resulting in vision loss in the left lower visual field quadrants for both eyes

Homonymous Left Superior Quandrantonopia

Quadrantanopia is loss of vision in the superior left quadrant of the visual fields for both eyes Geniculocalcarine fibers divide into the inferior and superior fascicles while traveling to the visual cortex Inferior fascicle of fibers carries info from the inferior retinal quadrants of the retina (upper visual field) Fibers sweep around inferior horn of lateral ventricle in the temporal lobe in the Myer loop Temporal lobe lesion on the right side of the brain, selectively interruption of the inferior fibers of the geniculocalcarine tract, causes blindness in the left upper quadrants of the visual fields of both eyes

Nasal Hemianopia

Refers to loss of vision in the nasal field of only one eye results from lesion of lateral edge of the optic chiasm selectively interrupts uncrossed fibers from the ipsilateral temporal portion of the retina nasal hemianopia in the corresponding eye the right field of the left eye or the left field of the right eye

Temporal retinal fibers

Retinal fibers that do not cross at the optic chiasm

optic nerve pathway [reflex activity]

Some fibres leave the optic tract (without synapsing in the lateral geniculate body) and travel to the superior colliculus of the midbrain for reflex activity related to vision Optic nerve to optic tract ipsilateral to the superior colliculi

abducen nerves and coordinated eye movements

Supplies the lateral rectus muscle that directs the eye outward.

macular sparing

The centre of the retina is represented at the 'pole' of the occipital lobe (where the medial and posterior surfaces of the occipital lobe meet posterior cerebral artery supplies this area, but there is some BACKUP circulation from a special branch of the middle cerebral artery for the macula therefore maybe macular sparing by the middle cerebral artery in spite of visual loss due to a lesion of the posterior cerebral artery If there is a lesion and sparing occurs in both hemispheres, this may also result in tunnel vision (since the centre of the retina, which relays information at the centre of the visual field is spared

lesions of coordinated eye movements

The clinical presentation is a result of the unopposed muscles that remain intact. For example if the muscle that turns the eye inward (medial rectus) is paralyzed, the eye will not remain in the midline, but be pulled outward by the unopposed intact lateral rectus muscle that turns it outward.

Lesion of abducens nerve

The eye directed inward (loss of lateral rectus to direct eye out unopposed intact medial rectus turns eye inward)

homonymous hemianopsia

The loss of the homonymous [right or left] visual field of vision in both eyes. Interruption of fibers at any point in the course of the optic tract, LGB, geniculocalcarine fibers results in homonymous (same field in both eyes) visual field losses lesion in right optic tract interrupts visual fibers from the retinas of both eyes, resulting in a left visual field defect for both eyes

contralateral homonymous hemianopia

The vision loss occurs in the field opposite the lesion EX: lesion of L optic tract = loss of R visual field (contralateral to the lesion) and is the right term if side is not indicated on the figure- it tends to be drawn from above and or below any lesion from the optic tract to the occipital cortex produces the same clinical picture

Nasal retinal fibers

These fibers cross to the opposite side of the brain.

pupillary light reflex clinical relevance

This reflex is the pupillary response to light - i.e. pupil constriction the parasympathetic part (the outer part) of the oculomotor nerve is very vulnerable to compression or stretch due to its being trapped between the posterior cerebral and superior cerebellar arteries any shifting of the brain due to a space occupying lesion (swelling, tumour, bleeding, trauma) the oculomotor nerve outer surface is easily damaged and fails to conduct the pupil response to light At this early stage of compression, the voluntary motor part of the nerve is not yet involved but would be with further compression.

lesions to visual pathway

a lesion posterior to the optic chiasm will lead to contralateral homonymous visual field deficits - as the nasal retinal fibers cross, but the temporal retinal fibers do not cross at optic chiasm - lesion in optic tract, lateral geniculate nucleus, optic radiation Damage to the central visual pathway, posterior to the optic chiasm, will cause loss of vision from half of the visual field from each eye

accommodation pupillary reflex

a reflex action of the eye, in response to focusing on a near object, then looking at distant object (and vice versa), comprising coordinated changes in vergence, lens shape and pupil size (accommodation).

accommodation on the lens in accommodation reflex

accommodation of the lens occurs by active contraction of the circular ciliary muscle that acts like the trampoline frame; lens 'likes to be fat' but is stretched (thinner) when your eyes are at rest Accommodation of the lens (lens changing shape) requires active contraction of the ciliary muscle to allow the lens to thicken, while the accommodation reflex (near reflex) requires the interplay of lens accommodation, pupillary constriction and convergence of the eyes in order to focus on near objects

optic nerve pathway

axonal projections from retina optic nerve optic chiasm optic tract (formed by fibers from optic chiasm pes pedunculi of midbrain lateral geniculate body visual cortex of occipital lobe [upper and lower calcarine fissure located on midsagittal surface of occipital lobe] receives projections from both eyes retina -> optic nerve -> optic chiasm -> optic tracts -> lateral geniculate nucleus (LGN) of thalamus -> visual radiations -> visual cortex

primary visual cortex

bilaterally located on midsagittal surface of occipital lobe each cortex receives info from both eyes brodmann area 17 wide cellular layer 4 Contains extra band of myelinated fibers Known to send cortical feedback projections to the LGB Myelinated structure = site of large geniculocalcarine input

common visual field defects

bitemporal hemianopsia homonymous hemianopsia homonymous superior quadrantanopia homonymous inferior quadrantanopia

reflexive modification of lens curvature

controlled by contraction of ciliary muscles through suspensory ligaments Parasympathetic contraction of ciliary muscles reduces tension in suspensory garments by pulling ciliary processes forward In absence of pull from ligaments (reduced tension), lens assumes rounded form acquiring greater refractive power while relaxed state of ciliary muscles exerts tension on suspensory ligaments → flattens lens by pulling it, reducing the refractive power permitting far vision

frontal eye field

controls voluntary movement of the eyes area 8 frontal lobe (considered part of premotor area/nearby area)

Bilateral visual cortical involvement

cortical blindness, marked by blindness w/ ability to follow light

droopy eyelid due to oculomotor nerve damage

damage to the oculomotor nerve from brain shifting doesn't tend to result in damage to the deeper voluntary axons and it only interferes with the pupillary light reflex. However, a major injury to the oculomotor nerve will also damage the voluntary muscles it innervates, including the eyelid, making the eyelid droop a little but you will also see loss of some eye movements and a dilated pupil.

optic nerve

extension of the brain, similar to olfactory nerve CN II special sensory: vision sensory receptor: retina CNS tract, myelinated by oligodendrocytes lesion of optic nerve - complete blindness in ONE eye

depth perception

eyes are offset at a 17-degree angle from the midline. offset gives two slightly different images, which are compared by our brain for depth perception

accommodation reflex pathway

follows light pathway and synapses in the lateral geniculate body The parasympathetic fibres synapse in the Edinger-Westphal nucleus and synapse again in the ciliary ganglion terminate in the constrictor pupillae muscle (for pupil constriction) AND the ciliary muscle (to reduce the pull on the suspensory ligaments and thereby relax the pull on the lens, allowing it to thicken) The voluntary fibres (to muscles that turn eyes inward) synapse in the oculomotor nucleus in the midbrain and travel to the orbit to stimulate left and right medial rectus muscles.

visual field defects notes

for testing, the visual field for each eye is divided into two: the nasal visual field and the temporal (near temples) visual field for each eye but when working out the source of light and potential lesions it is helpful to ignore the fact that the fields overlap and just work out the visual field for one eye at a time

temporal visual field

image of the temporal visual field is projected upside down and reversed on the nasal retinal field

accommodation reflex requires which centres to be intact?

intact area 8 Normal smooth-following movements of the eyes are due to unconscious coordinated eye movements and saccadic (jerky) following eye movements involve voluntary (conscious) following such as reading a book intact cerebral cortex it involves the conscious level (cerebral cortex), the patient needs to be conscious for this test

pupillary light reflex

involuntary (parasympathetic) pupil constriction Light in either retina sends a signal via CN II to pretectal nuclei in midbrain that activate bilateral Edinger-Westphal nuclei; pupils constrict bilaterally (consensual reflex). Result: illumination of 1 eye results in bilateral pupillary constriction.

rule #1 retinal representations of visual field

large part of visual field (binocular area) is covered by both eyes visual field has central and peripheral regions - central: small area in center projected on the macula of the retina and responsible for sharp vision, reading and face/object recognition - peripheral: surrounds central visual field is divided into two half fields -> 4 quarter fields - nasal and temporal - upper and lower quadrants retinal representation of visual field is reverse and inverted - light rays from temporal half of visual field project to nasal half of retinal field and vice versa - light rays from right visual field fall on nasal retina of right eye and temporal retina of left eye - light rays from left visual field fall on temporal retina of left eye and nasal retina of right eye

Left homonymous hemianopsia

lesion between right optic tract and right occipital cortex that leads to loss of vision in the left eye temporal visual field and right eye nasal visual field A complete lesion of anywhere from the right optic tract to the right occipital lobe will cause a LEFT homonymous hemianopsia

Right homonymous hemianopsia

lesion between the left optic tract and left occipital cortex which leads to vision loss in the left nasal visual field and right temporal visual field A complete lesion of anywhere from the left optic tract to the left occipital lobe will cause a RIGHT homonymous hemianopsia

three facts need to know

lesions described in terms of VISUAL FIELDS not RETINA fields lens inverts light information from inner retina crosses in the optic chiasm

Droopy Eyelid and Horner's Syndrome

levator palpebrae muscle raises the eyelid - both types of muscle - skeletal (voluntary- innervated by the oculomotor nerve) - smooth (involuntary, innervated by the sympathetic nerves) presenting with droopy eyelid - issue is whether this is due to damage to the innervation of the voluntary muscles (part of oculomotor nerve palsy) or damage to the smooth muscle (Horner's syndrome) IMPORTANT: The involuntary innervation of the eyelid is sympathetic innervation, not parasympathetic innervation of the oculomotor nerve!!!

retina

light is transduced to electrical energy by the retina

heteronymous hemianopia

loss of different visual half-fields in the two eyes (e.g. one loses nasal and other loses temporal)

how are objects seen?

objects in the left side of the body are in the left eye's temporal visual field and right eye's nasal visual field strikes the nasal retinal field in the left eye and right eye's temporal retinal field

oculomotor nucleus

oculomotor nucleus is for voluntary eye muscles

trochlear nerve and coordinated eye movements

only cranial nerve to leave the posterior surface of the brainstem travels around to the anterior surface of the brainstem where the other cranial nerves exit supplies the superior oblique muscle that directs the eye down and out HOWEVER, when damaged alone, other muscles look after the outward movement and the real loss is coordinated vision looking downward The peripheral nerve fibres cross just as they exit the brainstem; usually a lesion is outside the brainstem and therefore on the same side as the target for CN IV; only cranial nerve that does this

rule #2 retinal representations to the optic chiasm

optic nerve fibers from retinal ganglion enter cranial cavity to reach optic chiasm partial crossing occurs at chiasm 1) fibers from nasal halves of retina (rep temporal visual field for each eye) cross midline to project to opposite visual cortex 2) fibers from temporal halves of retina (rep nasal visual field for each eye) remain uncrossed and project to ipsilateral visual cortex accounts for processing and projection of right visual field -> left hemisphere and vice versa

retinal representation to visual cortex

optic radiations (geniculocalcarine fibers) enter the retrolenticular portion of the posterior internal capsule divides into dorsal (superior) and ventral (inferior) bundles superior bundle travels to cells in visual cortex, above the calcarine fissure and carries upper retinal quadrants inferior bundle forms Meyer loop, moves rostrally and laterally around inferior horn of lateral ventricle before travelling to cells in visual cortex, below calcarine fissure and caries lower retinal quadrant info

visual system

pathway of optic nerve sensory input of vision and cranial nerves that supply muscles that move the eyes - oculomotor (III) - trochlear (VI) - abducen (IV) interacts with vestibular system medial longitudinal fasciculus (MLF coordinates eye movement by connecting nuclei for motor signals to muscle controlling eye movements) reticular formation spinotectal and tectospinal tracts occipital cortices involved with conscious and subconsious appreciation and processing of visual info interacts with .. for visual scanning and reflex behaviours gaze centres in brainstem frontal eye field area (area 8) in frontal lobe

important characteristics of central vision mechanism

point-point rep of visual field from retina thorugh LGB to primary visual cortex projection from each eye to both cerebral hemispheres (binocular processing)

neural mechanism of pupillary light reflex response

pretectal area, edinger-westphal nucleus and fibers of the oculomotor nerve 1) Activated retinal ganglion cell send projections to the brain [following normal light pathway: retina -> optic nerve -> optic chiasm -> optic tract] 2) fibers leave optic tract before LGB and synapse on cells in the pretectal area 3) pretectal area, small area between superior colliculi, bilaterally projects to the edinger-westphal nucleus of the oculomotor nerve 4) preganglionic fibers from E-W nucleus travel in the oculomotor nerve and innervate ipsilateral ciliary ganglion in the orbit 5) postganglionic fibers from the ciliary ganglion provide parasympathetic projections to the circular fibers of the iris 6) constriction of circular fibers narrows aperture of pupil [miosis]

primary visual cortex map

primary visual cortex (brodmann 17) bilaterally located on midsagittal surface of occipital lobe each cortex receives info from both eyes two opercula, separated by calcarine fissure - inferior lip carries lower retina projections - superior lip carries upper retina projections central visual field (macular region of retina) occupies area in posterior partnear occipital lobe -- more than 50% of PVC) peripheral visual fields rep anterior portions of calcarine cortex

accommodation reflex

response to a stimulus of 'blurriness' presented to BOTH eyes, with a three-part response 1) accommodation of the lens 2) constriction of the pupil 3) convergence of the eyes (turning them inward to focus on the object) requires conscious input because signal must travel to the occipital cortex (conscious level) and then to the Frontal Eye Field (Area 8), before going to the brainstem Therefore, it synapses in the lateral geniculate body, unlike the pupillary light reflex. The occipital lobe must be intact; the frontal lobe area 8 must be intact AND the brainstem nuclei must be intact

frontal eye field lesion

stimulation causes deviation of the eyes away from the side being stimulated If Area 8 is damaged (e.g. stroke), the eyes remain deviated toward the side of the lesion (unopposed, therefore the intact opposite side takes over).

Pathway of pupillary light reflex

the afferent pathway of this reflex (which occurs when light is shone into the eye) tested in only one eye at a time. fast, subconscious reflex and does NOT involve the cerebral cortex until after the reflex occurs 1) The information travels through the first part of the light pathway but without synapsing in the lateral geniculate body!! 2) Instead it goes directly to the midbrain reflex centre where it synapses in the pretectal nucleus on BOTH sides and then travels on the outer part of BOTH oculomotor nerves 3) It then synapses a third time in the ciliary ganglion and terminates in constrictor pupillae (involuntary) muscle on both sides - cell body of the short ciliary nerve is in the ciliary ganglion (near the eyeball) 4) Response = constriction of both pupils.

nasal visual field

the image of the nasal visual field is projected upside down and reversed on the temporal retina.

retinal field

the reversed image of the visual field that is projected upon the retina

droopy eyelid due to sympathetic innervation damage

the symptoms will include a constricted pupil and a dry red face on that side, all due to sympathetic nerve loss

homonymous

the word 'homonymous' means 'from the same side of' each visual field

Visual association cortex lesion

visual agnosia Bilateral lesion involving association cortex (18 an 19) or extending to inferior temporal lobe areas (20, 21, 37) may result in visual agnosia

oculomotor nerve and coordinated eye movements

voluntary motor function innervate most of the eye muscles - medial rectus (eye inward) - superior rectus (eye upward) - inferior rectus (eye downward) - inferior oblique (eye up and out) muscles involved in the all-important pupillary light reflex nerve exits the brainstem between two arteries (posterior cerebral artery and superior cerebellar artery) susceptible to increased pressure due to shifting brain causing the parasympathetic fibres to be damaged, causing dilation of the pupil and interfering with the pupil response to light.

Gaze

you should know that all the nuclei of CNs III, IV and VI are interconnected (both sides) by the Medial Longitudinal Fasciculus (MLF) to coordinate eye movement MLF is a common target for demyelination and nystagmus in Multiple Sclerosis. It is connected to the vestibular nuclei. Only very severe damage to the vestibular system (usually with profound brainstem injuries and grave prognosis) causes 'doll's eye syndrome'.