Correlative Neuroanatomy of the Sensory System

OS 202B Integration and Control Systems Correlative Neuroanatomy of the Sensory System Lecturer: Jose Leonard Pascual, MD OUTLINE I. Introduction A. Sensory Modalities B. How We See Things C. Basic Layout of the Sensory System 1. Neurons 2. Dermatomes II. Pain A. Tracing the Pain Pathway B. How We Feel Pain C. Pain/Temperature Pathway from Body D. Pain/Temperature Pathway from Head III. Touch Note: This trans is based on Dr. Pascual’s slides. Italicized notes are lifted from 2019 trans. Learning Objectives  FORM: Identify the important neuroanatomical structures involved with the transmission of sensation (receptor, nerve, nerve root ganglion, ascending tract, thalamus, cerebral cortex)  FUNCTION: To be able to trace the pathways below from receptor to cortex

A. 1. 2. 3. 4. 

I. INTRODUCTION Sensory Modalities Pain and temperature - noxious stimuli; extreme situations Light touch - subtle stimulus Vibration sense - repetitive light touch Proprioception - position and movement Different sensory modalities each have their own receptors o Thermoreceptors – heat o Meissner’s corpuscle – touch o Nociceptor – pain o Pacinian corpuscle – pressure o Photoreceptors - light

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How We Sense Things Nerve receptor picks up the stimulus o Free nerve endings: PAIN (meaning they are naked dendrites) o Pain in the fingers must ASCEND to the brain so it’s AFFERENT  The stimulus is sent to the thalamus o Stimuli received by several receptors travel together within the peripheral nerves, reach their respective nerve ganglia (eg. dorsal root ganglia) and enter the CNS o Receptor  peripheralnerve  nerveganglia  CNS (via ascending tract)  The thalamus relays the stimulus to the cerebral cortex  The cerebral cortex understands the stimulus in its context C. Basic Layout of the Sensory System 1. Neurons a. First Order Neurons b. Second Order Neurons  Receives impulses from first order neuron  Location: o Neurons of the body: lies in the dorsal horn of the spinal cord o Neurons of the face: lies in the spinal nucleus of CN V  Crosses the midline and ascends into the thalamus

c.

Date: March 1, 2016 Trans: 03-06

01-01 Third Order Neurons  From the thalamus to the cerebral cortex

Figure 1. Organization of Sensory Pathways.

2. Dermatomes  Dermatomes: areas on the skin supplied by a single specific spinal nerve root  Each dermatome segment corresponds to one spinal nerve  There is NO C1 dermatome o C1 is a purely motor peripheral nerve  Two or three vertebral bodies separate the spinal nerve from its origin in the spinal cord 
 o e.g. a lesion affecting the site of origin of T10 is not necessarily at the level of T10 vertebra; it would most probably be at the level of T8 vertebra  C1-7 exit ABOVE their corresponding vertebral bodies  C8 exits ABOVE T1  T1 downwards exit BELOW their corresponding vertebral bodies  From the spinal cord exits a dorsal root (sensory), which further becomes a dorsal root ganglion and a ventral root (motor) Table 1. Easy-to-remember dermatomal segments.

Anterior C3 C6 C7 C8 T4 T10 L1 S2-3 L4 L5 S1

Front of neck (“lovebite”) Thumb Middle finger Little finger Nipple Umbilicus Inguinal area (bikini line) Genitalia Knee & big toe Shin Little Toe

Posterior Back of the head

C2

Also innervates the area at the angle of the jaw and earlobe. Do NOT include said area when testing for CN V function.

C5 S2-5

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OS202B Correlative Neuroanatomy of the Sensory System II. PAIN A. Tracing the Pain Pathway  This pathway mediates nociceptive stimuli o Two different modalities, one pathway = Pain and Temperature  Each of those sensations have corresponding nociceptors o Pain (pinprick): A-delta mechanical receptors o Deep pain (tissue damage): C-polymodal (free nerve endings o Heat/cold: Free nerve endings

Figure 2. Diagram of dermatomal segments.

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There is a difference between the length of the spine and length of the cord o Spine > cord o There is a difference between a vertebral level and a cord level o Cord will terminate at L1-L2, which is important when doing a spinal tap In every interval between each vertebral body, there is a gap where the spinal nerves will exit

B. How We Feel Pain  Unmyelinated nerve endings are preferentially stimulated (nociceptors) o If it’s myelinated, you will feel the pain very rapidly and very painfully  Impulse travels along the nerve assigned to the area affected (dermatome) o Each spinal nerve has a dorsal root ganglion (DRG) and each DRG is assigned a cord segment  Pain is carried from periphery into CNS by dorsal root ganglia o The trigeminal ganglion is structurally similar to a dorsal root ganglion  The stimulus is passed to clusters of neurons whose axons cross to the other side of the CNS o Dorsal horn grey matter and spinothalamic tract (spinal cord to thalamus)  Second order neuron = where decussation occurs  The tracts either ascend or descend to cross over to the contralateral side o Trigeminal spinal nucleus and tract  The spinal trigeminal tract will cross at the level of C2 segment and ascend together with the spinothalamic tract  All of the decussations must pass through the spinal cord

Figure 4. Spinal trigeminal and spinothalamic tract.

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Figure 3. Spinal nerves coming out of the roots.

The tracts terminate in the thalamus, which then processes the stimuli and relays them to the cerebral cortex Lesion at the spinothalamic tract area ABOVE the level of C2 will manifest as loss of pain and temperature sensation at the CONTRALATERAL half of the face and body

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OS202B Correlative Neuroanatomy of the Sensory System C. Pain/Temperature Pathway from the Body 1. First Order Neurons: Lies within the DRG  Dendrites travel within the spinal nerve, passing through the intervertebral foramen to reach the DRG  Axons from the DRG ascend or descend (more often) one or two spinal cord segments before synapsing on the dorsal horn  Axons of nociceptive receptor cells travel 1-2 spinal cord segments up or down within the dorsolateral fasciculus (Lissauer’s tract) and synapse within the second order neuron within the dorsal horn of the spinal cord

Figure 5. Lissauer’s tract (Red).

2. Second Order Neurons  Lies in the postmarginal nucleus within the outermost layer (lamina 1) of the dorsal horn  The second order neuron’s axons immediately decussate within the anterior commissure to ascend within the lateral fasciculus as the spinothalamic tract Going caudally to rostrally, the spinothalamic tract fibers  are pushed laterally by each spinal nerves decussating fibers o Tumor growing from outside = affects outermost fibers = ascending loss of pain and temperature sensation o Tumor growing from inside of spinal cord = affects innermost tract fibers = descending loss of pain and temperature sensation (i.e. cervical nerve fibers first before thoracic, lumbar, and sacral)  At the level of the medulla, the spinothalamic tract ascends within the reticular formation adjacent to the spinal tract and nucleus of the trigeminal nerve o If this is harmed, the patient will manifest with cross numbness (ipsilateral half of face and contralateral extremities)

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From the trigeminal ganglion, CN V arises and pierces through the middle cerebellar peduncle of the pons o Axons of nociceptive receptor cells within CN V enter the pons via the sensory root o Descends within the spinal tract of CN V to terminate on the pars caudalis of the spinal nucleus of CN V (second order neuron) CN IX (petrosal) and CN X (jugular) ganglion o The dendrites travel within the glossopharyngeal and vagus nerves, which enter the skull via the jugular foramen  Both penetrate the brain stem o Axons of the nociceptive receptor cells within CN IX and X enter the medulla o They join the spinal tract of CN V to terminate on the pars caudalis of the spinal nucleus of CN V (second order neuron) as well as in the dorsal horn of the upper cervical cord

2. Second Order Neurons Spinal Nucleus of CN V  o Pain and temperature o Decussates in the cervical cord (C2) to ascend as the trigeminal lemniscus together with the spinothalamic tract  Divided into: o Pars oralis o Pars interporalis o Pars caudalis  Histologically indistinguishable from the dorsal horn of the spinal cord

3. Third Order Neurons  The axons of the secondary order neuron synapse with the third order neurons within the ventroposterolateral (VPL) thalamus (“L” for limbs)

Figure 6. Spinal nucleus divisions.

D. Pain/Temperature Pathway from the Head  The segmental innervation of the somatosensory system of the body continues also in the head  Onion-skin pattern of innervation o Upper cervical cord and caudal medulla = more peripheral areas of face and head o Upper medulla = more central areas (nose, cheeks, lips) o Pons = mouth, teeth, pharynx

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1. First Order Neurons 1st order neuron: the free nerve endings are the tips of the  dendrites and its soma lies within the Gasserian/ trigeminal/semilunar ganglion (or in ganglia of VII/IX/X) CN V (trigeminal) ganglion  o Dendrites pass through respective foramina to reach trigeminal ganglion  Supraorbital fissure: V1  Foramen rotundum: V2  Foramen ovale: V3

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The second order neuron cell body lies within the pars caudalis of the spinal nucleus of CN V, whose axons decussate at the level of C2 and ascend as the trigeminal lemniscus

3. Third Order Neurons  Axons of second order neurons synapse with tertiary neurons within ventroposteromedial (VPM) nucleus of the thalamus (“M” for mukha) Postcentral gyrus Primary somatosensory cortex Brodmann’s areas 3, 2, and 1 The more important the area, the higher the number of neurons allocated o Face and hands have the biggest representation of the primary somatosensory cortex

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OS202B Correlative Neuroanatomy of the Sensory System

Figure 7. Primary somatosensory cortex and associated homunculus.

E. Pain Modulation  Within the spinal cord: substantia gelatinosa  Within the brainstem: o Periaqueductal gray of rostral brain o Periventricular gray of diencephalon o Reticular formation: nucleus raphe magnus III. TOUCH A. How We Feel Touch General Pathway 1. Touch receptors travel within nerves of their assigned dermatome 2. Touch impulses travel UNCROSSED to the brainstem and terminate in nuclei assigned for touch and vibration 3. Fibers from those nuclei CROSS over within the brainstem and ascend to the thalamus 4. The thalamus relays the touch stimulus to the ipsilateral sensory cortex and the rest of the brain Differences with the Pain Pathway  First order neuron always travels upwards uncrossed  Second order neuron is in the medulla  Only the second order neuron decussates B. 

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Tactile Pathway from the Body to the Head This pathway mediates the following sensations: o Touch (tactile), including  Two-point discrimination  Stereognosis (determine object by touching it)  Graphesthesia (determine what’s written on skin) Pressure (deep touch) Vibration Limb position Limb motion

Figure 8. Arrangement of fasciculi gracilis and cuneatus in the spinal cord. Note that f. gracilis is pushed medially by the f. cuneatus.

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Each of these sensations have corresponding mechanoreceptors:  Light Touch o Tactile (hairless skin): Meissner's corpuscles o Tactile (hairy skin): hair follicle receptors  Pressure o Merkel discs o Ruffini endings  Vibration: Pacinian corpuscles  Limb position & motion: muscle spindles 1.     

First Order Neuron Soma lies within the dorsal root ganglion Axon fibers from the leg enter lumbar/sacral spinal cord via the dorsal root and are funneled medially into the fasciculus gracilis (tract of Goll) Axon fibers from the arm enter cervical/thoracic spinal cord via the dorsal root and are funnelled medially into the fasciculus cuneatus (Tract of Burdach) Fibers coming from the upper body (including the arms) push the fibers from the lower body medially, leading to somatotopic organization of the posterior columns Together, the axons within the posterior columns ascend to the brainstem uncrossed

Second-order Neuron Within the nucleus gracilis / nucleus cuneatus Fasciculus gracilis and cuneatus terminate in their respective nuclei (nucleus gracilis and nucleus cuneatus), which lie under their respective tubercles (gracile tubercle and cuneate tubercle) Myelinated fibers from each nuclei reach the brainstem, travel up the midline, and cross anteriorly o The fibers are now seen as internal arcuate fibers in the medulla, forming a structure in the midline known as the medial lemniscus on the contralateral side o Lamination of the medial lemniscus (at the level of the medulla):  Fibers from gracile nucleus are in the anterior half  Fibers from cuneate nucleus are in the posterior half

Figure 9. Lamination of the medial lemniscus. Nucleus gracilis is blue and anterior; nucleus cuneatus is purple and posterior.

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At the level of the pons, the medial lemniscus undergoes dorsolateral rotation o The medial lemniscus now appears flattened and on a medial lateral axis o Fibers from the legs are now more lateral

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OS202B Correlative Neuroanatomy of the Sensory System Case 2 The security guard at the mall suddenly feels dizzy. You did a neurologic examination and found that he has:  No sensation of pinprick on the left side of his body  No sensation of pinprick on the right side of his face  Difficulty swallowing (may be problem of medulla) Answer: Cross numbness (see p. 3 under “Pain/Temperature Pathway to Head”)  Lesion at C2 level; CN V fibers have not decussated while spinothalamic tract fibers have already decussated

Figure 10. Dorsolateral rotation of the medial lemniscus; Laterally placed fibers from the leg.

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Third-order Neuron The medial lemniscus can now ascend without decussating to the VPL nucleus of the thalamus (similar to pain pathway)

C. 1.  

Tactile Pathway from the Head First Order Neuron Large pseudounipolar cells within the Gasserian ganglion Axons enter the pons via the sensory root of CN V and pass dorsomedially to tegmentum of pons Principal target of light touch for face is the principal sensory nucleus of CN V Counterpart of the gracilis and cuneatus

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Second Order Neuron Lies within the principal sensory nucleus of CN V Main sensory nucleus / principal nucleus o Analogous to posterior column nuclei of the cord o Concerned with proprioceptive sensations of the head o Decussates and joins contralateral medial lemniscus, which ascend to terminate in the thalamus (ventral trigeminal tract) o Axons of these second order neurons travel within the ascending trigeminothalamic tract and terminate on third order neurons within the VPM nucleus

Case 3 The fishball vendor suddenly feels his right hand is clumsy and has right sided weakness of his face, arm and leg. You do a neurological examination and find that he has:  No problems understanding you and follows what you say  Shallow nasolabial fold on the right  Weak right arm and leg  Walks like a drunk person and teeters to the right  He has lost his vibration sense and light touch on right side Answer: Lesion at L and stroke at the medial lemniscus at the medulla (affected spinothalamic tract). Slight involvement of the pyramidal (corticospinal tract) tract. Case 4 The banana-Q vendor suddenly slumps to the ground and snores loudly. You were nearby and did a neurologic examination and found that he has:  Very drowsy mental state  Cannot feel anything on left side of face and body (even with very painful stimuli) Answer: L-sided hemianaesthesia. Stroke at the R thalamus  damaged R spinothalic tract, disruption of ARAS. Snoring is sign of decreased sensorium and loss of consciousness. Corona radiata and internal capsule are affected. V. VISION This part is heavily based on sir Pascual’s lecture slides: https://drive.google.com/file/d/0B_x7FtVlOWK_OERzX0VvQjBf eVU/view It is recommended that you check the slides while you read the trans because there are a number of pictures there that may be useful and the trans might have too much pictures if they are included as they are part of a pathway. A.  

Figure 11. A more complete somatotopic representation (homunculus) can be seen for the tactile pathway at the level of the pons.

IV. 

LOCALIZING LESIONS IN SOMATOSENSORY PATHWAYS Look at the pattern of the sensory phenomena (e.g., pain, numbness, etc): o Peripheral nerve o Segmental dermatome o Spinothalamic tract o Postcentral gyrus

Case 1 Your friend was hurt in a vehicular accident. On your neurologic examination you found that he has:  Loss of sensation below the umbilicus o On the right side: cannot feel light touch/vibrations o On the left side: cannot feel pinprick Answer: Lesion at R T10  Below umbilicus = T10  Cannot feel light touch on R side = lesion on R side (no decussation of nerves for light touch pathway)  Cannot feel pinprick on L side = decussation of nerves

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How We See Light and color stimulate photoreceptors in the retina The visual impulse reaches the retinal ganglion cells, whose axons converge into the optic disc The axons continue on as optic nerve Half of the axons from each eye cross over to the contralateral optic tract within the optic chiasm Visual impulses within the optic tract reach the thalamus The thalamus sends forth optic radiations within the subcortical white matter, which terminate within the visual occipital cortex Summary: Light and color  Photoreceptors in the retina  Retinal ganglion cells axons  Optic disc (papilla)  Optic nerve  Optic tract  Thalamus

Figure 12. The ganglion cell neurons and axons that form the optic nerve.

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OS202B Correlative Neuroanatomy of the Sensory System B. 

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Visual Pathway Sight o What objects look like o Where objects are o These would be recognition and location which are primitive The optic nerve exits the orbit via the optic foramen and unite to form the optic chiasm Due to the refractive properties of the lenses of our eyes, the image that falls on our retinas will be inverted upside down and flipped horizontally:

Figure 13. Image seen by eye (left); image refracted in the retina (right).

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Visual stimuli from the temporal halves of the image fall on the nasal halves of the retina Visual stimuli from the nasal halves of the image fall on the temporal halves of the retina (That’s why at the level of the retina, visual fields in each optic nerve are represented as inverted mirror images). Figure 15. The Visual Pathway.

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Figure 14. Representation of image stimulus as received by the temporal and nasal halves of the eyes.

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Blindness can actually root from the optic nerve to the retina itself At the optic chiasm, the nasal fibers from each optic nerve decussate to the contralateral optic tract At the optic chiasm, the temporal fibers continue on the ipsilateral optic tract o Snakes and crocodiles have Ipsilateral Uncrossed Visual Fibers o This is actually needed to have front facing eyes and for eye-hand coordination o ALBINOS - Do not have front facing eyes (Optic Chiasm) thus they also have poor eye-hand coordination From the optic chiasm, optic tracts emanate from each side and pass posterolaterally along the surface of the hypothalamus and cerebral peduncles Axons of the retinal ganglion cells will terminate on the lateral geniculate nucleus (LGN) of the thalamus on each side

Third order neuron: Lies within the LGN o Sends projections to the primary visual cortex or cerebral (occipital) cortex (optic radiations a.k.a. geniculocalcarine/geniculostriate pathways) o Representation of the field of vision in the LGN  The area of the sharpest visual acuity is subserved by the macula of the retina (has large amount of neurons)  This small area is greatly represented within the LGN and in the visual cortex (seeing sharp image is very important to us and our brains) o The optic radiations enter the retrolenticular portion of the posterior limb of the internal capsule o From the internal capsule, the fibers of the optic radiation sweep to the lateral surface of the lateral ventricle  The more dorsal (parietal) fibers proceed directly posteriorly, through the parietal lobe and finally the occipital lobe  The more ventral (temporal) fibers loop anteriorly over the inferior (temporal) horn of the lateral ventricle (Also known as the Meyer’s Loop, which is the anterior most extension of the optic radiations to the temporal horn)

Figure 15. Location of Meyer’s Loop.

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OS202B Correlative Neuroanatomy of the Sensory System D. Localizing Lesions  At the level of the optic nerve

Figure 16. Location of Meyer’s Loop in imaging (left) and axial cut section (right) of the brain.

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Superior fibers of the optic radiation pass straight through the parietal lobe to the occipital cortex  Only the inferior quadrant of the image appears in the superior (parietal) optic radiations  The superior fibers terminate in the cuneus  Sees the ground and used for locating objects Inferior fibers must fist loop around the temporal horn of the lateral ventricle before going to the occipital cortex  Only the superior quadrant of the image appears in the inferior (temporal) optic radiations  The inferior fibers terminate on the lingual (medial occipitotemporal) gyrus Optic radiations as a whole will have the image appear as just the contralateral half

o o 

At the level of the optic chiasm (lateral)

o o o o 

Figure 17. The contralateral halves.

o o

Visual cortex: composed of primary visual (striate) cortex and the extrastriate visual cortical areas o Primary visual cortex (V1), also known as the striate cortex

C. Visual Acuity  As previously mentioned, the macula has a great representation of the visual cortex  While the occipital cortex receives most of its blood supply from the posterior cerebral artery (PCA), the macular vision area receives additional (collateral) blood supply from the middle cerebral artery (MCA)

o o o 

Damage to optic chiasm AWAY from midline structures can affect fibers that run through inferior fibers or Meyer’s loop Results to loss of superior ipsilateral visual field Also known as left superior quadranopia May result to macular loss of vision

At the level of optic chiasm (central)

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Damage to the retina or one of the optic nerves before it reaches the chiasm results in a loss of vision that is limited to the eye of origin Anterior to the chiasm - loss of one eye or monocular

Damage to the middle portion of the optic chiasm (often the result of pituitary tumors, pharyngeal tumors, rathke pouch tumors) results in damaged fibers of the nasal retina Intact temporal retina Resulting loss of vision is confined to the temporal visual field  images from the temporal visual field fall onto nasal retina Called bitemporal/heteronymous hemianopia (due to cut nasal fibers) Also includes loss of lateral aspect of body space Brain tumors or problems with the cavernous sinus  Pituitary gland and optic nerves are near

At the level of optic tract

o o o

Interruption of the L optic tract results in loss of sight in the R visual field (i.e., blindness in the temporal visual field of R eye and nasal visual field of L eye) Called homonymous hemianopia (in this case, R homonymous hemianopia) Pathway was disrupted before the chiasm

Figure 18. Blood supply of the primary visual cortex.

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OS202B Correlative Neuroanatomy of the Sensory System 

o o o o 

o o o o

At the superior fibers of the optic radiation crossing the parietal lobe

Results in loss of interior contralateral visual field Called Inferior right quadranopia “Pie on the floor”  lower quadrant “Pie on the sky” upper quadrant  temporal lobe problem

E. Case Your girlfriend has a vague headache the past few years, and recently it has gotten worse. She now tends to bump into people while walking into BSLR-East. Her neurological examination only showed this:

Transecting the whole optic radiations

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Similar to transecting an optic tract Manifests also as homonymous hemianopia

Transecting the striate cortex

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Occipital lobe lesion: black pigmentations Temporal lobe lesion: angel-like visions Micropsia: objects appear smaller than actual size Macropsia: objects appear larger than actual size

Results in the contralateral homonymous hemianopia with macular sparing Selecting preservation of foveal vision has not been clearly understood, although this is a common feature of damage in the striate cortex Loss of blood supply, PCA was blocked, occipital lobe lost blood supply but macular area is persistent

Damage to optic chiasm resulting to bitemporal hemianopsia. END OF TRANSCRIPTION

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In the primary visual cortex (“Cortical blindness”) o Blindness may not be due to eye damage, but cortical lesion  Pupils still react to light, but cannot see anything  Patient may experience hallucinations (Optic nerve is working but the brain is blind)  Results to visual field defects/scotoma APPENDIX A: Summary of the Lesions in the Visual Pathway

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