Midbrain

CN II (Optic Nerve) Function

Sensory nerve responsible for visual information and visual acuity.

Retinal Ganglion Cells

Neurons whose axons form the optic nerve and exit the eye via the optic canal.

Optic Chiasm

Brainstem structure where nasal retinal fibers undergo partial decussation (crossing).

Optic Tract

Pathway carrying visual information from the optic chiasm to the LGN, pretectal area, and superior colliculus.

Lateral Geniculate Nucleus (LGN)

Thalamic nucleus where optic tract fibers synapse before projecting to the primary visual cortex.

Primary Visual Cortex Location

Located in the occipital lobe around the calcarine fissure, specifically Brodmann Area 17.

Association Visual Cortex

Located in Brodmann Areas 18 and 19 in the occipital lobe.

Cuneus

Superior gyrus of occipital cortex above calcarine sulcus; termination point for superior optic radiations.

Lingual Gyrus

Inferior gyrus of occipital cortex below calcarine sulcus; termination point for inferior optic radiations.

Inferior Optic Radiations (Meyer's Loop)

Passes through the temporal lobe carrying superior visual field information to the lingual gyrus.

Superior Optic Radiations

Passes through the parietal lobe carrying inferior visual field information to the cuneus.

Nasal Hemi-retina

Medial portion of retina closest to nose; receives light from the peripheral visual field.

Temporal Hemi-retina

Lateral portion of retina closest to temples; receives light from the central visual field.

Right Visual Field Representation

Processed entirely by the left LGN and left primary visual cortex.

Left Visual Field Representation

Processed entirely by the right LGN and right primary visual cortex.

Monocular Visual Loss

Complete blindness in one eye caused by a total lesion to the ipsilateral optic nerve.

Bitemporal Hemianopsia

Loss of vision in both temporal (peripheral) fields caused by a lesion to the center of the optic chiasm.

Homonymous Hemianopsia

Loss of the same side visual field in both eyes (e.g., left visual field) caused by an optic tract lesion.

Contralateral Superior Quadrantanopsia

Loss of upper opposite visual field quadrant caused by a temporal lobe lesion damaging Meyer's loop.

Contralateral Inferior Quadrantanopsia

Loss of lower opposite visual field quadrant caused by a parietal lobe optic radiation lesion.

Pretectal Area Target

Rostral midbrain structure receiving optic tract fibers involved in pupillary light reflexes.

Superior Colliculus Target

Dorsal midbrain structure receiving optic tract fibers for visual tracking and eye-head alignment.

CN III (Oculomotor Nerve) Type

Mixed cranial nerve carrying both somatic motor and parasympathetic fibers.

CN III Medial Rectus Action

Adduction of the eye (turning pupil medially toward the nose).

CN III Superior Rectus Action

Elevation of the eye (turning pupil upward).

CN III Inferior Rectus Action

Depression of the eye (turning pupil downward).

CN III Inferior Oblique Action

Up-torsion and elevation of the eye.

CN III Levator Palpebrae Action

Lifts/elevates the upper eyelid.

CN III Parasympathetic Functions

Constricts the pupil (pupillary sphincter) and contracts ciliary muscle for lens accommodation.

Oculomotor Nucleus Location

Located in the midbrain tegmentum at the level of the superior colliculus.

CN III Brainstem Emergence & Route

Exits ventrally in interpeduncular fossa, traverses cavernous sinus, and enters orbit via superior orbital fissure.

Edinger-Westphal Nucleus

Midbrain parasympathetic nucleus providing pre-ganglionic fibers for CN III.

Ciliary Ganglion

Orbital ganglion where pre-ganglionic parasympathetic fibers from EW nucleus synapse.

Oculomotor Palsy Resting Eye Position

Eye rests in a 'down and out' position due to unopposed lateral rectus and superior oblique muscles.

Oculomotor Palsy Eyelid & Pupil

Presents with ptosis (drooping eyelid) and a dilated, unreactive pupil.

CN IV (Trochlear Nerve) Type & Muscle

Pure motor nerve innervating the Superior Oblique muscle.

Superior Oblique Muscle Action

Causes depression (downward gaze) and intorsion (inward rotation) of the eye.

Trochlear Nucleus Location

Located in the midbrain tegmentum at the level of the inferior colliculus.

Unique Anatomy of CN IV

Only cranial nerve to exit the brainstem dorsally (below inferior colliculus).

CN IV Trochlear Palsy Symptoms

Vertical diplopia (double vision) and hypertropia (affected eye elevated).

CN IV Palsy Compensatory Head Position

Patient tilts head AWAY from affected eye and tucks chin to align visual axes.

Trochlear Palsy Worst Diplopia Direction

Vertical diplopia is most severe when looking downward and inward (toward nose).

Afferent Branch of Eye Reflexes

Cranial Nerve II (Optic Nerve) senses visual and light inputs.

Efferent Branch of Eye Reflexes

Parasympathetic fibers of Cranial Nerve III (Oculomotor Nerve) execute pupillary constriction.

Pupillary Light Reflex Pathway

Light -> Retina -> Optic tract -> Pretectal nucleus -> Bilateral EW nuclei -> Bilateral CN III -> Ciliary ganglia -> Pupil constriction.

Direct Pupillary Reflex

Constriction of the pupil in the eye directly exposed to light.

Consensual Pupillary Reflex

Constriction of the pupil in the opposite eye not directly exposed to light.

Anisocoria

Condition characterized by asymmetrical pupil sizes.

Accommodation Reflex Components

Bilateral pupil constriction, ciliary muscle contraction (increased lens convexity), and bilateral eye convergence.

Convergence Mechanism

Bilateral contraction of medial recti muscles via pretectal inputs to align eyes on near objects (<20 cm).

Medial Longitudinal Fasciculus (MLF)

Brainstem pathway coordinating conjugate eye movements across CN III, IV, VI, and vestibular nuclei.

Diplopia Etiologies

Can be mechanical (orbital fracture), extraocular muscle disease, NMJ disorder, or CN III, IV, VI lesions.

Weber's Syndrome (Medial Midbrain)

Midbrain vascular syndrome from PCA occlusion causing ipsilateral CN III palsy and contralateral hemiparesis.