Projection of the eye onto the brain

The sensory homunculus is the topographical representation of body parts on the brain. In the figure in the link one can see that the hands have a disproportionately large representation compare to the eyes with respect to sensation but the perception of vision of course is in area 17 of the calcarine cortex .

The visual pathway is very well shown in brief in this link. The individual fibers can be related to the visual field and its representation in this link from the University of Western Ontario. The left side of the brain views the right visual field and the images are projected as upright compared to the visual field.

Interruption of the visual pathway results in defects in vision that are location specific. Ophthalmologists will need to know the anatomic correlate of each pattern in the visual field (see link).

Sensory Pathway of V.

Trace the pathway of sensory input from the iris or cornea to the brain. In summary sensory nerves from the cornea and iris travel as the long posterior ciliary nerves (combined of course with sympathetic fibers) exit the eye posteriorly and pass thru the ciliary ganglion to join the nasociliary nerve as the long sensory root. The nasociliary nerve exits the orbit thru the superior orbital fissure and enters the cavernous sinus lateral to the internal carotid artery. The nerve synapses in the trigeminal ganglion (also called semilunar or Gasserian ganglion) which is present in Meckel's cave. From the synapse in the trigeminal ganglion the fibers enter the pons and descend in the ipsilateral spinal trigeminal tract, synapsing in the most ventral portion (synapse 2). The fibers ascend and cross as the trigeminothalamic tract to the ventroposteromedial nucleus of the thalamus (3rd synapse) and onward to the internal capsule to the postcentral gyrus of somtosensory cortex (4th synapse).

Hutchinson's sign is the accompaniment of herpetic lesions on the tip of the nose with corneal involvement. This is possible because both are branches of the V1 the nasociliary nerve. The nasociliary nerve fibers enters the nose through branches that pass through the anterior ethmoidal foramen across the cribriform plate of the ethmoid bone, through a slit at the side of the crista galli, into the nasal cavity. The nasociliary nerve fibers supply internal nasal branches to the mucous membrane of the front part of the septum, lateral wall of the nasal cavity and as the external nasal branch, supplies the skin of the ala and apex of the nose.

The detailed explanation for the entire Cranial Nerve V is given from Gray's Anatomy as seen below with stunning diagrams in links.


Sensory Root.—The fibers of the sensory root arise from the cells of the semilunar ganglion which lies in a cavity of the dura mater near the apex of the petrous part of the temporal bone. They pass backward below the superior petrosal sinus and tentorium cerebelli, and, entering the pons, divide into upper and lower roots. The upper root ends partly in a nucleus which is situated in the pons lateral to the lower motor nucleus, and partly in the locus cæruleus; the lower root descends through the pons and medulla oblongata, and ends in the upper part of the substantia gelatinosa of Rolando. This lower root is sometimes named the spinal root of the nerve. Medullation of the fibers of the sensory root begins about the fifth month of fetal life, but the whole of its fibers are not medullated until the third month after birth.
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The Semilunar Ganglion (ganglion semilunare [Gasseri]; Gasserian ganglion) occupies a cavity (cavum Meckelii) in the dura mater covering the trigeminal impression near the apex of the petrous part of the temporal bone. It is somewhat crescentic in shape, with its convexity directed forward: medially, it is in relation with the internal carotid artery and the posterior part of the cavernous sinus. The motor root runs in front of and medial to the sensory root, and passes beneath the ganglion; it leaves the skull through the foramen ovale, and, immediately below this foramen, joins the mandibular nerve. The greater superficial petrosal nerve lies also underneath the ganglion.
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The ganglion receives, on its medial side, filaments from the carotid plexus of the sympathetic. It give off minute branches to the tentorium cerebelli, and to the dura mater in the middle fossa of the cranium. From its convex border, which is directed forward and lateralward, three large nerves proceed, viz., the ophthalmic, maxillary, and mandibular. The ophthalmic and maxillary consist exclusively of sensory fibers; the mandibular is joined outside the cranium by the motor root.
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Associated with the three divisions of the trigeminal nerve are four small ganglia. The ciliary ganglion is connected with the ophthalmic nerve; the sphenopalatine ganglion with the maxillary nerve; and the otic and submaxillary ganglia with the mandibular nerve. All four receive sensory filaments from the trigeminal, and motor and sympathetic filaments from various sources; these filaments are called the roots of the ganglia.
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The Ophthalmic Nerve (n. ophthalmicus) (Figs. 776, 777), or first division of the trigeminal, is a sensory nerve. It supplies branches to the cornea, ciliary body, and iris; to the lacrimal gland and conjunctiva; to the part of the mucous membrane of the nasal cavity; and to the skin of the eyelids, eyebrow, forehead, and nose. It is the smallest of the three divisions of the trigeminal, and arises from the upper part of the semilunar ganglion as a short, flattened band, about 2.5 cm. long, which passes forward along the lateral wall of the cavernous sinus, below the oculomotor and trochlear nerves; just before entering the orbit, through the superior orbital fissure, it divides into three branches, lacrimal, frontal, and nasociliary.
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The ophthalmic nerve is joined by filaments from the cavernous plexus of the sympathetic, and communicates with the oculomotor, trochlear, and abducent nerves; it gives off a recurrent filament which passes between the layers of the tentorium.
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FIG. 777– Nerves of the orbit, and the ciliary ganglion. Side view. (See enlarged image)

The Lacrimal Nerve (n. lacrimalis) is the smallest of the three branches of the ophthalmic. It sometimes receives a filament from the trochlear nerve, but this is possibly derived from the branch which goes from the ophthalmic to the trochlear nerve. It passes forward in a separate tube of dura mater, and enters the orbit through the narrowest part of the superior orbital fissure. In the orbit it runs along the upper border of the Rectus lateralis, with the lacrimal artery, and communicates with the zygomatic branch of the maxillary nerve. It enters the lacrimal gland and gives off several filaments, which supply the gland and the conjunctiva. Finally it pierces the orbital septum, and ends in the skin of the upper eyelid, joining with filaments of the facial nerve. The lacrimal nerve is occasionally absent, and its place is then taken by the zygomaticotemporal branch of the maxillary. Sometimes the latter branch is absent, and a continuation of the lacrimal is substituted for it.
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The Frontal Nerve (n. frontalis) is the largest branch of the ophthalmic, and may be regarded, both from its size and direction, as the continuation of the nerve. It enters the orbit through the superior orbital fissure, and runs forward between the Levator palpebræ superioris and the periosteum. Midway between the apex and base of the orbit it divides into two branches, supratrochlear and supraorbital.
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The supratrochlear nerve (n. supratrochlearis), the smaller of the two, passes above the pulley of the Obliquus superior, and gives off a descending filament, to join the infratrochlear branch of the nasociliary nerve. It then escapes from the orbit between the pulley of the Obliquus superior and the supraorbital foramen, curves up on to the forehead close to the bone, ascends beneath the Corrugator and Frontalis, and dividing into branches which pierce these muscles, it supplies the skin of the lower part of the forehead close to the middle line and sends filaments to the conjunctiva and skin of the upper eyelid.
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The supraorbital nerve (n. supraorbitalis) passes through the supraorbital foramen, and gives off, in this situation, palpebral filaments to the upper eyelid. It then ascends upon the forehead, and ends in two branches, a medial and a lateral, which supply the integument of the scalp, reaching nearly as far back as the lambdoidal suture; they are at first situated beneath the Frontalis, the medial branch perforating the muscle, the lateral branch the galea aponeurotica. Both branches supply small twigs to the pericranium.
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The Nasociliary Nerve (n. nasociliaris; nasal nerve) is intermediate in size between the frontal and lacrimal, and is more deeply placed. It enters the orbit between the two heads of the Rectus lateralis, and between the superior and inferior rami of the oculomotor nerve. It passes across the optic nerve and runs obliquely beneath the Rectus superior and Obliquus superior, to the medial wall of the orbital cavity. Here it passes through the anterior ethmoidal foramen, and, entering the cavity of the cranium, traverses a shallow groove on the lateral margin of the front part of the cribriform plate of the ethmoid bone, and runs down, through a slit at the side of the crista galli, into the nasal cavity. It supplies internal nasal branches to the mucous membrane of the front part of the septum and lateral wall of the nasal cavity. Finally, it emerges, as the external nasal branch, between the lower border of the nasal bone and the lateral nasal cartilage, and, passing down beneath the Nasalis muscle, supplies the skin of the ala and apex of the nose.
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The nasociliary nerve gives off the following branches, viz.: the long root of the ciliary ganglion, the long ciliary, and the ethmoidal nerves.
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The long root of the ciliary ganglion (radix longa ganglii ciliaris) usually arises from the nasociliary between the two heads of the Rectus lateralis. It passes forward on the lateral side of the optic nerve, and enters the postero-superior angle of the ciliary ganglion; it is sometimes joined by a filament from the cavernous plexus of the sympathetic, or from the superior ramus of the trochlear nerve.
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The long ciliary nerves (nn. ciliares longi), two or three in number, are given off from the nasociliary, as it crosses the optic nerve. They accompany the short ciliary nerves from the ciliary ganglion, pierce the posterior part of the sclera, and running forward between it and the choroid, are distributed to the iris and cornea. The long ciliary nerves are supposed to contain sympathetic fibers from the superior cervical ganglion to the Dilator pupillæ muscle.
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The infratrochlear nerve (n. infratrochlearis) is given off from the nasociliary just before it enters the anterior ethmoidal foramen. It runs forward along the upper border of the Rectus medialis, and is joined, near the pulley of the Obliquus superior, by a filament from the supratrochlear nerve. It then passes to the medial angle of the eye, and supplies the skin of the eyelids and side of the nose, the conjunctiva, lacrimal sac, and caruncula lacrimalis.

Pupillary Reflex Pathway

The pupillary reflex can be thought of as an afferent limb, a parasympathetic efferent limb, and a sympathetic efferent limb. A summary is shown in this interactive link.
Afferent limb- Light stimulates photoreceptors and the signal is conveyed to a special set of ganglion cells that send their nerve impulses thru the axons in a similar topographic distribution as those carrying other signals in the optic nerve. Decussation occurs for the nasal fibers. The big difference is that the afferent fibers do not enter the lateral geniculate body but instead exit and pass thru the brachium of the superior colliculus where they synapse on the pretectal nuclei (olivary and sublentiform nuclei). These nuclei project bilaterally to the Edinger Westphal nuclei.

Efferent parasympathetic response- The Edinger Westphal nuclei send fibers to join the oculomotor CNIII and follow that course on the dorsomedial surface of the nerve. This is important because this is an outer surface which lies adjacent to the posterior communicating artery and is exposed to forces of herniation. After coursing thru the cavernous sinus these fibers emerge to enter the orbit with the inferior oblique branch of CNIII. These fibers synapse at the ciliary ganglion and then enter the eye thru short posterior ciliary nerves to distribute fibers to the choroid, iris, and ciliary ganglion.

Efferent sympathetic response- This is believed to start in the hypothalamus and project in an uncrossed fashion with synapses in the mesencephalon and pons. These neurons project to and synapse upon the intermediolateral cell column from C8-T2 in the spinal cord. These exit the spinal cord and pass thru the stellate ganglion to synapse in the superior cervical ganglion. These fibers travel with the internal carotid artery, enter the cavernous sinus and travel with CNVI in the cavernous sinus to enter the superior orbital fissue with cranial nerve V. The fibers travel with the nasociliary branch of V, and pass thru the ciliary ganglion without synapsing. The fibers pass thru the long ciliary nerves to terminate on the dilator muscle. Some fibers diverge in the superior orbital fissue to innervate Muller's muscle.

These pathways are important for the clinician to understand the basis for a blown pupil with a third nerve palsy. The pupillary fibers are compressed and this is most likely due to an aneurysm that affects the 3rd nerve at the posterior communicating artery.

Cranial Nerve IV- Trochlear Nerve

The Trochlear Nerve (IV cranial) innervates the superior oblique muscle and contains only somatic motor fibers. CN IV originates from the trochlear nucleus, which is a small, oval mass situated in the ventral part of the central gray matter of the cerebral aqueduct at the level of the upper part of the inferior colliculus (see Figure link). The cells of the trochlear nucleus are large, irregular and yellowish in color. The nuclei of the two sides are separated by the raphé through which dendrites extend from one nucleus to the other. They receive many collaterals and terminals from the posterior longitudinal bundle which lies on the ventral side of the nucleus.
The nucleus then is located at the level of the pons but posteriorly. The axons from the nucleus pass downward in the tegmentum toward the pons, but turn abruptly dorsalward before reaching it, and pass around the cerebral aqueduct into the superior medullary velum, in which they cross horizontally, to decussate with the nerve of the opposite side, and emerges from the surface of the velum, immediately behind the inferior colliculus (click for illustration from the University of Michigan). The nerve passes between the posterior cerebral and superior cerebellar arteries but more lateral than the third cranial nerve (see Figure #4). The course of this nerve in the cranium is long~about 75 mm (at this link click on CN IV to see the course).

CN IV travels in the cavernous sinus and enters the orbit through the superior orbital fissure to innervate the superior oblique muscle (click for figure).

Optic Canal, Optic Chiasm

The opening to the optic canal is the optic foramen at the apex of the orbit in the less wing of the sphenoid bone. From its location at the junction of the roof and medial wall of the orbit the optic canal is directed medially, upward moving posteriorly to the middle cranial fossa. This direction is useful to distinguish the optic canal from the superior orbital fissure on CT scan. The optic canal is on the same horizontal plane as the upper portion of the superior orbital fissure so they will often be seen in sections of the CT scan together. The optic canal houses the optic nerve, ophthalmic artery and sympathetic fibers from the cartoid plexus. A line drawn from the top of the nose to the auditory canal will approximately form the inferior border of the optic foramen. The dura of the optic nerve merges with the periosteum as the nerve enters the optic canal. The optic canal measures about 12 mm in length and about 7 mm in width but there are quite a few variations (Akdemir et al).

Upon exiting the optic canal, the optic nerve now lies medial to the internal carotid artery as seen in the picture from Gray's anatomy (click for link) and in this elegant photo of the brain (numbers 3 and 2, respectively in the figure provided in this link to the University of Manitoba). The optic nerve lies above the ophthalmic arteries but below the anterior cerebral arteries and anterior communicating arteries, which join to complete the anterior Circle of Willis (see figure). The optic nerve passes over the cavernous sinus extending medially to join the fellow optic nerve and form the optic chiasm. The optic nerve and chiasm is therefore medial and superior to the cavernous sinuses. The optic chiasm lies anterior to the pituitary gland stalk (see figure link). These anatomic relationships become important when discussing the effect of aneurysms and tumors on the visual pathway.

The general and simplistic projection of visual fields in relation to the organization of optic nerve fibers as nasal fibers (temporal fields) cross through the optic chiasm. Therefore the right visual field is projected to the left side of the brain (see link). However, there are some important details of these anatomic relationships that the ophthalmologist needs to remember:
1. extramacular nasal and inferior retinal fibers (superotemporal fields) cross in the anterior portion of the chiasm (Wilbrand's knee red fiber in Figure). Hence the relationship to pituitary tumors.
2. Nasal macular fibers cross in the posterior part of the chiasm.
3. Temporal fibers remain uncrossed.
4. Macular projections are located centrally in the nerve and chiasm and account for most of the fibers!!!

Cavernous sinus

A nice diagram of the cavernous sinus can be seen in the following link by Netter: cavernous sinus.

From Gray's Anatomy:
The Superior Ophthalmic Vein (v. ophthalmica superior) begins at the inner angle of the orbit in a vein named the nasofrontal which communicates anteriorly with the angular vein; it pursues the same course as the ophthalmic artery, and receives tributaries corresponding to the branches of that vessel. Forming a short single trunk, it passes between the two heads of the Rectus lateralis and through the medial part of the superior orbital fissure, and ends in the cavernous sinus.
The Inferior Ophthalmic Vein (v. ophthalmica inferior) begins in a venous net-work at the forepart of the floor and medial wall of the orbit; it receives some veins from the Rectus inferior, Obliquus inferior, lacrimal sac and eyelids, runs backward in the lower part of the orbit and divides into two branches. One of these passes through the inferior orbital fissure and joins the pterygoid venous plexus, while the other enters the cranium through the superior orbital fissure and ends in the cavernous sinus, either by a separate opening, or more frequently in common with the superior ophthalmic vein.

FIG. 572– Veins of orbit. (Poirier and Charpy.) (See enlarged image)
The intercavernous sinuses (sini intercavernosi) (Fig. 570) are two in number, an anterior and a posterior, and connect the two cavernous sinuses across the middle line. The anterior passes in front of the hypophysis cerebri, the posterior behind it, and they form with the cavernous sinuses a venous circle (circular sinus) around the hypophysis. The anterior one is usually the larger of the two, and one or other is occasionally absent.
The superior petrosal sinus (sinus petrosus superior) (Fig. 570) small and narrow, connects the cavernous with the transverse sinus. It runs lateralward and backward, from the posterior end of the cavernous sinus, over the trigeminal nerve, and lies in the attached margin of the tentorium cerebelli and in the superior petrosal sulcus of the temporal bone; it joins the transverse sinus where the latter curves downward on the inner surface of the mastoid part of the temporal. It receives some cerebellar and inferior cerebral veins, and veins from the tympanic cavity.
The inferior petrosal sinus (sinus petrosus inferior) (Fig. 570) is situated in the inferior petrosal sulcus formed by the junction of the petrous part of the temporal with the basilar part of the occipital. It begins in the postero-inferior part of the cavernous sinus, and, passing through the anterior part of the jugular foramen, ends in the superior bulb of the internal jugular vein. The inferior petrosal sinus