Optic Pathways PDF

Pag. 1 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways NEUROAN #8 Pathways from the retina: the optic pathways Prof. Dellavia – 26/10/21 – Authors and revisers: Sohayla...

Pag. 1 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways NEUROAN #8 Pathways from the retina: the optic pathways Prof. Dellavia – 26/10/21 – Authors and revisers: Sohayla Zayan & Chiara Zilli 1. Optic pathways The optic nerve exiting from the optic disc is completely enveloped by the meningeal coverings which arise at the level of the aye. Note the dimension respect to the other nerves inside the orbit: the abducens and the trochlea (for oculomotion) and the ophthalmic branch of the trigeminal. These all have a similar dimension which is less than one tenth of the optic nerve. So, a large amount of information comes from the retina. The ciliary ganglion which is a parasympathetic ganglion can be seen too. This collects the preganglionic fibers from the oculomotor and then it has the post ganglionic fibers that are delivered through afferent bundles to either the ciliary body or the pupillary area. The optic nerve once it passes through a tendinous ring in which are the attachment of the muscles that then have to attach on the sclera, reaches the medial cranial fossa. Once it enters, it passes over the hypophysis and so the hypothalamo-hyphophysial peduncle. In this area there is the formation of the optic chiasma. The hypothalamic area can be divided in an infundibulum pre-chiasmatic area which is anterior and the tuberomammillary area which is posterior to the chiasma. In this area there is a partial decussation of the optic pathways. It is the medial part of the optic nerve which crosses. Considering the right eye in the image below, the fibers in the lateral half of the retina that collects the visual field from the left remain in the same side, so remain ipsilateral. While the fibers that collect visual field from the right-side pass oppositely, so move to the left. The fibers that do not decussate enter in the thalamus, at the level of the lateral geniculate body (meta thalamus) and then radiate, forming the optic radiation, to reach the primary visual cortex in the occipital lobe. At the primary visual cortex of the right hemisphere there is the arrival of what the right eye had collected from the left (that remained on the same side), plus what this optic pathway has received from the decussation and so what the left eye has collected from the left. For the left eye it is vice versa. So, in a hemisphere there is the opposite information of both eyes.  Pag. 2 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways The optic pathway Optic chiasm When fibers arrive at the level of the thalamus, they segregate before entering the geniculate body meaning that each bundle from each group of ganglion cells enter in one single layer rotating 90° because the thalamus is made by several layers that are not in vertical organization but in a medial one: the lateral geniculate body is stratified mediolaterally. So, fibers that are coming from the optic tract in cranial position are moving in a medial situation and the one that was inferior goes laterally and when they go to the retina they will come back to the initial organization. So, at the level of the cortex, they are upside down. The integration of these signals gives the possibility to create depth perception.  Pag. 3 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways As it is reported in the lower image, some of the fibers coming with the optic tract are not entering and synapsing inside the thalamus; the W is not entering the thalamus. Fibers can proceed the way and arrive in the pretectal area and the superior colliculus so in the tectum. This is the alternative way in respect to entering into the thalamus. - Axons from M ganglion cells form the magnocellular system and enter the thalamus medially, into the layers I and II (most medial portion). Layer I for contralateral system, layer II for ipsilateral system. These fibers arrive in the layers of the visual cortex 4-c-alpha, 4-beta and layer 6. - Axons from P ganglion cells form the parvocellular system, entering the thalamus in layers III, IV, V, VI. Layers III/V= ipsilateral, layers IV/VI contralateral. The lateral geniculate body Fibers then enter in the primary visual cortex, in the border of the calcarine fissure. Then it is in the secondary area that this information about the vision is connected with others coming from associative areas in order to create the recognition of what it is observed.  Pag. 4 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways The pathways are different depending on the system that originated at the level of the retina. - In the case of the magnocellular, form the primary visual cortex which is in the Broadman area number 17 fibers move to involve the secondarily area number 18 and number 19 and then as it is reported in the diagram below part of the fibers will then create a pathway (a stria) which will go as it is reported to the parietal 7a or into the ventral intraparietal and superior portion medially into the temporal lobe (5a). This magnocellular system is the one that provides information (as it is reported in the legend) about the shape of the object, not the color but its motion and the stereopsis. This means that a pathway is then created to recognize our location in relationships to the other structure. The information about the color is segregated in another pathway: the parvocellular system. - Fibers can be integrated into other pathways that originate in the small ganglion cells that carry information about the color. In order to perceive and recognize details about the colors like the shadows, fibers enter in columns of cells that are called blobs inside the primary visual cortex. In the portion of the cortex that is in between the blobs the information about the color is matched with the information about its shape and stereopsis. These fibers arrive in the inferotemporal area where very precise information of the object seen can be collected. 2. The visual cortex So, entering the thalamus data are separated but also integrated, then fibers enter different areas of the cortex. The primary cortex (are 17) is called striate since it is divided into two portions by the fibers. The primary cortex is along the calcarine fissure. In the superior border is the superior retina input, which means the upper part of the visual field, while in the inferior border is the inferior retina input. Remember that in the retina the image is upside down, so the upper portion of the visual field is impressed in the lower portion of the visual cortex and vice versa.  Pag. 5 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways The visual cortex Into the primary visual cortex there is a visuotopism. The largest portion is the one dedicated to the fovea foveola, in which the information comes from the center of the concavity of the fovea centralis. The other areas that are secondarily involved surround the primary area. The one that is immediately eccentric with respect to area 17 is area 18, that is called parastriate. Peripherally is the peristriate which is area 19. From the primary visual cortex fibers can move to associative areas (the cephalon-oculomotor center in the frontal lobe) to coordinate the oculomotion, the eye movements (extrinsic muscles), for example convergence, divergence, synchronization and so on. The fibers of the magnocellular system arrive in the first and second layer of the geniculate body. One layer is dedicated to one side and the other to the other side: fibers that enter layer one are contralateral while in layer two arrive ipsilateral fibers. The parvocellular system begins oppositely, meaning that the first layer, number 3, receives information coming ipsilaterally and alternate in the next layers, so it’s contralateral then ipsilateral and so on. In the image below, the stratification presents six layers but some of them are further subdivided. In some of these layers the formation where the elaboration of colors occurs can be found. The spread of the blobs (the columns of neurons) do not occupy entirely the layers but they alternate with interblobs that are portions that  Pag. 6 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways interpose between them. It can be observed also that the information coming ipsilaterally that derive in sort of stripes of the cortex along all the 6 layers are not really rectilinear but have an undulated organization. Blobs are found only in layer one and two, so synapses in these two are necessary to give the interpretation of the color. Only the parvocellular system enters into the blobs. Fibers from the magnocellular system enter into the portion of the cortex contralaterally and go through the layers to synapse in the 4 c-alpha which is an oblige station, from here fibers can project to the layer 4b then move away with the dorsal streams that go to the ventral intraparietal part of the superior portion of the temporal lobe. This pathway that from the visual cortex goes to the parietal is called the dorsal stream. Inside this system an important attribution that is peculiar to the system is found, which is the motion. While regarding the parvocellular system, layer 3 of the lateral geniculate body projects into the optic radiation in order to arrive at the fourth layer again, not to c-alpha but to c-beta. This projects to the third layer and so it can enter into the blobs and process the color information. This schematization is very similar also for the ipsilateral layer number 5 of the thalamus or for the contralateral system in layer number 4 and number 6. In the case of the parvocellular, fibers project to the temporal inferior portion and so it not formed a system that goes superiorly but that goes inferiorly: form the occipital lobe into the temporal. It’s a ventral stream, that in particular uses either the color or the detailed contour of the shape to interpret what we are observing exactly and not where we are topographically. So, two interpretations are given: “what is what we are observing and where is what we are observing”. These two systems that pass at the level of the thalamus need to go to the cortex using the optic radiation. The optic radiation then passes into the internal capsule, which has different limbs. The retro lentiform limb is the tangential passage to the lentiform nucleus of the basal ganglia of the corpus striatum. The radiation then moves to the occipital lobe at the level of the calcarine fissure. In the table below are illustrated all the bipolar ganglion cells both of the on and off system. - 100% of the parvocellular arrives at the level of the lateral geniculate body, enters layer 3 to 6 and goes to the cortex in order also to pass through the blobs. All the fibers need to enter the thalamus and from there to go to the cortex, and from there they create the ventral stream, going to associative areas into the inferior portion of both the temporal lobe. - The majority of the fibers (80%) of the magnocellular pathway coming from the bipolar ganglion cell on and off enters into the lateral geniculate body and from there go to the layers of the visual cortex that  Pag. 7 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways do not have the blobs. So the information built up regards the shape and the stereopsis to exactly locate the object. From there it’s created the dorsal stream. - The residual 20% of the fibers that escape the thalamus go to the superior colliculus of the lamina quadrigeminal, of the tectum. The superior colliculus, in fact, is involved in the visual system differently from the inferior one which is involved in the acoustic. The superior colliculus is important to create somatic reflexes, but also descending tracts (tecto spinal), important to adjust the posture in respect to what we are looking at. From the superior colliculus fibers can also go to the cerebellum, devoted to the coordination of the posture and movement. - All the fibers of the W which are in fact less represented in respect to the P and M, completely escape the thalamus and go to the pretectal areas. Even if they are not the majority they are important because these information are important for other types of reflexes, the visceral (smooth muscles). In this case the parasympathetic system is involved, differently from the somatic reflexes in which the skeletal muscles are controlled. These two different reflexes can also be combined. The optic pathway 3. ​The pupillary reflex The pupillary reflex controls the diameter of the pupil: this diameter depends on the contraction of the constrictor of the pupil which has its fibers that are circular along the hole inside the iris. In order to activate this reflex, the parasympathetic system must be activated since this part of the vegetative system controls the smooth muscle of the eye. Fibers of the W system that come from the medial side of the eye decussate at a certain point and after the chiasm they can go on the opposite tract; on the other hand, if the fibers come from the lateral aspect of the eye, they remain ipsilateral. They do not enter the lateral geniculate body and proceed their way in order to go more posteriorly and medially at the level of the midbrain in the pretectal area. Here there are nuclei and some fibers from these nuclei remain on the same side and then activate the Edinger-Westphal nucleus, the visceral motor nucleus of the oculomotor nerve. The oculomotor nerve has 2 motor nuclei: one for the somatic and one for the parasympathetic system. The Edinger-Westphal nucleus is in the parasympathetic system and it is close to the midline: it receives fibers from the same side or, using the posterior commissure which is located in between the two aspects of the lamina quadrigemina (posteriorly to the midbrain) it crosses and reaches the contralateral  Pag. 8 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways Edinger-Westphal nucleus. The aim is to simultaneously reduce the diameter of the pupil from right and left, so the fibers are divided in order to activate the same or the contralateral side. From the Edinger-Westphal nucleus, the oculomotor nerve with preganglionic fibers passes into the middle cranial fossa, reaches the superior orbital fissure and enters into the orbit. In the orbit these preganglionic fibers need to synapse with a new neuron which is the postganglionic and this synapse occurs into the ciliary ganglion which is parasympathetic. From the ciliary ganglion, the trigeminal nerve collects the postganglionic fibers and delivers them into the eye. The ophthalmic division (V1) of the trigeminal nerve arrives into the eye because the ophthalmic division has fibers for the nonspecific sensory innervation therefore for touch, thermal, and nociception of the eye. This somatic information that is general exteroceptive is innervated by the trigeminal nerve with this division. The nerve passes from the posterior portion of the orbit in order to reach the cornea: this portion is highly innervated and the trigeminal nerve delivers the fibers for the smooth muscle inside. This nerve, by going to the cornea, can deliver fibers to the ciliary body and the iris and in this case the postganglionic fibers from the ciliary ganglion have to reach the iris (and so the sphincter of the pupil). This is the circuit that has to be recreated in order to reduce the diameter of the pupil: this happens when the luminosity is too high for example since this is a protective system that acts in order to ensure that the luminosity does not damage the photoreceptors harming the circuits of the retina. This pupillary reflex is visceral, so there is the need to go to the pretectal area, involve W cells and arrive (in order) into the Edinger-Westphal nucleus, oculomotor nerve, ganglion of the parasympathetic (ciliary) and then trigeminal nerve 1st division that delivers the fibers (nasociliary nerve) to the orbit. Similar pathway (not identical) is for the reflex of accommodation. Schematic representation of the pupillary reflex  Pag. 9 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways Schematic representation of the pupillary reflex 4. The accommodation reflex This reflex is needed while looking at objects that are really close to the eyes. The ciliary muscle is used in order to relax the zonular fibers of the ciliary body. This reflex involves the fibers of the parasympathetic system. Here, like in the pupillary reflex, the Edinger-Westphal nucleus is involved into the output to the ciliary ganglion and the postganglionic fibers through the trigeminal nerve pass through the eye and deliver the information into the ciliary body. Together with the changes in the convexity of the lenses, there is the need of the pupillary reflex in order to reduce the quantity of light that enters through the pupil: if the object is near the eye, there is not the need to take all the information of the light inside the environment and the miosis phenomenon (=contraction of the pupil) takes place. Inside the accommodation reflex there is an additional reflex integrated which is the convergence. While looking at an object both eyes are used and in order to have a closer view the axis of the eyes must converge: there is the need to contract the extrinsic, medially located skeletal muscles for oculomotion that move the ocular lobe (medial rectus which is somatic, and that’s why there is the need to activate also the other component of the system that goes to the superior colliculus and so the M portion of the system that does not enter into the thalamus). To sum up, from the optic nerve there is a decussation and M and W cells carry different information. These enter into layer 1 and 2 of the lateral geniculate body but if only the reflex is considered fibers enter at the level of the cortex. Also from the cortex it is possible to create a modulation of the reflex: for example, from the primary visual cortex (area 17) it is possible to reach areas 18, 19 (secondary visual cortex) and from there modulate by going to the pretectal area. Just following the information that arrives to the tectum (superior colliculus), it is possible to reach the midbrain and here enter with M fibers into the nucleus of the oculomotor nerve (somatic motor nucleus) and from here give rise to direct motor fibers to the medial rectus. The presence of the Perlia’s nucleus between the Edinger-Westphal ones allows the simultaneous contraction of these two visceral and somatic nuclei, both controlled in order to have bilateral simultaneous activation.  Pag. 10 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways From the Edinger-Westphal therefore it is possible to follow the oculomotor nerve, arrive at the ciliary ganglion and either go to the sphincter of the pupil or to the ciliary body carrying the fibers using the trigeminal nerve. This is the combination of possible different reflexes: somatic and visceral. The reflexes can of course be used singularly. Schematic representations of the accommodation reflex 5. The blink reflex It is the contraction of the components into the palpebrae, and this system is activated in order to protect the eye. This reflex is activated if the cornea is touched (tactile receptors) and works together with lacrimation. Several fibers of the trigeminal nerve must be involved. Into the cornea there are tactile receptors that convey information at the level of the brainstem into the main nucleus of the trigeminal nerve but there may be a reflex that activates lacrimation (parasympathetic system,  Pag. 11 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways stimulated by the facial nerve, CN VII). The facial nerve has a nucleus for the preganglionic fibers which is inside the pons, and this is the so called muco-naso-lacrimal nucleus of the facial nerve: this nucleus contains the motor neurons preganglionic for the glands of the mucosa of the nose inside the nasal mucosa and the lacrimal gland which is inside the orbit. Inside the orbit there is a proper lacrimal apparatus composed of a main lacrimal gland and accessory glands inside the palpebrae. These glands in the palpebrae need to be stimulated by the muscle which is into the superior palpebrae (levator palpebrae) and then there is the orbicular muscle of the eye. These muscles are under control of the facial nerve but are skeletal. So, there is a reflex that has some visceral parasympathetic output and some fibers that go to the skeletal muscle in order to control the palpebrae. If there is the involvement of the parasympathetic system, one nucleus is in the pons (muco-naso-lacrimal) and the preganglionic fibers reach the pterygopalatine ganglion (in pterygopalatine fossa). The postganglionic fibers reach the orbit, and it is the trigeminal nerve that carries the fibers. The trigeminal nerve and its division carry the fibers of other nerves that the trigeminal nerve has connected at the level of the ganglia. For the lacrimal gland there is an anastomosis between the facial and the trigeminal nerve. The reflex can be stimulated by stimulating the cornea but also the conjunctiva: this is a consensual reflex. Newborns don’t have this reflex. The nerve pathway needs the ophthalmic division that collects the information also from the conjunctiva together with the cornea, arrives at the pons and comes back from the temporal zygomatic branches of the facial nerve. The post ganglionic fibers can also be delivered to the trigeminal nerve for the lacrimation. The blink reflex 6. Ocular annexes Under the cutis there is the subcutis and the orbicularis that is circularly organized. Then there is a septum of fibrous tissue (orbital septum): if this membrane is removed there is the skeleton of the palpebrae which is called the tarsus, one for the superior and one for the inferior palpebrae. This system is connected to the rest of the orbital septum. In the superior tarsus there is the arrival of the fibers of the levator superior palpebrae and the tendon of this muscle can be seen in the picture below. At the medial and lateral aspects the two tarsi are connected and there is a ligament that attaches the two palpebrae medially and laterally to the lateral wall of the orbit. The medial one closes a sort of sac, a large vesicula that contains the tears, the lacrimal sac.  Pag. 12 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways Ocular annexes 7. Conjunctiva By cutting in a sagittal plane (image on the left), the prominence of the cornea can be seen. There is a virtual space between the cornea and the inner surface of the palpebrae. The inner surface of the palpebrae is layered by the conjunctiva and not by the cutis. From the free margin of the palpebrae, the conjunctiva externally continues in the cutis and internally arrives until a certain point creating a fornix, a change of direction creating a fissure coming superiorly, attaches to the sclera and then continues into the cornea. Part of the conjunctiva protects the eye. In the fissure between the cornea and the conjunctiva there is a film of tears which is produced in part from the main lacrimal gland and in part from the accessory glands that are inside the thickness of the palpebrae. This portion of lacrimal film is very aqueous, while the portion of the proper lacrimal gland is denser. The lacrimal gland is situated supero-postero-laterally in the eye. The tears enter into different ducts between the conjunctiva and the cornea, and they are collected together with the tears produced by the accessory gland. Two lacrimal canaliculi deliver the tears to the lacrimal sac positioned into the medial aspect of the eye. By using a canal between the lacrimal bone, the ethmoid and the inferior conche it is possible to deliver the tears in the inferior portion of the nasal cavity, the so-called inferior meatus of the nasal cavity. Therefore, from the orbit tears pass between the medial wall of the orbit and the lateral wall of the nasal cavity and the canaliculi open in proximity of the floor of the nose, below the inferior nasal concha and here there is the possibility to enter into the flux of the airways.  Pag. 13 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways Conjunctiva, lacrimal glands and lacrimal sac 8. Correlation system All the reflexes are automatic, but from the tectum there is the possibility either to go into the cerebellum or enter into descending tracts. In the dorsal view of the midbrain below, starting at the level of the tectum, there are the superior colliculi that can receive fibers both from the retina and from the cortex. To modulate what happens at this level, fibers coming from the secondary visual cortex could be used. From there fibers either enter into the pretectal area or into the superior colliculus and then descend. From the superior colliculus it is possible to connect through the posterior commissure the two colliculi but also from the superior one it is possible to create the tectospinal system which decussates, as can be seen from the image below. The two colliculi talk together and descend contralaterally to the spinal cord ending in the motor neurons for the regulation of the posture (upper portion of the trunk). Part of the superior colliculus can enter the cerebellum: in this case it is possible to stop at the level of the basilar nuclei of the pons and use the middle cerebral peduncle to enter or go directly into the superior cerebellar peduncle. Both peduncles decussate: the right cerebral cortex enters into the hemisphere of the cerebellum on the left. Between the two superior colliculi, there is the passage of the medial longitudinal fasciculus recognizable into the brainstem and this continues into the spinal cord and here different typologies of information pass. This tract allows the connection between many nuclei of the brainstem and from there it is possible to connect this area to the vestibular nuclei together with the cochlear nuclei or also to connect to the facial and trigeminal nerves. This system can therefore be used also to create the circuit to reach the nuclei of the other cranial nerves, in particular the trigeminal and facial.  Pag. 14 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways Correlation system The parasympathetic fibers generate the reflexes for the miosis or the accommodation in terms of changes into the convexity of the lenses. Into the iris the myoepithelium is activated thanks to its radial fibers of smooth muscle that are inside the epithelium of the iris. This structure is normally relaxed and contracts only in extreme situations: fibers are carried at the level of dilator of the pupil (sympathetic fibers) and these are already postganglionic since the sympathetic generate at the level of the superior cervical ganglion. From there, the fibers follow the course of the internal carotid artery, which passes inside the pyramid of the temporal bone and exits at the level of the apex and then into the middle cranial fossa. The fibers of the sympathetic system create a plexus all around the internal carotid artery and so they can reach the middle cranial fossa and then the sympathetic follows the oculomotor and enters into superior orbital fissure and into the orbit. To go inside the eye, the fibers go parallelly the parasympathetic ones and they will follow the trigeminal nerve until reaching the iris. From the spinal cord, at the level of the trunk of the sympathetic system at level of cranial base (superior cervical ganglion), fibers follow the internal carotid artery, the oculomotor nerve and then first division of the trigeminal nerve. In the picture below, the blue lines are the trigeminal nerve, the black is the sympathetic and the short ciliary nerve that are the parasympathetic of the oculomotor that meet to join the trigeminal.  Pag. 15 a 15 International Medical School – NEUROAN #8 – prof. Dellavia –Pathways from the retina: the optic pathways Ciliary ganglion: short, long and sympathetic roots