FNC - Neuroanatomy #7 PDF

Pag. 1 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain FNC - Neuroanatomy #7 The eye and the brain Prof. Dellavia – 26/10/21 – Author: Neil Connolly – Reviser: Davide...

Pag. 1 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain FNC - Neuroanatomy #7 The eye and the brain Prof. Dellavia – 26/10/21 – Author: Neil Connolly – Reviser: Davide Pizzi 1. General structure of the of the retina The retina has a complex organisation made up of both neurons proper and supporting cells. It has a similar organisation to the CNS, because the retina can be considered an extension of the diencephalon. The first layer in the image below, the retinal pigmented epithelium (RPE), lies between the neural retina and the capillary lamina of choroid. It is a single layer of flat cells with microvilli. The outer segments of the photoreceptors are continuously substituted, and it is the role of the pigmented epithelium, using their microvilli, to eliminate the outer segments when they need to be replaced. The plexiform layers are designed to create synapses between the different cells of the retina. It is a complex network whose role it is to spread signals in both vertical and lateral directions and to ensure that a 3d network of communication exists inside the retina.  Pag. 2 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain 2. The layers of the retina Layer 1 is the pigmented epithelium as mentioned previously. Layer 2 is made up of the outer segment of the photoreceptors. (Photoreceptors are specialised cells with different portions that are inside different layers of the retina.) Layer 3 contains the inner segment of the photoreceptors. Layer 4 contains the nuclei of the photoreceptors. Photoreceptors have fibres that synapse into layer 5 (the outer plexiform layer) This outer plexiform layer creates a network of synapses between the photoreceptors and the deeper bipolar cells, whose nuclei lie in layer 6. Bipolar photoreceptor fibres travel in the inner plexiform layer, layer 7, to synapse with the ganglion cells whose nuclei lie in layer 8. The axons of the ganglion cells leave the retina as the optic nerve in layer 9 via the optic foramina and reach the middle cranial fossa At layer 10 there is the inner limiting membrane. This encloses the fibres that form the optic nerve, So there are three main groups of cells in the retina. Photoreceptors to capture the light, Bipolar cells and also ganglion cells which form the optic nerve. But there is also the plexiform layer and other supporting cells which can spread the signal either laterally or vertically. The left side shows the organisation of the retina, the right side shows the histology  Pag. 3 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain There are also limiting membranes which maintain the framework of the structure of the retina. The visual portion of the retina is attached to the choroid (vascular intermediate layer). In between the choroid and the pigmented epithelium we have the Bruch membrane, which is made of collagen and elastin fibres. It keeps the choroid attached to the pigmented epithelium and it also mediates the exchange of substances. The choroid supplies nutrients to the retina. The Bruch membrane has passages to allow capillaries pass from choroid to the photoreceptors and retina. 3. Support Cells Muller cells are attached to both the outer limiting membrane and to the inner limiting membrane. They are glial cells, not neurons, and they create the framework of the retina. They have protrusions which act as a scaffold where the neurons can be supported. As shown in the image below, light passes first through the inner limiting membrane, penetrates the layers of the retina and then lastly it meets the photoreceptors and activates them. In the opposite direction of the light there is the transmission of the signal/excitation, which goes from the photoreceptors to the ganglion cells. In between the layers of the 3 main groups of cells, we have other cells important for divergence or convergence of the signals. Horizontal cells are found in between photoreceptors and bipolar cells. They can connect multiple photoreceptors, even ones which are far apart, and integrate their signals, before passing the signal to a bipolar cell. Each bi-polar cell can also synapse with multiple photoreceptors, But only ones which are close together. Horizontal cells are needed to integrate signals from photoreceptors that are far apart. So bipolar cells can be used to diverge and converge signals but if there is a need to spread the signal more laterally horizontal cells are used.  Pag. 4 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain Amacrine cells are found in between bipolar and ganglion cells. They can receive signals from different bipolar cells and connect them with different ganglion cells. So like the horizontal cells, they ensure that there is not only a flow of information vertically, but also laterally which creates collaterals, and prevents there being only one to one connection (this is common across the CNS). 4. Different types of bipolar and ganglion cells Some bipolar and ganglion cells are excited by glutamate (On cells) and some are inhibited by glutamate (Off cells). Bipolar and ganglion cells can also be classified by size as Parvi (small), Magni (medium), Wide (large). They will be segregated in order to enter into different areas of the thalamus, and they have different properties, for example their conduction velocity and response to colour.  Pag. 5 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain 5. Photoreceptors The external portion of the photoreceptor is the outer segment, which absorbs the light. It is continuously changed (coloured orange in the image below). The inner segment last for life. Between the inner segment and the nucleus/cell body, there is the outer limiting membrane. There are two different typologies of photoreceptor, with different morphologies The two shapes are due to the outer segment being formed by different molecules which have different sensitivities to light. Rods contain rhodopsin, which only needs low intensity of light to be activated, and so works better in the dark. They have a low threshold of activation. Cones need much more light to be activated. Their outer segments contain photopsin. There are 3 types photopsin; S, M and L, which absorb light at different wavelengths. Depending on the spectrum of the light, different photopsin, are activated. Because cones have a higher threshold of activation, they can also recognize more details than rods. The organisation of the rods and cones in the retina is not uniform. Moving from the Ora Seratta to the posterior pole of the retina there is a progressive reduction of rods and increase in number of cones.  Pag. 6 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain 6. Fundus of the eye In the posterior area (fundus of the eye), there is an area which contains mainly cones. This is the fovea, which is concave, and the most concave, central part of it is the foveola. The foveola contains only cones. The cells in the foveola are orientated in an oblique way and so the transmission of the signal is angulated. In the foveola there is only photoreceptors, nothing else. In the fovea, there are the synapses between the photoreceptors contained in the foveola, and the bipolar cells. In the foveola because there is only photoreceptors. this is where there is maximum power of resolution, and this is where the light is focused to create the majority of the detail in an image.  Pag. 7 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain It is possible to find the location of the foveola because it is found in the part of the fundus of the eye which is yellow (macula lutea). This area is a few millimetres in the temporal direction, from the central posterior point of the eye.  Pag. 8 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain 7. Optic Disk On the other side of the central posterior point, in the nasal direction, there is the optic disk. This is also called the blind sport. In this thick area there is the convergence of axons of the ganglion cells in order to create the optic nerve. This is also where the vessels converge. There are no photoreceptors here. There are a bundle of nerves and vessels, so there is no vision. The margins are very clearly delimited. This makes it easy to see alterations which could be caused by vascular damage. So looking at the fundus of the eye can reveal a lot of information. Also, if there is a loss of transparency of the retina, it can highlight that there is a detachment of the retina from the choroid. 8. Blood supply The central retinal artery is inside the bundle of the optic nerve. It can also give collateral arteries to the bundle. The nerve and artery are completely covered by the meninges, which gives them protection, and this again highlights that this can be considered a portion of the brain/CNS. The central retinal artery comes from the ophthalmic artery. When it reaches the optic disk, it anatomises at the level of the choroid with the posterior ciliary artery. The ophthalmic artery is also covered by the meninges. They both travel inside the optic canal to enter the orbit.  Pag. 9 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain 9. Course of light rays: The cornea collects light rays and converges the rays using its convex shape → they then pass through the anterior and posterior chamber → they reach the lens which has double convexity and whose curvature is regulated by ciliary muscle → the light rays are then focused onto the fovea. 10. Image formation The eye field and the retina can be divided into four quadrants: upper, lower, medial (nasal) and lateral (temporal). Information that entered at the upper portion of our visual field, activates photoreceptors in the lower portion of the retina. Information entering the medial portion of our visual field will activate lateral photoreceptors. The lateral visual information reaching the eye will end up at the medial portion of the retina.  Pag. 10 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain 11. Visual Field It is important to know which area of the visual field is collected only by the left or the right eye, and which is the global field collected by both eyes. The right and left eye can see 60 degrees nasally. But nasally the two are also partially superimposed (purple area below). This superimposition gives 3D depth to vision. Laterally view up to angle of 100 degrees. The eye can view vertically up to 60 degrees above, and 75 degrees below. The right eye covers the area missed by the blind spot on the left and vice versa.  Pag. 11 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain 12. Optic nerve The below image shows the dimension of the optic nerve compared to the oculomotor nerve and the ciliary ganglion. They are less than one tenth the size of the optic nerve. The ciliary ganglion is a parasympathetic ganglion which collects preganglionic fibres from the oculomotor nerve, and delivers its post ganglionic fibres to the ciliary body. The ophthalmic branch of the trigeminal nerve is also shown. The nerves pass through the optic foramen into the middle cranial fossa. In the middle cranial fossa, the optic nerve passes over the hypophysis and in this area, there is the formation of the optic chiasm.  Pag. 12 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain 13. The optic pathways The chiasm is the area where there is the partial decussation of the optic pathways. It is the medial part of the optic nerve which decussates. In terms of the image below. For the right eye: The red line is what the right eye collects from the left part of its visual field. and it doesn’t decussate. The blue line is what the right eye collects from the right part of its visual field, and it does decussate. The fibres then enter the thalamus. The optic radiations then go from the thalamus to the primary visual cortex of the occipital lobe. So what the right eye collected from the right reaches the left side of the primary visual cortex, and what the right eye collected from the left, reaches the right side of the primary visual cortex. The right side of the primary visual cortex receives information that the right eye collected from the left of its visual field, and what the left eye collected from the left of its visual field. More simply: What the left eye collects from the left + what the right eye collects from the left → right side of the primary visual cortex When the fibres arrive at the thalamus, they segregate. Each bundle from each group of specific ganglion cells (PMW) enters into different layers of the thalamus. Also when the fibres reach the thalamus, they rotate 90 degrees. This is because the layers of the thalamus are not organized vertically, but are organised medial  Pag. 13 a 13 International Medical School – FNC - Neuroanatomy #3 – prof. Dellavia – The eye and the brain to lateral. The fibres then go back to original orientation when they go towards cortex. The optic nerve is only the portion of the fibres which goes from optic disk to the optic chiasm. After they pass the chiasm, it is called the optic tract. So from the optic chiasm to the thalamus it is called the optic tract. Then from the thalamus to cortex they are called optic radiations.