The Optic Nerve And Optic Neuritis
The optic nerve is actually more than one million individual nerves bundled together. Each nerve plays a significant role in sending information from the retina to the brain. Inflammation causes affected nerves to swell, which means they cannot work properly. The degree of visual impairment experienced with optic neuritis depends on how many nerve fibres are inflamed the more nerve fibres affected, the worse the symptoms. Generally, eyesight deteriorates over a few days rather than suddenly. The peak of vision loss usually happens about a week after the symptoms first appear.
How To Deal With Optic Nerve Damage
If you or someone you know is living with optic nerve damage, there are countless ways you can opt to make your life easier. Using corrective lens, adopting a healthy life style and incorporating proper exercise and healthy food will help immensely. Low vision aids like IrisVision have proven to remove a lot of roadblocks and allowed patients to do things they once thought they couldnt.
For inspiration, here is a story of Lannie showing how he is living with optic nerve damage.
Basic Building Blocksgrowth Cone Attraction Versus Repulsion
Among the earliest axon guidance molecules identified were extracellular matrix molecules such as laminin and fibronectin that promote axon growth. Analysis of the protein domain structure of these and other subsequently identified families of guidance molecules showed that guidance molecules in general all contain a number of common domain motifs such as immunoglobin-like repeats, EGF repeats, and fibronectin type III domains. Each family of guidance molecules is, however, also defined by its own distinctive domain . In addition to guidance molecules that promote axon outgrowth, an important contribution to our understanding of axon pathfinding was the discovery that a substantial number of guidance proteins control axons by inhibiting their ability to extend. Given that the nervous system is able to both encourage and inhibit axon growth, it would seem that one simple strategy for axon guidance is to use arrays of growth promoting and inhibitory guidance cues to steer developing axons along specific pathways to their targets. Indeed, as illustrated in the following examples, there are instances of RGC axon guidance that seem to reflect this strategy.
Model Systems For Comparison With Mammalian Visual System Regeneration
Fundamental insights about mechanisms limiting or promoting RGC regeneration and reconnecting to the brain may be inspired by examining similarities and differences with other model systems, mammalian , lower vertebrates and worms.
Spinal cord injury
There is a long and rich history of research in the field of spinal cord regeneration, a problem that shares many common features with RGC regeneration, as it is a CNS pathway and a relatively common site of traumatic injury . Intracellular signaling pathways that promote spinal cord regeneration often overlap with those in RGCs and KLF . Thus, large-scale screens for factors that promote spinal cord regeneration are likely also to be informative for RGC regeneration. Similarly, exogenous factors that inhibit spinal cord regeneration such as Nogo receptor ligands , may play a similar role inhibiting RGC regeneration but have only had limited investigation in the visual pathways.
There are also important lessons to be learned from studies of PNS regeneration. In an excellent use of gene profiling, bioinformatics, and systems biology, alterations in genes during CNS regeneration versus PNS regeneration were analyzed . From the information gleaned, a drug was identified that enhanced optic nerve regeneration.
What Connects The Eye To The Brain
The Optic Nerve.
The human optic nerve transports the data arriving into the retina in the eye to the visual cortex.
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How The Eye And The Brain Work Together
Nearsightedness , and Farsightedness
Near and farsightedness are the result of varying- shaped eyeballs that cause light to focus in front of or behind the retina.
Light is focused from near and far objects exactly on the retina.
|Light from near objects focuses behind the retina.||Light from far objects focuses in front of the retina.|
Compilation by Blind Babies Foundation, 1998
Aqueous a clear watery fluid that fills the space between the cornea and the vitreous. It is responsible for nourishing the cornea, iris, lens, and maintaining the intraocular pressure.
BLIND BABIES FOUNDATION
How Do We See
The wall of an eyeball has three layers, rather like the layers of an onion:
The space in the center of the eyeball is filled with a clear jelly-like material called the vitreous humor. This material allows light to pass through to the retina. It also helps the eye keep its round shape.
Vision is the process by which images captured by the eye are interpreted by the brain, and the visible part of the eye is where the process of sight begins. On the front surface of the eye is the see-through, circle-shaped cornea. You can’t see a person’s cornea the way you can see the colored part of the eye behind it the cornea is like a clear window that focuses light into the eye.
The Color System Of The Eye
Cone cells contain a pigment through which light must pass beforereaching the receptor. There are three pigments: One passes violet,with a wavelength of 430 nm; one passes blue-green, with a wavelengthof 530 nm; and the last pigment passes yellowish-green, with awavelength of 560 nm. In fact, these optical filters have filterskirts, meaning they pass light of other wavelengths, but with reducedsensitivity. Any monochromatic light actuallyactivates cone cells of multiple pigments, but at differentsensitivities. This also explains why we can see light with wavelengthsshorter than 430 nm, and longer than 560 nm.
No conecells, however, can truly perceive red. The closest we really get isyellowish-green. What we call red is really an opticalillusion, supplied by the brain by means of extrapolation. Oursensitivity to red is dramatically reduced compared to other colors,and our visual acuity in the red end of the spectrum is extremely bad.Everyone knows not to focus a projector using a redtest pattern. This is why the red gun in color-video equipment needsthe least resolution to be satisfactory .
Folk wisdom has many sayings about believing what you hear andbelieving what you see. The visual sense is just as prone to illusionas the auditory pathway, and equally filled with mystery andmisunderstanding. Maybe belief should rest not on the particularsensory pathway but rather on our understanding of the ways and meansthrough which we view the world.
Gross Anatomy Of The Eye
The eyeball is a sphere approximately 24 mm in diameter. There is asmaller bulge in the front, containing the structures that admit lightinto the eye. The eye socket is an opening in the skull called theorbit, where fatty tissues, connective tissues and muscles cushion andprotect the eyeball.
Movement of the eye is controlled by six muscles for each eyeball.One pair controls up-and-down movement, one pair controls side-to-sidemovement, and another pair controls diagonal movement. The eyes are inconstant involuntary motion, moving 30 to 70 times per second. Theseastonishing movements are called saccades. The eye is inconstant, restless search for parts of the visual field with thegreatest light-dark contrast. Amazingly, we are completely unaware ofthese movements. To prove that to yourself, stand before a mirror andgaze into the reflection of your left eye. Now shift your gaze to thereflection of your right eye. Notice two things: There was no blur asyour eyes moved, and you saw no reflection of moving eyes in themirror.
Behind thecornea is a chamber containing a clear, colorless liquid called theaqueous humor. This fluid is 99% water; the remainder is a kindof clear serum found in blood. The aqueous humor is also found behindthe iris and around the lens. It provides nutrients to the transparentparts of the eye. See Figure 1.
Ways Of Manipulating Hemispheric Dominance
|Any physical stimulus of the left hand side of the body is going to increase activity in your right brain hemisphere so a firm pinch on the left leg will encourage a larger right-brain contribution to the activity at hand. Similarly twisting a finger on the right hand is likely to produce an increase in the left hemisphere’s involvement in what is going on. Note that if the stimulus becomes too painful to bear, the brain will tend to move away from the source and will produce the opposite movement of a gentler, bearable stimulus.|
How Pituitary Adenoma Affects The Optic Chiasm
The most common disorder affecting the optic chiasm is a pituitary adenoma. Pituitary adenomas are benign tumors. In most cases, they have no impact at all, but in some cases, they can affect vision, sometimes causing vision loss. As they grow in size, pituitary adenomas can put pressure on important structures in the body, such as the optic nerve. Putting pressure on the optic nerve may cause blindness, so it is crucial for eye doctors to detect pituitary tumors before they cause damage to vision.
The pituitary gland is about the size of a bean and is attached to the base of the brain behind the nasal area. it sits right under the optic chiasm. Although small, the pituitary controls the secretion of many different types of hormones. It helps maintain growth and development and regulates many different glands, organs, and hormones. Changes in hormones can cause significant changes in our bodies. Besides vision changes such as double vision, drooping eyelids, and visual field loss, pituitary adenomas also may cause the following symptoms:
- Forehead headaches
Translational And Clinical Perspectives: Issues And Ideas
Interdisciplinary engagement between basic and clinical scientists
To speed progress, it seems prudent to bring basic and clinical vision scientists together to identify diseases and patient populations that are the easiest target for regenerative therapies. At the same time, scientists from diverse backgrounds , need to collaborate rather than work in isolation, to speed translational research and development of candidate therapies that could be quickly tested in clinical trials. This would likely require innovative models to support these interdisciplinary studies.
If the field selects a priority clinical disorder or disease model, as has been done when addressing photoreceptor regeneration, the focused effort may speed development of clinically relevant therapies. Acute optic nerve injury is not very common clinically, so it may be prudent to think about how findings on the basics of regeneration might be applicable to slowing degenerative diseases, such as glaucoma or Leber’s hereditary optic neuropathy. Optic nerve crush might have some relation to traumatic brain injury, but optic neuritis, a frequent manifestation of multiple sclerosis, affects a much wider population. Using optic neuritis , stroke , or some other more common model for regeneration studies could be especially useful.
Ask patients what they consider meaningful improvements in visual function.
Amblyopia is a good system for studying plasticity and recovery from vision loss in humans.
Growth Promotion And Axon Fasciculation
The first major pathfinding task for a newly born RGC is to extend an axon towards the optic nerve head. During development, ganglion cells are born in a central to peripheral gradient such that the oldest RGCs are closest to the optic disc and the younger RGCs are in more peripheral retina. Newly formed RGC axons are in contact with axons of older RGCs and travel along, or fasciculate with, these neighbouring axons to reach the optic nerve head. This fasciculation appears to be due to growth promoting molecules such as L1 on the RGC axons themselves. L1 is a member of the immunoglobulin family of cell adhesion molecules, and functions in a homophilic manner. Homophilic binding means that an L1 molecule on a given axon binds an L1 molecule on an adjacent axon. It is thought that such L1 homophilic interactions encourage retinal axons to grow in bundles, or fascicles, within the retina on their way to the optic disc. This model is supported by the finding that experimental blockade of L1 function, or the function of related Ig guidance molecules, causes RGC axons to wander in the retina instead of growing directly to the optic disc. Thus, RGC axon pathfinding to the optic disc appears to involve the ability of retinal growth cones to follow a trail of attractive axon guidance molecules.
A New Implant For Blind People Jacks Directly Into The Brain
Allí, says Bernardeta Gómez in her native Spanish, pointing to a large black line running across a white sheet of cardboard propped at arms length in front of her. There.
It isnt exactly an impressive feat for a 57-year-old womanexcept that Gómez is blind. And shes been that way for over a decade. When she was 42, toxic optic neuropathy destroyed the bundles of nerves that connect Gómezs eyes to her brain, rendering her totally without sight. Shes unable even to detect light.
But after 16 years of darkness, Gómez was given a six-month window during which she could see a very low-resolution semblance of the world represented by glowing white-yellow dots and shapes. This was possible thanks to a modified pair of glasses, blacked out and fitted with a tiny camera. The contraption is hooked up to a computer that processes a live video feed, turning it into electronic signals. A cable suspended from the ceiling links the system to a port embedded in the back of Gómezs skull that is wired to a 100-electrode implant in the visual cortex in the rear of her brain.
Using this, Gómez identified ceiling lights, letters, basic shapes printed on paper, and people. She even played a simple Pac-Manlike computer game piped directly into her brain. Four days a week for the duration of the experiment, Gómez was led to a lab by her sighted husband and hooked into the system.
Reconnecting Eye To Brain
- NIH, National Eye Institute
- Scientists have completed a comprehensive assessment of what scientists know about optic nerve development, regeneration, and reconnection.
Michael Crair, Yale University, and Carol Mason, Columbia University, have co-authored a report published online today in the Journal of Neuroscience. “Reconnecting Eye to Brain” is a comprehensive assessment of what scientists know about optic nerve development, regeneration, and reconnection. The report was based on input gathered during the Oct. 16, 2015, panel discussion, titled “Reconnecting Neurons in the Visual System,” sponsored by the National Eye Institute Audacious Goals Initiative . The panel comprised two dozen leading experts on neural development and regeneration.
The AGI is developing therapies to restore vision, lost through disease or injury, by regenerating the retina. A major AGI challenge is reconnecting retinal ganglion cell axons that project from the retina to the brain. Zebrafish do this naturally after injury. What might we learn from Zebrafish to inform us about the regeneration mechanisms in humans?
The NEI AGI is an effort to push the boundaries of vision science. By facilitating cross-disciplinary research, the AGI is tackling the most devastating and difficult-to-treat eye diseases. Learn more about the NEI AGI at www.nei.nih.gov/audacious.
The Vascular System Of The Eye
There are no blood vessels in the cornea or the lens of the eyebecause, of course, blood is not clear and light would not pass throughit well. The cornea and the lens are fed oxygen and other nutrientsthrough the aqueous humor and even through the tears. At the junctionwhere the sclera becomes transparent and becomes the cornea, there isthe ciliary body. This structure feeds the aqueous humor with needednutrients, in the area between the iris and the lens. This newlyrefreshed aqueous humor streams gently out the opening in the iris to the front cavity behind the cornea. Aqueous humor exhaustedof nutrients becomes cloudy, and this is drained through the canal ofSchlemm, leading ultimately to a vein at the rear of the eye. Failureof the canal of Schlemm causes the aqueous humor to become more andmore cloudy, a condition called glaucoma.
At the very rear of the eyeball there are four main arteries andmany more smaller veins bringing nutrients to the eye and carrying awaywaste products. A layer behind the retina, the choroid membrane, is afabric of interconnected blood vessels and connective tissues, smallerthan a postage stamp. This is an extraordinary membrane: At no placeelse in the body is there such a concentration of vessels and bloodflow. Physiologists speculate the choroid membrane not only bringsnutrients to the tissues of the eye, but may also control thetemperature at the back of the eye and prevent overheating of theretina by brilliant illumination.
Biological And Conceptual Barriers To Progress
There are a number of barriers preventing the functional regeneration of RGC connections to the brain . Overarching challenges that to date have few answers, but are crucial for implementing regeneration of RGCs, include the following:
RGCs encounter a number of barriers to regeneration following injury or trauma. In the eye, a range of factors, including cell-intrinsic transcription factors and receptors as well as exogenous growth factors, influence RGC survival and the ability of cells to generate axons and grow out of the eye and into the optic nerve. Some classes of RGCs show greater regenerative ability than others. In the optic nerve, supportive glia may not be present in the adult, and inhibitory influences associated with the scar block regeneration . At the optic chiasm, growth cones are often misrouted toward the hypothalamus , or grow back into the opposite optic nerve, or halt completely. In the LGN and SC, if axon guidance factors are sufficient to guide the axons to these targets, synapses may form in the wrong retinotopic area, layer, or target cell. Synapse strength may also be inappropriate to mediate functional connections. In the visual cortex , regenerated connections may be too few or too weak, and circuit plasticity may not be adequate to compensate to generate useful functional response.
The optic chiasm seems to be a blockade to successful axon growth to targets.
As discussed by
Better models are needed, especially for visual pathway regeneration.
The Cell Structure Of The Brain
The brain is made up of two types of cells: neurons and glial cells, also known as neuroglia or glia. The neuron is responsible for sending and receiving nerve impulses or signals. Glial cells are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin and facilitate signal transmission in the nervous system. In the human brain, glial cells outnumber neurons by about 50 to one. Glial cells are the most common cells found in primary brain tumors.
When a person is diagnosed with a brain tumor, a biopsy may be done, in which tissue is removed from the tumor for identification purposes by a pathologist. Pathologists identify the type of cells that are present in this brain tissue, and brain tumors are named based on this association. The type of brain tumor and cells involved impact patient prognosis and treatment.
Say What The Connection Between Hearing And The Brain
Did you know that March is a Brain Awareness Month? In honor of this, lets look at the connection between the brain and the ear. It is a partnership that is often overlooked, but something that has been researched for years.
Hearing essentially occurs in the brain. Think of the ear as a vehicle for getting sound to the brain. Dig back into your memory of high school science class and you might remember that there are three main parts of the ear: the outer ear, the middle ear, and the inner ear.
- The outer ear collects sound and transfers it down the ear canal to the eardrum which then vibrates.
- The middle ear starts at the eardrum. When it vibrates, it moves tiny bones which transfer the sound farther into the ear. By the way, these are smallest bones in your body.
- The inner ear contains tiny hair cells surrounded by fluid. When bones of the middle ear vibrate, it causes the inner ear fluid to move, which makes the hair cells wave just like beach grass moves with the ocean tides. This waving motion causes electrical impulses which transmit information to the brain.
Axon Guidance In The Human Visual System
Almost all of the studies discussed above were performed in the developing visual systems of lower vertebrates or rodents. Naturally, one question is how much of what we have learned about retinal axon guidance from these species can be applied towards understanding how the human visual system is assembled. Some insight can be obtained by considering the fact that the molecular basis of growth cone guidance is highly conserved throughout evolution. For example, homologues of many of the axon guidance molecules discussed here, such as netrins, slit, and semaphorins, have been identified in invertebrates such as the fruit fly Drosophilia and the nematode C elegans, where they also participate in axon pathfinding during neural development. Human homologues of netrins, slits, semaphorins, and ephrins have also been identified, but as yet, little is known about their patterns of expression or function during human development. However, it seems highly likely that these same gene families, and similar axon guidance principles, contribute to patterning the human visual system. A list of molecules contributing to axon guidance is given in Table 1.
Gaps In Knowledge Of Basic Features Of Rgcs And The Regeneration Landscape
A pressing question at the National Eye Institute Satellite meeting was whether the questions/gaps in our knowledge of how to stimulate RGC regeneration can and should be tackled through nonhypothesis driven research. The participants thought the following areas need to be addressed, both for recovery of function in the visual system and in damaged CNS in general.
Axon growth, guidance, and targeting
What are growth cone behaviors along different loci in pathways, especially in targets, in normal development and regeneration in the adult? Live imaging of RGC axons extending in the normal developing optic chiasm and along the visual pathway reveals that RGC growth cones advance more slowly in the chiasm, and repeatedly extend and retract before traversing the midline and entering targets . Live imaging of the chiasm after optic nerve injury would elucidate where the barriers are for regenerating RGC growth cones.
Studies in Drosophila and other model systems indicate that interactions of essentially every surface of a growth cone helps determine what it does. Adhesion and other molecules , some of them newly discovered, implement various steps in targeting and synapse formation, others involving overshooting, retraction, and changes in morphology, before synapses are made . Understanding growth cone surfaces and the molecules they express is critical to knowing whether they provide opportunities or obstacles for driving regeneration.