Tag Archives: optic nerve

The brain is far more wired up than we thought

The hardest thing in experimental psychology is being smart enough to think of something truly clever.  Here is a beauty, showing that the brain is far more wired together than we ever thought.

First some background.  You’ve probably heard of the blind spot (although you’ve never seen it).  It’s the part in your eye were all the never fibers from the sensory part of the eye (the retina) are collected together forming the optic nerve.  Through an ophthalmoscope it appears like a white oval (largest diameter under 2 milliMeters)  It’s white because it’s all nerve fibers (1,000,000 of them) with no sensory retina overlying it.  So if you shine a very narrow light on it, you’ll see nothing.   That’s the blind spot.

Have a look at https://en.wikipedia.org/wiki/Visual_system. Both eyes project to both halves of he brain.  Because the blind spot is off to one side in your visual field, the other eye maps a different part of the retina to the same area of the brain.  But if you patch that eye, on one side of the brain the blind spot gets no input.

 

 In the healthy visual system, the cortical representation of the blind spot (BS) of the right eye receives information from the left eye only (and vice versa). Therefore, if the left eye is patched, the cortex corresponding to the BS of the right eye is deprived of its normal bottom-up input.

Proc. Natl. Acad. Sci. vol. 117 pp. 11059 – 11067 ’20 https://www.pnas.org/content/pnas/117/20/11059.full.pdf

Hopefully you’ll be able to follow the link and look at figure 1 p. 11060 which will explain things.

Patching the left eye deprives that area of visual cortex of any input at all.

Here comes the brunt of the paper — within minutes of patching the left eye, the cortical representation of that spot begins to widen.  It starts responding to stimuli from areas outside its usual receptive field.

Nerves just don’t grow that fast, so the connections have to have been there to begin with.   So the brain is more wired together than we thought.  Perhaps this is just true of the visual system.

If not, the work has profound implications for neurologic rehabilitation.

I do apologize for not being able to explain this better, but the work is sufficiently important that you should know about it.

Addendum 4 June — here’s another shot at explaining things.

    • As you look straight ahead, light falls on the part of your retina with the highest spatial resolution (the macula). The blind spot due to the optic nerve is found closer to your nose, which means that in the right eye, the retina surrounding the blind spot ‘sees’ light coming from toward your ear. Light from the same direction ( your right ear) will NOT fall on the optic nerve of your left eye (which is toward your nose) so information from that area gets back to the brain (which is why you don’t see your blind spot).

      Now visual space (say looking toward the right) is sent back to the brain coherently, so that areas of visual space transmitted by either eye go to the same place in the brain.

      So if you now cover your left eye, there is an area of the brain (corresponding to the blind spot of the right eye) which is getting no information from the retina at all. So it is effectively blind. Technology permits us to actively stimulate the retina anywhere we want.. We also have ways to measure activity of the brain in any small area (functional MRI). Activity increases with visual input.

      Now with the left eye patched, stimulate with light directed at the right eye’s blind spot. Nothing happens (no increase in activity) in the cortical area representing that part of the visual field. It isn’t getting any input. So it is possible to accurately map the representation of the right eye’s blind spot in the brain in terms of the brain areas responding to it.

      Next visually stimulate the right eye with light hitting the retina adjacent to the right eye’s blind spot. Initially the blind spot area of the brain shows no activity, After just a few minutes, the area of the brain for the right eye’s blind spot begins to respond to stimuli it never responded to initially. This implies that those two areas of the brain have connections between them, that were always there, as new nerve processes just don’t grow that fast.

      To be clever enough to think of a way to show this is truly brilliant. Bravo to the authors.

 

Do axons burp out mitochondria?

People have been looking at microscope slides of the brain almost since there were microscopes (Alzheimer’s paper on his disease came out in 1906). Amazingly, something new has just been found [ Proc. Natl. Acad. Sci. vol. 111 pp. 9633 – 9638 ’14 ]

To a first approximation, the axon of a neuron is the long process which carries impulses to other neurons far away. They have always been considered to be quite delicate (particularly in the brain itself, in the limbs they are sheathed in tough connective tissue). After an axon is severed in the limbs, all sorts of hell breaks lose. The part of the axon no longer in contact with the neuron degenerates (Wallerian degeneration), and the neuron cell body still attached to the remaining axon, changes markedly (central chromatolysis). At least the axons making up peripheral nerves do grow back (but maddeningly slowly). In the brain, they don’t, yet another reason neurologic disease is so devastating. Huge research efforts have been made to find out why. All sorts of proteins have been found which hinder axonal regrowth in the brain (and the spinal cord). Hopefully, at some point blocking them will lead to treatment.

THe PNAS paper found that axons in the optic nerve of the mouse (which arise from neurons in the retina) burp out mitochondria. Large protrusions form containing mitochondria which are then shed, somehow leaving the remaining axon intact (remarkable when you think of it). Once shed the decaying mitochondria are found in the cells supporting the axons (astrocytes). Naturally, the authors made up a horrible name to describe the process and sound impressive (transmitophagy).

This probably occurs elsewhere in the brain, because accumulation of degrading mitochondria along nerve processes in the superficial layers of the cerebral cortex (the gray matter on the surface of the brain) have been seen. People are sure to start looking for this everywhere in the brain, and perhaps outside as well.

Where else does sort of thing this occur? In the fertilized egg, that’s where. Sperm mitochondria are activated in the egg (which is why you get your mitochondria from mommy).