If you cut your skin, it can e, but after a stroke or brain injury, you can't replace neurones. Why is it so? What might be the evolutionary significance of this?
If you cut your skin, it can e, but after a stroke or brain injury, you can't replace neurones. Why is it so? What might be the evolutionary significance of this?
“Why can’t neurons in the brain damaged by a stroke be replaced?”
We sometimes think of a neuron as a connection between two other neurons. This is true for some neurons, like those that run from the spinal cord to an extremity, and in some cases those do regrow by extending an axonal foot along the previous path.
The problem with neurons in the brain is that they have as many as 10,000 connections. And all the dendritic connections (perhaps half of those) store an activation level. When a neuron in the brain is lost, the specific connections are lost (meaning where they connect, both incoming and outgoing) and the learned activation levels are also lost. That information isn’t stored anywhere else, so when a neuron is lost a huge amount of information is lost.
Replacing a brain neuron would require, first, knowing the neuron was lost, but there is no central registry of neurons. Second, how many dendrites and axonal feet were lost (again, 10,000). Third, where those dendrites and axons were connected (and those places may have also been lost in the stroke.) Fourth, the activation levels of the dendrites (again no redundant copy).
The brain might have been designed to replace the neuron, but the cost of duplicating all the information that is lost with it was apparently not worth the evolutionary trouble. Instead, remaining neurons grow to replace some of the connections, as needed. And new memories in the hippocampus can partially offset the lost information with new learning.
Note that damage to the cerebellum can be overcome by neurogenesis there, but the new neurons still have to relearn all the connections that were lost, typically through repetition in therapy.
Brain damage is serious, and it can be overcome.
“Why can’t neurons in the brain damaged by a stroke be replaced?”
We sometimes think of a neuron as a connection between two other neurons. This is true for some neurons, like those that run from the spinal cord to an extremity, and in some cases those do regrow by extending an axonal foot along the previous path.
The problem with neurons in the brain is that they have as many as 10,000 connections. And all the dendritic connections (perhaps half of those) store an activation level. When a neuron in the brain is lost, the specific connections are lost (meaning where they connect, both incoming and outgoing) and the learned activation levels are also lost. That information isn’t stored anywhere else, so when a neuron is lost a huge amount of information is lost.
Replacing a brain neuron would require, first, knowing the neuron was lost, but there is no central registry of neurons. Second, how many dendrites and axonal feet were lost (again, 10,000). Third, where those dendrites and axons were connected (and those places may have also been lost in the stroke.) Fourth, the activation levels of the dendrites (again no redundant copy).
The brain might have been designed to replace the neuron, but the cost of duplicating all the information that is lost with it was apparently not worth the evolutionary trouble. Instead, remaining neurons grow to replace some of the connections, as needed. And new memories in the hippocampus can partially offset the lost information with new learning.
Note that damage to the cerebellum can be overcome by neurogenesis there, but the new neurons still have to relearn all the connections that were lost, typically through repetition in therapy.
Brain damage is serious, and it can be overcome.
