Nerve Cells have Trouble Regrowing Damaged Parts

Looking at the structure of a neuron, you will notice it has a cell body and several arms that it uses to connect and talk with other neurons (Figure 1, left). The really long arm that sends signals to other neurons is called axon, and axons can be really long. If an axon is damaged along its way to another cell, the damaged part of the axon will die (Figure 1, right), while the neuron itself may survive with a stump for an arm. The problem is neurons in the central nervous system have a hard time regrowing axons from stumps. Why do skin cells not have this problem? Skin cells are much simpler in structure. And because they can give birth to entirely new cells, they don’t face the problem of having to repair parts of their cells.

Figure 1

• Left: the structure of a brain cell. Note the branch-like arms that extend from the cell body (top left corner). These arms receive incoming signals. The really long arm that extends to the bottom right is called the axon, which sends signals to a receiving cell. The axon is enveloped by a myelin sheath (in darker violet), which helps signals travel faster along the axon to the receiving cell. Right: when an axon gets injured, the end part dies off and leaves an axon stump. Stumps have a hard time to grow back after injury.

So, why do damaged neurons have trouble regrowing axons?

First, they need motivation. There are special molecules that help activate growth in neurons. More of these motivating molecules are made when the neurons are active. So, if you keep your brain active, your neurons are more likely to grow. This is true both after injury and in the healthy brain.

Second, axons face a hostile environment that is full of molecular “stop signs” that signal “no trespassing” to axons. Some stop signs are part of the sheath, or covering, around neighboring axons, called myelin sheath (Figure 1, left). Some stop signs are part of a scar that gets built like a protective wall around an injury in an effort to keep the damage from spreading. These scars are made by brain cells called astrocytes (star cells, due to their star-like appearance). Scar-building astrocytes are just trying to help, but they also release a chemical into their environment that makes it hard for axons to grow (Figure 2).

Figure 2

• Growing axons (green in top right and bottom left corners) looking for new target cells to connect with have a hard time in an injury environment. This is partly due to star-shaped support cells (astrocytes, in yellow), which spit out chemicals (red). These chemicals stop axon growth.

But, there is good news here as well. Scientists are working on strategies to motivate injured neurons to grow by using special growth molecules and to eliminate stop signs for axons in order to make the injury environment more supportive for nerve cell growth [1].

The Immune Response in the Brain is Different from That in Skin

The immune response plays an essential role in any kind of repair after injury. In injured skin, immune cells will rush to the site of injury from the blood and help the resident immune cells clean up debris from dead cells. Once the clean up is done, the immune cells die and stop the fight. The brain has specialized resident immune cells as well, and they will become activated when they sense danger or damage. A common problem in the brain is that the activated immune cells often don’t know when to stop fighting. If they continue to spit out toxic chemicals over long periods, they can cause more harm than good, by killing healthy neurons. This is why scientists are trying to understand what switches brain immune cells on and off and trying to figure out how they can modify the response of these immune cells, so the cells can be helpful rather than harmful [2].

https://kids.frontiersin.org/article/10.3389/frym.2016.00022