Saturday, May 5, 2012

Axon regeneration speed

After damage axons can grow back with the average speed of  1-1,5 millimeters per day. This speed could be influenced by type of damage and health. In case of damage by crushing parts of nerve could shorten further centimeter within week. Regrowth is faster in case of cutting injury.
Remains of nerve stay usually near wound and continue growing from there as long as cell bodies survive. Most neuron cell bodies for humans are in spine or very close to spinal area so that injury to limb should keep them alive.
Growing axons may create unwanted connections that can cause pain or unprecise reactions. In case of facial nerve this can lead to tear glands activating together with salivary glands so eating can cause unwanted tear flow.
Regrowth can be further slowed by scar tissue that blocks former nerve path. 
Diabetes can slow the growth of axons or even shorten them.

Nerve growth factors have been tested for increasing nerve growth speed or for restoring brain function. In study with 8 Alzheimer patients they didn't stop the progression of dementia but these growth factors slowed measurable loss of cognitive skill during the 2 year study period by 50%.  
As one side effect they cause chronic pain.

Considering the slow growth speed of 1-1,5 mm/per day it's apparent that some places take long time to regain sensitivity and control.
In case of a penis transplant patient had it removed in 14 days because it did not regain sensitivity and felt too foreign. For every 10 cm he should have had waited ~100 days.
In case of leg paralysis it may take several years to gain control of limb over 1 meter away from cell bodies (cell bodies for leg neurons are in ribcage).

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