Nerve Regeneration After Injury

Scar tissue has accumulated after the inflammatory response has been activated at the injury site.

A broken bridge is an example of how communication

is disrupted after a spinal cord injury.

After injury occurs in the Central Nervous System (which refers to either the brain or spinal cord), special cells which are unique to this area of the body are activated.  Their names are Glial (meaning glue) cells and include Schwann cells, astrocyes, and microglia.  Within the nervous system these cells activate a wound healing inflammatory response known as gliosis.  As gliosis is triggered, cells that are damaged at the injury site undergo apoptosis which is known as cell death, this mechanism is then followed by the formation of dense scar tissue at the injury site (as shown in the first picture).

As show in the picture on the right, scar tissue disrupts connections of both nerves and axons, and like a broken bridge there is no longer communication.  This broken communication is the cause of paralysis after a spinal cord injury, after a stroke or any other traumatic injury in the brain.

As shown in the figure, CSPGs inhibit the growth cone of a nerve cell,

acting as a Chemorepulsion mechanism.

During scar formation, astrocytes have been found to release inhibitory molecules known as Chondroitin sulfate proteoglycans or (CSPGs), which then cause further damage (Yamaguchi, Y. 2005).  CSPGs cause further damage because they inhibit the growth cone  from crossing the injury site to reform the connection that has been broken.  A growth cone is located at the end of a nerve or axon of a neuron, its job is to guide the nerve or axon through the environment by detecting environmental cues.  This video shows the growth cone interacting with the environment: Growth Cone

As soon as the growth cone finds the perfect environmental conditions, a connection is made.  However if it encounters inhibitory environmental cues like CSPGs which are the molecules present within scar tissue, the growth cone collapses or turns around to continue the search for a Chemo-attracting environment (see video below).  Since the scar tissue is a great barrier, the growth cone never crosses to find a connection, and therefore paralysis can be permanent unless there is a treatment to overcome these barriers.

Citations:

Peripheral Nerves. http://www.vet.purdue.edu/cpr/peripheral2.html#null. access: 09/05/13

Majeq.Broken Bridge Speedy. http://majeq.deviantart.com/art/Broken-Bridge-Speedy-176766406. access: 09/08/13

Taken from Treloar HB, Bartolomei JC, Lipscomb BW, Greer CA. (2002) Mechanisms of axonal plasticity: lessons from the olfactory pathway. Neuroscientist. 7:55-63