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

Inexpensive Dialysis Method

Dialysis- Inexpensive Method

Dialysis is a technique that uses a porous membrane to move small enough solutes (micro or nano size) from an area of high concentration to an area of low concentration.  I used this technique in lab to remove ammonium hydroxide, acetic acid, or excess and unwanted dye from a sample.  There are many dialysis techniques, and the one I used is shown in the picture on the right. A mixture was placed in a small micro-centrifuge tube with a small hole on the lid, then a small square of dialysis membrane was cut and placed on the micro-centrifuge tube in between the lid, and was held tight after the lid was closed.  The hole on the lid will allow small solutes like ammonium hydroxide to pass through the porous membrane while keeping the sample in the tube.  The micro-centrifuge tube was then placed in a beaker containing solution of choice.  In my case, I wanted my sample of interest to stay in 95% ethanol, therefore I had a beaker full of 95% ethanol.  The tubes were then left to stir overnight, and within this time frame, solutes would move through the membrane out into the solution in the beaker (95% ethanol).  I found this specific dialysis technique to be less expensive compared to using a dialysis membrane bag,  this is because it uses less membrane, and the membrane itself is very expensive.  I also like this technique because after dialysis, I am able to safely remove all of my sample, compared to when using only a membrane bag where the sample usually gets trapped in the corners of the bag. Loosing sample can be an issue when your sample is pure, expensive or if present in a small amount.

The purpose of using dialysis, was to completely remove ammonium hydroxide.  To test the presence of ammonium hydroxide, I added copper nitrate to a tiny amount of my sample.  If after adding copper nitrate, my dialyzed solution turned purple, then ammonium hydroxide is still present. Dialyzing the same sample for a few more hours should remove the left over ammonium hydroxide.  But if after adding copper nitrate, the dialyzed sample turned cyan blue, then all the ammonium hydroxide was removed successfully.

Citations:
Zeelen, Johan. Dialysis of small volumes. Photograph. 01/22/04. http://www.xtal-protocols.de/prot/protein.html. Access: 09/09/13