TeachableMedicalNews article 12182021

Teachable moment in classrooms:

  1. nervous system chapter – neurons in sensory and motor nerves
  2. nervous system chapter – axons as extensions of neurons
  3. nervous system chapter – Schwann cells insulate axons, and create neurilemma

The news item:  Recently this report was published about a patient who received a double hand transplant:

First child to receive double hand transplant is thriving

At just 8 years old, Zion Harvey received two new hands, a surgery that has changed his life. According to previous reporting, the boy needed an amputation of both hands and feet when he was younge…

The article states that the patient received the transplant in 2015. The original hands of the recipient were amputated because of a spreading infection. The patient still receives ongoing physical therapy.

So, Why Do I Care??  Because successful hand transplants are examples of the regeneration of the nervous system, medical technology and treatments worked out for hand transplants may be helpful to stroke patients, or patients with physical trauma of the central nervous system where natural regeneration is very poor.

Plain English, Please!!! First, let’s talk about what is the major challenge with hand transplants. At first look it seems that all a successful hand transplant needs is the meticulous surgical connection of blood vessels, nerves and muscles of the donated hand to the blood vessels, nerves and muscles of the stump. While the connected blood vessels will then carry blood, the connected muscles will heal, however, but connecting the nerves doesn’t make them functional. That is because once the donated hand is cut from the donor, all nerves deteriorate inside of it.  Therefore, after surgical connection of nerves, the axons from the stump side need to grow into the transplanted hand.  Such “re-wiring” of the transplanted hand is the greatest challenge to the functioning of the transplant.

Second, let’s talk about how axons grow into the transplanted hand. The first act in “re-wiring” is that the axons have to grow sprouts from the stump. The new sprouts have to find the ends of the decayed axons in the transplant. The key to this is that while an axon in the transplant decays, the insulation of the axon lives on. The Schwann cells form the insulation in the transplanted hand, so the axon that sprouts from the stump move toward the chemical clues of those Schwann cells at the cut surface of the transplant. Just like flies move toward sugary substances, or like mosquitoes move toward carbon dioxide, the chemical clues of the Schwann cells attract the axon sprout.

Third, let’s talk about what guides the axons to their correct target. The second challenge in the “re-wiring” process is the guiding the axon sprout to functional destinations, such as skeletal muscles, sweat glands, or sensory structures in the skin of the transplant. Each sprout will be guided through a tunnel made up by thousands of Schwann cells. A living axon looks like a finger with 5-6 rings on it.  The equally spaced rings represent the Schwann cells wrapped around the axon. When the axon degenerates the finger decays, but the rings stay alive, so the new axon sprout moves into the tunnel of Schwann cells just like a finger moves through a tunnel of 5-6 rings. Because the original axon was connected to a destination, the sprout (the finger) can move through the tunnel of Schwann cells (now thousands of rings) to reach the same destination. The name of the tunnel is neurilemma, and it is a typical feature in the peripheral nervous system. In our central nervous system (brain and spinal cord), however, the insulation is provided by the oligodendrocytes, and those cells do not form a neurilemma. The consequence of that is that in case of a stroke, or the severing of the spinal cord, the regenerating axon sprouts have no guidance and virtually no re-wiring happens, leaving the injuries to the central nervous system without self-repair.

 

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