There are two nerve systems in the human body. The Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS is made up of the brain and spinal cord, and the PNS is everything else. I will do a briefe overview of the PNS but because this is about spinal cord injuries I will focus more on the central nervous system.
Histology
In the PNS there are three types of cells: Neurons, Satellite, and schwann cells. Neurons are what carry the electrical impulse, Satellite cells are thought to control the chemical environment surrounding the neurons, and Schwann cells surround and form sheaths around the nerve fibres. Schwann cells are similar to oligodendrocytes (see further down), however, they are vital in the peripheral nervous system’s ability to regenerate.
Neurons are made up of a cell body, dentrites and one or two axons. The chemicals causing the electrical impulse are received in the dentrites and are carried down the axon and out the synaptic knobs towards the next nerve cell’s dendrites. It is in this way that the electical impulse is carried from the brain to whatever, or from the stimulus to the brain. This can happen in a matter of miliseconds before you even have the chance to think about it.
In the CNS you have neurons and the supporting cells, which are collectively called neuroglia (or glia for short). Glial cells outnumber neurons in the CNS by nine to one. The most abundant glial cell is the astrocyte. These serve a more structural purpose with their many projections. They hold themselves to both the neurons and the blood supply ensuring the neurons stay well supplied with nutrients. The astrocytes also allow nutrients and waste to be passed through them to and from the neurons. In case your interested, astrocytes signal to each other through intracellular calcium pulses.
The next type of cell is the Microglia. These are important to the CNS as they act as “moniters” watching the neurons health. When microorganisms invade the central nervous system the microglia gather and form a type of macrophage (similar to white blood cells in the immune system). This is importand because cells in the immune system are not allowed access into CNS.
Then you have Ependymal cells. These vary in shape due to the fact that they line the cavities of the brain and spinal cord, and act as a semipermeable membrane between the cerebrospinal fluid and what supports the neurons.
Lastly you have the oligodendrocytes. These work simarly to schwann cells as they create a myolin sheath around the neuron extensions, acting as insulation and ensuring that the elecrical and chemical impulses travel along faster.
Now…onto the more interesting part…regeneration!
You are born with a certain amount of neurons. Once the are mature they no longer divide. Once a neuron is dead that is it. In the Peripheral Nervous System, if the damage to a nerve cell is severe, or too close to the cell body, then the cell will die. However, cut or compressed axons can regenerate. How? This is where Schwann cells come into play. Almost immediately after the damage is done the seperated ends of the nerve cell are sealed off and substances are collected at these sealed ends. Macrophages are sent to the trauma site where all the axon debris is all gotten rid of. Once this is done the macrophages omit a chemical which causes the Schwann cells to reproduce and create tubes. Growth factors are then released and the leftover axon in the nerve cell grows into the tubing. The Schwann cells also act as protection for the developing axon.
Why can’t this happen in the Central Nervous System? This is because of two reasons. Firstly, the astrocytes and microglial cells are the ones doing the clean up. The number of macrophages which are allowed to surround the damaged nerve is much lower, so the clean up works much more slowly. On top of this the oligodendrocytes surrounding the injured nerve tissue die, and are unable to guide the fibre regrowth. The second reason is because the neighbouring axons contain growth inhibiting proteins. This is mainly to keep the younger axons going in the right path.
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