The nervous system can be divided into its various components in many ways. One way of dividing it up is into its different cell types. At its most basic form, this division splits the nervous system into neuronal cells and non-neuronal cells. Neuronal cells, as the name implies, encompass those most recognisable of cells - the neurons. The flagship of the cytoarchitecture of the nervous system. But of course, to those who have read any of my previous posts - there’s a world of other cells in the nervous system, not least the cells that make up the blood vessels and meninges, but in particular the glia. Glial cells outnumber neurons in the human nervous system. They outnumber neurons by a long way, with some estimates putting the ratio of glia to neurons at 10:1.
Perhaps the most ubiquitous of all the glia is the astrocyte. I’ve written about it before - they’re vital for nervous system function. They keep neurons alive, and also help the transmission of impulses (action potentials) in neurons by releasing various factors in the synapse. You understand that astrocytes are pretty darn important.
Astrocytes can also be scumbags. They’re one of the main reasons that the central nervous system (the brain and spinal cord) is so bad at repairing itself after injury. This is particularly the case in the spinal cord which contains a lot of nerve fibre tracts (aka white matter).
Diagram of a neuron. The axon (with it’s respective myelin sheath) is usually really really really long, and diagrams like this don’t do it any justice.
The nervous system is often divided into ‘White matter’ and ‘Grey matter’. This rather simplistic division can be rather effective. The grey matter is the tissue which predominately contains the synapses and cell bodies of neurons, and white matter is tissue which is occupied by the axons of the neurons. So basically, the white matter is the wiring of central nervous system. It is white because of the insulation around the axons, known as the myelin sheath (which can be likened to wires, as they propagate the electrical impulse, or the action potential). In the central nervous system, the myelin sheath is formed by a type of glial cell known as an oligodendrocyte. The insulation is made up of fat and protein giving white matter its white colour, and helps axons to conduct their nervous impulses at faster speeds.
Grey matter and White matter in a cross section of the spinal cord
Anywho, the spinal cord contains really long stretches of axons, which are descending from their cell bodies in the brain. Accidents which result in these axons being cut result in paralysis as the electrical impulses can no longer travel to their muscles. The information input into the respective muscle or glands has been cut so it can no longer be controlled by the brain - potentially causing paralysis.
Descending motor pathways. These nerve fibre tracts (in pink) are bundles of axons from neurons whose cell bodies start in the brain. The axons descend down the spinal cord to the level of the muscle they need to innervate, where they synapse with another neuron in the grey matter of the spinal cord (labelled as the ‘anterior nerve roots’ in the diagram). This subsequent neuron goes on to control its respective muscle. Cutting axons in the spine causes a break in this neural wiring, removing muscle control.
So a motorbike accident which results in paralysis most likely does so because the axons in the spinal cord get damaged. The region of the axon downstream from the lesion dies away, but the region of axon still attached to its cell body remains alive and actually tries to grow back.
Degeneration of the axon following a lesion (a cut/damage). The downstream region dies off, leaving the upstream region intact and ready to regrow.
The key word is tries. Try as lesioned axons might, their regrowth is physically impeded by spinal cord astrocytes. At lesion sites, astrocytes release a glycoprotein called chondroitin sulphate proteoglycan (CSPG) which plugs the gap. In laboratory animals, some axon regrowth can be acheived by removing this CSPG. This CSPG, although has some benefit, doesn’t seem to be a great loss to the nervous system when removed from a lesion site. The response is most likely a vestige from our evolutionary past which we don’t really need anymore. Aside from this, axon regrowth is impeded by the oligodendrocytes which express proteins on their surfaces which halt axon growth. Bearing this knowledge of inhibitory astrocytes in mind, however, much work has gone into working around this biological caveat and therapeutic strategies for spinal repair are on the horizon.
Diagram showing how astroglial CSPG halts axon regrowth. And how remvoal of CSPG theoretically should allow regrowth of axons, thus allowing nervous regeneration!
Spinal glia really become bastards when someone breaks their back.