In the brain, there are over 100 billion neurons compressed into an organ the size of both your fists. Within these billions of neurons, there are a plethora of different types. And yet neurons are not the only cell that take up residence in your brain: non-neuronal cells are vital in the running of this magnificent organ.
Before we delve into the diversity of non-neuronal cells, I will give a brief overview of the generic structure of a neuron:
On the left side of the diagram you have the dendrites. These hairlike structures are branching fibres which receive electrical signals from neighbouring neurons. It is key to note here that the neighbouring neurons are not fused together, but rather there are minuscule gaps called synapses which link the two together.
The cell body, shown on the left of this diagram, holds the nucleus.
The axon is a long continuous fibre, much like a telephone cable, that conducts the electrical impulse away from the cell body.
Different types of non-neuronal cells within the CNS
Within the central nervous system there are hundreds of types of neurons. In addition to this extensive variety, there are many countless other cells which are essential to the functioning of the brain so should not be eagerly overlooked. The main type being the Neuroglia cells.
The Neuroglia cells
These cells are non neuronal, meaning they don’t transmit electrical impulses, so their function is to provide support and protection for the neurons. There are three main varieties:
1) The Astrocyte Oligodendrocyte
Astrocytes get their name from their star shaped structure. The astrocytes have a crucial role in maintaining the physical and nutritional support of neurons.
In terms of the nutritional support, astrocytes supply the nerve cells with glucose needed for nerve activity. The long extensions of the astrocytes bind to capillaries, where glucose can readily diffuse into the cell to be partially metabolized then sent onto the neurons. In addition, astrocytes can store glycogen as a fuel reserve, which can be metabolized to glucose and released to the neurons during periods of high demand. In this way, neurons can fire quickly without draining their glucose supplies.
Astrocytes are known to be highly permeable to potassium. During periods of high neuronal activity, excessive amounts of potassium are produced which can lead to hyperkalemia and epileptic activity. Since astrocytes are interconnected via ‘gap junctions’, this accumulation of potassium can pass through astrocytes in a wave like fashion to be evacuated to the capillaries.
Furthermore, due to their linkage via the gap junctions, the astrocytes function as a single entity, called an electrically coupled syncytium. Thus, the activity of one astrocyte will have repercussions on those which are distant. Unlike neurons, glia cells don’t communicate via electricity; instead they communicate using waves of calcium ions. The fact that astrocytes harmoniously joined to one another means that these calcium waves can propagate symmetrically to surrounding astrocytes. This communication plays a major role in modulating the nerve activities.
Another key function of astrocytes is nervous system repair: when injured, these cells flock to the impaired area to form a ‘glial scar’.
2) The Oligodendrocyte
The oligodendrocyte primarily functions as insulation around the axon. They wrap around the axon and provide the myelin sheaths, which reduce the leakage of ions which may disrupt the action potential and additionally decrease the capacitance of the cell membrane. The speed of action potentials are also increased with myelin coverage. However, the myelin does not envelope the entire axon. Rather it leaves small exposed sections (shown in the diagram) called nodes of Ranvier. Myelination speeds up neural conduction because of a process called saltatory conduction. The word ‘saltatory’ comes from the latin ‘to jump’. This is a fitting description, as the action potentials literally jump from neighbouring nodes of Ranviers which results in much faster transmission than is observed in continuous propagation.
As an example, down a non myelinated axon the speed of action potentials can be as slow as 0.5 metres per second, whereas due to myelin this speed can increase to 120 metres per second!
Microglia are the police of your brain: They are the native macrophages which constitute to the first line of defence within the brain (and spinal cord) against any foreign material.
These cells are also known for ‘scavenging’, that is clearing up all the random debris such as fragments of DNA and apoptic cells (cells that have committed suicide).
As well as destruction, microglia also serve an important role in repair. For example, when tissue is damaged the microglia strip the dendrites from the nerves near damaged tissue which promotes regrowth and repair.
There are many more interesting and weirdly shaped non-neuronal cells within the CNS and the peripheral nervous system, yet scientists still don’t have clue as to what this value actually is.