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It has been estimated that the human body consists of 37.2 trillion cells plus or minus around 0.81 trillion (Bianconi et al., 2013) and there are hundreds of different cell types (Mescher, 2016). The cells in one body have identical DNA but carry out a coordinated myriad functions to enable the maintenance of a near-stable internal environment. Only by communicating with one another can the necessary high level of coordination be possible. The two primary organizations for cell–cell communication are the NS, using neurotransmitters such as acetylcholine, and the ES, which transports neuromodulators and hormones (e.g., cortisol) around the entire body. Most cell–cell communication occurs using intracellular enzymes, molecules that speed up chemical reactions (Michael et al., 2017). We outline here the basic structure and functions of the NS.

There are two main cell types in the NS, neurones and glial cells. Both cell types are absolutely necessary for neurological health. Glial cells provide support and nutrition, maintain local homeostasis, produce myelin and participate in signal transmission. The total number of glial cells roughly equals the number of neurons. Of particular importance are microglial cells, a type of glial cell accounting for 10–15% of all cells found within the brain. Microglial cells are highly plastic and act as macrophage (‘big eater’) cells, the main form of active immune defence in the central nervous system (CNS).

As both unique immune cells and unique brain cells that constantly change shape and have numerous different functions, microglial cells could stake a claim to being the ‘smart’ cells of the body. Microglia travel independently, unattached to any structure, circling a territory with extended arms seeking suboptimal functioning. This constant system of microglial surveillance protects the brain against any microbe invaders, demyelination, trauma and cancerous or defective cells (Lieff, 2013). When glial cells go wrong, all sorts of chaos can break loose, including brain inflammation and neurodegeneration, which can cause chronic pain (McMahon et al., 2005), Alzheimer’s disease (Paresce et al., 1996), Parkinson’s disease (Kim and Joh, 2006) and, according to some research, myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) (Morris and Maes, 2014). It can be seen already that intimate connections exist between the immune and nervous systems, and so far we have only mentioned the ‘foot soldiers’ of the NS and not the command structures.

Neurones provide the main ‘wiring’ of the NS; they are communication devices that connect with other neurones, tissues, organs and muscles. How the neuronal communication works can best be explained by looking at the structure of the neurone (Figure 2.2).

The brain contains around 86 billion neurones, 20% in the cerebral cortex and 80% in the cerebellum (Lent et al., 2012). Each neurone can connect with up to 1–10,000 other neurons so there may be as many as 860 trillion synaptic connections in total. Each neurone consists of a cell body or ‘soma’, dendrites and an axon. Dendrites are thin structures that arise from the cell body. They may be hundreds of micrometres in length and branch multiple times to produce a complex ‘dendritic tree’. An axon, or ‘nerve fibre’ when myelinated, arises from the soma at the axon hillock and travels for a distance which can be as far as one metre in humans (connecting the toe to the spinal column).

Figure 2.2 A schematic neurone and synapse

Source: Yurana’s portfolio, IMG ID:214981837, acquired via Shutterstock

Most excitatory synapses are formed between the axon of one neuron and a dendritic spine on another. When two neurons on either side of a synapse are active simultaneously, that synapse becomes stronger, a form of memory. The dendritic spine also becomes larger to accommodate the extra molecular machinery needed to support a stronger synapse.

Myelin is a fatty white substance that surrounds the axon of some neurones, providing electrical insulation. Multiple sclerosis (MS) occurs when an abnormal IS response produces chronic inflammation, which damages or destroys myelin.