A brief outline of the structure and function of the human Nervous System

As the Greeks put it, the ability to rule lies in the brain (Diels, 1961). The human brain consists of approximately 100 billion neurones and about three times more glial cells (Herculano-Huzel, 2009, p.1). Neurones contain intracellular fluid (ICF) and are surrounded by extracellular fluid (ECF). The composition of the cerebrospinal fluid (CFS) affects the composition of the ECF and controls the extracellular environment of both neurones and glial cells. The brain is constantly supplied with blood from a dense network of blood vessels. Each neurone may synapse with thousand others and almost all synapses are chemical (Augustine, 2004, p.93). Neurones can maintain constant the potential difference across their membrane and change that difference according to the messages received. This allows them to fire action potentials (AP), created in the axon hillock (Coster and Siegelbaum, 2013, p.145) and transmitted along the axon in an ‘all or none’ fashion (Scott, 2007, p.655). The speed at which they travel depends upon neuronal type (Cummins and Dorfman, 1981, p.67). Axons are organised in nerves, neural networks and neural pathways. Neurones are organised in brain areas that serve different functions.

The various senses (vision, olfaction, taste, touch, hearing) are controlled by different parts of the nervous system. Information carried as APs can be transformed into exitatory postsynaptic potentials (EPSP) and inhibitory postsynaptic potentials (IPSP). The EPSPs and IPSPs that reach the same postsynaptic neurone are summed. If the total surpasses the threshold required, an AP is fired in the postsynaptic neurone (Gardner and Johnson, 2013, p.466). The type of messages and the type of the postsynaptic neurone, determine the type of integration of the postsynaptic potentials.

Apart from olfaction, all other sensory information reaches the thalamus. From the thalamus, information travels to the primary areas of the cerebral cortex. In these areas, information is transformed into the different sensory modalities. The primary visual cortex is located in the occipital lobe; the primary auditory cortex is located in the temporal lobe; the primary somatosensory cortex is located in the postcentral gyrus of the parietal lobe; the olfactory cortex and the gustatory cortex are located in the temporal lobe. Next, information is transmitted to other areas for further processing and comparison. This generates higher brain functions such as the perception of reality and the ability to think. Sensory information is also used by the primary motor cortex in the central sulcus, the premotor cortex, the basal ganglia and the cerebellum which collectively plan, organise and control movements. Each brain hemisphere controls afferent and efferent information from and to the contralateral side of the body.

It therefore seems that in order for the brain to understand the external environment, it has to transduce it into action potentials that travel in the corresponding pathways. In the same way, efferent stimuli are generated as a result of action potentials created in various brain areas. Eventually, efferent stimuli reach various organs and either chemical substances are secreted or muscle fibres are contracted and movement is produced.

References

1. Augustine, G., ed., 2004. Synaptic transmission. In: Purves, D., Augustine, G. J., Fitzpatrick, D., Hall, W.C., Lamantia, A. S., Mcnamara, J. O. and Williams, S. M., eds. 2004. Neuroscience, 3rd edn, Sinauer Associates, Massachusetts.

2. Coster, J. and Siegelbaum S. A., 2013. Membrane Potential and the Passive Electrical Properties of the Neuron. In: Kandel, E.R., Schartz, J.H., Jessel, T.M., Siegelbaum, S.A., Hudspeth, A.J., eds. 2013. Principles of Neural Science, 5th edn, McGraw-Hill, n.p.

3. Cummins, K. L. and Dorfman, L.J., 1981. Nerve fiber conduction velocity distributions: studies of normal and diabetic human nerves. Annals of Neurology, 9(1), p.67.

4. Diels, H. (1961). Die Fragmente der Vorsokratiker [ Αποσπάσματα των Προσωκρατικών ], Berlin: Weidmann.

5. Gardner E.P. and Johnson K.O., 2013. Sensory coding. In: Kandel, E.R., Schartz, J.H., Jessel, T.M., Siegelbaum, S.A., Hudspeth, A.J., eds. 2013. Principles of Neural Science, 5th edn, McGraw-Hill, n.p.

6. Herculano-Houzel, S., 2009. The human brain in numbers: a linearly scaled-up primate brain, Frontiers in human neuroscience, November, (3), p.1.

7. Scott, L. L., Hage T. A., and Golding N. L., 2007. Weak action potential backpropagation is associated with high-frequency axonal firing capability in principal neurons of the gerbil medial superior olive. Journal of Physiology, 583(2), p. 655.

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