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How do the cells of the nervous system carry out their functions? Do they normally function in isolation?

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Introduction

How do the cells of the nervous system carry out their functions? Do they normally function in isolation? The neuron is the structural and functional unit of the nervous system. In all, there are over 100 million neurons, and they are responsible for the reception, transmission, and processing of stimuli; the triggering of cell activities; and the release of neurotransmitters. These functions are conducted by electrical changes in the form of nerve impulses, and a neuron communicates chemically with other neurons at a synapse (where a neurotransmitter may be released). It is this creation, analysis and integration of impulses that allows the nervous system to carry out its two major functions- to maintain stable, constant internal conditions (carbon dioxide and oxygen levels, blood pressure, pH, hormone and blood glucose levels) and to initiate behavioural patterns (movement, reproduction, nutrition etc). A neuron can be said to consist of three main parts: a cell body or soma; a dendrite; and an axon. The soma contains the nucleus and associated cytoplasm, mitochondria, and a highly developed endoplasmic reticulum and Golgi apparatus for protein synthesis (e.g. certain neurotransmitters or their precursor molecules). When looked at under the light microscope, neuronal RER and ribosomes resemble granular areas called Nissl substance. There are also abundant neurofilaments in both the cell soma and processes. The filaments are seen as bundles due to the presence of special fixatives, and are thought to be for skeletal stability rather than for neuronal transport. ...read more.

Middle

The same neurotransmitter is released from all axon terminals of a single neuron, and all similar neurons (ones with the same origin, function or location) use the same neurotransmitter too. Acetylcholine is a ubiquitous transmitter, released at synapses between motor neurons and striated muscle, at autonomic ganglia, and by post-ganglionic parasympathetic neurons. Other neurotransmitters may be amino acids (GABA, glutamate); or monoamines (noradrenaline, dopamine, serotonin); peptides (enkephalin, substance P, cholecystokinin, somatostatin, dynorphin)- peptides are often found as co-transmitters, so are known as neuromodulators. Neurotransmitters may be broadly divided into two types: fast neurotransmitters, which operate through ligand-gated ion channels (e.g. glutamate, GABA) and slow neurotransmitters, which operate mainly through G-protein-coupled receptors (e.g. dopamine, neuropeptides). There are also numerous other neuromodulators (such as NO and arachidonic acid metabolites) that may be produced by non-neuronal cells too, and other mediators (cytokines, chemokines, growth factors, steroids) that perhaps control long-term changes in the brain (synaptic plasticity, remodelling), by affecting gene transcription. Although most synapses are chemical, as described, some cell synapses conduct impulses directly through gap junctions- these are known as electrical synapses. The principal mechanism of transmitter release in both the peripheral and central nervous systems (and also in many hormone-secreting cells) is exocytosis, initiated by the arrival of an action potential along the axon. The action potential depolarises the membrane, thus opening voltage-gated calcium channels, increasing the amount of calcium entering the cell. ...read more.

Conclusion

This feature allows astrocytes to respond to several different stimuli. They also have the ability to release metabolic substances and neuroactive molecules themselves, which means their role in neuronal survival is not to be underestimated, especially since one of the neuroactive molecules is known to be the potentially neurotrophic somatostatin. Their effects reach to other glial cells too, as they are linked via gap junctions, allowing astrocytes to interact with oligodendrocytes to influence myelin turnover, for example. Brain tumours most commonly originate from neuroglial cells, and in particular from astrocytes. This is due to the ability of glia to replicate (unlike neurons), and so are susceptible to neoplasia. Oligodendrocytes are neural tube-derived cells responsible for producing the myelin sheath that surrounds some neuronal axons, and provides electrical insulation and thus faster impulse propagation. Schwann cells have the same origin and role as oligodendrocytes, but are found surrounding axons in the PNS. With Schwann cells, one cell forms myelin around one axon, whilst oligodendrocytes have the ability to branch and serve more than one neuron at a time. Ependymal cells are low columnar ciliated epithelial cells that line the ventricular system of the brain. Cilia present on their free surface help to propel CSF through the ventricles. Microglia are small, elongated cells with short irregular processes. They are derived from bone marrow precursor cells, and are the nervous equivalent of phagocytic cells. In the adult CNS, microglia are responsible for inflammation and repair. ...read more.

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