The nervous system along with the endocrine system work together to coordinate all of the body systems. It does this by detecting, storing, transmitting and responding to information or stimui.
The nervous system can be anatomically subdivided into the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system consists of the brain and spinal cord and the peripheral nervous system consists of the spinal nerves and ganglia.
Neurons or nerve cells are the basic components of the nervous system and our bodies contain billions of them. Supporting and protecting the neurons are neuroglia cells which form a type of connective tissue around the nerve cells.
Neurons come in various shapes and sizes but they all contain a cell body and usually two processes; a dendrite and an axon. Dendrites are short, thin branched projections (the word dendrite is derived from the Greek word "dendron", which means tree) that receive signals and transmit them towards the cell body. They form synapses with other neurons and respond to neurotransmitters. Axons are long straight projections which transmit signals (action potentials) away from the cell body. Their ends branch to form presynaptic terminals which contain neurotransmitters to send signals away from the cell.
Neurons can be classified according to their structure or by the direction in which the action potentials travel.
Neuroglia are essential for the normal functioning of the nervous system. They have a number of supporting roles throughout the nervous system and there are 5 different types of neuroglia cells which carry out these functions.
The lipid rich membrane of the oligodendrites or schwann cells tightly wrap around a section of an axon several times like a swiss roll. It is this tightly packed membrane that forms the myelin sheath around an axon, which is now known as a myelinated fibre. Cells line up in rows along the axon and between each adjacent oligodendrite or schwann cell is a tiny gap called a node of Ranvier. The myelin sheath acts like as an insulator between the nodes of Ranvier, only allowing the action potential to leap from node to node rather than to travel along the entire length of the axon. This means that axons with a myelin sheath conduct action potentials quicker along their length than unmyelinated axons. The myelin sheath also prevents the action potential from being passed to adjacent neurons as well as protecting the fibre.
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In every part of our body are electrically charged particles known as ions which can be positively or negatively charged. Neurons rely on these differently charged ions to create and conduct electrical impulses (action potentials).
All cells have a 'resting potential', meaning when at rest the overall charge of ions inside the cell are negative compared to the ions outside the cell in the extracellular fluid. The difference in charge across the cell membrane of a neuron creates a potential electrical difference of about -70 milivolts (mV). The cell membrane maintains this resting potential by selectively allowing some ions to pass into the cell via special channels or gates and by blocking the entry of other ions. Due to the electrochemical gradients Na+ slowly diffuses into the neuron and K+ slowly diffuses out of the neuron. Because of this natural diffusion the resting neuron must actively pump Na+ out of the cell and take K+ in to maintain its resting potential of -70 mV.
When a neuron is stimulated a section of its membrane becomes depolarised by the exchange of ions across it. A section of the cell membrane opens its sodium channels allowing sodium ions to move inside the cell. The sodium ions are positively charged and are attracted into the cell by the negatively charged ions inside, as well as the lower sodium concentrations. The influx of positive ions reverses (depolarises) the resting potential and the inside of the neuron becomes more positively charged. When depolarisation reaches a certain level or threshold, i.e. the voltage inside the cell reaches at least -55 milivolts, it triggers the opening of more sodium channels which in turn triggers the opening of sodium channels in the adjacent cell membrane. Thus depolarisation is spread along the entire cell membrane in a wave; this is an action potential and conducts the nerve impulse along the axon. Once the action potential reaches the end of the axon the action potential is converted to a chemical signal by the release of a neurotransmitter. The inside of the cell continues to depolarise until the voltage peaks at about +35 milivolts, at which point the cell membrane closes its sodium channels so that no more Na+ can enter, and opens its K+ channels to allow the positively charged K+ ions to leave the cell. This process reverses the depolarisation (repolarisation) and allows the neuron to return to its original resting potential of -70 milivolts.