The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals betweendifferent parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous system of vertebrates (such ashumans) contains the brain, spinal cord, and retina. The peripheral nervous system consists of sensory neurons, clusters of neurons called ganglia, and nervesconnecting them to each other and to the central nervous system. These regions are all interconnected by means of complex neural pathways. The enteric nervous system, asubsystem of the peripheral nervous system, has the capacity, even when severed from the rest of the nervous system through its primary connection by the vagus nerve, tofunction independently in controlling the gastrointestinal system.
At the most basic level, the function of the nervous system is to send signals from one cell toothers, or from one part of the body to others. There are multiple ways that a cell can send signals to other cells. One is by releasing chemicals called hormonesinto the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides"point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling is capable of amuch higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second.
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