Nervous system:
The nervous system is the part of the animal's body that regulates its activity and transmits signals between different parts of the body. In vertebrates it contains two main components, called the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain and spinal cord. PNS consists mainly of nerves, which are the long lines that connect the CNS to all other parts of the body, but also include other things such as the peripheral ganglia, the sympathetic and parasympathetic ganglia, and the enteric nervous system, the part that represents the nervous system function its to control the digestive system.
At the cellular level, the nervous system is defined by the presence of a special type of cell, called a neuron, also known as a "nerve cell". Neurons have special features that allow them to send signals quickly and directly to other cells. They transmit these signals in the form of electrical waves that travel through tiny fibers called axons, which cause chemicals called neurotransmitters to be released at the interaction of other neurons, called synapses. A cell that receives a synaptic signal from a neuron (postsynaptic neuron) can be excited, blocked, or otherwise activated. Interactions between neurons form neural circuits that can create highly complex patterns of dynamic activity. Along with the nerves, the nervous system also contains other specialized cells called glial cells (or glia), which provide structural and physical support. Recent evidence suggests that glia could also play a major role in signing.
Nerve systems are found in almost all animals with many cells, but they vary greatly in appearance. The only animals with many cells that do not have a nervous system at all are sponges and small blob-like creatures called placozoans and masozoans. The sensory systems of ctenophores (comb jellies) and cnidarians (e.g., anemones, hydra, coral and jellyfishes) contain a diffuse nerve net. All other species of animals, with the exception of echinoderms and a few species of worms, have a sensory system consisting of a brain, a central nerve (or two parallel nerves), and nerves from the brain and central nervous system. The size of the nervous system ranges from a few hundred simple worm cells, to a sequence of 100 billion cells in humans.
At the most basic level, the function of the nervous system is to control the body's movement and affect the environment (e.g., with pheromones). This is achieved by sending signals from one cell to another, or from one part of the body to another. The excitement in the nervous system arises from the signals that travel to the muscle cells, triggering muscle activation, and from the signals that move to the endocrine cells, causing hormones to be released from the blood or other internal fluids. The input to the nervous system comes from very different types of nerve cells, which transmit bodily functions such as light and sound to neural functions. Internally, the sensory system contains complex logs of communication between nerve cells that allow it to produce activity patterns that rely solely on sensory input. The nervous system is also able to store information over time, by dramatically altering the power of communication between neurons, and other means.
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