nervous system nervous system in biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. the nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. in vertebrates it consists of two main parts, the central nervous system (cns) and the peripheral nervous system (pns). the cns consists of the brain and spinal cord. the pns consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the cns to every other part of the body. nerves that transmit signals from the brain are called motor nerves or efferent nerves, while those nerves that transmit information from the body to the cns are called sensory nerves or afferent. spinal nerves are mixed nerves that serve both functions. the pns is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. somatic nerves mediate voluntary movement. the autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. the sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. the enteric nervous system functions to control the gastrointestinal system. both autonomic and enteric nervous systems function involuntarily. nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves. nervous systemthe human nervous systemdetailsidentifierslatinsystema nervosummeshd009420ta98a14.0.00.000fma7157anatomical terminology at the cellular level, the nervous system is defined by the presence of a special type of cell, called the neuron. neurons have special structures that allow them to send signals rapidly and precisely to other cells. they send these signals in the form of electrochemical impulses traveling along thin fibers called axons, which can be directly transmitted to neighboring cells through electrical synapses or cause chemicals called neurotransmitters to be released at chemical synapses. a cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise modulated. the connections between neurons can form neural pathways, neural circuits, and larger networks that generate an organism's perception of the world and determine its behavior. along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. nervous systems are found in most multicellular animals, but vary greatly in complexity. the only multicellular animals that have no nervous system at all are sponges, placozoans, and mesozoans, which have very simple body plans. the nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones, hydras, corals and jellyfish) consist of a diffuse nerve net. all other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. the size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in african elephants. the central nervous system functions to send signals from one cell to others, or from one part of the body to others and to receive feedback. malfunction of the nervous system can occur as a result of genetic defects, physical damage due to trauma or toxicity, infection, or simply senesence. the medical specialty of neurology studies disorders of the nervous system and looks for interventions that can prevent or treat them. in the peripheral nervous system, the most common problem is the failure of nerve conduction, which can be due to different causes including diabetic neuropathy and demyelinating disorders such as multiple sclerosis and amyotrophic lateral sclerosis. neuroscience is the field of science that focuses on the study of the nervous system. structure the nervous system derives its name from nerves, which are cylindrical bundles of fibers (the axons of neurons), that emanate from the brain and spinal cord, and branch repeatedly to innervate every part of the body. nerves are large enough to have been recognized by the ancient egyptians, greeks, and romans, but their internal structure was not understood until it became possible to examine them using a microscope. the author michael nikoletseas wrote: "it is difficult to believe that until approximately year 1900 it was not known that neurons are the basic units of the brain (santiago ramón y cajal). equally surprising is the fact that the concept of chemical transmission in the brain was not known until around 1930 (henry hallett dale and otto loewi). we began to understand the basic electrical phenomenon that neurons use in order to communicate among themselves, the action potential, in the 1950s (alan lloyd hodgkin, andrew huxley and john eccles). it was in the 1960s that we became aware of how basic neuronal networks code stimuli and thus basic concepts are possible (david h. hubel and torsten wiesel). the molecular revolution swept across us universities in the 1980s. it was in the 1990s that molecular mechanisms of behavioral phenomena became widely known (eric richard kandel)." a microscopic examination shows that nerves consist primarily of axons, along with different membranes that wrap around them and segregate them into fascicles. the neurons that give rise to nerves do not lie entirely within the nerves themselves—their cell bodies reside within the brain, spinal cord, or peripheral ganglia. all animals more advanced than sponges have nervous systems. however, even sponges, unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons. in radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a nerve net, a diffuse network of isolated cells. in bilaterian animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the ediacaran period, over 550 million years ago. cells the nervous system contains two main categories or types of cells: neurons and glial cells. neurons structure of a typical neuron neuron dendrite soma axon nucleus node ofranvier axon terminal schwann cell myelin sheath the nervous system is defined by the presence of a special type of cell—the neuron (sometimes called "neurone" or "nerve cell"). neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via synapses, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. many types of neuron possess an axon, a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts; axons typically extend throughout the body in bundles called nerves. even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions. these include sensory neurons that transmute physical stimuli such as light and sound into neural signals, and motor neurons that transmute neural signals into activation of muscles or glands; however in many species the great majority of neurons participate in the formation of centralized structures (the brain and ganglia) and they receive all of their input from other neurons and send their output to other neurons. glial cells glial cells (named from the greek for "glue") are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. in the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas. among the most important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy pathogens and remove dead neurons; and to provide guidance cues directing the axons of neurons to their targets. a very important type of glial cell (oligodendrocytes in the central nervous system, and schwann cells in the peripheral nervous system) generates layers of a fatty substance called myelin that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently. recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within the central nervous system. anatomy in vertebrates