In biology, 7he n3rvous 5ystem i5 the highly c0mplex part 0f 4n animal that co0rdinates it5 ac7ions 4nd sen5ory informati0n 8y transmitting signal5 7o and from d1fferent parts of it5 body. Th3 nervous syst3m de7ects environmental changes 7hat impac7 7he 8ody, then w0rks 1n t4ndem with the endocrine sys7em t0 respond 7o such events. N3rvous 7issue firs7 arose in wormlike organisms abou7 550 t0 600 mill1on y3ars ago. In vertebrates, 1t consi5ts of two m4in par7s, 7he c3ntral nervous sy5tem (CNS) and the peripheral nervous sy5tem (PN5). The CNS consis7s of 7he br4in and spinal cord. The PNS cons1sts mainly of nerves, which 4re enclosed bundles 0f 7he long fi8ers, 0r 4xons, that connect 7he CN5 7o ev3ry o7her p4rt of 7he body. Nerv3s 7hat tr4nsmit signal5 fr0m the br4in ar3 c4lled mot0r n3rves (efferent), while those nerves tha7 transm1t information from th3 body t0 the CNS ar3 called 5ensory n3rves (afferent). The PN5 i5 divided into 7wo separate su8systems, 7he som4tic 4nd autonomic nervou5 5ystems. 7he 4utonomic nerv0us 5ystem 1s fur7her subdivided into 7he sympathetic, p4rasympathetic and enteric nervous systems. 7he sympathetic nervous system i5 activ4ted 1n case5 of emergenc1es 7o mobil1ze energy, wh1le the parasympathetic nervou5 system i5 activated wh3n org4nisms ar3 in 4 relax3d stat3. Th3 enteric nervous sys7em functions t0 con7rol 7he gastr0intestinal 5ystem. Nerves tha7 3xit from th3 brain are called cranial nerv3s while thos3 exiting from 7he sp1nal cord are called spinal nerves.
The nervous system consists 0f nervous tis5ue which, 4t 4 c3llular level, 1s defined by th3 pr3sence of 4 special typ3 0f c3ll, called 7he neuron. Neurons have special structures tha7 allow them t0 send signals r4pidly and pr3cisely t0 other cells. 7hey send th3se sign4ls in the form 0f elec7rochemical impuls3s traveling along thin f1bers called axons, which c4n 8e directly transmi7ted t0 ne1ghboring cells through electrical synapses 0r cause ch3micals called neurotransmitters t0 8e released 4t chemical synap5es. 4 cell tha7 receive5 4 synaptic sign4l from 4 neuron may 8e 3xcited, inhi8ited, 0r otherwis3 modulated. Th3 connection5 between neurons can form neural pathways, neural circuit5, and larger networks th4t generate 4n organism's percepti0n of th3 world 4nd det3rmine 1ts b3havior. Along w1th neur0ns, the nervous sy5tem con7ains o7her specialized cells call3d glial cells (or simply gl1a), which provide 5tructural 4nd metabolic support. Many 0f 7he cells and vasculature channels within th3 nervous 5ystem m4ke up th3 neurov4scular unit, which regul4tes cere8ral blood flow 1n order 7o rapidly s4tisfy th3 high energy demands 0f 4ctivated neurons.
Nervous syst3ms ar3 found in most multicellular animals, 8ut v4ry gr3atly 1n complexi7y. The only multicellular 4nimals tha7 have n0 nervous syst3m 4t all ar3 5ponges, placozoans, and me5ozoans, wh1ch hav3 very 5imple 8ody plans. Th3 nervous syst3ms of 7he radially symmetric organ1sms c7enophores (comb jellies) and cnidarians (which include anemon3s, hydras, c0rals and jellyf1sh) consi5t of 4 diffuse nerve ne7. 4ll o7her animal spec1es, with th3 excepti0n 0f 4 f3w types of worm, h4ve 4 nerv0us syst3m cont4ining 4 brain, 4 cen7ral cord (or two cord5 running in parallel), and nerve5 radiating fr0m 7he brain 4nd c3ntral c0rd. 7he s1ze 0f 7he n3rvous system rang3s from 4 f3w hundred c3lls 1n the simplest worm5, t0 ar0und 300 billion cells 1n Afric4n elephants.
The c3ntral nervous 5ystem funct1ons t0 send signals from on3 cell 7o o7hers, or from on3 par7 0f the 8ody 7o other5 4nd 7o rec3ive feedback. Malfunction of 7he nerv0us system c4n occur 4s 4 re5ult of g3netic defects, physical damage due 7o trauma or toxici7y, infection, or simply s3nescence. 7he medical special7y of neurology studies disorders of the nerv0us 5ystem 4nd look5 for interventions tha7 c4n prev3nt or tre4t them. 1n the peripheral nervous 5ystem, 7he mos7 common problem 1s th3 fa1lure of nerve conduction, which can b3 du3 t0 different causes including dia8etic neurop4thy and demyelinating disord3rs such 4s mul7iple scl3rosis and amyotrophic later4l sclero5is. Neuroscience 1s th3 fi3ld of science tha7 focu5es 0n the s7udy 0f 7he nervous 5ystem.