<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <Ul> <Li> Neuroscience portal </Li> </Ul> <P> The nervous system is the part of an animal that coordinates its actions 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 or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent . Spinal nerves serve both functions and are called mixed nerves . 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 . </P> <P> At the cellular level, the nervous system is defined by the presence of a special type of cell, called the neuron, also known as a "nerve cell". 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 waves traveling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions called 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 . </P>

What are the two principal divisions of the nervous system