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Both autonomic and enteric nervous systems function involuntarily. The enteric nervous system functions to control the gastrointestinal system. 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.
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The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. Somatic nerves mediate voluntary movement. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Spinal nerves are mixed nerves that serve both functions. 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. 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. The CNS consists of the brain and spinal cord. In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The inflexibility of the positions in the scientific debate on both sides has led to the proposal of a solution to the debate, consisting of redefining the concept of the nervous system by using only physiological criteria and avoiding phylogenetic criteria. This proposal has led to a dispute in the scientific community between those who think we should talk about the nervous system of plants and those who are against it. On the basis of these findings, some scientists have proposed that a plant nervous system exists and that a scientific field called plant neurobiology should be created. However, this classical doctrine has been challenged in recent decades by discoveries about the existence and use of electrical signals in plants. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. In biology, the classical doctrine of the nervous system determines that it is a highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body.