In biology, the classical doctrine of the<unk>  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. The<unk>  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. However, this classical doctrine has been challenged in recent decades by discoveries about the existence and use of electrical signals in plants. On the basis of these findings, some scientists have proposed that a plant<unk>  exists and that a scientific field called plant neurobiology should be created. This proposal has led to a dispute in the scientific community between those who think we should talk about the<unk>  of plants and those who are against it. 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<unk>  by using only physiological criteria and avoiding phylogenetic criteria. In vertebrates it consists of two main parts, the central<unk>  (CNS) and the peripheral<unk>  (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<unk> s. Somatic nerves mediate voluntary movement. The autonomic<unk>  is further subdivided into the sympathetic and the parasympathetic<unk> s. The sympathetic<unk>  is activated in cases of emergencies to mobilize energy, while the parasympathetic<unk>  is activated when organisms are in a relaxed state. The enteric<unk>  functions to control the gastrointestinal system. Both autonomic and enteric<unk> s function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves. At the cellular level, the<unk>  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 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<unk>  contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support.<unk> s are found in most multicellular animals, but vary greatly in complexity. The only multicellular animals that have no<unk>  at all are sponges, placozoans, and mesozoans, which have very simple body plans. The<unk> s 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 worms, have a<unk>  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<unk>  ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants. The central<unk>  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<unk>  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<unk>  and looks for interventions that can prevent or treat them. In the peripheral<unk> , 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<unk> .
Nervous system
Peripheral nervous system
Central nervous system
Integumentary system
Cranial nerves
Motor system
Urinary system
Reproductive system