Nervous System

The nervous system is an organ system containing a network of specialised cells called neurones that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous system contains the brain and spinal cord. The peripheral nervous system consists of sensory neurones, clusters of neurones called ganglia, and nerves connecting them to each other and to the central nervous system. These regions are all interconnected by means of complex neural pathways. The enteric nervous system, a subsystem of the peripheral nervous system, has the capacity, even when severed from the rest of the nervous system through its primary connection by the vagus nerve, to function independently in controlling the gastrointestinal system.

Neurones send signals to other cells as electrochemical waves travelling along thin fibres called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. A cell that receives a synaptic signal may be excited, inhibited, or otherwise modulated. Sensory neurones are activated by physical stimuli impinging on them, and send signals that inform the central nervous system of the state of the body and the external environment. Motor neurones, situated either in the central nervous system or in peripheral ganglia, connect the nervous system to muscles or other effector organs. Central neurones, which in vertebrates greatly outnumber the other types, make all of their input and output connections with other neurones. The interactions of all these types of neurones form neural circuits that generate an organism's perception of the world and determine its behaviour. Along with neurones, the nervous system contains other specialised cells called glial cells (or simply glia), which provide structural and metabolic support.