All other types of animals, with the exception of echinoderms and a few types of worms, have a nervous system 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 nervous system ranges from a few hundred cells in the simplest worms, to on the order of 100 billion cells in humans.
The connections between neurons form neural circuits that can generate very complex patterns of dynamical activity.
Glial cells (named from the Greek word for "glue") are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system (Allen, 2009).
In the human brain, it is currently estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas (Azevedo et al., 2009).
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 to other neurons, called synapses.
A cell that receives a synaptic signal from a neuron (a postsynaptic neuron) may be excited, inhibited, or otherwise modulated.
The nervous system is the part of an animal's body that coordinates its behavior and transmits signals between different body areas.
In vertebrates it consists of two main parts, called the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS consists mainly of nerves, which are long fibers that connect the CNS to every other part of the body, but also includes other components such as peripheral ganglia, sympathetic and parasympathetic ganglia, and the enteric nervous system, a semi-independent part of the nervous system whose function is to control the gastrointestinal system.
Recent evidence suggests that glia may also have a substantial signaling role.
Nervous systems are found in almost all multicellular animals, but vary greatly in complexity.
A microscopic examination shows that nerves consist primarily of the axons of neurons, along with a variety of membranes that wrap around them.
The neurons that give rise to nerves do not generally lie within the nerves themselves — their cell bodies reside within the brain, central cord, or peripheral ganglia.
All animals more derived than sponges have nervous systems.