Nervous system

The nervous system of an animal coordinates muscle movement, monitors the organss, constructs and processes impressions from the senses, and initiates actions. In animals with brains, it also generates and conducts thoughts and emotions. Thus it is the system that animates "animals" (sponges are the exception). Chemicals that target the activity of nerves generally are the most rapidly acting toxins, typically causing paralysis and/or death.

The nervous system consists basically of two types of cells:

• Neurons
• Glia

Neurons are the primary cells of the nervous system, in that they more or less independently conduct the system's moment-to-moment activities, whereas glia play what at least appears to be a more supporting role, such as nourishment and structural support.

Rapid signalling within the nervous system occurs by two primary mechanisms:

• Within nerves and neurons by way of action potentials
• Between neurons by way of neurotransmitter diffusion across synapses.

For heuristic and other purposes, the nervous system is divided into the central and the peripheral nervous systemss, of which the autonomic nervous system is part.

The nervous system varies greatly among living animals. Animals belonging to the phylum Porifera, mainly sponges, do not have nerves. Cnidarians, such as sea anemones, have a nerve net but no association neurons. In addition, their nerves are bidirectional- both afferent and efferent messages are transported in the same nerve. Animals from the phylum Platyhelminthes have no brain while Chordates, Annelids have ganglia, which are "bunches" of nerves.