Nervous System

The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and processes input from the senses, and initiates actions. (see Central Nervous System).

In animals with brains, the nervous system also generates and conducts thoughts and emotions. Thus it is the system that animates "animals" (sponges are an exception). Chemicals that target the activity of nerves generally are the most rapidly acting toxins, typically causing paralysis and/or death.

Anatomy

The nervous system consists basically of two types of cells:

Neurons are the primary cells of the nervous system
Glia are secondary cells involved in nourishment and structural support.
Rapid signalling within the nervous system occurs by two primary mechanisms:

Within neuronal nerve fibers by way of action potentials
Between neurons by way of neurotransmitter diffusion across synapses

Vertebrate nervous systemsThe nervous systems of vertebrate animals are often divided into a central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists of all other nerves and neurons that do not lie within the CNS. The large majority of what are commonly called nerves (which are actually axonal processes of nerve cells) are considered to be PNS. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.

The somatic nervous system is responsible for coordinating the body's movements, and also for receiving external stimuli. The autonomic nervous system is the involuntary part of the nervous system where all of the internal maintenance is taken care of.

The autonomic nervous system is then divided into the sympathetic division and parasympathetic division. The sympathetic nervous system responds to impending danger or stress, and is responsible for the increase of one's heartbeat and blood pressure, among other physiological changes, along with the sense of excitement he feels. The parasympathetic nervous system, on the other hand, is evident when a person is resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and geniturinary systems.