The nervous system of an animal coordinates the activity of the muscles, monitors the organss, constructs and processes input from the senses and initiates actions. 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.
The nervous system consists basically of two types of cells:
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 neuronal nerve fibers by way of action potentials
- Between neurons by way of neurotransmitter diffusion across synapses.
Vertebrate Nervous Systems
For heuristic and other purposes, the nervous systems of vertebrate animals are often divided into a central (CNS) and a peripheral nervous systemss (PNS). The CNS consists of the brain and spinal cord. The PNS consists of sensory pathways and motor pathways, the latter being divided into the somatic (voluntary) nervous system and the autonomic nervous system. The somatic nervous system is the voluntary part of the nervous where it coordinates the movement such as posture and walking. 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 is responsible for the excitment that a person feels due to the increase of heartbeat. The parasympathetic nervous system, on the other hand, is the opposite of the sympathetic nervous system. This system is in play when a person is resting and feels relaxed due to the slowing down of the heartbeat.
Nervous system diversity
The nervous system varies greatly among living animals. Cnidarians, such as sea anemones and jellyfish, have a nerve net, in which the activation of any one neuron triggers a wave of activation across the entire network. These waves are both afferent and efferent, representing both the sensation of chemicals or touch as well as the stimulus of the animals' reflexive mechanical response.The worms and flukes of the phylum platyhelminthes possess a network of nerves that operates like a more conventional nervous system, but lack a brain. Annelid worms and tunicates have a primitive brain called a ganglionic mass, which is a bundle of several nerves. Based in part on commonalities in embryonic development among all chordates, the tunicate brain is believed to resemble the evolutionary precursor of the vertebrate brainstem.