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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.
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:
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 systems (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.
See also
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