The nervous system has three functions; sensory input, integration, and motor
output.
- Sensory input is the conduction of signals from sensory receptors. Sensory
receptors are structures that detect changes in the external and internal
environment due to stimuli. They detect stimuli such as light, heat, pressure, and
chemicals.
- Integration is the responsibility of the central nervous system, brain and
spinal cord. It includes interpretation and determining the appropriate response
i.e..
motor output.
- Motor output is the conduction of signals from the processing center to
effector cell that carry out the body's response. Nerves that conduct in this way
are
grouped together in the peripheral nervous system.
Evolution
Neurons function in a similar manner throughout the animal kingdom, yet,
each nervous system is diverse and unique.
Hydra are among the least complicated animals in the animal kingdom.
their nervous system is comprised of a nerve net. The nerve cells are
evenly spread throughout the body. When triggered, the impulse is carried
by an electric charge that runs in both directions on the neurons. Hydra has no
centralization of nerves.
As animals become more advanced so do their nervous systems. In
planarian we see the first evidence of cephalization, the definition of a
central area of the nervous system or a brain. In planarian the nerve
impulse is directed from the brain along the parallel central nerve cords
to the body.
Arthropods, such as the grasshopper, have even greater cephalization.
Earthworms have also developed a well defined brain and a single ventral
nerve cord that controls the impulses.
Vertebrates have the most defined and developed nervous system. The
vertebrate nervous system is composed of a brain and a dorsal hollow
nerve cord, as well as a peripheral nervous system. Below is a map/ flow
chart of the human nervous system.
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