The complexity of nervous systems differs from organism to organism. In the simplest of organisms, the nervous system may consist of little more than a random collection of neurons. Such systems are known as a nerve net. An example of an animal with a nerve net is the hydra, a cylinder-shaped freshwater polyp. Hydra respond to stimuli such as heat, light, and touch, but their nerve net is not a very effective way to transmit messages. Their responses tend to be weak and localized.
In other organisms, neurons are bunched together in structures known as ganglia (single: ganglion). Flatworms, for example, have a pair of ganglia that function like a simple brain. The ganglia are attached to two nerve cords that run the length of the worm's body. These two cords are attached to each other by other nerves. This kind of nervous system is sometimes described as a ladder-type nervous system.
The most complex nervous systems are found in the vertebrates (animals with backbones), including humans. These nervous systems consist of two major divisions: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord, and the peripheral system of all neurons outside the central nervous system. The brains of different vertebrate species differ from each other in their size and complexity, but all contain three general areas, known as the forebrain, midbrain, and hind-brain. These areas look different, however, and have somewhat different functions in various species.
The peripheral nervous system consists of two kinds of neurons known as sensory neurons and motor neurons. Sensory neurons are located in the sensory organs, such as the eye and ear. They are able to detect stimuli from outside the organism, such as light or sound. They then pass that information through the peripheral nervous system to the spinal cord and then on to the brain. Motor neurons carry messages from the brain, through the spinal cord, and to the muscles. They tell certain muscles to contract in order to respond to stimuli in some way or another.
The peripheral nervous system can be subdivided into two parts: the somatic system and the autonomic system. The somatic system involves the skeletal muscles. It is considered to be a voluntary system since the brain exerts control over movements such as writing or throwing a ball. The autonomic nervous system affects internal organs, such as the heart, lungs, stomach, and liver. It is considered to be an involuntary system since the processes it controls occur without conscious effort on the part of an individual. For example, we do not need to think about digesting our food in order for that event to take place.
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