An animal’s nervous system is critical for transmitting signals within its body and coordinating voluntary and involuntary actions. Generally the nervous system includes sensors (afferent neurons), an integrating center (brain and spinal cord) and effector organs (efferent neurons). Of course, across different animal groups the complexity of the nervous system varies incredibly, but as a general rule, more complex systems are comprised of more neurons (nerve cells).
An important concept when discussing the nervous system is cephalization. It is a term related to the development of nerve tissue being concentrated in one end of the body, eventually giving rise to a head region and sensory organs. In more complex organisms, such as vertebrates, especially higher level ones, cephalization is very apparent.
In simpler invertebrates, such as those with bilateral symmetry such as arthropods (insects) and platyhelminths (flatworms), neurons tend to be grouped near the bodies anterior end (mouth region), but also a bit throughout the body. Neurons grouped throughout the body are called ganglia. Other animals with different symmetry, such as echinoderms which are radially symmetrical, have a neural net across their bodies which are made up of nerve cells.
Many invertebrates do not have a brain, such as star fish and jelly fish, whereas all vertebrates do have a brain. Throughout evolutionary time the size and sophistication of the brain has grown considerably. Furthermore, vertebrates have a unique nervous system which is highly cephalized and has a hollow dorsal nerve cord. Cartilage or bone (depending on the animal) encloses part of the vertebrate nervous system.
Additionally, the vertebrate nervous system is comprised of two parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (every other part). It is the peripheral nervous system which contains the afferent neurons (sensory branch) and the efferent branch. The efferent branch contains the autonomic nervous system (sympathetic, parasympathetic and enteric systems) and the motor nervous system.
This discussion solely acts to provide a brief introduction on how diverse animal nervous systems are. Throughout the BrainMass website there are different postings which further detail concepts related to this topic.
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