![]() But neurons are also highly specializedâdifferent types of neurons have different sizes and shapes that relate to their functional roles. Most neurons share the same cellular components. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors. Despite these very different numbers, the nervous systems of these animals control many of the same behaviorsâfrom basic reflexes to more complicated behaviors like finding food and courting mates. A human brain contains around 86 billion neurons. This number compares to 75 million in the mouse and 300 million in the octopus. The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. There are four major types of neurons, and they share several important cellular components. There is great diversity in the types of neurons and glia that are present in different parts of the nervous system. Although glia have been compared to workers, recent evidence suggests that they also usurp some of the signaling functions of neurons. Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. A neuron can be compared to an electrical wireâit transmits a signal from one place to another. The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. Watch this video of biologist Mark Kirschner discussing the âflippingâ phenomenon of vertebrate evolution. ![]() There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the invertebrate body plan arrangement somehow âflippedâ during the evolution of vertebrates. One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally whereas the vertebrate spinal cords are located dorsally. While there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS that contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves. Sweeney, NOAA credit f: modification of work by NIH)Ĭompared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. (credit e: modification of work by Michael Vecchione, Clyde F.E. In (f) vertebrates, the brain and spinal cord comprise the central nervous system, while neurons extending into the rest of the body comprise the peripheral nervous system. Mollusks such as squid and (e) octopi, which must hunt to survive, have complex brains containing millions of neurons. In addition to a brain, (d) arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. In animals exhibiting bilateral symmetry such as (c) planarians, neurons cluster into an anterior brain that processes information. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In (a) cnidarians, nerve cells form a decentralized nerve net. Nervous systems vary in structure and complexity. Octopi may have the most complicated of invertebrate nervous systemsâthey have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species. These ganglia can control movements and behaviors without input from the brain. It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). The insect nervous system is more complex but also fairly decentralized. Flatworms of the phylum Platyhelminthes have both a central nervous system (CNS), made up of a small âbrainâ and two nerve cords, and a peripheral nervous system (PNS) containing a system of nerves that extend throughout the body. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a ânerve net.â Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves. Some organisms, like sea sponges, lack a true nervous system. Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure.
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