Diversity of Nervous Systems

Learning Outcomes

  • Differentiate between the nervous systems of different animals

Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure 1. Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves.

Flatworms of the phylum Platyhelminthes have both a central nervous system (CNS), made up of a small “brain” and two nerve cords, and a peripheral nervous system (PNS) containing a system of nerves that extend throughout the body. The insect nervous system is more complex but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can control movements and behaviors without input from the brain. Octopi may have the most complicated of invertebrate nervous systems—they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.

Illustration A shows the nerve net of a hydra, which resembles a fish net surrounding the body. Illustration B shows the nervous system of a sea star. A nerve ring is present in the center of the body. Radiating out from this ring into the five arms are radial nerves. Illustration C shows the nervous system of a planarian, or flatworm. The flatworm has centralized ganglia, or brains, around each eye in the anterior end, and two nerve cords that run along the sides of the body. Transverse nerves connect the nerve cords together. Illustration D shows the nervous system of a bee. The central ganglia, or brain, is located in the head. The ventral nerve cord runs along the lower part of the body. Bumps of nerve cell bodies, called peripheral ganglia, occur periodically along the nerve cord. Illustration E shows the nervous system of the octopus, which consists of a large brain located between the two eyes, and nerves that run into the body and arms. Two large ganglia exist in the nerves located in the body. Illustration F shows the nervous system of a human, which consists of a central nervous system composed of the brain and spinal cord, and a peripheral nervous system composed of the nerves running into the rest of the body.

Figure 1. Nervous systems vary in structure and complexity. In (a) cnidarians, nerve cells form a decentralized nerve net. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In animals exhibiting bilateral symmetry such as (c) planarians, neurons cluster into an anterior brain that processes information. In addition to a brain, (d) arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. Mollusks such as squid and (e) octopi, which must hunt to survive, have complex brains containing millions of neurons. 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. (credit e: modification of work by Michael Vecchione, Clyde F.E. Roper, and Michael J. Sweeney, NOAA; credit f: modification of work by NIH)

Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. 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. 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. There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates.

Watch this video of biologist Mark Kirschner discussing the “flipping” phenomenon of vertebrate evolution.

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