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By the end of this section, you will be able to:
  • List and describe the functions of the structural components of a neuron
  • List and describe the four main types of neurons
  • Compare the functions of different types of glial cells

Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in [link] . 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.
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.

Questions & Answers

what is biology
Dada Reply
The scientific study of life.
juanita
the virus that causes mumps in humans is composed of a protein outer Shell containing a core of DNA
Daniel Reply
Basic science and applied science question about cancer
Joyce Reply
what are the importance of ATPs
Olatunji Reply
How can biology be studied from a microscopic approach to a global approach
Joyce Reply
The large central opening in the poriferan body is called
Chynna Reply
You go for a long walk on a hot day. Give an example of a way in which homeostasis keeps your body healthy.
Joyce Reply
You sweat.
juanita
sweating is your bodies way of keeping you from overheating.
juanita
Thank you
Joyce
what is biology
Neya Reply
biology is the study of life
IYANUYIMIKA
Biology is the study of Life
Brianna
is the branch of science which deals with the of living things.
Peter
what is metabolism
Sangam Reply
hi palz, what's the ?
Gbolajobi Reply
hi plz, what's the.
Atta
pls come a again
Gyamfi
Describe the steps and results of reintroducing wolves to Yellowstone National Park.
Natalia Reply
tag and release wolves into Yellowstone. wolves eventually reproduce and the pack grows. as wolves hunt they cull the sick and weak prey. the carcass that is left provides food for other species (scavengers and insect.. etc). this heals the circle of life and contributes to the biodiversity...
Will
before you know it species that are critical to the eco system return. having apex predators is crucial to an ecosystem... it helps run the deer and elk , etc around.
Will
example: there was a species of shrub/ plant that grows along river banks that moose love to eat.. the moose have no predator so they decimate that food source which also helps prevent erosion. when the wolves were reintroduced this changed. oddly enough this plant species started to repopulate in
Will
the areas where wolf feces sat and decayed
Will
which of the following statements about the parts of an egg are false?
Israel Reply
Monotremes include...?
Israel
medicinal plants including microbs
vijay Reply
,medicinal plants including microbes
vijay
what is biology
Siyanbola Reply
study of living organisms...
R0se
study of plants and animals
Divine
what is abiotic and biotic factors?
Hira Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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