![]() An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. The cell body contains a specialized structure, the axon hillock that integrates signals from multiple synapses and serves as a junction between the cell body and an axon. Once a signal is received by the dendrite, it then travels passively to the cell body. Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections. Although some neurons do not have any dendrites, some types of neurons have multiple dendrites. Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. ![]() Neurons also contain unique structures, illustrated in Figure 16.3 for receiving and sending the electrical signals that make neuronal communication possible. Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. Sweeney, NOAA credit f: modification of work by NIH) (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 systems-they 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 “brain” 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 “nerve net.” 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 16.2.
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