These gaps are called nodes of Ranvier and are sites where the signal is “recharged” as it travels along the axon. Along the axon there are periodic gaps in the myelin sheath. The myelin sheath is not actually part of the neuron. This insulation is important as the axon from a human motor neuron can be as long as a meter-from the base of the spine to the toes. Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon, greatly increasing the speed on conduction. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Chemicals released at axon terminals allow signals to be communicated to these other cells. These terminals in turn synapse on other neurons, muscle, or target organs. 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 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. 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. 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 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. Compare the functions of different types of glial cells.List and describe the four main types of neurons.List and describe the functions of the structural components of a neuron.

By the end of this section, you will be able to:

Motor neuron dendrite cell body