13 Chapter 13: Nervous System
Chapter Outline
- 13.1 Neurons and Glial Cells
- 13.2 How Neurons Communicate
- 13.3 The Central Nervous System
- 13.4 The Peripheral Nervous System
Introduction to the Nervous System
13.1 Neurons and Glial Cells
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. A neuron can be compared to an electrical wire-it transmits a signal from one place to another. 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. Although glia have been compared to workers, recent evidence suggests that also take on some of the signaling functions of neurons.
13.1.1 Neurons
The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. This number compares to 75 million in the mouse and 300 million in the octopus. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors-from basic reflexes to more complicated behaviors like finding food and courting mates. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors.
Most neurons share the same cellular components. But neurons are also highly specialized-different types of neurons have different sizes and shapes that relate to their functional roles.
13.1.2 Parts of a Neuron
Like other cells, each neuron has a cell body that contains a nucleus, mitochondria, and other cellular components. Neurons also contain unique structures, illustrated in Figure 13.2 for receiving and sending the electrical signals that make neuronal communication possible. Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses.
Once a signal is received by the dendrite, it then travels passively to the cell body. 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. An axon is a tube-like structure that propagates (carries) the signal to specialized endings called axon terminals. These terminals in turn synapse on other neurons, muscle, or target organs. Chemicals released at axon terminals allow signals to be communicated to these other cells. Some axons are covered with myelin, which acts as an insulator to minimize loss of the electrical signal as it travels down the axon; minimizing this loss of signal greatly increasing the speed on conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter, such as the neurons that travel from the base of the spine to the