Chapter 2

### OBJECTIVES

• 1) Delineate general principles underlying neuronal function and cellular signaling, including passive and active propagation of electrical potentials, communication between neurons, and how information is encoded for transmission through the nervous system

• 2) Describe the sequence of events for a neuron to receive, integrate, and transmit information, including EPSPs, IPSPs, action potentials, and synaptic transmission

• 3) Describe the role of glia in establishing and maintaining homeostasis in the CNS

• 4) Synthesize the role of microglia, astrocytes, and oligodendrocytes in motor learning and activity-dependent plasticity

• 5) Describe the factors that allow glia to communicate with each other and with neurons

### INTRODUCTION

In a multicellular organism, cells communicate with each other. This is true of all cells, and the broad name used in biology to describe this phenomenon is cellular signaling. Neurons are cells specialized for cellular signaling so that information can be received, processed, transmitted, stored, and retrieved.

As described in Chapter 3, neurons receive information from receptors in the body. These can be sensitive to internal signals (interoceptors) or external signals (exteroceptors). As described in Chapter 4, sensory neurons that receive certain kinds of input will transmit that information to the spinal cord where it can be processed to produce an outgoing neural command for a reflexive response. And for the reader of this chapter, it is hoped that some of the information can be stored and later retrieved from the brain.

How do neurons do this? This chapter will explain basic mechanisms of neural structure and function. In addition, the structure and function of the glial cells, cells in the nervous system that support the functions of neurons, will also be described.

#### A Typical Neuron

A typical neuron has five main parts. First, there is the dendritic tree. This is a structure of processes resembling the arbor of a tree that emanates from the cell body of the neuron. The dendritic tree is typically where information from other neurons is received (Figure 2-1, A, C; blue). Next is the soma, the main part of the neuron’s cell body (Figure 2-1, black). As with any cell, a neuron has a cell nucleus and the usual complement of intracellular organelles. These are contained in the soma, so from a biological perspective, the soma is the metabolic center of the cell. In terms of information processing, the soma is thought of as an integrator for most neurons (Figure 2-1, A, C, green). From a certain point of view, all the information coming into the cell, at any point in time, is ultimately represented as one number: the voltage level of the cell membrane at the soma. So, the soma integrates all that input to form one result. The next part of the neuron is called the axon hillock (Figure 2-1, gold), ...

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