Learning Objectives
By the end of this section, you will be able to:
Explain how neurons and glial cells work together to perform and support the nervous system functions.
- Describe the basic structure of a neuron and how these structures function in a neuron
- Identify the different types of neurons on the basis of shape
- List the glial cells of the CNS and describe their function
- List the glial cells of the PNS and describe their function
Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to communicate between each other and with target cells. Glial cells, or glia or neuroglia, are much smaller than neurons and play a supporting role for nervous tissue. Glial cells maintain the extracellular environment around neurons, improve signal conduction in neurons and protect them from pathogens. Ongoing research also suggests that glial cell number matches neuron number and that they even can send signals themselves.
Neuron Anatomy
Neurons are nucleated cells with specialized structural properties. Some neurons have a single long extension (axon) that reaches great distances, others are very small, star shaped cells without obvious axons (See Figure 12.2.1 – add to image the term axon, reference cells without one).
Though neuron shapes vary greatly, every neuron houses its nucleus in a region known as the cell body (also called soma) from which cellular activity like repair or cell membrane recycling is controlled. Associated with the nucleus, neurons also have many rough endoplasmic reticula, called Nissl bodies (these can be seen in neurons using a light microscope). The nucleus, Nissl bodies and golgi apparatuses together produce the many ion channels and pumps that reside in the cell membrane. These transmembrane proteins are neccessary for neurons to send electrical signals (graded potentials and action potentials, see section 12.4). In addition, neurons consume much ATP and typically have many mitochondria.
In figure 12.2.2, the cell body shows both many short projections and one long projection emerging from the cell body. These short projections are dendrites which receive most of the input from other neurons or stimuli in the extracellular environment; the location of the dendrites on the neuron marks the receptive region of the neuron. Dendrites are usually highly branched processes, providing locations for other neurons to communicate with the neuron. Neurons have polarity—meaning that information flows in one direction through the neuron. In the figure 12.2.2 neuron, information flows from the dendrites, across the cell body, and down the large axon emerging from the cell body at the axon hillock (axon hillock is an anatomical term to describe where the cell body and axon meet). The first section of the axon where an action potential is generated is called the