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After studying this chapter, you should be able to:

  1. Draw a typical neuron and identify the role played by soma, dendrites, axon, and initial segment in impulse generation and conduction.

  2. Explain the basis for the resting membrane potential of a neuron and the effect of hyperkalemia and hypokalemia on the resting potential.

  3. Explain the ionic fluxes that occur during an action potential.

  4. Compare and contrast how unmyelinated and myelinated neurons propagate impulses.

  5. Compare the conduction velocity and other properties of different types of sensory and motor nerve fibers.

  6. Explain the importance of orthograde and retrograde axonal transport.

  7. Compare the functions of the various types of glia found in the nervous system.

  8. Identify neuropathologies related to dysfunction of myelin proteins or the loss of myelin.

  9. Describe the function of neurotrophins.


The basic working unit of the central and peripheral nervous system is the nerve cell or neuron. Neurons are identified as excitable cells because they have the ability to be electrically excited resulting in the generation of action potentials. Other examples of excitable cells are skeletal, smooth, and cardiac muscle cells (Chapter 5) and secretory cells of the pancreas. Neurons come in many different shapes and sizes, but they share features that impart in them an ability to receive, process, integrate, and transmit information from external and internal sources to initiate most physiological behaviors. This chapter describes the ionic mechanisms that enable neurons to generate and conduct impulses; Chapter 6 explains how neurons communicate with other neurons and effector organs (synaptic transmission). This chapter also describes the roles played by neuroglia cells and neurotrophins in physiological and pathophysiological processes.


Figure 4–1 shows the basic components of a neuron for the prototypical spinal motor neuron. The cell body (soma) contains the nucleus that is the metabolic center of the neuron and stores the hereditary material or DNA. Neurons have several processes called dendrites that extend outward from the cell body and arborize extensively to aid their role in receiving incoming signals, processing the information, and then transmitting the information to the soma of the neuron. A typical neuron also has a long fibrous axon that originates from a thickened area of the cell body (axon hillock). The first portion of the axon is called the initial segment. The axon divides into presynaptic terminals, each ending in a number of synaptic knobs that are also called terminal buttons or boutons. They contain granules or vesicles in which the synaptic transmitters released by the nerves are stored. Based on the number of processes that emanate from their cell body, neurons can be classified as unipolar, bipolar, pseudounipolar, and multipolar (Figure 4–2).


Motor neuron with a myelinated axon. A motor neuron is composed of a cell body (soma) with a nucleus, several ...

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