Passive Electrical Flow in Neurons

Neurons are among the most important and interesting cells in the body. They are the fundamental building blocks of the central nervous system and hence responsible for motor control, cognition, perception, and memory, among other things. Although our understanding of how networks of neurons interact to form an intelligent system is extremely limited, one prerequisite for an understanding of the nervous system is an understanding of how individual nerve cells behave.

There is a great deal of experimental data indicating that parts of neurons conduct electricity in a passive manner. Thus, there has been developed an extensive body of theory describing the flow of electricity in neurons using the theory of electrical flow in cables. A cable is any structure that provides a one-dimensional pathway for communication via an electrical signal. Neurons are among the most abundant cells with a cable structure, although skeletal muscle and cardiac cells also have cable-like features.

A typical neuron consists of three principal parts: the dendrites; the cell body, or soma; and the axon. The structure of some typical neurons is shown in Fig. 8.1. Den-drites are the input stage of a neuron and receive synaptic input from other neurons. The soma contains the necessary cellular machinery such as a nucleus and mitochondria, and the axon is the output stage. At the end of the axon (which may also be branched, as are the dendrites) are synapses, which are cellular junctions specialized for the transmission of an electrical signal (Chapter 7). Thus, a single neuron may receive input along its dendrites from a large number of other neurons, which is called convergence, and may similarly transmit a signal along its axon to many other neurons, called divergence.

The behaviors of the dendrites, axon, and synapse are all quite different. The spread of electrical current in a dendritic network is (mostly) a passive process that can be well described by the diffusion of electricity along a leaky cable. The axon, on the other

Passive Processes Neurons

Figure 8.1 Structure of typical neurons. The motor neuron is from a mammalian spinal cord and was drawn by Dieters in 1869. The other cells were drawn by Ramon y Cajal. The pyramidal cell is from mouse cortex, and the mitral cell from the olfactory bulb of a cat. (Kuffler et al., 1984, Fig. 1, p. 10.)

Figure 8.1 Structure of typical neurons. The motor neuron is from a mammalian spinal cord and was drawn by Dieters in 1869. The other cells were drawn by Ramon y Cajal. The pyramidal cell is from mouse cortex, and the mitral cell from the olfactory bulb of a cat. (Kuffler et al., 1984, Fig. 1, p. 10.)

hand, has an excitable membrane of the type described in Chapter 4, and thus can propagate an electrical signal actively. At the synapse (Chapter 7), the membrane is specialized for the release or reception of chemical neurotransmitters. In this chapter we discuss how to model the behavior of a cable, and then focus on the passive spread of current in a dendritic network; in the following chapter we show how an excitable membrane can actively propagate an electrical impulse, or action potential.

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Responses

  • aman
    How is the structure of the skeletal muscle similar to an electrical cord?
    8 years ago
  • costante
    What is passive electrical signal in neurons?
    4 years ago

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