Biomedical Engineering Reference
In-Depth Information
12.1 INTRODUCTION
Chapter 3 briefly described the nervous system and the concept of a neuron. Here, the
description of a neuron is extended by examining its properties at rest and during excita-
tion. The concepts introduced here are basic and allow further investigation of more sophis-
ticated models of the neuron or groups of neurons by using GENESIS (a general neural
simulation program; see suggested reading by J.M. Bower and D. Beeman) or extensions
of the Hodgkin-Huxley model by using more accurate ion channel descriptions and neural
networks. The models introduced here are an important first step in understanding the
nervous system and how it functions.
Models of the neuron presented in this chapter have a rich history of development. This
history continues today as new discoveries unfold that supplant existing theories and
models. Much of the physiological interest in models of a neuron involves the neuron's
use in transferring and storing information, while much engineering interest involves the
neuron's use as a template in computer architecture and neural networks. New develop-
ments in brain-machine interfacing make understanding the neuron even more important
today (see [4] and [1] for additional information). To fully appreciate the operation of a
neuron, it is important to understand the properties of a membrane at rest by using stan-
dard biophysics, biochemistry, and electric circuit tools. In this way, a more qualitative
awareness of signaling via the generation of the action potential can be better understood.
The Hodgkin and Huxley theory that was published in 1952 described a series of experi-
ments that allowed the development of a model of the action potential. This work was
awarded a Nobel Prize in 1963 (shared with John Eccles) and is covered in Section 12.6.It
is reasonable to question the usefulness of covering the Hodgkin-Huxley model in a text-
book today, given all of the advances since 1952. One simple answer is that this model is
one of the few timeless classics and should be covered. Another is that all current, and per-
haps future, models have their roots in this model.
Section 12.2 describes a brief history of bioelectricity and can be easily omitted on first
reading of the chapter. Section 12.3 describes the structure and provides a qualitative
description of a neuron. Biophysics and biochemical tools that are useful in understanding
the properties of a neuron at rest are presented in Section 12.4. An equivalent circuit model
of a cell membrane at rest consisting of resistors, capacitors, and voltage sources is
described in Section 12.5. Section 12.6 describes the Hodgkin-Huxley model of a neuron
and includes a brief description of their experiments and the mathematical model describ-
ing an action potential. Finally, Section 12.7 provides a model of the whole neuron.
12.2 HISTORY
12.2.1 The Evolution of a Discipline: The Galvani-Volta Controversy
In 1791, an article appeared in the Proceedings of the Bologna Academy, reporting
experimental results that, it was claimed, proved the existence of animal electricity.
This now famous publication was the work of Luigi Galvani. At the time of its publica-
tion, this article caused a great deal of excitement in the scientific community and sparked