Chapter 7 - Stimulation Of Excitable Tissues

STIMULATION OF EXCITABLE TISSUES

An electrically excitable cell in its resting state is essentially a charged capacitor. The cell
membrane is the dielectric, the ionic solutions on either side of the membrane constitute
the plates, and differences in the concentrations of ions on each side generate a potential
difference of about -70 to -90 mV (measured inside the cell against a reference in the
extracellular fluid). To generate an action potential, the membrane capacitance must be
discharged by about 15 mV in a small region. This results in a brief sequence of openings
and closings of sodium and potassium channels in the membrane, which results in the flow
of the action current. The action current depolarizes and then repolarizes adjacent regions
of the cell membrane, giving rise to the action potential.

Excitable cells can be activated by a variety of stimuli, which include burning, mechanical
trauma, electrical currents, and very intense variable magnetic fields. If sufficiently
strong, any of these stimuli can depolarize the membrane of the excitable cells to a threshold
voltage level at which the regenerative mechanisms of the action potential take over.
However, the most common method of stimulating excitable tissue artificially is to pass an
electrical current through the target tissue.

Hodgkin and Huxley’s classical experiments on excitable cells were carried out by placing
electrodes inside the cells under study. They wisely chose a huge cell membrane (at
least as far as cells go), the giant squid axon, to make it easier to manipulate the electrodes
without destroying cells. To analyze the nonlinear properties of ion conductances underlying
action potentials, Hodgkin and Huxley [1952] used the voltage-clamp technique¹
developed by Kenneth Cole. As shown in Figure 7.1, space-clamp experiments usually
involve inserting two electrode wires into the axon, one for recording the transmembrane
voltage and the other for passing current into the axon. In the voltage clamp, the same

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¹Cell electrophysiology is outside the scope of this book. However, if you are interested in the subject, we would
like to refer you to what we consider is the most no-nonsense source of information on cell electrophysiology
and biophysics techniques: Axon Instruments Inc. publishes The Axon Guide for Electrophysiology & Biophysics,
which is a practical laboratory guide covering a broad range of topics, from the biological basis of bioelectricity
and a description of the basic experimental setup (including how to make pipette microelectrodes) to the principles
of operation of advanced electrophysiology lab hardware and software. Best of all, you can download the
complete guide free from Axon’s Web site at www.axon.com/MR_Axon_Guide.html.

STIMULATION OF EXCITABLE TISSUES

An electrically excitable cell in its resting state is essentially a charged capacitor. The cell
membrane is the dielectric, the ionic solutions on either side of the membrane constitute
the plates, and differences in the concentrations of ions on each side generate a potential
difference of about -70 to -90 mV (measured inside the cell against a reference in the
extracellular fluid). To generate an action potential, the membrane capacitance must be
discharged by about 15 mV in a small region. This results in a brief sequence of openings
and closings of sodium and potassium channels in the membrane, which results in the flow
of the action current. The action current depolarizes and then repolarizes adjacent regions
of the cell membrane, giving rise to the action potential.

Excitable cells can be activated by a variety of stimuli, which include burning, mechanical
trauma, electrical currents, and very intense variable magnetic fields. If sufficiently
strong, any of these stimuli can depolarize the membrane of the excitable cells to a threshold
voltage level at which...