Electrical Equipment Background
In this lab, you are measuring the voltage of individual neurons.
We assume that you are familiar with the concept of voltages, which is the only electrical measurement you
are going to make. If you have forgotten or never really learned about voltages, please consult a basic
physics book to familiarize yourself. You should also know that the electronic instruments that are
used in electrophysiological experiments have been vastly simplified here.
The Set Up:
In real life, you would place the dissected tissue in a salt solution deep enough
to cover it. This solution simulates the internal fluid of the animal, with the
proper osmorality, pH, and ionic concentrations. The purpose of the solution is
two-fold: (1) it prevents the tissue from drying up and dying, and (2) because
a salt solution conducts electricity, it makes the extracellular space electrically continuous.
Why should you care about the extracellular space? "I thought I was measuring the voltage of the
inside of the cell, not the outside." As you recall from your recent readings of the introductory
physics text, voltage is a measure of potential difference. So, to measure the voltage of
the inside of the cell, you need a reference point against which to compare. This is achieved by
placing a reference electrode (not shown in this lab) in the
solution and connecting it to the ground (the third prong of an electrical outlet, defined as 0
volts); all your measurements are made with respect to this reference electrode. In effect,
when your microelectrode is measuring the potential
of the inside of a neuron, you are actually measuring the potential difference across the neuron's
membrane. This is called the transmembrane potential, or simply the membrane potential.
The glass microelectrode you use to penetrate the neurons is mounted on a micromanipulator.
Because it is manufactured from
glass, not an electrical conductor, the microelectrode is insulated at all points except at the tip
and the opening at the back end. The microelectrode is filled with an electrolyte
so that the back end of the electrode is in electrical continuity to the tip. The back end is then connected
to a special amplifier that is not shown in this lab. This amplifier
compares the voltage at the tip of the microelectrode to the reference electrode.
The output of the amplifier is connected to an oscilloscope.
An oscilloscope is basically a
sophisticated voltmeter, but many people have a problem understanding its function, so we will
give a quick tutorial on the oscilloscope next.
Oscilloscope Tutorial:
You may be familiar with analog or digital voltmeters or multimeters. They are instruments
that can measure voltage. An oscilloscope is also a voltage measuring instrument. The
difference is that while a voltmeter displays the voltage at a given time, an oscilloscope
is specially designed to display changing voltages. It's true that if the voltage
is changing slowly, you can look at the voltmeter reading and write down the changing values
quickly, say every second, but you will soon get tired of it. An oscilloscope can display
voltages that change on the scale of one thousandth or even one millionth of a second or faster.
The oscilloscope display can be thought of as a graph, with the y-axis representing voltage
and the x-axis representing time. The graph is drawn by an electron beam which travels from
left to right and produces a green line on the screen. If the voltage doesn't change, a
horizontal line will be displayed. In this lab, the oscilloscope is set so that when the
beam reaches the extreme right of the display, it will immediately start on the left edge
again, thus drawing a graph of the monitored voltage over and over. In a real oscilloscope,
there are many controls, but we have omitted those for simplicity.
This is what you see when you start the electrophysiological portion of the experiment.
You can tell that it's a steady voltage because it's a flat line. But what is the actual
voltage? At the start of the experiment, the tip of the electrode is still outside any cell,
and the reference electrode is also outside the cell. Thus, there is no voltage difference,
so it is zero volt (0 V). We have adjusted the vertical position of the flat line representing
0 V to be in the upper portion of the display because most of the voltages you will encounter
in neurons are negative and would appear below this line.
When your electrode tip penetrates a neuron, the display changes to this view (usually).
You are now measuring the membrane resting potential. Notice
that it is a steady voltage and that it is negative. We don't provide you with the y-axis scale, which
can be adjusted in a real oscilloscope, but typical resting potentials are around -60 mV (millivolt =
one thousandth of a volt) in neurons. You've encountered nothing but steady voltages so far. You could
have measured everything so far with a simpler and cheaper voltmeter, but wait...
When a neuron fires an action potential, this is what the
screen looks like. You can't see this with a voltmeter. To find out how fast an event this action
potential is, you will have to look at the x-axis scale, yet another adjustable control in a real
oscilloscope. In this lab, the width of the oscilloscope display represents about 500 ms (millisecond = one thousandth of a second).
So there you have it, a quick tutorial on the electrical equipment used in this lab. I hope you will
eventually have an opportunity to perform this or a similar exercise in real life and appreciate how
much more difficult yet exciting the real thing is. If you do have that chance, I hope that this
little tutorial was of some help to you in achieving success in your exercise.