Cognitive Psychology: Connecting Mind, Research and Everyday Experience, 3rd Edition

28 • CHAPTER 2 Cognitive Neuroscience

Adrian recorded electrical signals from single neurons using microelectrodes—small shafts of hol-

low glass fi lled with a conductive salt solution that can pick up electrical signals at the electrode

tip and conduct these signals back to a recording device. Modern physiologists use metal microelec-

trodes. The electrode is lowered into tissue until the tip of the electrode is positioned near a neuron.

This electrode, called the recording electrode, is connected to a recording device and to another

electrode, called the reference electrode, which is located outside of the tissue (● Figure 2.5a).

The key principle for understanding how electrical signals are recorded from neurons is

that we are always measuring the diff erence in charge between the recording and reference

electrodes. The diff erence in charge between these two electrodes is displayed on an oscilloscope,

which indicates the diff erence in charge by the vertical position of a small dot that creates a

line as it moves across the screen. For example, the record in Figure 2.5b indicates that the

diff erence in charge between the recording and reference electrode is −70 mV (mV = millivolt =

1/1,000 volt) and the dot continues to move along this −70 mV line as long as no electrical

signals are being transmitted in the neuron. However, when an electrical signal, called a nerve

impulse or action potential, is transmitted down the axon, the dot is defl ected up (as the

neuron becomes more positive) and then back down (as the charge returns to its original level),

all within 1 millisecond (1/1,000 second), as shown in Figure 2.5c. Figure 2.5d shows action

potentials on a compressed time scale, so an action potential like the one in Figure 2.5c appears

to be a vertical line. Each line in this record is an action potential, so the series of lines indicates

that a number of action potentials are traveling past this electrode. There are other electrical

signals in the nervous system, but we will focus here on the action potential, because it is the

mechanism by which information is transmitted throughout the nervous system.

0

+40

–70

Time Time Time

1/1,000 second 1/10 second

1 millisecond

To computer

Oscilloscope

Axon

(a)

(b) (c) (d)

Recording

microelectrode

Reference

electrode

Difference in charge between recording

and reference electrode (millivolts)

● FIGURE 2.5 Recording

from a single neuron.

(a) The diff erence in charge

between the recording

and reference electrodes is

displayed on the oscilloscope

screen. (b) A small dot moves

across the screen, which

briefl y leaves a trail. In this

situation, electrical signals

are not being transmitted by

the axon, so the diff erence

in charge remains at –70

millivolts. (c) When an action

potential travels down the

axon, it causes a brief positive

pulse, like the one shown

here, as the potential passes

the recording electrode.

(d) A number of action

potentials are displayed on

an expanded time scale,

so a single action potential

appears as a “spike.”

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