Human Physiology, 14th edition (2016)

The Central Nervous System 213

chemicals known as MRI contrast agents are sometimes used

to increase or decrease the signal in different tissues to improve

the image.

Scientists can study the functioning brain in a living person

using a technique known as functional magnetic resonance

imaging (fMRI). This technique visualizes increased neuronal

activity within a brain region indirectly, by the increased blood

flow to the more active brain region (chapter 14; see fig. 14.22).

This occurs because of increased release of the neurotransmitter

glutamate, which causes vasodilation and increased blood flow

in the more active brain regions. As a result, the active brain

regions receive more oxyhemoglobin (and thus less deoxyhe-

moglobin, which affects the magnetic field) than they do when

resting. This is known as the BOLD response (for “blood oxy-

genation level dependent contrast”).

Magnetoencephalogram (MEG) recordings provide images

of brain activity on a millisecond time scale that can be more accu-

rate than EEG recordings (discussed next). Because postsynaptic

currents produce weak magnetic fields, thousands of these together

generate magnetic fields that can be detected by sensors surround-

ing the head. The sensors are hundreds of SQUIDS (supercon-

ducting quantum interference devices) cooled in liquid helium to

4 degrees above absolute zero. Techniques for visualizing the

functioning brain are summarized in table 8.2.

Electroencephalogram

Synaptic potentials (chapter 7, section 7.3) produced at the

cell bodies and dendrites of the cerebral cortex, action poten-

tials produced by deeper axons, and even potentials produced

by glia summate to induce electrical currents in the extracel-

lular medium. These generate a potential (measured in volts

with respect to a reference potential) that is referred to as an

electroencephalogram ( EEG ) when measured by electrodes

on the concept that protons (H^1 ), because they are charged
and spinning, are like little magnets. A powerful external mag-
net can align a proportion of the protons. Most of the protons
are part of water molecules, and the chemical composition of
different tissues provides differences in the responses of the
aligned protons to a radio frequency pulse. This allows clear
distinctions to be made between gray matter, white matter, and
cerebrospinal fluid ( figs. 8.8 and 8.9 ). In addition, exogenous

Figure 8.9 An MRI scan of the brain. Gray and white
matter are easily distinguished, as are the ventricles containing
cerebrospinal fluid.

Third

ventricle

White

matter of

cerebrum

Lateral

ventricle

Gray

matter of

cerebrum

Table 8.2 | Techniques for Visualizing Brain Function

Abbreviation Technique Name Principle Behind Technique

EEG Electroencephalogram Neuronal activity is measured as maps with scalp electrodes.

fMRI Functional magnetic resonance

imaging

Increased neuronal activity increases cerebral blood flow and oxygen consumption

in local areas. This is detected by effects of changes in blood oxyhemoglobin/

deoxyhemoglobin ratios.

MEG Magnetoencephalogram Neuronal magnetic activity is measured using magnetic coils and mathematical

plots.

PET Positron emission tomography Increased neuronal activity increases cerebral blood flow and metabolite

consumption in local areas. This is measured using radioactively labeled

deoxyglucose.

SPECT Single photon emission computed

tomography

Increased neuronal activity increases cerebral blood flow. This is measured using

emitters of single photons, such as technetium.

CT Computerized tomography A number of x-ray beams are sent through the brain or other body region and are

sensed by numerous detectors; a computer uses this information to produce

images that appear as slices through the brain.

Source: Burkhart Bromm “Brain images of pain.” News in Physiological Sciences 16 (Feb. 2001): 244–249.