Biomedical Engineering Reference
In-Depth Information
the radioactive labels are at trace levels (
M or less). Most PET
methods use radioactive isotopes that are relatively short-lived
(
μ
30 min), but as in
14
C-DG autoradiography, require long accu-
mulation times and/or repeated scans (5-30 min) for radioactiv-
ity to build up. PET provides spatial resolution appropriate for
human brain imaging (3-5 mm) and is well suited for clinical
use. Recent advancements have improved PET resolution further
(1-3 mm) to image brains of smaller animals
(66)
.
The techniques shown in
Figure 1.2
include a variety of
electrophysiological, optical, and magnetic resonance methods
which are now widely used in many neuroscience laboratories,
both in animals and in humans. These methods dynamically mea-
sure everything from changes in neuronal activity (e.g., mem-
brane potential, ion flux, neurotransmitter flux) to the coupled
alterations in energy metabolism (e.g., glucose consumption, oxy-
gen consumption, creatine kinase flux) and hemodynamics (e.g.,
blood oxygenation, blood flow, blood volume).
<
4.1.
Electrophysiology
Electrophysiology deals with the study of the electrical proper-
ties of cells and tissues
(2)
. Classical electrophysiological meth-
ods, using different types of microelectrodes, measure changes in
voltage (or current). Microelectrodes are inherently designed to
cover a few specific regions because it is impractical to perforate
very large areas of the brain. Microelectrode arrays cover slightly
larger (e.g., 4
4mm
2
) superficial regions of the brain.
Intracellular recordings involve direct access into the intra-
cellular milieu so that membrane potential can be measured with
microelectrodes. The obvious advantage with intracellular record-
ing is the unrivaled information about the individual cell, but the
disadvantage is the difficulty in sampling large number of neu-
rons. Extracellular recordings are made with the microelectrode
tip (1-3
×
m) situated next to cell bodies so that the extracellu-
lar voltage can be measured. Depending on the impedance and
location of the electrode tip, extracellular recordings can capture
the activity from either a single neuron (i.e., single-unit activity
(SUA) recording) or from several neurons (i.e., multi-unit activity
(MUA) recording). The clear benefit of extracellular recording
is that several individual neurons can be assayed simultaneously.
However, only the largest signals from neurons closest to the
microelectrode tip can be reliably identified individually.
Since microelectrodes are designed to measure the high fre-
quency action potentials (or spikes), the data are collected with
high temporal resolution (
μ
s). The high signal-to-noise
ratio of action potentials can be sorted out from the lower ampli-
tude signals (by spike sorting). The extracellular signal, however,
is also susceptible to the slower waves representing the local field
potential (LFP) which may arise from graded events at the nerve
terminal as shown in
Figure 1.1
. Thus, appropriate filtering
<
100
μ
