Biomedical Engineering Reference
In-Depth Information
40
20
0
−20
−40
−60
−80
0
5
10
15
20
Time (msec)
Figure 9.1:
Effect of stimulus frequency on charging and action potential firing.
of neurotransmitter will slowly rise, leading to a sustained post-synaptic current. Recall that the post-
synaptic current may either depolarize (excite) or hyperpolarize (inactive) the post synaptic membrane.
9.2 NEURALDECODING
The purpose of neural decoding is to listen in on many neurons to understand how they are passing
information to one another. As it is the timing of action potentials that is important, the extracellular
potentials,
φ
e
, contain all the information needed to decode neural messages. From Ch. 8, we know that
as an action potential approaches an extracellular electrode,
φ
e
becomes positive. As the action potential
is moving away from the electrode,
φ
e
will become negative.Therefore, if an electrode is placed very close
to an axon, the
biphasic
deflections in
φ
e
(as in Fig. 9.2) will reflect a single-action potential.
9.2.1 One Electrode, Many Recordings
When an extracellular electrode is very close to a single axon, other surrounding neurons contribute very
little to
φ
e
and the recording will reflect only one neuron. In reality, neurons form a complex web and we
would not know the exact location of our recording electrode in the web. Therefore, whenever one action
potential fires and propagates down an axon, a corresponding deflection will be observed in the recording.
Further complicating the extracellular potential time course is the fact that axons are not straight cables
and may be at any distance or angle relative to the electrode. The result is that
φ
e
is no longer biphasic.
A somewhat more challenging problem is to distinguish between the many neurons that may
contribute to a single
φ
e
recording. Figure 9.3 shows the relatively simple case of two neurons being
recorded by two electrodes. Notice that the deflections are not biphasic because of the orientation of the
electrode relative to the axon. Furthermore, because of the geometry, the firing of cell 1 will appear as a
different deflection at electrode 1 and electrode 2.This situation becomes even more complicated if many
neurons are firing. Lastly, consider how difficult it would be to separate the firing of cell 1 and 2 if they
fired at nearly the same time.
