Biomedical Engineering Reference
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signal that has a known amplitude at all recording locations on
neuronal processes
(29)
. In a general case, such a signal is not
available. However, the action potential amplitude in mitral cells
is constant along the entire length of the primary dendrite, as
established by direct electrical measurements
(30-32)
and, thus,
can be used as a calibration signal.
A typical threshold EPSP recorded from the dendritic tuft
is shown in
Fig. 3.4-II
C. Trace 1 shows the EPSP that was
suprathreshold for spike initiation in the control solution and an
EPSP alone after spike was blocked with QX-314 to facilitate aver-
aging synaptic potentials. The amplitude and the shape of the
EPSP were mostly unaltered by the drug. The EPSP was mon-
itored optically and calibrated in terms of membrane potential
along a 260
m primary dendrite. The optical signals are shown
on a voltage scale in
Fig. 3.4-II
D. The recordings from 8 loca-
tions on the primary dendrite (traces 1-8), including the termi-
nal tuft, and two sites on oblique dendrites (traces 9 and 10) are
shown together with the electrical recording from the soma (trace
11). The calibration of the EPSP in the tuft revealed that the peak
amplitude was 18 mV. The EPSP amplitude in the soma, deter-
mined from optical data, was 13.2 mV (panel D, trace 8). The
direct electrical recording of the EPSP in the soma (panel D, trace
11) showed amplitude of 12.5 mV. This result indicates that the
error in calibrating optical signals can be very small. A series of
measurements of this type provided information about the char-
acteristics of the EPSPs at the synaptic sites and its attenuation
along the primary dendrite. On average, the EPSP attenuated by
35
μ
m. The mean apparent “length constant” in the
primary dendrites of mitral cells was unusually long, correspond-
ing to approximately 1200
±
9% in 300
μ
μ
m
(24)
.
It is often useful to monitor and correlate regional dendritic
membrane potential signals and the associated local calcium tran-
sient, the two signaling pathways that control a variety of func-
tions in individual neurons. We combined voltage imaging and
Ca
2
+
imaging from the dendritic arbor of individual neurons
loaded with two indicators. In this approach, changes in mem-
brane potential and in [Ca
2
+
]i are recorded sequentially from the
dendritic tree of the same neuron as fractional changes in fluores-
cence intensity using two different filter sets
(12)
.
Figure 3.5
is
an example of typical measurements carried out to correlate
V
m
signals with corresponding changes in [Ca
2
+
]
i
during the EPSP-
AP pairing protocol that typically induces long-term potentiation
(LTP) in hippocampal CA1 pyramidal neurons.
V
m signals and
[Ca
2
+
]i transients were monitored during three stimulation pro-
tocols (APs alone, EPSPs alone, paired stimulation) from multi-
ple sites on a dendritic tree. The recordings from one location
on an oblique dendrite close to a stimulating electrode revealed a
marked localized supralinear increase in the [Ca
2
+
]
i
signal evoked
by pairing a train of EPSPs with a burst of two backpropagated
