Topologically Defined Neuronal Networks Controlled by Silicon Chips

CHAPTER 2. NETWORKS OF DEFINED TOPOGRAPHY
Figure 2.20: Left: Synaptically coupled cell pair on a UV-patterned glass substrate. No scale bar is
available, somata have an estimated diameter of about 30µm. Right: Electrophysiological recordings
with impaled microelectrodes. Top row, cell 1 is presynaptic, bottom row, cell 2 is presynaptic. Top
traces, injection of a depolarizing current of 0,15nA; bottom traces, hyperpolarizing current injection
with -0.15nA. Coupling coefficients are displayed for each direction of signal transfer and the synapse
conductance is shown. The transients were corrected for electrode artifacts.
cluster neurons need not necessarily establish synaptic connections in vitro, especially if they have not
been connected in vivo.
Studies with Helisoma neurons indicate the importance of both neurons being in a state of active growth
to form a synapse [41]. Time lapse photographs of developing networks suggest a similar situation in
the case of Lymnaea stagnalis [79].
A third, yet very speculative reason might be a negative influence of the SU-8 topographic structures
on synapse formation. It is supported by studies with UV-patterned substrates, where 73% (n=15) of
the pairs were coupled, as compared to just 60% here. However, due to the small number of networks
tested, the statistical relevance is low. The difference could also arise from a more careful selection of
neurons, for example.
Electrophysiological recordings of coupled cell pairs
The following paragraph discusses signal transfer between coupled neuron pairs on the basis of three
example networks.
Fig. 2.20 shows two nerve cells growing neurites collinearly on a track of intact conditioning factors
on a UV-patterned substrate. As illustrated by the electrophysiological recordings on the right, a
functional synaptic connection has been formed. Transients can not be assigned to a specific cell in
the micrograph, because they were not labelled during the experiment. This is of little relevance to the
following discussion, however.
Injecting a hyperpolarizing current of -0.15nA into cell 1 hyperpolarized its membrane potential by
about 25mV. The signal passed via the synapse and also hyperpolarized the postsynaptic cell 2 by
10mV. Determination of the coupling coefficient according to eq. 2.24 yields k1,2 = 0.4 (the actual
value given in the diagram is averaged over several measurements with currents of different amplitude
injected). Switching the roles of the two neurons, such that cell 2 was now presynaptic and cell
1 postsynaptic, resulted in similar recordings, but with a somewhat smaller coupling coefficient. The
difference in values k1,2 and k2,1 is attributed to the varying soma conductance of the respective postsynaptic
neuron as described by eq. 2.24.
The behavior found with this cell pair, i.e. signal transfer in both directions across the synaptic connection
and transmission of hyperpolarizing currents, is characteristic for electrical synapses. They were
34