The neocortex is a complex structure, with
each neuron receiving thousands of synaptic inputs from
other elements of the network. Dr. Williams aims to explore
at a mechanistic level: 1) how individual neurons of the
neocortical network integrate synaptic input; and 2) how
the pattern of action potential output influences forward
transmission of information through neocortical networks.
To this end he has developed techniques that enable the
recording of postsynaptic potentials (PSP5) from sites
throughout the apical dendritic arbor of a class of layer
5 neocortical pyramidal neurons.
Dr. Williams and his laboratory found that
single excitatory PSPs (EPSPs) generated at sites remotely
in the dendritic arbor have relatively little direct influence
on action potential output, due to a uniformity of synaptic
conductance. In contrast they found that trains of EPSPs,
generated from distal dendritic sites, provide powerful
drive action potential output through the engagement of
active dendritic spiking mechanisms. Interestingly, they
found that identical inputs generated from somatic and
distal apical dendritic sites generate action potential
output patterns that are statistically distinct and that
may be broadly classified as simple single action potential
firing and action potential burst firing, respectively.
To explore how such output patterns are
propagated through the neocortical network, the lab made
multineuronal whole-cell recordings. To their surprise
they found that the dynamics of synaptic transmission
between layer 5 neocortical pyramidal neurons are tuned
to allow the reliable signaling of action potential burst
discharges, but not single spikes. Dr. Williams concluded
that distal excitatory synaptic inputs decisively control
the excitatory synaptic output of layer 5 neocortical
pyramidal neurons and, therefore, are a powerful influence
on network activity in the neocortex.
