The mystacial vibrissae in rodents provide the major peripheral input to primary
somatosensory cortex via a topographically arranged ascending system to cortex.
The cortex in turn integrates this information and provides the major input to the
basal ganglia and thalamus. Whilst the ascending system and the cortical integration
have been studied extensively the efferent projections from cortex have not. The aim
of this study was to investigate the corticostriatal pathways in a highly organised and
topographically arranged cortical area using an intracellular in vivo method for the
recording, filling and tracing of single corticostriatal neurons.
In order to do so a successfull intracellular recording protocol had to be developed
(from an existing extracellular method) to ensure maximum neuronal filling. This
process was an ongoing and the various stages of development are highlighted.
Alongside this a suitable biocytin method had to be developed for the visualisation
of terminal fields arising from single neuronal fills. The various stages in the
development of this method are also described.
This study has resulted in recordings from all four types of layer V pyramidal
projection neurons described in vitro . Whilst the morphology was identical to that
observed in vitro for the IB and RS neurons the physiological responses to whisker
deflection were more complex, especially in IB neuronal firing patterns. In general
there was a lot more background synaptic activity observed in all the cellular
records. The IB neurons tended to show more complex firing patterns, firing either
as mainly single action potentials with occassional burst or predominantly bursts
with occassional action potentials. Only rarely did they fire the typical burst firing
observed in vitro.. It was apparent from this study that the RS2 and IB 1 neurons
were very similar in both their physiology and morphology and were more difficult
The study revealed two different classes of ipsilateral corticostriatal neurons. One
class is similar to the innervation pattern seen in motor cortex. This innervation arose
from a lateral branch of the axons of pyramidal tract neurons as they run through the
fibre bundles within the striatum. In this case the neuron has the physiology and the
morphology of an RS2 neuron. The second class of corticostriatal innervation arose
from a typical IB 1 neuron that innervated the striatum in a topographic manner
similar to that described in anatomical studies of rat primary somatosensory cortex.
The topography appears to be arranged within rows. In the ascending vibrissal
system, and within cortex, there appears to be an in -row preference generated in
cortex by intracortical connections. The IB input may also be arranged in rows as it
may obey the general rules observed in monkey corticostriatal connections where
more heavily interconnected cortical regions are more likely to overlap and innervate
the same regions of the striatum whilst still retaining a topographically arranged
innervation pattern. The conclusions drawn are that the RS2 innervation may result
in the initial short latency activation of the striatum whereas the topographic IB
innervation may provide a longer latency and more detailed input to the striatum