The complex dynamics of the membrane potential endows neurons with a wide
repertoire. Even the least complex feature, the steady state membrane
potential, can attain two distinct values, which is indicative of
For this bistability to be of functional significance, a control mechanism that
transforms the cell's membrane potential from one state to the other is
Here we show that a bistable neuron, a Purkinje cell (PC), behaves like a
toggle switch, where a single synaptic input is the lever that shifts
between the two
states. We demonstrate that PC bistability, which is typically attributed to
in-vitro conditions, is retained in anaesthetized guinea pigs and that the
fibre input switches the cells between the two states in both directions.
We propose a general dynamic mechanism to explain this toggling effect,
and discuss its possible computational role.
Simple cells in layer 4 of the primary visual cortex are the first neurons
in the visual pathway showing orientation and direction selective
responses. The precise role of intracortical excitatory and inhibitory
connections in generating these properties is still unclear. Intracortical
inhibitory processes have been shown to be crucial to the generation of
direction selective responses. In vivo, excitatory and inhibitory layer 4
cells differ in their receptive field properties: excitatory (regular
spiking) neurons are orientation- and direction-selective whereas
inhibitory (fast spiking) neurons are orientation, but poorly direction
tuned. This difference in direction tuning could be due to differences in
intracortical inhibitory synaptic input patterns.
To address this question we have optically recorded orientation and
direction maps from ferret primary visual cortex (n=11, age: postnatal
days 37 - 41). Subsequently the imaged brain region was removed and
tangential slices (400 mm thickness) prepared. Whole cell patch clamp
recordings from individual layer 4 neurons were done and synaptic inputs
were scanned by local photolysis of Nmoc-caged glutamate (1 mM).
Postsynaptic cells were filled with biocytin and histological sections
were aligned with the synaptic input maps and the optical images obtained
in vivo to determine the spatial distribution of presynaptic inputs. We
recorded from n = 12 spiny (excitatory) and n = 10 aspiny (inhibitory)
stellate cells. The majority (68 %) of excitatory inputs to both spiny
(excitatory) and aspiny (inhibitory) stellate cells originated from
cortical regions preferring the same orientation and direction as the
postsynaptic cell. However, the inhibitory input patterns were
significantly different for the two cell populations: excitatory layer 4
cells received two populations of inhibitory inputs, about 50 % originated
in iso-direction domains whereas the remaining inputs originated in
cortical regions preferring the opposite direction of stimulus motion.
Inhibitory layer 4 neurons did not receive inhibitory synaptic inputs
tuned to the opposite direction. This indicates that specific inhibitory
connections originating in regions tuned to the opposite direction are
important for direction tuning of cortical neurons and that differences in
response properties in different populations of cortical neurons might be
explained by their different intracortical connectivity patterns.