Cross-correlation between neurons: a source of information about the nervous system

Multiple firing patterns in deep dorsal horn neurons of the spinal cord: computational analysis of mechanisms and functional implications

Deep dorsal horn relay neurons (dDHNs) of the spinal cord are known to exhibit multiple firing patterns under the control of local metabotropic neuromodulation: tonic firing, plateau potential, and spontaneous oscillations. This work investigates the role of interactions between voltage-gated channels and the occurrence of different firing patterns and then correlates these two phenomena with their functional role in sensory information processing. We designed a conductance-based model using the NEURON software package, which successfully reproduced the classical features of plateau in dDHNs, including a wind-up of the neuronal response after repetitive stimulation. This modeling approach allowed us to systematically test the impact of conductance interactions on the firing patterns. We found that the expression of multiple firing patterns can be reproduced by changes in the balance between two currents (L-type calcium and potassium inward rectifier conductances). By investigating a possible generalization of the firing state switch, we found that the switch can also occur by varying the balance of any hyperpolarizing and depolarizing conductances. This result extends the control of the firing switch to neuromodulators or to network effects such as synaptic inhibition. We observed that the switch between the different firing patterns occurs as a continuous function in the model, revealing a particular intermediate state called the accelerating mode. To characterize the functional effect of a firing switch on information transfer, we used correlation analysis between a model of peripheral nociceptive afference and the dDHN model. The simulation results indicate that the accelerating mode was the optimal firing state for information transfer.

Efficacy of retinal spikes in driving cortical responses

How does a single retinal ganglion cell (RGC) affect the firing of simple cells in the visual cortex? Although much is known of the functional connections between the retina and the lateral geniculate nucleus (LGN) and between LGN and visual cortex, it is hard to infer the effect of disynaptic connections from retina to visual cortex. Most importantly, there is considerable divergence from retina to LGN, so cortical neurons might be influenced by ganglion cells through multiple feedforward pathways. We recorded simultaneously from ganglion cells in the retina and cortical simple cells in the striate cortex with overlapping receptive fields and evaluated disynaptic connections with cross-correlation analysis. In all disynaptically connected pairs, the retinal receptive field center and overlapping cortical subregion always shared the same sign (either both ON or both OFF). Connected pairs were similar in other respects, such as relative position and timing of their receptive fields, and thus obeyed the same rules of connectivity found previously for retinothalamic and thalamocortical connections. We found that a single RGC directly contributed on average to approximately 3% of the activity of its cortical target. The relative timing of pairs of spikes from the retinal cell affected their efficacy in driving the cortical cell. When two retinal spikes were closely spaced (<10 msec), the second spike was several times more likely to drive the cortical target. The relative magnitude of this disynaptic paired spike enhancement was considerably larger than has been found previously for retinogeniculate and geniculocortical connections. The amplified paired spike enhancement from retina to cortex ensures that signal transmission from retina to cortex is particularly effective when the retina fires a series of closely spaced action potentials.

Dynamic balance of metabotropic inputs causes dorsal horn neurons to switch functional states

Sensory relay structures in the spinal cord dorsal horn are now thought to be active processing structures that function before supraspinal sensory integration. Dorsal horn neurons directly receive nociceptive (pain) signals from the periphery, express a high degree of functional plasticity and are involved in long-term sensitization and chronic pain. We show here that deep dorsal horn neurons (DHNs) in Wistar rats can switch their intrinsic firing properties from tonic to plateau or endogenous bursting patterns, depending upon the balance of control by metabotropic glutamate (mGlu) and GABA(B) receptors. We further show that this modulation acts on at least one common target, the inwardly rectifying potassium channel (Kir3). Finally, we found that these firing modes correspond to specific functional states of information transfer in which dorsal horn neurons can faithfully transmit, greatly enhance or block the transfer of nociceptive information.

An analysis of the effect of retinal ganglion cell impulses upon the firing probability of neurons in the dorsal lateral geniculate nucleus of the cat

This study examines the probabilistic way in which LGN cells produce impulses. Simultaneous extracellular recordings were made from a single lateral geniculate nucleus (LGN) relay cell and the one retinal cell that triggered most of its impulses during vigorous responses. Impulses in the retinal train were classified as 'successful' in triggering an LGN impulse, or 'unsuccessful'. Similarly, the impulses in the LGN train were either 'Triggered' by a successful retinal impulse, or were 'Anonymous'. These impulses delimited various intervals whose distributions were compared to the relevant distribution of all intervals to determine whether short or long intervals tended to dominate in each case. Intervals between unsuccessful and successful impulses tended to be shorter than other retinal intervals, with their probability declining exponentially with duration. These data imply a decaying excitation produced by each impulse, but with a short refractory period following each Triggered impulse. Short intervals between Anonymous impulses were relatively common; Anonymous impulses thus lack the same refractoriness and tend to occur in bursts. The exponential excitation following an unsuccessful retinal impulse also facilitates Anonymous impulses, while Anonymous impulses (during visual stimulation) render the LGN slightly refractory for subsequent retinal impulses.