02/09/2010 by admin.

PLoS One has a paper, A Conserved Behavioral State Barrier Impedes Transitions between Anesthetic-Induced Unconsciousness and Wakefulness: Evidence for Neural Inertia, by Friedman and others here.

The abstract:

One major unanswered question in neuroscience is how the brain transitions between conscious and unconscious states. General anesthetics offer a controllable means to study these transitions. Induction of anesthesia is commonly attributed to drug-induced global modulation of neuronal function, while emergence from anesthesia has been thought to occur passively, paralleling elimination of the anesthetic from its sites in the central nervous system (CNS). If this were true, then CNS anesthetic concentrations on induction and emergence would be indistinguishable. By generating anesthetic dose-response data in both insects and mammals, we demonstrate that the forward and reverse paths through which anesthetic-induced unconsciousness arises and dissipates are not identical. Instead they exhibit hysteresis that is not fully explained by pharmacokinetics as previously thought. Single gene mutations that affect sleep-wake states are shown to collapse or widen anesthetic hysteresis without obvious confounding effects on volatile anesthetic uptake, distribution, or metabolism. We propose a fundamental and biologically conserved concept of neural inertia, a tendency of the CNS to resist behavioral state transitions between conscious and unconscious states. We demonstrate that such a barrier separates wakeful and anesthetized states for multiple anesthetics in both flies and mice, and argue that it contributes to the hysteresis observed when the brain transitions between conscious and unconscious states.

There are a number of pointers in this paper to the nature of consciousness. First is the indication that consciousness is not restricted to humans, or primates, or mammals or even vertebrates. Some of the molecular machinery involved in losing and gaining consciousness probably pre-dated the split between our line and that of the fruit fly. Consciousness, at some level, is likely very old and very general in animals.

Second, the hysteresis between being conscious and being unconscious may be functional. The brain is protected from small fluctuations in the system that might cause repeated fluctuations between consciousness and unconsciousness so that both are relatively stable states. The brain appears to be almost bistable with a fairly clean transition between the two states.

Third, the fall into unconsciousness is quicker (steeper) than the re-establishment of consciousness. This may indicate that consciousness is easier to disrupt than to initiate; it is a more complex state.

This work was done with anesthetics on animals and therefore cannot be applied directly to the sleep-wake cycle in humans. There are parallels though between hysteresis in anesthesia in mice and sleep inertia in humans. It is really hard to wake up, even if it is not Monday!

Friedman EB, Sun Y, Moore JT, Hung H-T, Meng QC, et al. (2010). A Conserved Behavioral State Barrier Impedes Transitions between Anesthetic-Induced Unconsciousness and Wakefulness: Evidence for Neural Inertia.
PLoS ONE 5(7) DOI: 10.1371/journal.pone.0011903

Posted in Uncategorized | 1 Comment »

01/09/2010 by admin.

There is a posting in MIT Technology Review (here) about an electronic device called a memristor. It acts like a resister with a memory and was first produced about a year ago.

…it turns out that the synapses between neurons behave exactly like memristors. That raises the possibility that memristors can be connected together in a way that truly mimics the wiring of human brains.

One of the defining features of the connections between neurons is that they become stronger when neurons fire together; hence the phrase “neurons that fire together, wire together”, a phenomenon otherwise known as Hebbian learning. Various experiments have shown that this effect is most pronounced early in the learning process, when the increase in connection strength is greatest. Later learning merely reinforces the links

(there have been problems with memristor curcuits) Merrikh-Bayat and Bagheri have a simple solution: use two memristors in series. Choosing their memristance carefully allows them to reproduce Hebbian-type synapse strengthening more or less exactly.

That may turn out to be a useful insight. The first neuromorphic chips to use memristance to mimic synapse behaviour are already being built. A small change in their design may make a significant difference.

This is gives an approximation of a neuron for research but still is a long way from actual neurons. I think an electronic neuron may have to react to electrical and magnetic fields and also to many chemical gradients (or have a way to simultate these) before it can mimic a real neuron’s actual behaviour.

Posted in Uncategorized | 1 Comment »