October 05, 2008

Neurons: Digital or Analog?

Schematic representation of Schrodinger's cat in a superposition of both dead and aliveIts very hard to stamp neurons solely as either an analog device or a digital device. Actually its both. I'll tell you why it is analog. When an impulse arrives at the cell body of neuron, it creates an EPSP or excitatory post synaptic potential. During an EPSP, the sensitivity of the post synaptic ending to another stimulus is increased, hence the name. At this phase, stimuli add up resulting in spatial or temporal summation. Thus individual stimulation may not elicit a propagating action potential, but together they can cause enough inflow of sodium ions to produce an action potential. So from this point of view, we can see that the neuron is not following an all or none law; and the actions may best be described as analog.

Once an action potential has occurred, it always spreads down the axon to the synaptic knob. Actually, when a neuron is stimulated at any point in its course, the nerve depolarizes in both the directions. Since the cell body of a neuron is devoid of the neurotransmitter machinery of a synapse, the impulse is not chemically forwarded to the adjacent neuron. Thus this part of the neuron is rather like a diode in electronics. On the whole, the axonal and synaptic part of a neuron resembles a Boolean machine, a ‘0 or 1’ response. So this part of the neuron is undoubtedly digital.

The story doesn’t stop here. There’s a quantum touch to a neuron too. Quantum mechanics can be foxy. You can imagine of a cat as either alive or dead. In quantum mechanics, the cat (called the Schrodinger's cat) may have another state: BOTH dead and alive at the same time. Quantum superposition is thought to exist in the brain too and even Bose-Einstein condensate may form among the neural proteins. But how an interaction like this forms in the warm (thermal noise) environment of the brain and how the brain avoids decoherence at this temperature, has been studied by Stuart Hameroff and Roger Penrose. Their proposed the Orch OR (orchestrated objective reduction) hypothesis addresses the issue of ‘binding’ of consciousness and the mechanisms of the thalamocortical circuitry by quantum interactions.

The neuron thus seems to behave in analog, Boolean and quantal ways, so far, until string theory comes up with another!
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