[Joe L. Ogan (OP)]

If the human being thinks by electrical impulse, why can't we think at the speed of light.  Is it because of resistance of the brain and other tissue that slows the thought process? Thanks for comments.  Joe L. Ogan

[Bored chemist]

The brain uses chemical impulses. Those chemicals generate a voltage which affects other cells, and so the speed is limited by the speed of the chemical reactions.

[chris]

Hi Joe. BC (above) is spot on. Although neurotransmission is an electrical process involving the flow of regularly-regenerated currents along the insides of nerve fibres, for one nerve cell to talk to another requires the release, at the point of connection between the two neurones, of a chemical nerve "transmitter". This diffuses from the first to the second nerve cell where it must engage with chemical receptors which must, in turn, activate other biochemical processes in order to change the activity of the recipient cell.

These synaptic connections, as they are known, apply a delay of the order of a millisecond or so, which holds up the formerly much faster electrical transmission process.

So why not use electrical coupling to connect two cells? Well this does exist in the form of "gap junctions", which are small regions where the membranes of two adjacent cells are fused to create a communication between the cells. These "connexons" as they are also known can carry electrical signals and are used in some parts of the body to transmit information between cells. However, the refinements and control offered by chemical synapses means that the brain has adopted this system as the mainstay of neuronal circuitry.

Chris

[CliffordK]

Even the conduction of impulses along an axon is significantly slower than the speed of light because it is far different than one might encounter along an electrical wire.

Neurons actively pump Potassium ions (K⁺) inside of the cells, and Sodium Ions (Na⁺) outside of the cells.  An action potential will first open the sodium channels to allow more sodium (+ charges) into the cell, followed by opening the Potassium channels to return the cell to the resting state.  This happens successively down the axon, significantly slowing the propagation of the action potential.

Myelination allows a mix of pure electrical signal propagation and ion signal propagation, and significantly speeds up the overall signal propagation.

Axons can also be overstimulated, at which point they loose the Sodium/Potassium gradient and can no longer fire until the ion pumps are able to regenerate the ion gradients.  As it is, the brain is a very energy expensive organ.  It is likely that if signal transmission was sped up, the brain would saturate and there would be huge problems of inactive neurons due to overstimulation.

[JP]

However, the refinements and control offered by chemical synapses means that the brain has adopted this system as the mainstay of neuronal circuitry.

What are the "refinements of control," you mention, Chris?  To me, it naively seems that evolving a computer-like electrical brain would be the best for survival because of superior reaction times, but I take it there's an advantage to chemical synapses that I'm missing here...

[CliffordK]

There are multiple neurotransmitters that could have an overall positive or negative or moderating effect on the synapse. 

A dendritic tree integrates the incoming signals from multiple neurons both in time and place, and only propagates the signal if a certain threshold is met.

In some ways, a direct electrical connection might speed up the process, but it may make it far more complicated to create the information integration.  And, as mentioned, faster transmission would also require more ions to be pumped, more energy, and a greater risk of everything becoming overloaded, and the ion gradient becoming depleted. 

[JP]

But can't you program a neural network on a computer to do much the same thing, and wouldn't that be faster?

[chris]

Actually, Clifford, the propagation inside an axon is the flow of an electrical current; but unlike a wire, axons are leaky and the signal drops with distance and needs to be regenerated by sodium influx. In a myelinated neurone this occurs at the nodes of Ranvier, but between the nodes the conduction is electrical, rather like a Newton's cradle.

chris