Hi, dakels
You are right in terms of that the relative degree of heat produced by the mitocondria is the same at all times. It is the level of body activity and thus the degree of mitocondria activation that determines how much heat that is produced. As correctly mentioned we need to sweat to cool down or shiver to increase the temperature in order to regulate the body temperature.
The only exception is in the so called "brown fat" which is prevalent in infants only. In that perticular fatty tissue the mitocondrial membranes are short-circuited and no ATP is created, only 100% heat. This is a protective mechanism against hypothermia. Unfortunately this tissue is absent in adults. Imagine going into a cold room and just watching the fat melt away. The process of shivering to increase body temp is not at all as effective as this method.
The so called "electric "signal is actually not a real current produced by the nerves. The "signal" produced by nervous cells is actually a brief depolarisation of the chemical potential that exists in all living cells. The chemical potential is due to different concentrations of mainly Na,K, Ca extracellularly vs intracellularly. The depolarisation is the result of an opening of ion channels in the cellmembrane allowing selective amounts above all K and Ca to move through the membrane. The depolarisation triggers different events in the body, for instance release of neurosubstances in the neurons and contraction of muscles in muscular tissue.
I believe that it could be possible in the future to create circuits that respond to these signals and possibly even biocomputers or organic neural networks. The major cons of using neural fibers are the vulnerability of the fibers and the fact that the process of depolarisation and repolarisation of membranes are extremely energy consuming. In comparison: 1/5 of of the blood flow of the heart goes through the brain in order to supply it with adequate amounts of sugar and O2, eventhough it only weighs about 1/50 of the total bodyweight.
With this in mind you quickly realize that you can't transport electricity, only a 1/0 signal.
Compare this to the loss of energy when transporting information using an ordinary copper cable or fiber optic cable which is nominal.
The answer is like Occam's principle. The easisest solution is the viable one.
/Matt