The search for answers to timeless questions continues, says Todd Whitcombe. Sledge hammer or feather?
Last time, I wrote about the way that neurons work. They are like electrical wires with the current or electricity carried from the cell body of the neuron down to the end of the axon using chemical compounds – the sodium and potassium ions. This is true for all neurons.
The magnitude of the action potential or voltage that results is a consequence of the concentration difference between the sodium ion outside the axon and the potassium ion inside the axon. When the neuron fires, the voltage is passed down the axon in the form of a pulse. The firing of a neuron is like successive waves rippling down the surface of the axon.
How do we get really intense nerve interactions? That is, what differentiates the feeling you get from dropping a feather on your toe compared to a sledge hammer? It’s not the intensity of the neuron’s firing but the frequency that differentiates these two feelings.
Picture a little kid, tugging on your sleeve very rapidly. This is bound to get your attention and the more rapidly the tugging, the faster you will respond.
Neurons firing rapidly demand attention from the brain.
But the really interesting stuff doesn’t happen along the axon. It happens at the end of the axon – in the synaptic cleft. It is here that the signal from one neuron is passed to the next through chemical intermediates called neurotransmitters. And it is here that different signals can be recognized and manipulated.
The intensity of the sensation is still a consequence of the frequency of action potentials but each pulse of electricity releases a small pocket of chemicals into the synaptic cleft.
The more pulses, the more molecules floating around and the bigger the response. However, there is also the question of which chemicals are going to be released and how are they going to be felt on the other side of the synapse.
I should perhaps back up a bit and re-iterate that, in a synapse, there is a sending neuron which emits the neurotransmitters, and a receiving neuron which detects the neurotransmitters once they have been released. Each chemical transmitter is passed to one of many receptor sites on the receiving neuron. The more receptor sites that get filled, the more rapidly the receiving neuron will fire.
There are not that many compounds that act as neurotransmitters.
In total, maybe 50 have been identified. I say “maybe” because it is tricky work to identify neurotransmitter amongst all of the chemicals in the brain.
However, some of these compounds have been known for a number of years, such as adrenaline (or epinephrine) and dopamine. These two neurochemicals have almost the same chemical structure and consequently do much the same thing. They stimulate the receiving neuron to fire. In doing so, they make the body more alert and ready to either fight or flee. The receptor site for these neurotransmitters is also the target of such compounds as cocaine and amphetamines.
Another neurotransmitter is serotonin, which is a complex molecule that has the exact opposite effect on the brain to dopamine or epinephrine. It induces sleep.
Serotonin is the neurotransmitter that gets out of sync when we fly across time zones – producing jet lag – or when the seasons change – generating “seasonal affective disorder” (SAD).
The chemicals of the brain – the neurotransmitters – are powerful compounds which can affect many of the actions of our minds.
And it is here that scientists search for the answer to the question, “Where does the brain end and the mind begin?”