Last week, we were discussing the essential features of neurons and nerve transmission.
The central concept is neurons are unidirectional with information flowing in one end and out the other.
Neurons transmit signals via electrical pulses which travel down the cellular membrane through the controlled flux of sodium and potassium ions. These electrical pulses are structured in such a way as to ensure the signals only travel in one direction.
The strength of a signal is not controlled by the size of the electrical pulses. That is, a stronger signal doesn't produce a higher voltage. Rather, it is the rapidity with which the pulses are transmitted which indicates the strength.
A slight touch might generate a half dozen signals per minute whereas a painful squeeze might generate a hundred pulses per second. And even that is a vast oversimplification.
Since each neuron is connected to a myriad of other neurons and makes connections to as many others, a lot depends upon how many incoming dendrites are stimulated and how the connections are made.
Consider a simplistic model with a single neuron, say A, which has 100 dendrites. If all of the incoming dendrites on A are connected to a second neuron B, then if B sends signals to A, it could be activating any number from one to 100. The number of dendrites B is able to stimulate depends on the inputs it receives.
Last week, we mentioned the axon hillock. Without enough stimulation, the potential will not pass the hillock and B would be silent. Hence, A would hear nothing. Mild stimulation of B might trigger a potential which would stimulate most of the dendrites connecting to A but it would take a strong signal for B to fully connect with A.
Now consider A as it is connects its 100 dendrites to two neurons - say 50 from B and 50 from C - it would require both B and C to fire to generate a strong pulse from A especially if A's hillock requires at least 60 receiving dendrites in order for a signal to pass.
This structure generates what is called an "and" gate. Stimulation must come from both B and C in order for A to transmit a signal. It is easy to model on a binary basis.
That is, B+C -> A but B, C or neither, generate no response. This is one of the key circuit structures in digital computers.
But in the case of the brain, a neuron can have any number of other neurons sending in signals.
For our neuron A with its 100 receiving dendrites, each one could be connected to a single other neuron. Now it wouldn't simply be B+C but B+C+D+E+... and so on. In this case, over half of those neurons would need to be simultaneously stimulated in order to activate A.
Any neuron must integrate all of the inputs it receives and only when enough inputs are received will it pass the message along. In human terms, this is like a juicy piece of gossip.
If you hear it from one person, you might not repeat it but when you are hearing it from 20 people, you might just begin to think it is fact and pass it along.
The same situation applies for the neurons A connects to next in the chain. Simply put, the more neurons it projects to, the more neurons it can influence but the smaller its average influence on each target neuron. It is a biological trade-off.
Again, in human terms, if you are living isolated with only one other person, you will each have a great deal of influence over one another but living in a city of 10,000, your influence might not be very much at all.
We have also only been talking about the case of stimulating the receiving neuron to send a signal.
Not every neural interaction is intended to stimulate. Some are inhibitory, shutting down a pathway.
Consider, for example, a three neuron circuit. A connects to both B and C. B pass the signal along while C connects back to A and shuts it down. If neuron A receives enough stimulation, it might transmit a signal to both B and C.
B stimulates the next neuron or neurons but C stops the stimulation from A with the result that only a brief burst of activity is registered.
This sort of connection might happen in the case of sharp stabbing pain.
The initial signal goes through and tells the rest of the brain but the backwards signal from C stops the pain from continuing. Or, at least, it inhibits A from firing again for a short while.
We feel this as anything from a quick twinge to an intense needle-like stabbing.
Other variations on inhibition and stimulation allow for more complex circuits and ultimately allows us to control our bodies.
