Thanks - I have been studying this stuff for a long time - since the 1970s.
You are mixing different parts of neural theory in places where they don’t apply so I am not surprised that it seems confusing. I will take a stab at placing these items in the correct place and give some context in how they fit together.
In no particular order:
Position in receptive field
The reach of the individual dendrite establish the scope of the receptive field. Synapse are distributed along the dendrite and are the point where the interaction between the axon and dendrite occurs. The action of the synapses are summed along the dendrite and while the action of the synapse is binary - the action of the dendrite is to sum the total action of the synapses over a length of dendrite. Synapse closer together are more likely to initiate depolarization (see below) than those further apart. This spacing bit of neural theory is generally ignored in the HTM canon.
depolarization
This is a small but critical part of the action potential. The cell acts to pump chemicals across the cell membrane and this establishes a sort of chemical battery that makes a voltage across the cell membrane. Different inputs can lower that voltage at “hotspots” in the cell membrane. A key fact in understanding depolarization is that firing the cell is about the same as shorting out this battery at some point; this discharge spreads along the cell membrane in a traveling wave. This is the primary effect of a synapse. One synapse by itself usually can’t fire the whole nerve - it usually take a gang of them to pull this off. It is a form of voting - each one cause some depression in the voltage - depolarization. It there is enough this starts the wave called the action potential.The closer together or the closer to the soma, the more effect a synapse can exert. Quoting George Orwell: “All animals are equal, but some animals are more equal than others.” This works for synapses too!.
*Action Potential
I really can’t do better than this wiki entry:
Neurotransmitters.
One of the oldest preserved functions from our origins as layers of pond scum is our use of chemical messengers. This predates defined neural structures and for that matter was well before cell walls were a thing. These messengers have been improved and incorporated into the nervous system as it has evolved.
A rouges gallery include many different chemicals and methods of action.
Neurotransmitters that fall into the category of amino acids
✦ Glutamate
✦ Aspartate
✦ Glycine
✦ D-serine
✦ Gamma-aminobutyric acid (GABA)
Monoamines or other biogenic amines considered as neurotransmitters
✦ Serotonin
✦ Norepinephrine
✦ Epinephrine
✦ Histamine
✦ Melatonin
Neurotransmitters that fall into the category of peptides
✦ Beta-endorphin
✦ Opioid peptides
✦ Somatostatin
✦ Calcitonin
✦ Vasopressin
✦ Oxytocin
✦ Glucagon
» Apart from these, there are several other important neurotransmitters, such as acetylcholine, dopamine, adenosine, and nitric oxide. So far, about 50 neuroactive peptides have been discovered.
» Sometimes, neurotransmitters are also classified as excitatory and inhibitory. This classification is based on their actions on the neurons. Excitatory neurotransmitters are those that excite the neurons and stimulate the brain, while inhibitory neurotransmitters are known for having a calming effect on the brain.
» Neurotransmitters like, GABA and serotonin come under the category of inhibitory neurotransmitters, while epinephrine and norepinephrine are the excitatory neurotransmitters. Dopamine on the other hand, can act as an excitatory, as well as an inhibitory neurotransmitter.
» However, the effect of a neurotransmitter on a postsynaptic cell depends on the receptors present in it. For some neurotransmitters, like glutamate, the important receptors have excitatory properties. On the other hand, most of the important receptors produce an inhibitory effect for GABA. But there are some neurotransmitters, for which both types of receptors exist.
It is very hard to make sweeping statement on how these work.
Some of these are passed from cells at the cleft between synapse and axon.
Some are circulated in the bloodstream.
Some are released into the inter-cellular space.
Some are fast acting, some sort of “set the mood.”
Some leave a longer term effect by either enhancing or suppressing future firing of the target neuron. The time scale varies depending on the exact mechanism of action.
This wiki page is generally correct but is very specific on what is considers a neural chemical messenger. The brain is not so neat about these things.
Inhibitory vs excitatory
Let’s try to separate what is going on in “Hebbian learning” and “chemical signalling.”
As I stated - the action of the chemical messenger can either facilitate depolarization or inhibit it. This effect can range from a mild influence to initiation of immediate effect.
The cells used in in the HTM theory are of both excitatory and inhibitory types, but the are not mixed together in the same parts of the system. This thread is focused on the SP part of the theory but in the larger HTM universe we use both types of cells.
The cells used in SP are excitatory when summing inputs. As the columns are summing the input field these triggers the inter-neurons to form an inhibitory effect where the strongest responding column suppresses the action of more weakly resounding columns. HTM uses the MAX function (k-winner) to select the strongest column in an area and suppresses the rest; in the biology this is performed with interactions with inhibitory inter-neurons. In both cases this enforces sparsity between macro-columns. The resulting effects are similar but not exactly the same.
In the TP portion of the theory, the mini-column that has been primed by a partial depolarization in the previous cycle fires faster and triggers shorter range inhibitory inter-neurons to suppress the rest of the macro-columm. Ya - prediction!
Hebbian learning
Hebbian learning is a proposed mechanism that relates the firing timing between two cells and the effect on the long term response of the receptive cell. Learning is a different thing from a nerve just firing. A cell firing is a one time event that transforms whatever influences are acting (Inhibitory or excitatory) on a cell into an action potential event. Learning is a long term change in the cell - normally thought to be some change in synapses - that changes the cells response to the influences to the cell. This influence could act to grow or shrink the strength of a synaptic connection.
Positive/Negative Hebbian reinforcement.
The simplest description is that if the cells fire at about the same time in a certain order the synapse becomes stronger and if the order is reversed the synapse becomes less effective. If this synapse is not used over a long time this synapse loses effectiveness (gets weaker/forgets)
@Falco - The effect of the spatial pooler input is always excitatory but it is more or less so based on this learning. There is no negative input or summation.
This has very little to do with the type of the neurotransmitter involved and it’s nominal Inhibitory vs excitatory effects. The outcome if this factoid is that the some of the input bits may be the compliment of the expected polarity of the input channel. That is outside scope of the Spatial Pooler and more correctly associated with the nature of the signal being sampled. (the encoder)