Artificial Consciousness/Neural Correlates/Synaptic Models/Ion Channel Model of Synaptic Function
Ion Channel Model of Synaptic Function[edit | edit source]
There are two main types of ion channels, although there are many types of ion channels that pump many types of ions. The ion channels can be separated into those ion channels that work with the electrostatic gradient across the membrane, and those ion channels that work against the electrostatic gradient, and so pump ions in a direction that the electrostatic gradient resists. The latter must overcome the resistence of the gradient, and as a result, require an energy source to overcome the electrostatic gradient. It is the difference between a pump and a filter, in either case only specific ions are transferred across the membrane but in the latter case, they need to be assisted with energy from within the cell, usually in the form of ATP.
Some Ion channels are voltage dependent, and only start pumping once the voltage in the cell either increases beyond a particular point, or drops beyond another point. Ion channels existed in the cell, long before some cells learned to be neurons, and as a result, have automatic functions that operate despite of the state of the synapse they may be connected to. One such type of ion channel, is the Potassium Ion channel, which causes the unique overshoot, or spike that causes the neuron to fire. This ion channel has a role in keeping the cell from allowing its internal voltage to drop low enough that the membrane loses its capacitance, and thus the ability to filter out unwanted ions from the extracellular fluid.
The Potassium Ion channels kick in when the membrane depolarizes, causing the cell to quickly gain voltage despite the depolarization. This is, it is believed a defensive reaction against depolarization in an attempt to keep the cell Activation Potential from dropping too low, despite the loss of integrity locally in the membrane caused by the disruption of the protein-lipid-lipid-protein bonds that act to keep the polarity of the cell membrane.
Other ion channels like the NMDA ion channel, named for the presence of the chemical NMDA in the channel structure, only begin to operate when the synapse is active, and the cell is ready to fire, This results in an increase in readiness to firing called Potentiation, which is the active mode that creates Long Term Potentiation (LTP). Potentiation results in an increase in the willingness of the cell to fire. It is interesting to note that the NMDA ion channel pumps Calcium ions into the cell, and Potassium Ions out of cell, thus reducing the overshoot at time of firing, and facilitating the firing of the cell so that it doesn't habituate as easily. This allows the cell to fire in trains of signals instead of just firing once.
Before Neurons evolved, ion channels existed to do mainly two different things, attract ionic nutrients from the nutrient rich extracellular fluid, and secrete ionic wastes into the extracellular fluid so that they could be carried away from the cell. Neurons specialized this capability, in order to transfer signals between cells. As a result, ion channels in neurons also act on the cellular voltage or action potential of the cell, named for the fact that it is thought that the voltage is the factor that causes the cell to "Fire". As a result of this specialization, Neurons are more sensitive to some environmental conditions, and require special caretaker cells called Glial cells to help them maintain their own survival. The more active a neuron is, the more glial cells it needs to support its survival.
Ion channels, in Neurons do mostly 4 things related to signal propagation, Some ion channels increase the voltage of the cell, this is called exciting the cell. Some ion channels inhibit the growth of the increase in voltage of the cell, these are called inhibitive ion channels. Some inhibit the growth of the voltage in the cell so strongly as to short it out, there are called shunt ion channels, and some ion channels modify the way the cell works in some manner, often with the use of a secondary messenger chemical, which triggers chemical reactions within the cell that cause some change in the way it works. These are called either Modifying or Moderating ion channels depending on how they cause the cell to react.
It is interesting to note that the brain works in a negative logic pattern, in the sense that inhibition of signals is the operative mode. The cell operates like a wired OR circuit, which means that if two excitory signals are detected, they both act to increase the chance the cell will fire, but if two inhibitive signals are combined within the neuron it can be partially demorganized to form a NAND like function that allows it to work to select outputs in much the same way that AND gates in a computer select outputs. However to do so requires a three NT minimal system, and while neurons have been shown to have as many as 5 different types of synapse sensitive patches, there is no guarantee that all neurons with 3 or more different types of synapses form the same logical functions, especially since the 3NT pattern of operation is a superset of logic that operates on an continuum between NAND and OR functions depending on the strength of the second inhibitive Neuro-Transmitter.
Despite this complexity to the actual neurons, The net effect of synapses being activated by Neuro-transmitters, is the modification of the cellular voltage. While it is controversial whether it is the actual firing of the neuron, or the voltage in the Axion buds before firing, that determines the amount of Neuro-Transmitter that gets into the extracellular fluid at the synaptic gap, It is recognized that it is the Action Potential, that is the main factor that determines the signal strength at the pre-synaptic bud.
According to Hebbs Model of the Synapse, it is the leverage between the number of permease molecules that detect Neuro-Transmitter and the change in voltage that they create, that determines how much one synapse will affect the Action Potential of a Neuron. This leverage is created by the number of Ion channels that are triggered, each time a permease signals. The Weight, or multiplier however, is not a set value, but changes with the learning rules. I interpret this to mean that the make up of the sensitive patch changes with the Weight, causing more ion channels to become part of the same synaptic sensitive patch as the weight increases and fewer ion channels to become part of the same synaptic sensitive patch as the weight decreases. If this is true, then somehow the synapse signals the cell to increase the number of ion channels in a particular sensitive patch when a synapse is more important to the function of the neuron, and signals the cell to reduce the number of ion channels in a particular sensitive patch when the signal is less important to the function of the cell. Thus, the learning rules, which adjust the weights in a Hebbian Neuron, actually act to adjust the number of ion channels in a synapse sensitive patch.