You may want to refer to this diagram in conjunction with the following
Functionally most large nerve cells are divided in two parts, each of which performs a different task. The axon and telodendria form a communications unit. Their job is to carry information from the part of the nervous system in which the soma is located, to some distant part where postsynaptic cells will use the information. So while a cell might have its body (soma) in the thalamus, its axon might reach out of the thalamus to make contact with another cell whose soma lies inches away in the cerebral cortex. The axon is like a cable connecting two computers. It delivers the output of the first computer to the second computer for use in its own calculations.
If communication is the function of the axon of the neuron, computing is the task of the other half, the cell body and dendrites. Each soma/dendrite unit receives inputs from thousands of terminals scattered over its surface. These terminals come from hundreds of presynaptic cells, each of whose soma could be located in any one of a dozen or more places dotted around the nervous system. Second by second, floods of information arrive at each postsynaptic cell in the form of an ever-changing pattern of presynaptic impulses.
Maybe you can imagine thousands of twinkling lights pulsing over the surface of the dendrites and soma, the pattern changing every millisecond, always in flux. All across the surface of the cell, vesicles are pouring transmitter into synaptic clefts while ion channels open and close. Dozens of EPSCs flash into existence each moment, spreading their effects across the cell membrane towards the axon hillock.
The excitatory postsynaptic current (EPSC) is a tiny transmembrane current of inward-flowing sodium ions and outward-flowing potassium ions which occurs at the postsynaptic membrane. Just like transmembrane currents that constitute a nerve impulse, it is accompanied by electronic currents that spread across the surrounding cell membrane. These electronic currents can spread all the way from the tip of a dendrite branch, through the soma to the axon hillock (initial segment). If it is strong enough to depolarize the initial segment the sodium gates there will open, initiating a nerve impulse. EPSCs are, therefore, the triggers for nerve impulses.
The main difference between EPSCs and impulses is in the nature of the channels they flow through. The gates on the sodium channels of the axon are held closed by a voltage between the inside and the outside of the membrane. When depolarization occurs, the gates open and the transmembrane currents flow. So the channels of the nerve impulse are voltage regulated. The ion channels in the membrane of the dendrites and soma, on the other hand, are not opened by a drop in voltage but by a neurotransmitter. So the channels responsible for EPSCs are transmitter regulated.
It is at the hillock that our microscopic biological computer does its most important adding and subtracting. Because each individual EPSC is too small to reach the axon threshold and trigger a nerve impulse by itself, the firing of an impulse has to wait until enough EPSCs arise at the same time. The electronic currents from various EPSCs amass in a process called summation. Only if there is sufficient summation is the axon threshold achieved and the sodium gates there open to produce a nerve impulse.
When an impulse does eventually happen in the axon of the postsynaptic cell it is a signal that the soma has received a particular pattern of presynaptic impulses. It is into patterns of this nature that information is coded in the nervous system. Thoughts, perceptions, and emotions all seem to be patterns of nerve impulses in the multi-billion neurons of the nervous system.
When a nerve impulse arrives at a synapse, one of two things can happen. This impulse may simply add its EPSC to the existing summated current without providing the additional voltage necessary to exceed the axon’s threshold (facilitation), or the impulse may be the last one needed to boost the summated current over the threshold and fire the axon (excitation).
Not all receptors open sodium channels and produce EPSCs associated with facilitation and excitation. Some create an inhibitory postsynaptic current (IPSC). For instance, a neuron junctions with striate muscles. Acetylcholine (Ach) opens sodium channels and created an excitatory current. But in the heart muscle, Ach binds with a receptor type that controls potassium channels, and the result is inhibition. The EPSC/IPSC mechanism allows the organism to perform decision-making and choice. (Whether these decisions are conscious, subconscious or unconscious will depend on where, within the brain, they are being made.) Without that mechanism we should all be simple robots, utterly at the mercy of our stimulus-response circuits. Each cell body in the nervous system is a choice maker, totting up the information it receives in the form of EPSCs and IPSCs from various sources, and producing sequences of impulses in its axon representing all these sources, rather than just one.
The combined summation of excitatory and inhibitory inputs is called integration. If the integration is sufficient to cause the axon to fire, it sends a signal to other parts of the brain saying that it is responding to some condition which exists, either in the outside world, or within the brain itself.
At the neurobiological level the aim of all psychotherapy, including hypnosis and NLP, and much non-therapeutic human behaviour, is to influence EPSCs and IPSCs so that their summation and integration produces different decisions in the face of real or imagined aversive stimuli, so that instead of “Spider –> Eek,” one gets “Spider –> So what”.
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