Basic Nerve Cell Communication
As psychotherapists, we are primarily concerned with processes of the mind—what a client is thinking, and how that thinking affects behaviour, relationships and wellness. But what about the ‘hardware’, or rather, ‘wet-ware’, that underlies and embodies the processes of the mind? For the most part, the ‘biology of the mind’ has not formed the theoretical basis for psychotherapy and counselling, and in some respects this may be justified. But with scientific research advancing as it is, we are gaining many helpful insights into the interplay between our thoughts and our biology. With ever more sophisticated technology to capture the brain in action, our understanding of neural processes is on an exponential rise. So too is our understanding of which neural networks are responsible for certain processes of thought, memory, emotion, and action, and how these networks integrate with one another. The recent fifth edition of Principles of Neural Science is a testimony to our growing knowledge of the biological underpinnings of the mind. Its 1,700 pages, by 79 of the world’s preeminent neuroscientists, take us on a gripping journey through what is probably the most sophisticated and complex system on the planet. It would be fair to say that we have only just begun to understand what these 100 billion nerve cells and their trillions of connections are doing. I believe we should take advantage of this explosion of knowledge about the biology of the mind and hone our therapeutic skills with a deeper understanding of the mechanisms that fundamentally modulate a healthy mind and psychopathology.
With this in view, I will be taking you through a series of short articles in an easy-to-digest language and style. I understand that not every therapist will be wanting to become a neuroscientist, but any worth his or her salt will be excited to learn the broad strokes of recent neural science that has the potential to significantly improve counselling outcomes. So while the budding neuroscientist may not find all he is looking for in this series (I intend to present only the basic concepts in simplified form), the rest of us, I trust, will come away with a basic grasp of the workings of the brain, and find the insight helpful in our personal development as health care professionals.
Basic Nerve Cell Communication
Let’s begin with the fundamentals. What is a nerve cell, and how does it communicate with other cells? In the nervous system there are two main divisions of cells: nerve cells (we will call them neurons from here on in), and glial cells (also called glia). Glial cells are recognised as a sort of ‘support network’ for neurons, but we won’t go into detail about them here, as our focus will be on neurons. The function of a neuron is determined by where it is in the brain, how it is connected with neighbouring cells, and its individual functional character. It’s very much like us: our function in society is determined by where we are, who we are connected to and how we interact with others and the environment.
There are different types of nerve cells for differing functions, but for the sake of simplicity we will be talking about a generic neuron—a model—that represents the fundamentals of all neurons. The basic schematic below illustrates the main components of our generic neuron. (Please note that it represents nothing like the scale and complexity of a real neuron, but only serves to illustrate some basic components, as well as my limitations as a graphic artist.)
Neurons communicate using two processes: an electrical signal within the neuron, and chemical signals between neurons. We’ll start by looking at the chemical signals between neurons before moving on to how this communication can initiate an electrical impulse within a cell.
Neurons use various chemicals to transmit signals across the very small gap between cells in an area know as a synapse. The communication chemicals are called neurotransmitters. Most neurons can send and receive signals by different types of neurotransmitters, and differing neurotransmitters work at different speeds. We will get to know some of the major neurotransmitters as well as the effects and speed of various chemical signals as we go through this series.
The schematic below is a radically simplified representation of a synapse. The upper part shows a presynaptic terminal of one cell’s axon, and the lower part the postsynaptic dendrite of another cell. Communication flows from the end of one cell’s axon, at these terminations known as presynaptic terminals, to the dendrites of another cell. Dendrites are like the branches of a tree that spread out to reach other cells. They are extensions of the cell body.
The synaptic vesicles shown above are packets of neurotransmitters that migrate to special release sites called active zones. These packets come to the surface of the presynaptic terminal and are released (this is called exocytosis) into the synaptic cleft. We will talk about what causes this release a little later. The chemicals diffuse across the gap and some molecules are taken up by receptors on the opposite-facing dendrite. The receptor sites on the dendrite bind to specific neurotransmitters, and this binding will have either an inhibiting or excitatory effect on the receiving cell. This happens by an opening of ion channels in the membrane of the cell that essentially produces a membrane potential in the dendrite (a more positive or more negative charge within the cell). The more positively charged a neuron becomes, the more likely it will pass a certain threshold and ‘fire’.
To put it in very simple terms, some neurotransmitters will cause the receiving dendrite to become more positive and others will cause the dendrite to become more negative. The receiving dendrite ‘calculates’ each of the incoming chemical signals and from that calculation (the resulting synaptic potential, or the resulting charge in the dendrite) sends a signal down to the main body of the cell. All of these little ‘calculations’, or synaptic potentials, are eventually ‘summed’ at the beginning of the axon (called the axon hillock). If the resulting charge rises above −55mV (depolarization), then from the axon hillock an electrical signal flows down the axon to its presynaptic terminals. When the charge reaches the postsynaptic terminals, it causes the membrane of the each terminal to open up channels to let calcium into the cell. This in turn causes the synaptic vesicles (those little packets full of neurotransmitters) to release their chemicals into the synaptic cleft.
Admittedly this is an extreme simplification of the process, but I just want to get across the main point: neurotransmitters are like a lot of little ‘on’ and ‘off’ signals, and if the ‘on’ signalling is strong enough—i.e., louder than the ‘off’ signal—then the receiving cell will turn ‘on’, or initiate an action potential.
The above diagram also shows some of the neurotransmitters being reabsorbed into the presynaptic terminal. Uptake, via plasma membrane transporters, serves two purposes: recapturing the chemicals for reuse, and terminating the synaptic action of the cell. Drugs used to inhibit the re-uptake of neurotransmitters will in effect keep the neurotransmitter in the vicinity of the postsynaptic dendrite, and when receptors are available for that particular chemical, they will bind with it. This is how an antidepressant, designed to inhibit serotonin re-uptake, can keep more serotonin in the synaptic cleft (assuming there is sufficient serotonin being released by the presynaptic terminal).
It should be further noted that receptor sites are either fast-acting ionotropic types, that primarily activate or inhibit, or slower-acting N-methyl D-aspartate (NMDA) types, that facilitate a strengthening of the synapse. We won’t go into how these different receptor systems work; suffice it to say that some neurotransmitters are involved in fast acting on/off functions and others are involved in strengthening neural connections and thus neural networks.
Next in part 2 we will look at neurotransmitters in a little more detail, as they become vitally important in the modulation of the whole brain.
For an in-depth coverage of all things neuron, the authoritative Principles of Neural Science is the volume to visit. However, our basic look at the brain would be well covered by any recent neuroscience text, and the humble wikipedia.org is home to a wealth of information and plenty of references to other online resources on the subject.
Principles of Neural Science, 5th ed, edited by E.R. Kandel, J.H. Schwartz, T.M. Jessell, S.A. Siegelbaum, A.J. Hudspeth (2013). New York, McGraw-Hill.
Images have been drawn by myself and may be reproduced for study purposes.