How Brain Cells Talk

Wow.  I just wrote a post that was almost completely about dopamine receptors, without actually explaining how neurotransmitters work.

Bruce Banner admitting that that was mean.

I’ve lost my link for this gif so I can’t cite it properly. If you know where its’ from, please let me kow.

That’s okay, I can fix it.

The first thing to know about neurotransmitters is how they fit into the electrochemical signalling between neurons.

A cartoon synapse.

Figure 1. Note how the neurotransmitters move across the synapse (the little gap) and stick to the receptors, pay attention to those. From lecture notes Dyck, R. (2010, Personal Communication).

Signalling occurs when neurotransmitter molecules bind to receptors (Figure 1).  Don’t take it to heart when magazine articles refer to neurotransmitters as having specific functions like “cuddle hormone” or “reward chemical” it’s the receptors which do all the interesting work.  This particular science article trope is one of my least favourite things; it’s meant to be a helpful simplification but it’s actually just straight up wrong.

The function of a neurotransmitter depends on which receptor it binds to and where it is in the brain.  This is why dopamine can both create and prevent attraction in voles by binding to different dopamine receptors.

Receptors can be classified based on either what they make the cell do, or how they work.

Ionotropic receptors (Figure 2A) are directly attached to ion channels so when they are activated they let ions in and out of the cell which is what generates the electrical current in the neurons

Metabotropic receptors turn on a g-protein, a member of a group of small molecules that transmit signals inside the cell.  G-proteins can open and shut ion channels just like ionotropic receptors but when they do they act more slowly and last for longer (Figure 2B).  They can also activate signalling cascades; chains of chemical reactions which alter the overall behaviour of the cell (Figure 2C).  These cascades don’t create electrical signals immediately, but they can change how the cell responds to subsequent signals.  They might, for instance, cause the cell to have a larger response to a specific neurotransmitter the next time it binds.

Different receptor types

Figure 2. Three different kinds of neurotransmitter receptor. From lecture notes: Dyck, R. (2010) Personal Communication

When receptors open or shut ion channels they can be excitatory, in which case the cell will fire a signal of its own or they can be inhibitory, in which case the cell will be prevented from firing.  The receptors which alter the cell’s behaviour in other ways, by activating different signalling cascades, they are referred to as neuromodulatory.  Neuromodulatory effects include things like changing the number or type of receptors on a cell’s surface, which doesn’t make the cell fire immediately but changes how it responds to the next round of neurotransmitter that is released; more receptors will create a bigger response to a neurotransmitter, for instance.

But those are just individual cells, what does opening ion channels have to do with adorable vole romance?  Well, these individual cells are organized into much larger networks, and it is the activity in networks in the brain, groups of cells communicating with other groups, sometimes in distant regions of the brain, that produces thoughts and behaviours.  The activity of specific cell types in specific networks is what creates behaviour.

So, to put it all together

  1. Neurotransmitters turn on neurotransmitter receptors – each neurotransmitter can bind to several different receptor types.
  2. Neurotransmitters change the way their cells behave – different receptor types create different changes, but a receptor typically only does a set thing, like making it easier for a cell to fire.
  3. The activity of big groups of cells in the brain come together to create behaviour – the same transmitter, binding to the same receptor, will create different results depending on which cell is being affected, and where it is.

Now go forth, and view lifestyle articles about neurotransmitters with the same grumpy skepticism I do, quite frankly I could use the company.

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Zap-Poof

If the job of the brain is to process information and generate thoughts, feelings and behaviour, how does it work?  The brain is made of two kinds of highly specialized cells, neurons and glia.  Neurons communicate with each other to process information.  Glia, broadly speaking, are support cells, helping neurons function.  It is the communication between the neurons that is primarily responsible for what the brain does. Neurons have a very specific structure (Figure 1) which is related to how they function.  The bushy dendrites take in information from other neurons.  The architecture of the dendrites is carefully organized to allow the neuron to get input from where it needs it, and is referred to as its dendritic tree.  Information travels through the neuron in the form of small electrical signals called membrane potentials, these can be either excitatory, making the cell more likely to generate a signal of its own, or inhibitory, making it less likely.  These signals travel down the dendrites and through the cell body.  At the base of the cell body (the axon hillock) all these membrane potentials are summed together, if the resulting signal excites the cell above a set threshold the then the neuron will fire a signal of its own called an action potential which travels down the axon (the difference between an action potential and a membrane potential is related to their electrical properties).  The myelin wrapped around the axon acts like an insulator so the electrical signal can bounce between the nodes of Ranvier, and travels much faster than it would otherwise.

Figure 1. Anatomy of a typical neuron from http://www.daviddarling.info/encyclopedia/N/neuron.html

While schematic neurons, like the one here, are a convenient way to see all the parts of a neuron different types of neurons actually all look slightly different and their shape is determined by their function. Neurons that gather input from all over the brain will have immense dendritic trees and small nearly invisible axons (Figure 2).  The pyramidal motor neurons which carry the information used to signal the muscles to move have small trees, but very large axons needed to transmit information down the body (Figure 3).   Typically neurons which transmit information out to the body are called motor neurons, those which carry information from the body to the brain are called sensory neurons, and those within the brain and spinal cord which process and transfer information are referred to as interneurons.

Figure 2. A thalamic interneuron, one of the classes of neurons which receives and directs information all over the brain, from http://berkeley.edu/news/media/releases/2008/12/11_winerobit.shtml

Figure 3. A motor neuron, with part of its very long axon shown, from http://anatomy.ucsf.edu/Anatomy103/Neurohistology/nissl-slides.html

Signalling between neurons involves first a burst of electricity and then a puff of chemicals called neurotransmitters, so it is referred to, imaginatively, as electrochemical signalling.  When the electrical signal reaches the axon terminals, they release a small burst of neurotransmitters.  Individual neurons are separated by a small gap called a synapse.  The neurotransmitters cross the synapse and bind to special receptors on the dendrites of the next neuron which is what creates the membrane potentials.  The combination of a neurotransmitter and a receptor is what determines whether a signal is inhibitory or excitatory each one can bind to multiple receptors, meaning that each transmitter can cause excitation or inhibition by binding to different receptors.  Different receptor types occur in different parts of the brain, which help control the effects of neurotransmitters, and determine the function of the different brain regions.


You Are Here

Since you are reading this post it’s a pretty safe assumption that you are a human being, and have a brain.

What is a brain?  The brain is a located inside the skull, it weighs approximately 3 pounds, on average, and has the consistency of Jello (a fact I have never personally tested).  The large wrinkly surface is called the cerebrum, this is the largest and most complex part of the human brain.  The smaller wrinkly knob at the bottom is called the cerebellum, and the stalk is the brainstem, which tapers down into the spinal cord.  Nerves from through the body run into and out of the spine (with the exception of a few who run directly into the brain), and carry information in and out.  Under the cortex, things get much more complicated.  Your brain isn’t a homogenous mass, but, instead, is made of dozens of specialized structures, all with different functions.  The cortex is also made of many different functional areas, although they can’t be differentiated by the naked eye).

What does a brain do?  The main job of the brain is information processing.  All the information about the outside world, and your internal environment is picked up by your nerves and carried to your brain, which interprets it and integrates it.  This is where sound waves turn into words and music, photons turn into pictures, and then they both get combined to give you the show you are watching on TV.

The brain is also the source of thoughts and feelings about all the information you feed it by seeing, hearing, smelling, etc.  Your brain generates emotions (aided and abetted by your hormones and immune system), remembers things to use later and generates new thoughts and ideas.  Finally, the brain also turns thoughts into behaviour, generating movement, speech, facial expressions.

All of your thoughts, sensations, feelings, impressions, memories, ideas and general you-ness, are in your brain.  Of course, it’s more complicated than that (in biology, everything is more complicated than that).  The body isn’t an army, the brain doesn’t take in information and give out orders, it gives and receives constant feedback to and from the rest of the body more like a conversation and the other participants in the conversation contribute to how the brain functions.