How Brain Cells TalkPosted: November 17, 2013 | |
Wow. I just wrote a post that was almost completely about dopamine receptors, without actually explaining how neurotransmitters work.
That’s okay, I can fix it.
The first thing to know about neurotransmitters is how they fit into the electrochemical signalling between neurons.
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.
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
- Neurotransmitters turn on neurotransmitter receptors – each neurotransmitter can bind to several different receptor types.
- 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.
- 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.