Nervous System Basics, Part 2: Excitatory vs Inhibitory

Continued from Part One

Nervous system cells, whether they are in the brain, the spinal cord, or the peripheral nerves, communicate with each other via a group of chemical messengers called neurotransmitters.  One nerve cell releases a neurotransmitter in order to create some kind of effect on the next nerve cell in line.

There are two major types of effects a neurotransmitter can have on a nerve cell: excitatory and inhibitory.

Excitatory refers to any stimulus that either causes a nerve cell to fire, or simply makes it more likely to fire (aka more likely to send a signal).

Inhibitory refers to any stimulus that makes a nerve cell less likely to fire (aka less likely to send a signal).

You will often find the words excitatory and inhibitory in scientific articles, whether those articles are discussing how pain works or how pain medication works.

For example, excitatory can refer to something that would encourage the nervous system to send more pain signals, more frequently. One nerve cell can have an excitatory effect on another, making that second cell want to fire.

On the other hand, some nerve cells communicate with other nerve cells and tell them not to fire. This is what is referred to as an inhibitory effect.

Many pain medications work in this way, by inhibiting the activity of nerve cells that transmit messages about pain.

But the body also has its own built-in inhibitory system when it comes to the nervous system sending pain signals.

Scientists are still discovering how many of these pathways work, and they are incredibly complex. I read about some of them for my neuroscience class last summer, and was blown away by how intricate they are; how many different neurotransmitters and hormones are involved; how many different types of nerve cells and receptors on those cells.

But the gist of the matter is that some nerves can have an inhibitory effect on the activity of other nerves, slowing the overall transmission of pain signals.

This is one way in which the body regulates your experience of pain. Sometimes your body decides it’s important for you to feel pain– like when you are home alone and get a papercut. You look down, see you’re bleeding, and realize you need to be more careful with the papers you’re handling.

Yet sometimes, the body decides it’s more important for your survival to block out those pain signals– for example, if you’re a soldier in battle who has been badly wounded, but still needs to get out of the zone of fire. In those crisis situations, people can feel no pain at all, because of the nervous system’s own inhibitory mechanisms.

Of course, these systems do not always work perfectly. Research suggests that in people with fibromyalgia/chronic pain, the inhibitory pathways might not be working properly, which is why they seem to experience more pain in response to non-harmful stimuli than do other people.

It’s not only that the nerves sending messages about pain are working overtime (which they definitely can!). But the other piece of the puzzle is that the nerve cells responsible for reducing some of those pain signals are underactive.

I’ll be discussing this more in Part 3. Stay tuned!

The beautiful photo at the top was available through a Creative Commons license thanks to Charis Tsevis/Harrison & Star

Medication picture courtesy of Steve Smith

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