Neurotransmitters and neuromodulators
A note for Lesson no. 2, "Your Brain Is a Network," in Lisa Feldman Barrett.
Some context from page 36 is:
These chemicals, such as glutamate, serotonin, and dopamine, are called neurotransmitters, and they make it easier or harder for signals to pass across synapses. [...] some of these chemicals, such as serotonin and dopamine, can also act on other neurotransmitters to dial up or dial down their effects. When brain chemicals act in this way, we call them neuromodulators.
Neurotransmitters are neurochemicals that ferry sense data throughout the brain and send motor commands to control the body. They have a speedy, local effect on other neurons, working their magic in milliseconds. They are released into a synapse and bind to fast-acting dendritic receptors (called ionotropic receptors). They are quick to cause axonal firing and are just as quickly disabled after release, either because they degrade or because they are taken back up into the axon, a process called “reuptake.” If you prevent the reuptake of a neurochemical such as serotonin with a drug, you prolong the influence of a neuron on other neurons. This is how some anti-depressant medications work.
In the book, I say colloquially that neurotransmitters "make it easier or harder for signals to pass across synapses." In fact, the neurotransmitters themselves are the signals that pass across synapses.
Neuromodulators are neurochemicals that magnify or diminish the impact of neurotransmitters like a volume control. They have wide-ranging effects because they release diffusely into the surrounding tissue (called neuropil) rather than titrate into specific synapses. They bind to slower-working receptors (called metabotropic receptors), having their effect over seconds or minutes. Neuromodulators tune the sensitivity of wide swaths of neurons over longer periods of time, which is why they are partly responsible for how sleepy or alert you feel and how easy it is to learn something new.
Some neurochemicals, such as dopamine and serotonin, serve double duty, acting as neurotransmitters or neuromodulators. The combination of neurotransmitters and neuromodulators allows the brain to use both analog and digital signals.
For two accessible essays that discussion neurochemicals, see these references:
For more technical information, see these references:
- Linden, David J. 2018. "Our Human Brain was Not Designed All at Once by a Genius Inventor on a Blank Sheet of Paper." In Think Tank: Forty Neuroscientists Explore the Biological Roots of Human Experience, edited by David J. Linden, 1–8. New Haven: Yale University Press.
- Snyder, Solomon H. 2018. "The Brain Harbors Many Neurotransmitters." In Think Tank: Forty Neuroscientists Explore the Biological Roots of Human Experience, edited by David J. Linden, 88–93. New Haven: Yale University Press.
- Siegelbaum, Steven A., David E. Clapham, and James H. Schwartz. 2013. "Modulation of Synaptic Transmission: Second Messengers" In Principles of Neural Science, edited by Eric R. Kandel, James H. Schwartz, Thomas M. Jessell, Steven A. Siegelbaum, and A.J. Hudspeth, 236–259. New York: McGraw Hill Medical.
- Schwartz, James H., and Jonathan A. Javitch. 2013. "Neurotransmitters" In Principles of Neural Science, edited by Eric R. Kandel, James H. Schwartz, Thomas M. Jessell, Steven A. Siegelbaum, and A.J. Hudspeth, 289–306. New York: McGraw Hill Medical.
- Bargmann, Cornelia I. 2012. "Beyond the Connectome: How Neuromodulators Shape Neural Circuits." Bioessays 34 (6): 458–465.