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Chapter 13 endnote 4, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

...from one moment to another, your billions of neurons continually reconfigure themselves from one pattern into another. Chemicals called neurotransmitters make this possible. [...] Neurotransmitters change how efficiently your neurons communicate and more.

Neurotransmitters change how efficiently your neurons talk to each other, which neurons are passing information back and forth, and how easily a neuron can change its firing pattern.[1] When the neurotransmitter(s) from a single neuron influences many different neurons at the same time, this chemical is called a neuromodulator. (Neurotransmitters are also called neuropeptides or neurochemicals). There is also some evidence that glial cells communicate with one another and with neurons using neurotransmitters.[2] Examples of commonly studied neurotransmitters (listed with one function; each neurotransmitter can have more than one function[3]):

Neurons can switch neurotransmitters they release and release more than one neurotransmitter a time.[3] This increases your brain's complexity (i.e., its ability to make many different neural patterns with the same set of neurons). For example, an earthworm’s nervous system has 302 neurons, 1/3 sensory, 1/3 motor, and 1/3 association neurons, but its genome codes for over 200 neurotransmitters, which gives it tremendous complexity. There is evidence that one behavior can be caused by several circuits (degeneracy). But more importantly, one neuron with one neurotransmitter can affect two different behaviors depending on the other neurons active (i.e., depending on the neural context). Think about that the next time you see a little slimy worm on the sidewalk in the rain. There is a lot going on inside that little slimy body.

Notes on the Notes

  1. Bargmann, Cornelia I. 2012. "Beyond the connectome: how neuromodulators shape neural circuits." Bioessays 34 (6): 458-465.
  2. Fields, R. Douglas and others. 2014. "Glial Biology in Learning and Cognition." The Neuroscientist, 20(5): 426-431.
  3. 3.0 3.1 Gjorgjieva, Juliajana, G. Drion and Eve Marder. 2016. "Computational implications of biophysical diversity and multiple timescales in neurons and synapses for circuit performance." Current Opinion in Neurobiology, 37: 44-52.