Neuroscience Breakthroughs Since Graduate School - Part 4: Neurons
Updated: Mar 24
The brain uses a sophisticated communication and command-and-control system that monitors and manages interactions between roughly 100 billion neurons, each with 5,000-10,000 synaptic connections, for as many as 500 trillion synapses in adults. Neuroscientists have learned a great deal more about neuronal function since graduate school. The most important findings are that the adult human brain creates new neurons, silent synapses may mediate neuroplasticity in adulthood, the lymphatic system extends to the brain, networks of neurons exhibit mirroring properties, neurons can release more than one neurotransmitter (NT), release NTs outside of a synapse, conduct two-way conversations, modulate NT release and action, talk to the astrocytes that enclose the synapse, and electrically communicate almost instantaneously. Research has expanded our understanding of the roles of astrocytes in regulating neuron growth, function, signaling, information processing, synapse formation and elimination, and brain waves.
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The Chemical Synapse
Neurons communicate through the release of over 200 neurochemicals and ions. Axon terminal buttons release neurochemicals across a 20-50-nm fluid-filled gap between presynaptic and postsynaptic structures called a synaptic cleft and into the extracellular fluid surrounding the neuron (Bear et al., 2020). Chemical synapses produce short-duration (millisecond) and long-duration (seconds to days) changes in the nervous system. Synapse animation without sound © 3Dme Creative Studio/Shutterstock.com.
They are functionally asymmetrical because the presynaptic neuron sends a chemical message and the postsynaptic neuron receives it. They are structurally asymmetrical because the presynaptic element (axon) contains vesicles containing neurotransmitters, and the postsynaptic element (dendrite) doesn’t. Neurotransmitter (NT) release from a terminal button is called exocytosis (Breedlove & Watson, 2020). Chemical synapse graphic © rob9000/Shutterstock.com.
In the graphic below, an axon terminal button releases NTs into the synaptic cleft. Neurotransmitters briefly form covalent bonds with receptors on a dendritic spine and then disengage after they initiate small graded potential changes (e.g., EPSPs or IPSPs) or more diverse, gradual, and long-lived actions (e.g., creating second messengers inside the target neuron). Chemical synapse graphic © nobeastsofierce/Shutterstock.com.
Neuroscientists have learned a great deal more about neuron-to-neuron communication since graduate school. The most important findings are that the adult brain creates new neurons, silent synapses may mediate neuroplasticity in adulthood, the lymphatic system extends to the brain, neuronal networks exhibit mirroring properties, and neurons can release more than one NT, release NTs outside of a synapse, conduct two-way conversations, modulate NT release and action, talk to astrocytes that enclose synapses, and electrically communicate almost instantaneously.
Neuroscience has challenged the doctrine that the adult human brain does not create new neurons. There is a consensus that neurogenesis occurs in the hippocampus (Eriksson et al., 1998) and olfactory bulb (Lim & Alvarez-Buylla, 2016). However, neurogenesis outside the hippocampus remains controversial. Animal research has yielded evidence of functionally significant neurogenesis in the amygdala, caudate nucleus and putamen (striatum), cortex, hypothalamus, and substantial nigra (Jurkowski et al., 2020). The graphic below was retrieved from sci.news.
Note. Neural stem cells giving rise to neurons are green, and an adult hippocampal neuron is red.