This axon terminal releases chemical messengers known as neurotransmitters (blue dots), into the space between the two cells, called the synaptic cleft. When a nerve cell communicates with another nerve cell, the message is transmitted from the tip of an axon, the long slender arms that extend from the cell’s main body. These neurotransmitters tell the receiver cell to either activate its own electrical charge, which sends the signal to the next neuron in the chain, or tell the receiver cell to stay quiet. If an electrical signal passes down an axon, its tip releases chemicals called neurotransmitters into the synapse. When the axon tip of a transmitter connects to a receiver, that’s a synapse. A second neuron, the receiver, can receive contacts along its main body, or along strands that branch out like a tree, called dendrites. So one neuron, the transmitter, uses a really thin strand called an axon. A brain cell, or a neuron, has a large main body, with small strands sticking out. It’s basically a connection: one cell talking to another. What exactly is a synapse, and what happens there? The links between neurons are called synapses. This conversation has been edited for length and clarity. She spoke with Knowable Magazine about key discoveries in the study of brain networks, and new work revealing their importance in disease. Now director of the Center for Neuroscience at the University of California, Davis, McAllister continues to investigate how the brain’s nerve cells - called neurons - find each other, connect and disengage. As a graduate student in the 1990s studying developmental neurobiology, she was drawn to the question of how the brain is built: how individual brain cells in a growing fetus somehow organize themselves into an organ capable one day of pondering the mysteries of life. Kimberley McAllister has been fascinated by the human brain since college. By sending electrical signals from nerve cell to nerve cell within a great network of connections, the brain creates thoughts as mundane as “Where are my keys?” or as profound as “I think, therefore I am.” That’s because it’s the connections between those cells that make the brain so amazing. No thoughts, no worries, no wonder or awe. If you were to take a human brain and toss it in a blender - not that you should - the resulting slurry of cells wouldn’t be special in the way that the human brain is.

Neurons firing in the brain