5 In support of their hypothesis, Steward and Levy provided electron microscopy data showing clusters of ribosomes called polyribosomes residing far away from the cell body, floating around the base of spines.
Instead, they argued that it made more sense for protein synthesis to be regulated locally based on need. In 1982, Oswald Steward and William Levy, then neuroscientists at the University of Virginia School of Medicine, published a study proposing that it was inefficient for the neuron to produce proteins in bulk when there is no clear demand. Synaptic construction sites can be hundreds of microns away from protein factories in the cell body, and the time it would take for a message to reach these factories, produce new proteins, and ship them back out did not align with the rapid demands that synapses place on the neuron. These findings suggested that activated synapses could use proteins already lying around while waiting an hour or so for new proteins to arrive from the cell body. 4 This activity lasted for an hour before gradually decaying back to baseline. However, the mechanisms by which the cell body communicates with its distal compartments to regulate gene expression remained an area of contention until the late 1990s.Įric Kandel, a neuroscientist at Columbia University, found that squirting a drug that blocks protein synthesis onto neurons from the sea slug Aplysia did not prevent a short-term spike in activity in response to a chemical stimulus measured in electrophysiological recordings. LTP results from repeated stimulation and induces new protein production and new synaptic connections. 3,4 Shortly after, scientists identified and characterized a cellular analogue of long-term memory: long-term potentiation (LTP), the persistent strengthening of a synaptic connection, and therefore communication, between two neurons. In the 1960s, scientists studying memory formation in goldfish and rodents demonstrated that long-term but not short-term storage of memories requires new protein production. (top) Gilles Laurant (bottom) University of California Los Angeles A heretical hypothesis When Schuman started studying synapses in the 1980s and 1990s, she was curious how neurons, with their tiny cell bodies and expansive volumes, maintained thousands of independent connections, many of which were far from the cell body. “We estimate that around 80% of the volume of a neuron is not in the cell body but rather in the dendrites and the axons,” said Schuman. 2Ī single neuron can receive messages through several thousand independent synapses spread across the dendritic arbor. Researchers have demonstrated how, at its most basic level, memory is modulated by strengthening weakening synaptic connections. Dendritic branches are densely populated with even smaller budding protrusions called spines, which make synapses with other neurons to receive important messages. Axons and dendrites sprout from the cell body, branching out in different directions in search of connections. Unlike most cells, neurons expand beyond their spherical cell bodies into a complex labyrinth of protrusions that evoke visions of naked trees in winter. -Erin Schuman, Max Planck Institute for Brain Research Neurons are in the business of interacting and connecting. “Neurons are in the business of interacting and connecting,” said Erin Schuman, a neuroscientist at the Max Planck Institute for Brain Research. Intercellular communication is not unique to neurons, but the sheer scale of their interactions distinguish neurons from other cells. Our three-pound brains are home to nearly 100 billion neurons (almost equivalent to the number of stars in the milky way) that communicate across short and long distances, shaping our behavior and identities.
1 Since then, neuroscientists have been trying to answer how neurons and the networks they weave capture something so complex, so illusory, as a memory. From these, he posited that neurons were the main building blocks of the brain and that memories might be encoded in the connections between neurons. More than a century ago, the anatomist Santiago Ramón y Cajal unveiled his incredibly detailed drawings of neurons. ABOVE: MODIFIED FROM © ISTOCK.COM, koto_feja
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