a general belief that the
circuitry of young brains has robust flexibility but eventually gets
"hard-wired" in adulthood. Researchers have learned, however, that
adult neurons aren't as rigidly glued in place as they assumed.
Investigators led by Johns Hopkins neuroscientist David Linden recently took advantage of a new technique known as "two-photon microscopy" that lets them literally see living neurons going about their business in the intact brain. The researchers injected fluorescent dye into the brains of mice to light up a subset of neurons and then viewed these neurons through a window constructed in the skull of living, anesthetized mice.
They examined neurons that extend fibers (called axons) to send signals to a brain region called the cerebellum, which helps coordinate movements and sensory information. Like a growing tree, these axons have a primary trunk that runs upward and several smaller branches that sprout out to the sides.
But while the main trunk was firmly connected to other target neurons in the cerebellum (stationary, as adult axons are generally thought to be), the side branches swayed like kite tails in the wind,
Over the course of a few hours, individual side branches would elongate, retract and morph in a highly dynamic fashion. These side branches also failed to make conventional connections, or synapses, with adjacent neurons. Furthermore, when a drug was given that produced strong electrical currents in the axons, the motion of the side branches stalled.
Why the brain would want such motile, non-connected branches is the next mystery to tackle. Linden thinks they may present a second mechanism for conveying information beyond traditional synapses or assist in nerve regeneration, quickly forming synapses should nearby nerves get damaged.
Source: John Hopkins University
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