A new study has given fresh insight into the basic units of memory by discovering a molecular "recycling plant" that allows nerve cells in the brain to perform two seemingly opposing functions - changeable enough to record new experiences, yet permanent enough to maintain these memories over time.
The finding of this molecular recycling plant provides new understanding of how the basic units of learning and memory function. Individual memories are "burned onto" hundreds of receptors that are continuously in motion around nerve synapses i.e. gaps between individual nerve cells crucial for signals to travel throughout the brain.
According to the study's leader, Duke University Medical Center neurobiologist Michael Ehlers, M.D., Ph.D., these receptors are constantly moving around the synapse and often they fade away or escape.
Ehlers found that a particular set of molecules catch these elusive receptors, take them to the recycling plant where they are reprocessed and returned to the synapse intact.
"These receptors constantly escape the synapse and are in a perpetual state of recycling. This process occurs on a time scale of minutes or hours, so the acquisition of new neurotransmitter receptors and their recycling is an on-going process. Memory loss may result from receptors escaping from the synapse," said Ehlers.
He adds that all this activity takes place on millions of tiny "nubs," or protrusions in the synapses known as dendritic spines, and the recycling plants are located within the body of these dendritic spines.
"We believe that the existence of this recycling ability explains in part how individual dendritic spines retain their unique identity amidst this constant molecular turnover. The system is simultaneously dynamic and stable," Ehlers said.
Ehlers believes that while this knowledge could help neurobiologists in their endeavour to understand the molecular foundations of learning and memory, it could also be helpful in explaining what happens in certain neurological disorders, such as Alzheimer's disease, schizophrenia, or learning disorders like autism.
For example, it appears that in animal models of the early phases of Alzheimer's disease, often before any symptoms become apparent, the dendritic spines gradually lose their ability to transport and recycle the receptors.
"If the receptors don't get recycled, you see a gradual loss of synaptic function that is associated with reduced cognitive ability. These dendritic spines are where learning and memories reside. These are the basic units of memory," Ehlers said.
The study is published early online in the journal Neuron.