Above-ground nuclear testing, 1955-1963, produced a marker for repopulating neurons devoted to memory
Episodic memory is managed by dentate gyrus of the hippocampus
Episodic memory recalls specific events of a person’s life. It is the long-term memory that people worry most about losing with age and disease. A special part of episodic memory is a person’s memory of those events that make up their own life’s history.
The hippocampus region of the brain is where new episodic memories begin. And, it is the only place in the human brain where birth of new neurons has been confirmed in the adult.
Episodic memory formation is a complex process. Often new episodic memories incorporate parts of old memories. For example, if you revisit your old school for a reunion, memory of the reunion will incorporate earlier events from the past when you attended classes at the school.
Continual incorporation of new neurons into the brain pathways that support memory adds a new perspective for those who theorize about the brain’s infrastructure for memory. It is accepted that the hippocampus orchestrates storage of the the details of previous memories and is critical for their recall.
But, the various particulars of long-term episodic memory such as color, place, people involved, sound etc. are not stored in the hippocampus. Rather they are stored as bits and pieces in various parts of the outer layer of cells covering the brain, the cerebral cortex.
There are very active incoming and outgoing neural pathways between the hippocampus and the cerebral cortex that are used to bring the details of previous events back for incorporation into new memories.
Birth of new neurons in the adult brain is a modern discovery
In adult mammals, the first brain stem cell population capable of developing new neurons was identified in the early 1990s in the mouse brain. The first indication that neuron producing stem cells also existed in the human brain was published in 1998.
In the 1998 study, postmortem histology of a cancer patient’s brain confirmed the presence of neurons created after initiation of chemotherapy to treat the cancer. The particular drug used against the cancer was only able to incorporate into the nuclear DNA of newly formed cells.
Neurons possessing the drug marker in their DNA were only observed in the patient’s hippocampus. There was no indication of new neuron formation during the treatment in other brain structures.
How above-ground nuclear testing was used to reveal the rate at which new neurons form in the adult human brain
In 2013, a multinational collaborative study [Spalding KL, et al., Cell 153:1219-1227, 2013 CellPress Open Access] retrospectively dated birth of neurons in human postmortem brain specimens by measuring the amount of 14Carbon present.
Above-ground nuclear testing significantly increased the amount of 14Carbon, a radioactive form of carbon, present in the atmosphere. Before nuclear testing there were only trace amount of 14Carbon in Earth’s atmosphere, about 1 part 14Carbon per trillion non-radioactive 12Carbon molecules. The natural occurrence of 14Carbon in the atmosphere is a result of comic ray action upon nitrogen gas.
14Carbon in the air reacts with oxygen to form 14CO2. Plants cannot distinguish 14CO2 from 12CO2. Since the nuclear testing period, 14CO2 has been incorporated into plant glucose to a greater extent than it was before 1955. Plants being the primary food source for man and animals, 14Carbon is now distributed throughout human bodies. And some of the excess 14Carbon from nuclear testing became incorporated into the DNA of cells created after 1955.
Rate at which new neurons are formed in the dentate gyrus of the hippocampus
Spalding et al. used several mathematical models to estimate the rate of new neuron formation based upon 14Carbon in DNA of neuron cell populations from various areas of the brain.
As 14Carbon decays to 12Carbon, it emits a low energy beta particle that can be detected. The half-life of the decay process for 14Carbon is 5,730+/-40 years. That is, after about 5,700 years half of the original material will have become 12Carbon. The level of 14Carbon measured in the postmortem DNA was compared to the amount of 14Carbon in plants before 1955. Tree rings are a good source of plant levels before 1955.
With these methods Spalding et al. concluded that a sub population of adult human hippocampus neurons, corresponding to the majority of dentate gyrus neurons, possess an annual turnover rate of 1.75%. This corresponds to 700 new neurons added daily in each hippocampus, right and left hemisphere.
Based upon the oldest subject in the study investigators also concluded that the new neurons develop in the adult human brain at least into the 5th decade of life. And, they only found a modest decline in new neuron formation during normal aging.
Theories about the relevance of young neurons to episodic memory formation
In the 2013 study, the half-life of dentate gyrus new neurons was 7.1 years. This is ten times shorter than the half-life of the non-renewing neurons of the rest of the hippocampus. Studies of the characteristics of new dentate gyrus neurons in rodents suggest, at least in those animals, that young neurons have a greater capacity for creating new synapses for connection to other neurons.
Other studies demonstrate that episodic memory formation is also associated with increased synapse creation. Theories propose, therefore, that the birth of new neurons may be required for efficient episodic memory management.
Current thought is that new dentate gyrus neurons may be required for the ability to store similar experiences as distinctly different events. Another theory is that the non renewing neurons of the remainder of the hippocampus are necessary to associate similar memories with each other.
It will require more study and new methods, or creative use of existing opportunities, to confirm or deny the current ideas about the importance of new adult brain neurons to the infrastructure on episodic memory.
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