Functional Neurogenesis

A fundamental property of the hippocampus is its ability to rapidly encode memories while simultaneously keeping them distinct. Recording from hippocampal neurons one can clearly see that different populations of neurons are active as a rat explores two environments. This is thought to be one mechanism by which information is kept distinct in the brain.

For the last 15-20 years it has been thought that the dentate gyrus (DG), a major subfield of the hippocampus, serves to take small changes in incoming sensory information and orthogonalize them (i.e. make them more different). This idea was built in part on the fact that there are many more DG neurons than upstream cortical neurons. Thus, the DG could use completely different populations of neurons to represent different sets of incoming information and then pass on these representations to CA3, which may bind them into coherent events/memories (the interconnectedness of CA3 neurons, via “recurrent collatorals”, is thought to be a mechanism by which the different components of a memory are bound together).

However, a “problem” arose when Leutgeb et al. found that it is always the same population of dentate granule neurons (~1% of the total population) that are active as an animal explores different environments, even very different ones. This was a bit of a surprise. Still consistent with the proposed role of the DG in orthogonalizing information, however, was the fact that the DG neurons fired (i.e. generated action potentials, which transmit information from neuron to neuron) at different rates/frequencies in the different environments. Thus, changes in sensory information were represented by changes in patterns of activity within the same population of cells, not by recruiting different populations of cells. This is but one study – the question of how the DG encodes and extracts information is far from settled (e.g. what are the other 99% of granule neurons doing? Surely there is a situation in which they are active, no?). But the findings were robust and raise many questions, namely: How does the same population of DG neurons activate different populations of downstream CA3 neurons, during different experiences? Read the rest of this entry »

Dendrites are the extensions of neurons that receive incoming information. Neurons have primary dendrites that further split off into secondary and tertiary dendritic branches. On each of these branches are thousands of synaptic connections with axons of neurons carrying incoming information. The result is a dendritic tree that is capable of receiving and integrating a wide array of information within a single neuron. This is one of the neurobiological mechanisms by which different components of a memory are thought to be joined.

Neurons are not born with dendrites and spines – they are acquired during a developmental process that takes many weeks (see here & here). During early development, the pattern of formation of dendrites and spines are sculpted by experience, as might be expected if dendrites and spines are anatomical structures involved in processing and storing sensory information. While a body of work has emerged suggesting adult-born neurons are involved in memory and behavior, no one has yet investigated whether experience is capable of altering the dendritic development of these new neurons. This paper by Tronel et al. is therefore very important because it is the first to look at this phenomenon. They show a dramatic acceleration of dendritic development in response to learning, suggesting a potentially powerful role for new neurons in storing and processing information.
Read the rest of this entry »