Tag Archives: place cells

How does the brain pick which neurons to use?

ResearchBlogging.orgWiring. That’s one answer to this question. We know this from topographic maps in the thalamus and neocortex, where the basic units of sensory information are neatly represented in spatially-arranged populations of neurons – the various body parts are represented in specific locations, as are the different frequencies of sound, the different parts of the retina, and different odors and tastes. This basic sensory information has to be represented (i.e. we all need a faithful representation of visual elements, we all need to hear the various frequencies of sound that make up human speech etc.) so why not hard-wire it and make its representation the same for all of us?

It’s often thought that things change as you move into parts of the brain that represent more complex and abstract concepts. For example, in the hippocampus, many neurons receive the same inputs so it’s generally assumed that different neurons are equally capable of representing a given piece of information. While wiring between neurons must play a role in determining which neurons are activated, the diffuseness of the wiring means that related information need not be stored in spatially neighboring neurons as in the sensory regions of neocortex. Indeed, if you look at hippocampal neurons activated by a given experience they don’t appear to have any particular spatial arrangement but are randomly distributed, anatomically. Alternatively, it could be that certain hippocampal neurons are hard-wired to respond to specific stimuli, it’s just that we don’t understand the wiring. Continue reading How does the brain pick which neurons to use?

SFN2010 Itinerary Pt. 1

I have 62 items in my itinerary and I expect to add to it in the following weeks. There are always great presentations I find out about last minute and undoubtedly others that never see the light of (my) day. So fill me in if you have any tips. It’s not always the data that’s the most interesting part but often the presenter themselves, their ideas, methods, or the fact that you’ve known them since undergrad and you want to see baby pictures. My plan here is to share some of the potentially (you never know til you’re there) interesting presentations in my itinerary, bit by bit, over the next couple weeks.

1) 99.8/JJJ44 – The hippocampus is required for social recognition but not object recognition in Octodon degus
1Doshisha Univ., Kyotanabe City, Japan; 2RIKEN BSI, Lab. for Biolinguistics, Wako City, Japan

Huh? Octogon what? They’re rodents unlike any other rodent. They perform communal digging, nurse each other’s young, are born with their eyes open, and are intolerant of sugar and get diabetes. Even more interesting is the fact that they can switch their circadian rhythms between nocturnal or diurnal patterns and they’re apparently able to use tools (at least according to Wikipedia). So, basically I want to chat about Degus with these guys.

2) 100.19/KKK35 – Hippocampal granular neuron recruitment during the evocation of a recent or remote object recognition memory
1Neurobiologia Conductual y Cognitiva, Inst. de Neurobiologia, Univ. Nacional Autonoma de Mexico, Queretaro, Mexico; 2Neurobiologia Conductual y Cognitiva, Inst. de Neurobiologia UNAM, Queretaro, Mexico

Victor Ramirez-Amaya has done some interesting work on learning-related structural plasticity in the hippocampus and he’s also shown that the plasticity-related protein Arc is expressed in a delayed fashion after experience, probably as a part of the process of memory consolidation. Since there’s an emerging role for the dentate gyrus and neurogenesis in long-term memory I’m interested in this poster, which looks at activity-dependent Arc expression in the dentate gyrus and in young neurons after recent and remote(ish) memory retrieval.

3) 101.6/KKK46 – Spatial representation along the proximo- distal axis of CA1
1Kavli Inst. Sys Neurosci, Ctr. Biol of Memory, NTNU, Trondheim, Norway; 2Univ. Arizona, NSMA, Tuscon, AZ

The hippocampus is a convenient structure to study because its anatomical boundaries are distinct – the dentate gyrus doesn’t abut any other principal cell layers. CA3 and CA1 are easily distinguished from each other based on cell packing density. Probably for this reason there has emerged the assumption that these subregions are homogeneous in function. However, at least for the dentate gyrus it has become clear that it’s two blades are very different. And now, this abstract reports that the CA1 neurons that border region CA3 carry more spatial information than those at the other end, near the subiculum. I’m not entirely sure why I’m interested in this, but it may be because it suggests a certain level of care must be taken in future experiments (e.g. being consistent in where measurements are made) and it argues for better reporting in methods sections as to how measurements were made (because we now know that not all areas of CA1 are equivalent).

What IS the dentate gyrus doing to CA3?

Calbindin expression in the dentate gyrus/hippocampus is variable, and particularly weak in young neurons

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? Continue reading What IS the dentate gyrus doing to CA3?