Author: Donald Doherty

Not so very long ago brain scientists believed that dendrites were entirely passive. It’s become clear over the past 10 years or so that dendrites are much more complex and include regenerative voltage phenomena. One of these is the back-propagation of action potentials from the axon initial segment, through the soma, and into the dendrites. Voltage-gated sodium channels are responsible for much of the action potentials positive current and are known to be highly concentrated in the axons.

The recent paper “Molecular Identity of Dendritic Voltage-Gated Sodium Channels” published May 14, 2010 in Science looked at voltage-gated sodium channel density throughout the dendritic tree of pyramidal cells in the cornu ammonis 1 (CA1) of the hippocampus.

They found that the Nav1.6 subunit, a building block of voltage-gated sodium channels, was key to enabling dendritic excitability.

They measured voltage-gated sodium channel density at:

• 5 channels per square micrometer plasma membrane in soma and proximal apical dendrites
• 3 channels per square micrometer plasma membrane in proximal oblique dendrites
• 2 channels per square micrometer plasma membrane in distal apical dendrites

These measures will be very useful in computer models of these pyramidal cells.

Observation after observation shows that complex interactions of genes and environment occur continuously and throughout the life of an organism. Within this context, how must we interpret the two papers recently published in Science on the development of place memory? (The papers are “Development of the Hippocampal Cognitive Map in Preweanling Rats” and “Development of the Spatial Representation System in the Rat” published June 18, 2010 in Science.)

The hippocampus and entorhinal cortex are important for an animal’s sense of where they are. Place cells in the hippocampus signal when an animal visits a particular place. Neurons in Entorhinal cortex known as grid cells, head-direction cells and border cells are also involved in animal navigation through space. (For these neurons’ relevance in humans see my recent blog post “Do You Know Where You Are? Place Memory.”)

The two research groups that published the papers asked how the responses of place cells, grid cells, and head-direction cells in animals exploring the environment outside their nests for the first time compared with responses in adults. Both teams showed that place cells exhibit adult responses from the earliest time points recorded. Both also observed head-direction cells with adult responses from the earliest ages recorded.

The teams disagreed on the appearance of grid cells. This seemed more due to different operational definitions of grid cells than differences in observations. To me the data look identical. The interpretations may be reconciled by working out the mechanisms that transform the immature (or less mature) grid cell to exhibit mature adult characteristics.

Nature versus nurture – is the system defined by genetics (nature) or is it sculpted through experience (nurture) – continues to stand as a useful and testable hypothesis that drives experimentation. However, we know that the environment, genetics, and the various systems at every level in between interact continuously and in complex ways.

Both research teams observed an increase in the number of cells exhibiting adult characteristics over time. One team observed environmental influences on characteristics of some cells but stated that aging without influence by the environment was dominant. These observations may be the key to going beyond the nature versus nurture debate. The genes and environment are in a continuous dance. Sometimes one leads. Sometimes the other.

Other related blog posts:

Do You Know Where You Are? Place Memory

Non-human animal research has demonstrated the importance of a structure known as the hippocampus in processing a map of the animal’s environment stored in memory. Specific brain cells (neurons) in this structure, often referred to as place cells, fire when the animal is in a specific location. However, the contribution of these cells in human spatial memory has been uncertain.

A recent paper looked at the role of human hippocampal neurons in place learning using brain imaging techniques in patients with a rare condition known as Transient Global Amnesia.

Patients with Transient Global Amnesia have nearly total lack of memory of past events except for things of high importance like personal identity and are unable to form new memories. These episodes usually last about 6 to 10 hours and otherwise these patients are alert and seem normal. Relatively small areas of the part of the hippocampus thought to be important for place memory become temporarily disrupted (lesion) and can be detected with brain imaging techniques 1 to 3 days after the onset of a patient’s loss of memory.

These patients were tested in a place memory task while they experienced memory loss. Later the associated brain lesion was imaged. Patients with Transient Global Amnesia episode were significantly impaired in place memory tasks compared with normal subjects.

The paper, titled “Focal Lesions of Human Hippocampal CA1 Neurons in Transient Global Amnesia Impair Place Memory” and published June 11, 2010 in Science, demonstrates that at least some of the large amount of research done on the hippocampus’ role in place memory in non-human animals is applicable to humans.