Tag: Brain Science

It’s become clear that adult brain anatomy is far from static and inflexible. New neurons are born from stem cells. Old connections break and new connections are made. Within this context a research team asked what the detailed circuit dynamics might be in rodent whisker related cerebral cortex. They published their results in the recent paper “Axonal Dynamics of Excitatory and Inhibitory Neurons in Somatosensory Cortex” ( June 2010 in PLoS Biology).

Not familiar with the whisker system? See my blog post Wiggling Whiskers for a Living? or some of the other posts listed below under “Other related blog posts.”

The paper takes a look at the horizontal connections of layer 2/3 pyramidal cells and inhibitory interneurons in whisker related cerebral cortex of mice during normal and modified experience using genetically engineered viruses and two-photon microscopy.

Connections (synapses) between nerve cells (neurons) in the brain are dynamic during normal experience. Old connections are broken and new connections are made. The research team observed that excitatory axons in adult brains with normal experience have a constant rate of connection turnover of 6% per week (6% are broken and 6% are new providing a no net gain of connections). Inhibitory axons broke connections at 10% per week and made new connections at 8% per week (with a net loss of 2% per week!).

To (dramatically) modify whisker sensory experience in their mice the research team plucked two full rows of whiskers (D and E rows) every day (whiskers grow back). They observed massive and rapid reorganization of the axons of both excitatory and inhibitory neurons along with a transient increase in the number of connections between neurons.

Within hours of whisker plucking, inhibitory neurons in sensory deprived cortex began extending new axons into sensory non-deprived cortex. This was followed by the extension of excitatory neuron axons from sensory non-deprived cortex into sensory deprived cortex.

After whisker plucking the turnover in connections between neurons increased dramatically. In non-deprived cortex excitatory synaptic boutons were added at rates of 43% and eliminated at 29% per month with a net gain of connections. Inhibitory interneurons in non-deprived cortex added new connections at rates of 31% every 2 days and eliminated 23% every 2 days with a net gain of connections.

In sum, this research demonstrated dramatic reciprocal affects of sensory deprivation on excitatory and inhibitory neuron connections within layer 2/3 somatosensory cortex. Some of the circuit connectivity data remain a puzzle to me. In particular, what should we make of the normative inhibitory connection data that show a constant loss of about 2% of the connections every week?

Other related blog posts:

Wiggling Whiskers for a Living?

Wiggling Whiskers: Directional Tuning

Whisker Related Brain Anatomy Data for Building Simulations

Wiggling Whiskers: Neurons in the Barrel Cortex and Object Localization

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