Adult Brain is Continually Modified by Experience: Demonstration in the Whisker System

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