Synchronous activity is perhaps one of the clearest examples of signal organization in the brain. When electrical impulses known as action potentials appear in different neurons at nearly the same time, the brain is exhibiting synchronous activity. Research has implicated electrical connections between neurons known as gap junctions in synchronous brain activity and, as we have seen in an earlier blog post (see “Other related blog posts” below), inhibitory interneurons have also been shown to be involved. Research reported in the recent paper “Submillisecond firing synchrony between different subtypes of cortical interneurons connected chemically but not electrically” (published March 2, 2011 in the Journal of Neuroscience) specifically investigates the role of inhibitory chemical synapses in synchronous activity within a plus or minus 1 millisecond time window.
In this research, simultaneous recordings were made of electrical activity from inside two neurons residing in layer 4 of the cerebral cortex (mouse somatosensory cortex slice setup). When one of the neurons spiked (elicited an action potential) the other neuron was considered to show synchronous firing if it spiked within 1 millisecond before or afterwards. The research team focused on fast spiking and somatostatin-containing inhibitory interneurons. Most important, the neurons they reported on in this paper had little or no electrical connections between them.
The team saw significant synchronous firing in the 44 neuron pairs with little or no electrical connections that they reported on. Knocking out excitatory chemical synapses had no effect on synchronous activity. However, knocking out inhibitory chemical synapses significantly decreased synchronous activity. In short, this paper appears to be the most direct evidence to date that chemical inhibitory synaptic activity influences synchronous activity in the brain.
Other related blog posts:
The Identity of Inhibitory Interneurons Driving Gamma Oscillations in the Brain