How is your brain like a crowd at a soccer match? Substitute basketball for soccer, if you prefer. Keep reading.
The cerebral cortex has been thought of as the part of the human brain in which conscious thought is processed. It would be expected that the cortex would be less active when a patient is under general anesthesia. A new study reports, however, that under general anesthesia, just some of the cortical cells record less activity. Other cells increase activity and synchronize.
These findings may lead to improvements in anesthetic drugs and better surgical outcomes.
The work of Professor Botond Roska and his group of researchers at the University of Basel, Switzerland, reveals how different cell types in the cerebral cortex change in activity during general anesthesia. This new information increases understanding about induction of unconsciousness via anesthetic drugs.
It has been known for the last 100 years that some cells in the cortex are active, alternating between periods of high and low activity, during general anesthesia. Attaching EEG electrodes to the scalp has been one of the few means available to detect cortical activity, but it doesn’t allow identification of the cells which are active.
The cortex is composed of different cell types; each type serves different functions. Different general anesthetics act on different receptors, located on different types of neurons throughout the brain. All general anesthetics, however, ultimately have the same effect – loss of consciousness.
“We were interested in finding if there is a common neuronal mechanism across different anesthetics,” says Dr. Martin Munz, co-leader of the study, in a statement.
To address the question, researchers used genetic tools, and mice with variable characteristics bred just for the study, to label individual cortical cell types. They found that in contrast to what had previously been suspected, only one specific cell type within the cortex, labeled “layer 5 cortical pyramidal neurons,” showed an increase in activity when the animals were exposed to different anesthetics.
“Each anesthetic induces a rhythm of activity in layer 5 cortical pyramidal neurons. Interestingly, these rhythms differed between anesthetics. Some were slower, and some were faster,” explains Dr. Arjun Bharioke, another co-leader of the study. “However, what was common across all anesthetics was that they all induced an alignment of activity. When they were active, all layer 5 cortical pyramidal neurons were active at the same time. We called this ‘neuronal synchrony.’
“It seems that instead of each neuron sending different pieces of information during anesthesia, all layer 5 cortical pyramidal neurons send the same information,” he continues. “One could think of this as when people in a crowd transition from talking to each other, for example, before a soccer or basketball game, to when they are cheering for their team, during the game. Before the game starts, there are many independent conversations. In contrast, during the game, all the spectators are cheering on their team. Thus, there is only one piece of information being transmitted across the crowd.”
Adds Dr. Alexandra Brignall, third co-leader of the study and a veterinarian: “Anesthetics are very powerful, as anyone who has been in a surgery can attest to. But they are also not always easy to use. During a surgery, one has to continuously monitor the depth of the anesthetic to ensure that the patient is not too deep or too shallow. The more we know how anesthetics work and what they do in the brain, the better. Maybe this will help researchers develop new drugs to specifically target the cells in the brain associated with unconsciousness.”
Findings are published in the open access journal Neuron.