Neurons in the human brain receive electrical signals from thousands of other cells, and long neural extensions, called dendrites, play a crucial role in delivering all of this information so that the cells can respond appropriately. In the field of human brain tissue, MIT neuroscientists have now discovered that human dendrites have different properties exhibit as other species. Their studies show that electrical signals weaken more as they flow along human dendrites, resulting in a higher degree of electrical compartmentalization, meaning that small dendrite segments can behave independently of the rest of the neuron.
These differences may contribute to the increased computing power of the human brain, say the researchers.
"It's not just that people are smart because we have more neurons and a larger cortex ̵
Harnett, who is also a member of MIT's McGovern Institute, Brain Research and Sydney Cash, an assistant professor of neurology at Harvard Medical School and Massachusetts General Hospital, are the lead authors of the study, which appeared in the Oct. 18 issue of Cell appears. The lead author of the article is Lou Beaulieu-Laroche, a PhD student in the Department of Brain and Cognitive Sciences at MIT.
Dendrites can be considered as an analogy to transistors in a computer that performs simple operations with electrical signals. Dendrites receive inputs from many other neurons and carry these signals to the cell body. When enough is stimulated, a neuron fires an action potential – an electrical impulse that then stimulates other neurons. Large networks of these neurons communicate with each other to generate thoughts and behavior.
The structure of a single neuron often resembles a tree, with many branches introducing information that is far removed from the cell body. Previous research has shown that the strength of the electrical signals arriving at the cell body depends, in part, on how far they move along the dendrite to get there. As the signals propagate they become weaker, so that a signal far removed from the cell body has less impact than one that arrives near the cell body.
Dendrites in the cortex of the human brain are much longer than those in rats and most other species because the human cortex has developed much thicker than that of other species. In humans, the cortex accounts for about 75 percent of the total brain volume, compared to about 30 percent in the rat brain.
Although the human cortex is two to three times thicker than that of rats, it retains the same overall organization consisting of six distinct layers of neurons. Layer 5 neurons have dendrites that are long enough to reach layer 1, which means that human dendrites need to lengthen as the human brain evolves and electrical signals travel much further.
The MIT team wanted to investigate how these differences in length can affect the electrical properties of the dendrites. They were able to compare electrical activity in rat and human dendrites by using small pieces of brain tissue removed from epilepsy patients undergoing surgical removal of part of the temporal lobe. In order to reach the diseased part of the brain, surgeons must also remove a small part of the anterior temporal lobe.
With the help of MGH employees Cash, Matthew Frosch, Ziv Williams and Emad Eskandar, Harnett's lab was able to obtain samples of the anterior temporal lobe, each about the size of a fingernail.
Evidence suggests that the anterior temporal lobe of epilepsy is unaffected and the tissue appears normal when examined with neuropathological techniques, says Harnett. This part of the brain appears to be involved in a variety of functions, including speech and visual processing, but is not critical to a function; the patients may function normally after removal
After the tissue had been removed, the researchers placed it in a solution very similar to cerebrospinal fluid, perfused with oxygen. This allowed them to keep the tissue alive for up to 48 hours. During this time, they used a technique known as patch-clamp electrophysiology to measure how electrical signals travel along dendrites of pyramidal cells, which are the most common type of excitatory neurons in the cortex.
These experiments were carried out mainly by Beaulieu. Laroche. Harnett's lab (and others) have previously done this type of rodent dendritic experiment, but his team is the first to analyze the electrical properties of human dendrites.
The researchers found that because human dendrites cover longer distances, a signal that flows along a human dendrite from layer 1 to the cell body in layer 5 is much weaker when it flows arrives as a signal flowing along a rat dendrite from layer 1 to layer 5.
They also showed that human and rat dendrites have the same number of ion channels that regulate the flow of current, but these channels occur at a lower density in human dendrites as a result of dendritic extension. They also developed a detailed biophysical model that shows that this density change can explain some of the differences in electrical activity between human and rat dendrites, says Harnett.
The question remains, how do these differences affect human intelligence? Harnett's hypothesis is that because of these differences, which allow more regions of a dendrite to influence the strength of an incoming signal, individual neurons can perform more complex computations of the information.
"If you have a cortical cleft that has a chunk of human or rodent cortex, you'll be able to do more calculations faster with human architecture versus rodent architecture," he says.
There are many other differences between human neurons and those of other species, adds Harnett, making it difficult to pinpoint the effects of dendritic electrical properties. In future studies, he hopes to further explore the exact implications of these electrical properties and how they interact with other unique properties of human neurons to generate more computing power.
The research was funded by the National Sciences and Engineering Research Council of Canada, the David Mahoney Neuroimaging Grant Program of the Dana Foundation, and the National Institutes of Health.