Neurons are the cells responsible for transmitting information to and from the brain. It is the core component of the nervous system. Each of these neurons are interconnected through synapses. Synapses are structures similar in function to telephone cables, that allow the passing of electrical or chemical signals between the neurons.
The image on the left (courtesy of Don Arnold), shows a living neuron in culture. The green dots indicate the excitatory synapses and the red dots indicate inhibitory synapses. An excitatory synapse is a synapse that increases the chance of an action potential occurring in a postsynaptic cell. An inhibitory synapse is the opposite, it decreases the chance of an action potential.
Neurons need a lot of the body's resources to perform efficiently. The metabolic requirements for these require about 15% of the output of the heart (cardiac output), 20% of the body's oxygen consumption, and 25% of the body's glucose utilization. The brain only takes energy from glucose.
There are about 80 to 100 billion neurons in the human brain. And there are about 100 trillion synapses connecting the neurons together. Looking at a portion of the brain, the size of a pinhead, one would find around 30,000 neurons in it.
The most number of neurons a person could ever have is during the first trimester as a fetus. Neurons are not made or replaced during one's life. Current developments in medicine have shown that neurons can be made and repaired using stem cell technology.
Visualizing Memory Formation
Oscar Wilde called memory "the diary that we all carry about with us." Now a team of scientists has developed a way to see where and how that diary is written.
The team, led by Don Arnold and Richard Roberts of USC, engineered microscopic probes that light up synapses in a living neuron in real time by attaching fluorescent markers onto synaptic proteins – all without affecting the neuron's ability to function.
The fluorescent markers allow scientists to see live excitatory and inhibitory synapses for the first time – and, importantly, how they change as new memories are formed.
The synapses appear as bright spots along dendrites (the branches of a neuron that transmit electrochemical signals). As the brain processes new information, those bright spots change, visually indicating how synaptic structures in the brain have been altered by the new data.
"When you make a memory or learn something, there's a physical change in the brain. It turns out that the thing that gets changed is the distribution of synaptic connections," said Arnold, associate professor of molecular and computational biology at the USC Dornsife College of Letters, Arts and Sciences, and co-corresponding author of an article about the research that will appear in Neuron on June 19.
The probes behave like antibodies, but bind more tightly, and are optimized to work inside the cell – something that ordinary antibodies can't do. To make these probes, the team used a technique known as "mRNA display," which was developed by Roberts and Nobel laureate Jack Szostak.
Video: Anatomy of a Neuron
"Using mRNA display, we can search through more than a trillion different potential proteins simultaneously to find the one protein that binds the target the best," said Roberts, co-corresponding author of the article and a professor of chemistry and chemical engineering with joint appointments at USC Dornsife and the USC Viterbi School of Engineering.
Arnold and Roberts' probes (called "FingRs") are attached to GFP (green fluorescent protein), a protein isolated from jellyfish that fluoresces bright green when exposed to blue light. Because FingRs are proteins, the genes encoding them can be put into brain cells in living animals, causing the cells themselves to manufacture the probes.
The design of FingRs also includes a regulation system that cuts off the amount of FingR-GFP that is generated after 100 percent of the target protein is labeled, effectively eliminating background fluorescence – generating a sharper, clearer picture.
These probes can be put in the brains of living mice and then imaged through cranial windows using two-photon microscopy.
The new research could offer crucial insight for scientists responding to President Obama's Brain Research Through Advancing Innovative Neurotechnologies (BRAIN) Initiative, which was announced in April.
Modeled after the Human Genome Project, the objective of the $100 million initiative is to fast-track research that maps out exactly how the brain works and "better understand how we think, learn, and remember," according to the BRAIN Initiative website.
University of Southern California
Brain Research Through Advancing Innovative Neurotechnologies (BRAIN)
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