The brain is the center of the nervous system, responsible for planning, executing, and directing activity. The brain governs thought and emotion, memory and sensory processing, movement, and cognition.
Neuroscience is the study of the brain and the nervous system. Different branches of neuroscience study wide-ranging aspects of the nervous system and how it functions. For example, behavioral neuroscience examines the relationship between brain activity and how we act and behave. Molecular neuroscience examines the biochemical processes happening at the cellular level to enable brain function.
Model Organisms
Studying the brain in humans can be tricky and dangerous. Most neuroscientists thus turn to model organisms, species of lab animals that have been extensively studied so that their behavior, brains, and genomes are well understood. Using these examples, scientists can begin to answer questions on everything from basic biology to complex human diseases.
Model organisms can be vastly different in appearance and complexity. But they all share the same fundamental drives to find food, reproduce, and survive. Though their brains are structured differently, they all contain neurons—brain cells that transmit information through electrical signals and small molecules. When a neuron produces an electrical signal, it is said to be firing.
Commonly studied model organisms:
Nematodes are tiny worms that do not have a centralized brain; rather, they have 302 neurons distributed throughout their approximately 1-millimeter-long bodies. This relatively small number of cells to study means that researchers can precisely characterize what each cell does.
Fruit flies have been studied so extensively by neuroscientists that the entire fly brain's "wiring diagram" has been characterized at a very detailed level. Additionally, scientists can easily turn on and off the genes in fruit flies to conduct experiments.
Zebrafish are unique in that they are transparent, enabling researchers to literally see directly into their brains using special microscopes that can peer deep into the brain. Scientists can also manipulate brain activity with light through a process called optogenetics (see "Tinkering" below).
Mice, being mammals, are more complex, but the structure of their brains is also more similar to humans. They also can exhibit more complicated behaviors such as anxiety.
Reading Minds: Tools to Study the Brain
Scientists have developed a number of tools and methods to observe brain activity in organisms. Some are noninvasive, meaning that they require little to no surgery.
Calcium imaging is a technique based on the fact that levels of calcium increase in neurons when they fire. The organism of interest, for example a mouse, is genetically engineered so that specific neurons contain fluorescent proteins that sense when calcium ions are present. This experiment doesn't change the animal's behavior or how its brain works, but in this way, as calcium levels increase and decrease in neurons, specialized microscopes can see brain activity as increases in fluorescent glow.
Functional magnetic resonance imaging (fMRI) is the least invasive method to study the brain, but it is less precise than others. It works by measuring alterations in blood flow to certain regions of the brain. Individual neurons cannot be seen with this method, just regions of brain activity.
Electrophysiology is a somewhat more invasive process. It involves inserting electrodes into brain regions scientists wish to study in order to measure the electrical signals of neurons firing. However, this method requires invasive surgery to implant electrodes, which can sometimes affect normal brain function or behavior.
Photoacoustic computerized tomography is a newer technology in development for noninvasive study of the brain. It uses ultrasound and laser light to measure precise changes in blood flow in the brain, which is correlated with neural activity
Tinkering: Tools to Probe the Brain
Experimentation is at the heart of science—making changes to a system and seeing what happens. Several tools exist to make precise manipulations to how the brain functions.
Optogenetics is a method in which a model organism is genetically engineered so that its neurons are activated by light. In this way, scientists can pulse focused beams of light at specific neurons and brain regions, turn on brain activity, and observe what happens to neural activity and behavior.
Chemogenetics is similar to optogenetics but involves engineering an organism's neurons to be sensitive to specific drugs to which the brain is normally insensitive.
Genetic engineering is a broad field of study to add, remove, or alter an organism's genes.
The Embodied Brain
Neuroscience is not limited to the brains located in our heads. The brain also connects to neurons throughout the spinal cord and nerve cells throughout the body. An emerging area of study is the gut–brain connection, or how signals in the gut are communicated to the brain and vice versa.
