Brain Primer

Mapping and Imaging Neural Anatomy

  • Reviewed27 Feb 2023
  • Author Susan Rojahn
  • Source BrainFacts/SfN
Person in an MRI machine via Portra

Our brains aren’t so simple. But that hasn’t stopped us from trying to understand their complexity: Cell atlases in development are attempting to map the nearly 200 billion cells within the human brain.

Anatomy is the study of structure — most often, the structure of biological organisms. For the brain, anatomy tends to center around the structure of neurons, which are among the most complex and diverse cell types in our bodies. Scientists were first able to observe neurons in the late 19th century, thanks to histological techniques that start with a very thin slice of brain tissue to which scientists apply stains or other compounds that add contrast or color to specific structures. They then view the tissue with a light microscope, which passes visible light through the thin slice and lenses that make the structures look up to 1,000 times larger than they do with the naked eye.

Histology is the study of how cells form tissues. Histological techniques can reveal changes in the density of cell types or the presence of molecules that can suggest a particular disease. These techniques have helped illuminate the brain changes underlying some neurodegenerative disorders. For example, histological methods have shown that an enzyme that breaks down acetylcholine is associated with the brain plaques and tangles of Alzheimer’s disease. And in the brains of Parkinson’s disease patients, histology has revealed the death of neurons that normally control movements through dopamine signaling.

Long after light microscopes gave scientists their first glimpses of neurons, a debate bubbled in the scientific community: Are neurons individual cells or a mesh of physically interconnected cell bodies? Neurons are so densely packed that the answer wasn’t clear until the 1950s, after the development of a new technology called electron microscopy. Electron microscopes can produce useful detailed images of cellular structures magnified many 100,000s of times by directing a beam of electrons through very thin slices of tissue, then enlarging and focusing the image with electromagnetic lenses. With this technology, researchers were finally able to see that neurons are not physically continuous but, instead, are individual cells.

Although they are individual cells, neurons do act in networks, communicating across small gaps called synapses, where the axon terminal of one cell meets a dendrite or cell body of another cell. One method for mapping the signaling pathways within these networks involves injecting radioactive molecules or “tracers” into the cell body of a neuron. Researchers monitor the movement of radioactivity down the neuron’s axon, showing where that neuronal path leads. A similar technique involves tracers that can actually travel across synapses, from one neuron to the next. Scientists have used such tracers to map the complex pathways by which information travels from the eyes to the visual cortex.

Another technique for examining brain anatomy is magnetic resonance imaging, or MRI. Developed in the 1980s, MRI is widely used by researchers and doctors to view a detailed image of brain structure. MRI equipment uses radio waves and strong magnets to create images of the brain based on the distribution of water within its tissues. MRI is harmless and painless to the person being scanned, although it does require sitting or lying in a narrow tube, and the procedure can be quite noisy. With an MRI scan, researchers can tell the difference between the brain’s gray matter and white matter. Gray matter consists of the cell bodies of neurons, as well as their dendrites and synapses. White matter mostly contains axons wrapped in the fatty myelin coating that gives these regions their white color. Based on the distribution of water in the tissues, MRI images clearly differentiate between cerebrospinal fluid, the water-rich cells of gray matter, and fatty white matter.

Adapted from the 8th edition of Brain Facts by Susan Rojahn.



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