Understanding the Brain

Neuroanatomy

From Cognopedia
Jump to: navigation, search

Neuroanatomy pertains to the organs and processes of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists merely of an isolated network of cells, animals with bilateral symmetry have a defined nervous systems, and thus we can begin to speak of their neuroanatomy.

Anatomy of the human brain.

In vertebrates the routes that the myriad nerves take from the brain to the rest of the body (or "periphery"), and the internal structure of the brain in particular, are both extremely elaborate. The delineation of distinct structures and regions of the brain has figured centrally in investigating how it works. For example, much of what neuroscientists have learned comes from observing how damage or "lesions" to specific brain areas affects behavior or other neural functions.

History

The first known written record of a study of the anatomy of the human brain is the ancient Egyptian document the Edwin Smith Papyrus.[1]The next major development in neuroanatomy was some thousand years later when the Greek Alcmaeon determined that the brain and not the heart ruled the body and that the senses were dependent on the brain.[2]

After Alcmaeon’s findings, many scientists, philosophers, and physicians from around the world continued to contribute to the understanding of neuroanatomy, notably: Galen, Herophilus, Rhazes and Erasistratus. Herophilus, and Erasistratus of Alexandria were perhaps the most influential Greek neuroscientists with their studies involving dissecting the brains.[2] For several hundred years afterward, with the cultural taboo of dissection, no major progress occurred in neuroscience. However, Pope Sixtus IV effectively revitalized the study of neuroanatomy by altering the papal policy and allowing human dissection. This resulted in a boom of research in neuroanatomy by artists and scientists of the Renaissance.[3]

In 1664, Thomas Willis, a physician and professor at Oxford University, coined the term neurology when he published his text Cerebri anatome which is considered the foundation of neuroanatomy.[4] The next four hundred some years has produced a great deal of documentation and study of the neural systems.

Tools

Modern developments in neuroanatomy are directly correlated to the technologies used to perform research. Therefore it is necessary to discuss the various tools that are available.

Many of the histological techniques used to study other tissues can be applied to the nervous system as well. However, there are some techniques that have been developed especially for the study of neuroanatomy. Here are four examples:

1) The Golgi stain uses potassium dichromate and silver nitrate to stain dendrites and cell bodies in brown and black, allowing researchers to trace their thin filament paths in a slice of nervous tissue.

2) By expressing variables amounts of red, green, and blue fluorescent proteins in the brain, brainbow allows the combinatorial visualization of many different colors. This tags neurons with enough unique colors that they will almost certainly be able to distinguished from their neighbors with fluorescence microscopy.

3) Nissl staining uses dyes to intensely stain the rough endoplasmic reticulum, which is abundant in neurons. This allows researchers to distinguish between different cell types (such as neurons and glia) in various regions of the nervous system.

4) Connectomics uses macroscale magnetic resonance imaging and microscale 3d electron microscopy to map the connections of the brain. These techniques rely heavily upon computers to reconstruct the connections in an understandable way, thus this is related to computational neuroscience.

The above is of course far from an exhaustive list, but it gives a sense of what type of tools can be used to probe the structure of the nervous system.

Model systems

Aside from the human brain, there are many other animals whose brains and nervous systems have received extensive study. For example, the neuroanatomy of the zebrafish has been well defined, in part so that studies on its neurodevelopment can be standardized to the same adult nervous system.[5] The nematode C. elegans is also widely studied, in part because the majority of its connectome (i.e., synaptic connections) has been mapped. The fruit fly Drosophila melanogaster is widely studied, in part because it is a eukaryotic organism whose genetics is well understood.[6] The octopus, although relatively less well studied, has some intriguing properties as well. Of its 500 million neurons, 120 to 180 million lie outside of the brain capsule in the optic lobes, and 330 million or so lie in the nervous system of the arms, which are fairly autonomous. Only 40 to 50 million lie in the actual “brain”, which is usually less than 8 cubic cm. Thus it presents an unusual case of neuroanatomy which is not so brain-centric.[7]

Composition

At the tissue level, the nervous system is composed of neurons (see List of animals by number of neurons) and glial cells. Both neurons and glial cells come in many types (see, for example, the nervous system section of the list of distinct cell types in the adult human body). Glial cells maintain homeostasis, form myelin, and provide support and protection for the brain's neurons. Some glial cells (astrocytes) can even propagate intercellular calcium waves over long distances in response to stimulation and release gliotransmitters in response to changes in calcium concentration. The extracellular matrix also provides support on the molecular level for these cells.

At the organ level, the nervous system is composed of brain regions, such as the hippocampus in humans or the mushroom bodies of Drosophila melanogaster.[8] These regions are often modular and serve a particular role within the general pathways of the nervous system. In many brains, there are also discrete lobes, which are typically separated by a sulcus, a depression or fissure on the surface of the brain. Many organisms also contain nerves, which are cylindrical bundles of fibers that originate from the brain and central cord, and branch repeatedly to innervate every part of the body. Nerves are made primarily of the axons of neurons, along with a variety of membranes that wrap around and segregate them into nerve fascicles.

Human nervous system

Para-sagittal MRI of the head in a patient with benign familial macrocephaly.

The human nervous system is divided into the central and peripheral nervous systems. The central nervous system consists of the brain and spinal cord, and plays a key role in controlling behavior. The peripheral nervous system is made up of all the neurons in the body outside of the central nervous system, and is further subdivided into the somatic and autonomic nervous systems. The somatic nervous system is made up of afferent neurons that convey sensory information from the sense organs to the brain and spinal cord, and efferent neurons that carry motor instructions to the muscles. The autonomic nervous system also has two subdivisions. The sympathetic nervous system is a set of nerves that activate what has been called the "fight-or-flight" response that prepares the body for action. The parasympathetic nervous system instead prepares the body to rest and conserve energy.

References

  1. Atta, H. M. "Edwin Smith Surgical Papyrus: The Oldest Known Surgical Treatise". American Surgeon, 1999, 65(12), 1190-1192.
  2. 2.0 2.1 Rose, F., "Cerebral Localization in Antiquity". Journal of the History of the Neurosciences, 2009, 18(3), 239-247.
  3. Ginn, S. R., & Lorusso, L., "Brain, Mind, and Body: Interactions with Art in Renaissance Italy". Journal of the History of the Neurosciences, 2008, 17(3), 295-313.
  4. Neher, A., "Christopher Wren, Thomas Willis and the Depiction of the Brain and Nerves". Journal of Medical Humanities, 2009, 30(3), 191-200.
  5. Wullimann, Mario F.; Rupp, Barbar;, Reichert, Heinrich (1996). Neuroanatomy of the zebrafish brain: a topological atlas. ISBN 3764351209. http://books.google.com/books?id=B5QVXvbOb1YC&pg=PA1&lpg=PA1&dq#v=onepage&q&f=false. 
  6. http://web.neurobio.arizona.edu/Flybrain/html/index.html
  7. http://www.ncbi.nlm.nih.gov/pubmed/16801504
  8. http://web.neurobio.arizona.edu/Flybrain/html/atlas/structures/mushroom.html

See also

Neurology Connectome

External links