Neuroanatomy-A Primer

by Kayt Sukel

July 27, 2011

The human brain is a unique structure that boasts a complex three-dimensional architecture. Neuroscientists are only beginning to understand how the different parts of this intricate configuration work together to produce behavior. In the numerous neuroimaging studies that are published weekly, researchers use common neuroanatomical terms to denote location, organization, and, at times, implied function. Though a complete discussion of neuroanatomy is worthy of a thick textbook full of elaborate illustrations, common terminology used in neuroscientific research is highlighted below.

The basics

Perched on top of the spinal column, the brain is the epicenter of the human nervous system. It is the largest part of the central nervous system (CNS) and made up of three general areas: the brain stem, the cerebellum, and the cerebral cortex. The brain stem is involved with autonomic control of processes like breathing and heart rate as well as conduction of information to and from the peripheral nervous system, the nerves and ganglia found outside the brain and spinal cord. The cerebellum, adjacent to the brain stem, is responsible for balance and coordination of movement. Resting above these structures, the cerebral cortex quickly perceives, analyzes, and responds to information from the world around us. It handles sensory perception and processing as well as higher-level cognitive functions like perception, memory, and decision-making. These three areas work together seamlessly in healthy individuals, allowing the brain to coordinate necessary functions and behaviors from breathing to spatial navigation.

The cerebral cortex is divided into two hemispheres connected by the corpus callosum, a bridge of wide, flat neural fibers that act as communication relays between the two sides. While several popular books suggest this lateralization is important to function (i.e., the right side of the brain is the creative side while the left hemisphere dabbles more in analytical processing), most cognitive processes are represented by activation on both hemispheres. The exception is language—both Broca’s Area, an area important to language syntax, and Wernicke’s Area, a region critical to language content, reside on the left side of the brain. Otherwise, the two hemispheres are nearly symmetrical and each one is further subdivided into four major lobes: the occipital, the temporal, the parietal, and the frontal.

Brain anatomy
Credit: Nucleus Medical Art, Inc./Getty Images

 Click image for larger view

These lobes are mostly used to denote general anatomical location. But they are also frequently spoken about in terms of function. The occipital lobe, located at the back of the brain, is the seat of the primary visual cortex, the brain region responsible for processing and interpreting visual information. Reaching from the temple back towards the occipital lobe, the temporal lobe is a major processing center for language and memory. Above the temporal lobe and adjacent to the occipital lobe, the parietal lobe houses the somatosensory cortex and plays an important role in touch and spatial navigation. Finally, the frontal lobe, extending from behind the forehead back to the parietal lobe, is the brain region that separates humans from our primate cousins. This large brain lobe is the seat of so-called executive function, with a hand in reasoning, decision-making, integration of sensory information, and the planning and execution of movement.[i]

Folds and grooves

The cortex is, at its most basic, an extended pane of neural tissue that is gathered and pleated to fit inside the skull cavity. The bump in each pleat is called the gyrus, while the groove made by the fold is called the sulcus. No two human brains are folded in the same exact way. Yet several of these folds are large and pronounced enough to merit specific names. They are used to specify location—but also may be referred to in discussions of function.

For example, the lateral sulcus is the inner fold that separates the temporal lobe from the frontal lobe. Adjacent to the lateral sulcus is the temporal gyrus. Both this groove and fold house the primary auditory cortex, where the brain processes sound information. Wernicke’s area, a brain region critical to understand language, also resides on the temporal gyrus.[ii]

Similarly, different studies may refer to specific activations in the superior frontal, middle frontal, and inferior frontal gyri in the frontal lobes. And, in studies of motor function, mentions of primary motor cortex may also refer to a location between the precentral gyrus and the central sulcus at the top of the brain. Contrary to popular lay-press usage, the terms lobe and gyrus are not interchangeable. References to gyri and sulci can help give a more specific location on a particular lobe of the cortex.

Brodmann areas and Talairach coordinates

Researchers sometimes refer to specific locations on the human brain by a number, or Brodmann area. Korbinian Brodmann was a German neurologist who studied the brain in the early part of the twentieth century. He created maps of the brain based on cytoarchitecture, or how the cells were functionally organized. His classification system is still widely used today—though the borders of some areas have been refined over time.

Although Brodmann areas are frequently cited in neuroscientific literature, it is not the only classification system available. The same brain area may be referred to by different names depending on the study. For example, neuroscientists who study visual perception may refer to primary visual cortex as V1, Brodmann area 17, or simply as part of the occipital cortex.[iii]

Specific brain areas may also be denoted by Talairach coordinates, as defined by French neurosurgeon, Jean Talairach. Talairach used two key anatomical landmarks, the anterior commissure and the posterior commissure (parts of the corpus callosum) as the basis of his coordinate system. Today, many neuroimaging analysis programs spatially normalize the brains of each study participant to fit a standard reference brain based on Talairach’s original brain atlas.[iv]

The importance of white matter

Neurons, or brain cells, are made up of cell bodies, axons, and dendrites. The cells mainly connect to one another through synapses (small junctions between brain cells where neurotransmitters and other neurochemicals are passed). Synapses are often found between the axons and dendrites, which allows the cells to signal to one another. Current estimates suggest the brain has approximately 86 billion neurons.[v]

The brain is made up of two types of matter: gray and white. Gray matter consists of the cell bodies and dendrites of the neurons, as well as supporting cells called astroglia and oligodendrocytes. White matter, however, is made up of mostly of axons sheathed in myelin, an insulating-type material that helps cells propagate signals more quickly. It’s the myelin that gives the white matter its lighter color.

For many years, neuroscientists believed white matter was simply a support resource for gray matter. However, recent studies show that white matter architecture is important in processes like learning and memory.[vi]

It’s all about connection

The frontal lobes are often referred to as the neocortex as they are the most recent parts of the brain to evolve—and their size and structure is unique to humans. But the neocortex works closely in concert with areas of the so-called reptilian brain, or subcortical brain areas residing close to the brain stem, to help us make sense of the world around us. Subcortical structures like the thalamus and basal ganglia (responsible for integrating sensory information and processing risk and rewards, respectively) are strongly connected to the neocortex and share information in both a bottom-up and top-down fashion. In fact, modern neuroimaging research is no longer focused on functional segregation, or the localization of function to a single area. Today, researchers are using new techniques to follow tracts of neurons that connect networks of brain areas to better understand how they work together to determine human behavior.[vii] 

[i] Johnson KA and Becker JA. The Whole Brain Atlas.
[ii] Ogar, JM, Baldo JV, Wilson SM, Brambati SM, Miller BL, Dronkers NF and Gorno-Tempini ML. Semantic demential and persisting Wernicke’s aphasia: linguistic and anatomical profiles. Brain and Language. April 2011, 117(1): 28/33.
[iii] Bernal B and Perdomo P. Brodmann’s Interactive Atlas.
[iv] Talairach Space.
[v]Azevedo FA, Carvalho LR, Grinberg LT, Farfel JM, Ferretti RE, Leite RE, Jacob Filho W, Lent R and Herculano-Houzel S. Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. The Journal of Comparative Neurology. 2009, 513 (5): 532–41
[vi] Scholz J, Klein MC, Behrens TEJ and Johansen-Berg H. Training induces changes in white matter architecture. Nature Neuroscience. 2009, 12: 1370-1371.
[vii] Friston KJ. Functional and effective connectivity in neuroimaging: A synthesis. Human Brain Mapping. 1994, 2: 56-78.