Chapter 2 Basic Brain Anatomy Where this icon appears, visit The Brain http://go.jblearning.com/ManascoCWS to view the corresponding video. The average weight of an adult human brain is about 3 pounds. That is about the weight of a single small To understand how a part of the brain is disordered by cantaloupe or six grapefruits. If a human brain was damage or disease, speech-language pathologists must placed on a tray, it would look like a pretty unim- first know a few facts about the anatomy of the brain pressive mass of gray lumpy tissue (Luria, 1973). In in general and how a normal and healthy brain func- fact, for most of history the brain was thought to be tions. Readers can use the anatomy presented here as an utterly useless piece of flesh housed in the skull. a reference, review, and jumping off point to under- The Egyptians believed that the heart was the seat standing the consequences of damage to the structures of human intelligence, and as such, the brain was discussed. This chapter begins with the big picture promptly removed during mummification. In his and works down into the specifics of brain anatomy. essay On Sleep and Sleeplessness, Aristotle argued that the brain is a complex cooling mechanism for our bodies that works primarily to help cool and The Central Nervous condense water vapors rising in our bodies (Aristo- tle, republished 2011). He also established a strong System argument in this same essay for why infants should not drink wine. The basis for this argument was that The nervous system is divided into two major sec- infants already have Central nervous tions: the central nervous system and the peripheral too much moisture system The brain and nervous system. The central nervous system (CNS) in their heads. None- spinal cord. consists of the brain and spinal cord. The peripheral theless, thanks to ad- Peripheral nervous nervous system (PNS) consists of the nerve tracts vancements in the system The nerve tracts outside of the central that connect the rest of the body to the central ner- fields of science, med- nervous system that work vous system. An easy way to differentiate the CNS icine, and the birth of to connect the central and the PNS is to remember that the CNS is entirely the fields of psychol- nervous system to the rest of the body. encased in bone whereas the PNS is not (Figure 2-1). ogy and neurology, we 94170_02_6272.indd 13 1/8/13 5:57 PM 14 Chapter 2 Basic Brain Anatomy Cranial Nerves (12 Pairs) Brain PNS CNS Spinal Nerves Spinal Cord (31 Pairs) Figure 2-1 The central and peripheral nervous systems. now know that the brain is much more than an inert mass of head stuffing. And though we know Aristotle was correct in stating that infants should not drink Cerebrum wine, we know that it is not because infants are born with copious amounts of head vapors. We know now that the brain is the organ that coordinates and regu- lates all functions and other organs in our bodies. It is Cerebellum the seat of consciousness, intelligence, and language. Midbrain The brain has three gross divisions, the cerebrum, the Brain Pons stem brain stem, and the cerebellum (Figure 2-2). Medulla oblongata Spinal cord When asked to describe a brain, most individuals describe the cerebrum of the brain. This is the most Figure 2-2 Cerebrum, brain stem, and recently evolved and most easily recognizable part of cerebellum. the brain. The cerebrum is the rounded gray section of the brain riddled with tiny ridges and valleys that makes so many appearances in zombie movies. The as produce language and cognition and organize cerebrum rests on top of the brain stem. The cere- body movements. The surface tissue of the cerebrum brum houses our highest Cerebrum The rounded is known as the cerebral cortex. The cortex is the gray section of the brain and most complex cog- most superficial layer of the cerebrum (Figure 2-3). with gyri and sulci. nitive functions. This is Cerebral cortex The where our consciousness The cerebral cortex is gray and folded and marked most superficial layer of originates as well as our by various ridges and valleys. The cerebrum is folded the cerebrum. ability to do things such in on itself so that more neural tissue can be packed 94170_02_6272.indd 14 1/8/13 5:57 PM The Central Nervous System 15 connecting different areas and structures of the brain to one another and enabling them to communicate. The myelinated axons of neurons make white mat- ter appear white and the lack of myelin makes gray matter gray. There are three primary types of white matter tracts in the CNS: association fibers, com- missural fibers, and projection fibers. Association fibers are the pathways of white matter that connect different structures and areas of the brain in a single hemisphere. Commissural fibers are white matter pathways that connect analogous areas between the Figure 2-3 Cerebral cortex. two cerebral hemispheres. Projection fibers are white matter tracts that project from the brain to the spinal cord and transmit motor (movement) signals from into a much smaller space. The gray coloration of the the CNS out to the PNS and sensory signals from the surface of the cerebral cortex results from the gray PNS back up through the lower CNS to be processed color of the cell bodies of the neurons and is therefore in the brain. said to consist of gray matter. Gray matter is where Certain diseases attack and break down the myelin processing and regulating of information occurs in sheath of white matter. These are known as demyelin- the CNS. Gray matter is found in the cortex and some ating illnesses. Demyelinating illnesses degrade the subcortical structures such as the cerebellum, thala- potential of neurons to conduct neural impulses. This mus, basal ganglia (to be discussed), in addition to degradation of the myelin inhibits appropriate neural the spinal cord. This is in contrast to white matter. functioning by disrupt- The white matter of the CNS consists of the axons Association fibers ing communication be- of neurons that are covered in a white sheath of a White matter pathways tween different areas of that connect different protein and fatty sub- Gray matter the CNS or PNS, or structures and areas of Unmyelinated neurons stance called myelin. the brain within a single both. Amyotrophic lat- responsible for the Myelin insulates the hemisphere. processing and regulating eral sclerosis (ALS) and axons of neurons (like Commissural fibers of information within the multiple sclerosis are nervous system. a rubber sheath insu- White matter pathways two such de myelinating that connect analogous lates the copper wire in White matter diseases. See the areas between the two Myelinated neurons electrical wiring) and al- cerebral hemispheres. video, Living with Mul- responsible for the trans­ lows electrical chemical mission of impulses from tiple Sclerosis for a dis- Projection fibers White impulses of the nervous matter pathways that one area of the brain to cussion of this disease as another. system to be conducted project from the brain well as an interview with to the spinal cord at approximately 100 Myelin An insulating an individual living with and transmit motor layer of protein and fatty meters per second. Un- (movement) signals from multiple sclerosis. substances that forms a myelinated neurons are the CNS out to the PNS layer around the axons and also transmit sensory of certain neurons, which able to conduct neural The folds on the cere- signals from the PNS back allows for the fast and impulses only at about 1 bral cortex are created up through the lower CNS effective transmission of meter per second. White by small convolutions to be processed in the neural impulses. brain. matter is responsible for and furrows, or ridges 94170_02_6272.indd 15 1/8/13 5:57 PM 16 Chapter 2 Basic Brain Anatomy and valleys. The ridges are known as gyri (singu- Clinical Note lar: gyrus). The valleys are known as sulci (singular: sulcus). (See the following Clinical Note on lissen- Lissencephaly cephaly for a discussion of the clinical relevance of As mentioned, the human cerebrum is folded in on the gyri and sulci.) Deeper grooves that make more itself to allow far more cortical tissue to be packed pronounced divisions in the anatomy of the brain into the skull. If you smoothed out the gyri and sulci are referred to as fissures. Knowledge of the major of the human cortex, the surface area would be far gyri, sulci, and fissures is important to learning about greater than that of the inside of the skull. This sys- tem of folding allows the body to fit as much corti- the rest of the anatomy of the brain because they cal tissue within the skull as possible. It is this great are important landmarks for describing the location amount of cortex that allows for normal cognitive of many other areas and functioning. However, there is a group of congenital Gyri Ridges forming the visible portion of the structures of the brain. malformations categorically referred to as lissen- cerebral cortex. The following sections cephaly, or smooth brain syndrome, in which the Sulci Inward folds of the on the cerebral hemi- brain develops in the mother’s womb without these folds in the cortex (Figure 2-4). There are many dif- cerebral cortex. spheres and the major ferent etiologies that account for the varying types Fissure A deep groove cerebral lobes revisit that creates major of lissencephaly.