The brain is composed of two cerebral hemispheres, the brainstem, and the cerebellum. These structures develop from the embryonic neural tube, which ultimately differentiates into five vesicles: telencephalon, diencephalon, mesencephalon, metencephalon, and the myelencephalon (). The telencephalon is made up of the cerebral cortex and related subcortical structures. The diencephalon is made up of the thalamus, hypothalamus, epithalamus, and subthalamus. The mesencephalon (midbrain) is composed of several structures located in close proximity to the cerebral aqueduct such as the mesencephalic reticular area, red nucleus, substantia nigra, and motor and sensory pathways among others. The metencephalon includes the pons and cerebellum. The myelencephalon is made up of the medulla along with associated nuclei and pathways. The mesencephalon, metencephalon, and myelencephalon collectively make up what is known as the brainstem.
Fig. 5.1 (a) Undifferentiated neural tube. (b) Three primary vesicles develop from the undifferentiated neural tube. (c) Five secondary vesicles develop from the primary vesicles. (d) The secondary vesicles eventually differentiate into adult structures. Note: The cavity of the neural tube also differentiates at the same time resulting in the formation of the four ventricles (I–IV) and the connecting as well as the aqueduct which leads to the central canal of the spinal cord. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy. Second Edition, Vol 3. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
Rostrocaudal: Early in development, the human embryo has a linear axis. In this model, rostral means the front of the brain and caudal indicates the back of the brain. However, due to flexures that are produced in the developing central nervous system (CNS), the axes change. In humans, a prominent flexure develops at the level of the midbrain that changes the axis from linear to a more curved orientation. This means that at or below the level of the midbrain, rostral is toward the cortex and caudal is toward the sacrum.
Dorsoventral: Rostral to the midbrain, dorsal indicates the top of the brain and ventral refers to the bottom of the brain. At the level of the flexure/midbrain or inferiorly, the term superior is often used rather than dorsal, and inferior rather than ventral. At the level of the lower medulla/spinal cord, the neuroaxis is once again linear so ventral (anterior) and dorsal (posterior) are appropriate.
Fig. 5.6 Cerebrum. Left lateral view. The surface anatomy of the cerebrum can be divided macroscopically into four lobes: frontal, parietal, temporal, and occipital. The surface contours of the cerebrum are defined by convolutions (gyri) and depressions (sulci). (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
Fig. 5.7 Frontal lobe lies anterior to the central sulcus and superior to the lateral sulcus. Gyri includes: superior, middle, and inferior frontal gyrus. (Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
The cerebral hemispheres are the largest component of the human brain. The outer layer is called the cortex, which is made up of neurons and supporting cells. It is commonly referred to as the gray matter due to its gray color. Deep to the gray matter is the white matter which is composed of myelinated tracts. It is the myelinated fibers that give it the white-ish color.
Gyri and sulci are somewhat variable between individuals; however, there are several that are fairly consistent and will be discussed in this chapter, particularly those that define the lobes of the cortex.
The central, parieto-occipital, and lateral sulci as well as the preoccipital notch are used as boundaries to demarcate the frontal, parietal, temporal, and occipital lobes of the cerebral hemispheres () ().
The precentral gyrus is somatotopically arranged creating a motor homunculus. Somatotopy refers to specific areas of the motor strip that are functionally associated with specific and distinct areas of the body ().
Lies immediately anterior to the precentral gyrus and has many of the same connections as the motor strip. Most of the output from the premotor cortex is to the precentral gyrus with some projections to the brainstem and spinal cord. It receives input from the sensory cortex as well as the basal ganglia by way of the thalamus. The premotor cortex is critical for the planning of movement and selection of appropriate responses based on learned associations.
Extension of the premotor cortex that reaches to the medial aspect of the hemisphere. Outputs from the SMA project to the precentral gyrus, basal ganglia, and thalamus. It also has connections to the contralateral SMA. Function includes complex motor tasks and coordinating movements of both hands. Studies have shown that this area becomes active prior to movement and is thought to be involved in the initiation of movement.
Part of the inferior frontal gyrus on the dominant side (usually the left). It is involved in the motor aspects of speech () (Clinical Correlation Box 5.1).
It is very well developed in humans and continues to develop postnatally. It deals with activities such as decision-making, planning complex cognitive behavior, personality expression, and social behavior (executive function).
The parietal lobe lies posterior to the central sulcus and continues posteriorly to the parietal-occipital sulcus located on the medial surface. From the lateral surface, the posterior border is an “imaginary line that is drawn from the parieto-occipital sulcus on the superior margin to the preoccipital notch on the inferior margin of the hemisphere.” The inferolateral boundary is the lateral sulcus, which separates it from the temporal lobe ().
The postcentral gyrus (S1) is posterior to the central sulcus and parallels it. It extends posteriorly to the postcentral sulcus. It is the primary somatosensory cortex and receives sensory information from the body and viscera. It also receives proprioceptive information.
The inferior parietal lobule contains Wernicke’s area, which is important in comprehension of language and reading (language input) (Clinical Correlation Box 5.2).
Wernicke’s area also encompasses the posterior part of the superior temporal gyrus of one hemisphere (usually the left). Thus, Wernicke’s area is made up of part of the inferior parietal gyrus and portions of the superior temporal gyrus.