During the third week of development the outermost layer of the embryo—the ectoderm—thickens to form a neural plate (eFig. 9.2A). This plate develops a longitudinally running neural groove, which deepens so that it is flanked on either side by neural folds (eFig. 9.2B). These folds further develop and eventually fuse during a process called neurulation to form a long tubelike structure called the neural tube with an inner lumen called the neural canal (eFig. 9.3). Fusion of the tube starts at the midpoint and extends cranially and caudally so that the tube is fully formed by the fourth week. Continued proliferation of the cells at the cephalic end cause the neural tube to dilate and form the three primary brain vesicles (eFig. 9.4): the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain), which later give rise to the structures of the brain. Caudally, the neural tube lengthens and narrows to form the spinal cord. The neural canal forms the cavities of the ventricular system in the brain and central canal of the spinal cord (eTable 9.1). The peripheral nervous system consists of cranial nerves, spinal nerves, spinal ganglia, the enteric system, and autonomic ganglia. The peripheral nervous system is formed by nerve fibers that extend out of the central nervous system and by neurons and their fibers that develop from migratory neural crest cells (eFig. 9.4A). Like the neural tube, neural crest cells originate from surface ectoderm and initially lie on each side of the developing CNS.
Several terms are used to identify the orientation and location of neural structures. The orientation nomenclature is quite simple in organisms such as fish and reptiles, which have a linear nervous system. For these animals, ventral (Latin for “belly”) is oriented toward the ground, dorsal (Latin for “back”) toward the sky, rostral (Latin for “beak”) toward the snout, and caudal (Latin for “tail) toward the tail (eFig. 9.5). Because humans are bipedal and maintain an erect posture, the nervous system makes an obligatory bend of 80 to 90 degrees at the midbrain–diencephalic junction. Because of this, directional references such as ventral, dorsal, rostral, and caudal have different meanings along different locations of the CNS structures (eFig. 9.6A). An additional set of terms that remain constant in their reference to orientation of nervous system structures are anterior, posterior, superior, and inferior.
When studied through imaging or in histopathology, the nervous system is observed in sections cut from one of three different planes: a coronal plane, which divides the nervous system into anterior and posterior parts; the sagittal plane, which is oriented at a right angle to the coronal plane and divides the nervous system into left and right parts; and a horizontal (also referred to as axial or transverse) plane, which divides the nervous system into superior and inferior parts (eFig. 9.6). Note that a sagittal plane passing through the midline may also be referred to as a midsagittal section, whereas a section taken just lateral to the midline is referred to as a parasagittal section.
Nerve cells (neurons) and glial cells are the primary cellular components of the nervous system. Neurochemical signaling is predominantly carried out through a complex series of physiological connections between adjoining neurons. Glial cells participate in a constellation of functions that are vital for proper brain function. Their historically appreciated contribution to neuronal function has expanded to include recognition of their role in regulating the content of the extracellular space and regulation of neurotransmitters at the synaptic junction.