These nerves make synaptic connections in the spinal cord or in the medulla oblongata and ascend to subcortical nuclei. The spinal cord is an essential link between the peripheral nervous system and the brain it conveys sensory information originating from different external and internal sites via 31 pairs of spinal nerves (Figure 1.5). The gray matter is composed of neurons and their supporting cells and is enclosed by the white matter that is composed of a dense layer of ascending and descending nerve fibers. The spinal cord is an elongated cylindrical structure lying within the vertebral canal, which includes the central canal and the surrounding gray matter. The caudal end of the myelencephalon develops into the spinal cord. The myelencephalon (spinal cord-like) includes the open and closed medulla, sensory and motor nuclei, projection of sensory and motor pathways, and some cranial nerve nuclei. The metencephalon includes the pons and the cerebellum. Lateral view of the metencephalon and a spinal cord section with ventral and dorsal root fibers, and dorsal root ganglia. The cerebral cortex consists of various types of cortices (such as the olfactory bulbs, Figure 1.2B) as well as closely related subcortical structures such as the caudate nucleus, putamen, globus, amygdala and the hippocampal formation (Figure 1.2C). The telencephalon includes the cerebral cortex (cortex is the outer layer of the brain), which represents the highest level of neuronal organization and function (Figures 1.2A and 1.2B). The mesencephalon, metencephalon, and the myelencephalon comprise the brain stem. These three vesicles further differentiate into five subdivisions: telencephalon, diencephalon, mesencephalon, metencephalon, and the myelencephalon (Figure 1.1B).

The brain and the spinal cord arise in early development from the neural tube, which expands in the front of the embryo to form the three primary brain divisions: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain) (Figure 1.1A). The adult spinal cord is approximately 40 to 50 cm long and occupies about 150 cc. It occupies a volume of about 1400 cc - approximately 2% of the total body weight, and receives 20% of the blood, oxygen, and calories supplied to the body. The adult human brain weighs between 1,200 to 1,500 g and contains about one trillion cells. Lateral view of human embryo at the beginning of the 3rd (A) and 5th (B) week of gestation.

The somatic nervous system is