The brain stem is not clearly separated into regions of gray matter and white matter. The nuclei of the cranial nerves located in the brain stem, however, appear as islands surrounded by more or less distinct tracts of white matter. The nuclei contain the cell bodies of the motor neurons of the cranial nerves and are both the morphologic and functional counterparts of the anterior horns of the spinal cord.
FIGURE 12.27 • Nerve cells in intracortical cerebral circuits.
This simple diagram shows the organization and connections between cells in different layers of the cortex contributing to cortical afferent fibers (arrows pointing up) and cortical efferent fibers (arrows pointing down). The small interneurons are indicated in yellow.
In other sites in the brain stem, as in the reticular formation, the distinction between white matter and gray matter is even less  evident.
Organization of the Spinal Cord
The spinal cord is a flattened cylindrical structure that is directly continuous with the brain stem. It is divided into 31 segments (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal), and each segment is connected to a pair of spinal nerves. Each spinal nerve is joined to its segment of the cord by a number of rootlets grouped as dorsal (posterior) or ventral (anterior) roots (Fig. 12.29; see also Fig. 12.3).
In cross-section, the spinal cord exhibits a butterfly-shaped grayish-tan inner substance surrounding the central canal, the gray matter, and a whitish peripheral substance, the white matter (Fig. 12.30). The white matter (see Fig. 12.3) contains only tracks of myelinated and unmyelinated axons traveling to and from other parts of the spinal cord and to and from the brain.
The gray matter contains neuronal cell bodies and their dendrites, along with axons and central neuroglia (Plate 31, ventral ramus of spinal nerve
FIGURE 12.28 • Cytoarchitecture of the cerebellar cortex. a.
This diagram shows a section of the folium, a narrow, leaflike gyrus of the cerebellar cortex. Note that the cerebellar cortex contains white matter and gray matter. Three distinct layers of gray matter are identified on this diagram: The superficially located molecular layer, the middle Purkinje cell layer, and the granule cell layer adjacent to the white matter. b. Purkinje cell layer from rat cerebellum visualized using double-fluorescent–labeling methods. Red DNA staining indicates the nuclei of cells in molecular and granular cell layer thin section. Note that each Purkije cell exhibits an abundance of dendrites. 380. (Coutesy of Thomas J. Deerinck.) page 398). Functionally related groups of nerve cell bodies in the gray matter are called nuclei. In this context, the term nucleus means a cluster or group of neuronal cell bodies plus fibers and neuroglia. Nuclei of the CNS are the morphologic and functional equivalents of the ganglia of the PNS. Synapses occur only in the gray matter.
The cell bodies of motor neurons that innervate striated muscle are located in the ventral (anterior) horn of the gray matter.
Ventral motor neurons, also called anterior horn cells, are large basophilic cells easily recognized in routine histologic preparations (see Fig. 12.30 and Plate 31, page 398). Because the motor neuron conducts impulses away from the CNS, it is an efferent neuron.
The axon of a motor neuron leaves the spinal cord, passes through the ventral (anterior) root, becomes a component of