Projection cells and local interneurons are the two main categories of neurons found in most nuclei and cortices. Projection cells are responsible for sending their axons to other regions, thus forming the tracts of the nervous system. On the other hand, local interneurons have axons that terminate within the same neural structure as their cell of origin and are involved in local computations rather than transmitting signals between regions. It is important to note that some neurons have axons that both give off local branches and project to distant regions.

The regional anatomy of the central nervous system (CNS) involves the subdivision of the spinal cord into different regions named after the vertebrae where their nerve roots enter or leave. These regions include the cervical, thoracic, lumbar, sacral, and coccygeal segments. The spinal cord maintains its tubular appearance, although the spinal canal may not always remain open. Within the gray matter of the spinal cord, the dorsal horn receives and processes sensory information from the dorsal roots, while the ventral horn primarily functions as a motor structure housing the motor neurons whose axons project via the ventral roots. The white matter surrounding the gray matter consists of various tracts that connect different levels of the spinal cord and facilitate communication with the brain. Notable tracts include the lateral corticospinal tract (motor), spinothalamic tract/anterolateral system (sensory), and dorsal column-medial lemniscus pathway (sensory).

Moving on to the brainstem, it comprises the medulla, pons, and midbrain. In addition to the longitudinal pathways that interconnect with the spinal cord, the brainstem contains nuclei and additional pathways that differ at each level. These structures serve various functions, including conveying sensory information and motor commands similar to those in the spinal cord, as well as other brain functions such as cardiac control and regulation of consciousness. The brainstem also receives input and sends motor output via cranial nerves, as shown in Table 4.1.

Dorsal to the pons and medulla sits the cerebellum. It receives inputs from the spinal cord, brainstem, and cerebral cortex, and it projects back to these same structures. Although the cerebellum plays a critical role in motor coordination, it is increasingly recognized for its involvement in other cognitive functions.
In most nuclei and cortices, one can classify neurons into two broad categories: projection cells and local interneurons. Projection cells are neurons that send their axon to another region and thus are the origins of the various tracts of the nervous system. In contrast, local interneurons have axons that terminate in the same neural structure as their cell of origin and are involved with local computations rather than conveying signals from one region to another. These categories are not exclusive; many neurons have axons that both give off local branches and project to one or more distant regions.
• Fig. 4.7 Lateral view of the human brain showing the left cerebral hemisphere, cerebellum, pons, and medulla. Note the division of the lobes of the cerebrum (frontal, parietal, occipital, and temporal) and the two major fissures (lateral and central). (From Nolte J, Angevine J. The Human Brain in Photographs and Diagrams. 2nd ed. St Louis: Mosby; 2000.)
Regional Anatomy of the CNS
The spinal cord can be subdivided into a series of regions (see
Fig. 4.6A ), each composed of a number of segments named for the vertebrae where their nerve roots enter or leave: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each portion maintains its tubular appearance, although its lumen, the spinal canal, may not remain patent. Within the gray matter the dorsal horn receives and processes sensory information from the dorsal roots, whereas the ventral horn is primarily a motor structure and contains the motor neurons whose axons project out via the ventral roots (
Fig. 4.8
The surrounding white matter consists of many tracts interconnecting spinal cord levels and for communication with the brain. Three major ones are the lateral corticospinal tract (motor), spinothalamic tract/anterolateral system (sensory), and dorsal column-medial lemniscus pathway (sensory) (see
Fig. 4.6F
The brainstem consists of the medulla, pons, and midbrain (
Fig. 4.9
Fig. 4.6 ). In addition to the longitudinal pathways interconnecting with the spinal cord, the brainstem contains nuclei and many additional pathways that vary by level. These structures have many functions, some of which are analogous to those of the spinal cord (e.g., conveying basic sensory information and motor commands) and others related to a variety of other brain functions, such as cardiac control and state of consciousness. The brainstem also receives input and sends motor output via cranial nerves (
Table 4.1
The cerebellum sits dorsal to the pons and medulla. It receives inputs from spinal cord, brainstem, and cerebral cortex and projects back to many of these same structures. The cerebellum is critical for motor coordination but is increasingly recognized as having key roles in other cognitive functions.