The conveyance of signals from the central nervous system (CNS) to their corresponding effector targets is accomplished by somatic and autonomic motor neurons. Somatic motor neurons are responsible for innervating the skeletal muscles throughout the body. Their cell bodies are situated in the ventral horn or equivalent brainstem nuclei, and they project out of the CNS via a ventral root or cranial nerve. The specific details regarding their connection to muscles are discussed in another section. On the other hand, the autonomic motor pathway regulates the functioning of organs, smooth muscles, and glands. It operates as a two-neuron pathway, and its characteristics are covered in Chapter 11.

The CNS is comprised of the spinal cord and brain, constructed from the aforementioned cellular elements (as depicted in Figure 4.6A). These elements are interconnected in intricate ways to form subsystems that underlie the numerous functions performed by the CNS. While the physiology of these systems is explored elsewhere, a fundamental understanding of CNS anatomy is necessary to comprehend systems physiology and will be briefly discussed here.

Regions of the CNS that contain a high concentration of axon pathways (with minimal neurons) are referred to as white matter due to the high refractivity of axonal myelin sheaths to light. In contrast, regions with high densities of neurons and dendrites are known as gray matter. It is important to note that axons can also be found in gray matter. These axons may be involved in local processing, either originating from or terminating on local neurons, or they may serve as fibers of passage. Therefore, damage to a specific area can result in the loss of local function or disconnection from remote regions that were connected by fibers passing through the damaged area.

Axons within the CNS often travel in bundles or tracts, with their names typically indicating their origin and termination. For instance, the spinocerebellar tracts transmit information from the spinal cord to the cerebellum. The term "pathway" is similar to "tract" but is generally used to imply a specific function, such as the auditory pathway, which consists of a series of neuron-to-neuron connections across multiple synapses for the conveyance and processing of auditory information.

In the CNS, gray matter exists in two primary configurations. A nucleus refers to a group of neurons within the CNS (while a similar grouping in the peripheral nervous system is called a ganglion). Examples include the thalamic, cerebellar, and cranial nerve nuclei. On the other hand, a cortex consists of neurons organized into layers and is typically found on the surface of the CNS. The most notable cortices are the cerebral and cerebellar cortices, which cover the surfaces of the cerebral hemispheres and the cerebellum, respectively (as shown in Figure 4.7).
Somatic and autonomic motor neurons convey signals from the CNS to their respective effector targets. The somatic motor neurons innervate the skeletal muscles throughout the body. Their cell bodies lie in the ventral horn (or equivalent brainstem nuclei) and project out of the CNS via a ventral root or cranial nerve. The details of their relationship to muscles is covered in . The autonomic motor pathway is responsible for controlling the functioning of organs, smooth muscle, and glands. It is actually a two-neuron pathway, and its properties are covered in
Chapter 11.
The Central Nervous System
The CNS is built from the cellular elements just described and includes the spinal cord and brain (
Fig. 4.6A ). These cellular elements are connected in a variety of complex ways to form the subsystems that underlie the multitude of functions performed by the CNS. The physiology of these systems is covered in ; however, a basic knowledge of CNS anatomy is needed to understand systems physiology and will be briefly discussed here.
Regions of the CNS containing high concentrations of axon pathways (and very few neurons) are called white matter because the axonal myelin sheaths of the axons are highly refractive to light. Regions containing high concentrations of neurons and dendrites are by contrast called gray matter. Note that axons are also present in gray matter. These axons may be related to local processing (i.e., either originating from local neurons or terminating on them) or may be fibers of passage. Thus effects of damage to an area may reflect either loss of local function or disconnection
CHAPTER 4 The Nervous System: Introduction to Cells and Systems pallidus Fimbria of Temporal (fornix) Caudate Inferior horn nucleus of lateral (tail) Mammillary body ventricle
OpticB Cerebral peduncle of hypothalamus tract Hippocampus •Fig. 4.6 A,SchematicofthemajorcomponentsoftheCNSasshowninalongitudinalmidlineview.B–F,Representativesectionsthroughthebrainandspinalcord,withthemajorlandmarkslabeled.B,Cerebrumandthalamus;C,midbrain;D,pons;E,medulla;F,cervicalspinalcord.Notethatmanypathways(e.g.,corticospinalfibers)crosssides(decussate)astheytravelthroughtheCNS,butthesedescussationsarenotindicatedinthefigure(seeChapters7and9fordetailsonthemotorandsensorypathwaycrossings).(A,FromHainesDE[ed].Fundamental Neuroscience for Basic and Clinical Applications. 3rded.Philadelphia:ChurchillLivingstone;2006.) of remote regions that had been linked by fibers of passage through the area that was damaged.
In the CNS, axons often travel in bundles or tracts. The names applied to tracts usually describe their origin and termination. For example, the spinocerebellar tracts convey information from the spinal cord to the cerebellum. The term pathway is similar to tract but is generally used to suggest a particular function (e.g., the auditory pathway: a series of neuron-to-neuron links across several synapses that convey and process auditory information).
Gray matter exists in two main configurations in the CNS. A nucleus is a group of neurons in the CNS (in the PNS such a grouping is called a ganglion). Examples include the thalamic, cerebellar, and cranial nerve nuclei. A cortex is neurons that are organized into layers and usually found on the surface of the CNS. The most prominent are the cerebral and cerebellar cortices, which cover the surface of the cerebral hemispheres and the cerebellum, respectively (
Fig. 4.7