Porth's Essentials of Pathophysiology, 4e

835

Organization and Control of Neural Function

C h a p t e r 3 4

The general visceral efferent (GVE) neurons are structurally and functionally divided between either the sympathetic or the parasympathetic nervous systems of the ANS (discussed later in this chapter). Their axons project through the segmental ventral roots to innervate smooth and cardiac muscle and glandular cells of the body, most of which are in the viscera. The general somatic efferent (GSE) neurons supply skeletal muscles of the body and head, including those of the trunk, limbs, and tongue and the extrinsic eye muscles. Because they transmit the commands of the CNS to peripheral effectors, the skeletal muscles, they are considered the “final common pathway neurons” in the sequence leading to motor activity. They are often called lower motor neurons (LMNs) because they are under the control of upper motor neurons (UMNs) that have their origin in the CNS. The pharyngeal efferent (PE) neurons innervate the muscles of mastication and facial expression, as well as the muscles of the pharynx and larynx. Pharyngeal efferent neurons also innervate the muscles responsible for moving the head. Longitudinal Tracts The gray matter of the cell columns in the CNS is sur- rounded by bundles of myelinated axons (i.e., white matter) and unmyelinated axons that travel longitu- dinally along the length of the neural axis. This white matter can be divided into three layers: inner, middle, and outer (Fig. 34-9). The inner layer contains short fibers that project for a maximum of approximately five segments before reentering the gray matter. The fibers of the middle layer project to six or more seg- ments. The outer layer contains large-diameter axons that can travel the entire length of the nervous system (Table 34-2). Suprasegmental is a term that refers to higher levels of the CNS, such as the brain stem and cerebrum and structures above a given CNS segment. The middle and outer layer fibers have suprasegmental projections. The longitudinal layers are arranged in bundles, or fiber tracts, that contain axons that have the same desti- nation, origin, or function (Fig. 34-10). These longitudi- nal tracts are named systematically to reflect their origin and destination; the origin is named first, and the desti- nation is named second. For example, the spinothalamic tract originates in the spinal cord and terminates in the

Gray matter (dorsal horn)

Tract fibers synapsing on lower motor neurons

Archilayer tracts

Paleolayer tracts

Neolayer tracts

thalamus and the corticospinal tract originates in the cerebral cortex and ends in the spinal cord. The Inner Layer. The inner or archi layer of white mat- ter contains the axons of neurons that connect neighbor- ing segments of the nervous system. Axons of this layer permit the pool of motor neurons of several segments to work together as a functional unit. They also allow the afferent neurons of one segment to trigger reflexes that activate motor units in the same or a neighboring segment. The inner layer is the first of the longitudinal layers to become functional, and is the most primitive. Its circuitry may be limited to reflex movements, such as those of the fetus (i.e., quickening) that begin during the fifth month of intrauterine life. The inner layer of the white matter differs from the other two layers in one important aspect. Many neurons in the embryonic gray matter migrate out into this layer, resulting in a rich mixture of neurons and local fibers called the reticular formation . The circuitry of most reflexes is contained in the reticular formation. In the brain stem, the reticular formation becomes quite large and acts in vital reflexes controlling respiration, cardio- vascular function, swallowing, and vomiting. A func- tional system called the reticular activating system (RAS) FIGURE 34-9. The three concentric subdivisions of the tract systems of the white matter of the spinal cord. Migration of neurons into the archilayer converts it into the reticular formation of the white matter.

TABLE 34-2 Characteristics of the Concentric Subdivisions of the Longitudinal Tracts in theWhite Matter of the Central Nervous System Characteristics ArchilayerTracts PaleolayerTracts NeolayerTracts

Intersegmental ( < 5 segments)

Suprasegmental ( ≥ 5 segments) Multisynaptic but fewer than archilayer tracts

Segmental span

Suprasegmental

Number of synapses

Multisynaptic

Monosynaptic with target structures

Conduction velocity Examples of functional systems

Very slow

Fast

Fastest

Flexor withdrawal reflex circuitry

Spinothalamic tracts

Corticospinal tracts