The reticular formation, RF, is the name for a group of neurons found throughout the brainstem. Using the ventral view of the brainstem, the reticular formation occupies the central portion or core area of the brainstem from midbrain to medulla (see also brainstem cross-sections in Figure 65-Figure 67).
This collection of neurons is a phylogenetically old set of neurons that functions like a network or reticulum, from which it derives its name. The RF receives afferents from most of the sensory systems (see next illustration) and projects to virtually all parts of the nervous system.
Functionally, it is possible to localize different subgroups within the reticular formation:
• Cardiac and respiratory "centers": Subsets of neurons within the medullary reticular formation and also in the pontine region are responsible for the control of the vital functions of heart rate and respiration. The importance of this knowledge was discussed in reference to the clinical emergency, tonsillar herniation (with Figure 9B).
• Motor areas: Both the pontine and medullary nuclei of the reticular formation contribute to motor control via the cortico-reticulo-spinal system (discussed in Section B, Part III, Introduction; also with Figure 49A and Figure 49B). In addition, these nuclei exert a very significant influence on muscle tone, which is very important clinically (discussed with Figure 49B).
• Ascending projection system: Fibers from the reticular formation ascend to the thalamus and project to various nonspecific thalamic nuclei. From these nuclei, there is a diffuse distribution of connections to all parts of the cerebral cortex. This whole system is concerned with consciousness and is known as the ascending reticular activating system (ARAS).
• Pre-cerebellar nuclei: There are numerous nuclei in the brainstem that are located within the boundaries of the reticular formation that project to the cerebellum. These are not always included in discussions of the reticular formation.
It is also possible to describe the reticular formation topographically. The neurons appear to be arranged in three longitudinal sets; these are shown in the left-hand side of this illustration:
• The lateral group consists of neurons that are small in size. These are the neurons that receive the various inputs to the reticular formation, including those from the anterolateral system (pain and temperature, see Figure 34), the trigeminal pathway (see Figure 35), as well as auditory and visual input.
• The next group is the medial group. These neurons are larger in size and project their axons upward and downward. The ascending projection from the midbrain area is particularly involved with the consciousness system. Nuclei within this group, notably the nucleus giganto-cellularis of the medulla, and the pontine reticular nuclei, caudal (lower) and oral (upper) portions, give origin to the two reticulo-spinal tracts (discussed with the next illustration, also Figure 49A and Figure 49B).
• Another set of neurons occupy the midline region of the brainstem, the raphe nuclei, which use the catecholamine serotonin for neurotransmission. The best-known nucleus of this group is the nucleus raphe magnus, which plays an important role in the descending pain modulation system (to be discussed with Figure 43).
In addition, both the locus ceruleus (shown in the upper pons) and the periaqueductal gray (located in the midbrain, see next illustration and also Figure 65 and Figure 65A) are considered part of the reticular formation (discussed with the next illustration).
In summary, the reticular formation is connected with almost all parts of the CNS. Although it has a generalized influence within the CNS, it also contains subsystems that are directly involved in specific functions. The most clinically significant aspects are:
• Cardiac and respiratory centers in the medulla
• Descending systems in the pons and medulla that participate in motor control and influence muscle tone
• Ascending pathways in the upper pons and mid-brain that contribute to the consciousness system
Ascending reticular activating system (ARAS)
Lateral group Medial group Raphe nuclei
Lateral group Medial group Raphe nuclei
FIGURE 42A: Reticular Formation 1 — Organization
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