The efferent connections of reticular formation are carried out by both ascending and descending long axons that arise from the nuclei of this brainstem structure. Thus, the reticular formation sends nerve fibers to five main regions of the central nervous system: the thalamus, spinal cord, basal ganglia, other brainstem nuclei, and the cerebellum. The cell groups that give off ascending axons to the thalamus are located somewhat more caudally than those projecting descending axons. There are also many interneurons connecting different parts of the reticular formation. Therefore, there is a close cooperation between the parts of the reticular formation that act on the cerebral cortex and those that act on the spinal cord. Meanwhile, collateral projections of ascending and descending axons mediate actions on brainstem nuclei.
The different parts of the reticular formation form premotor networks, with some of them controlling rhythmic movements, such as locomotion, respiration, and cough and vomiting reflex. In the pons and midbrain, these networks control and coordinate the activity of the eyeball muscle nuclei.
The descending efferent nerve fibers of the reticular formation run in the ventral portion of the lateral funicle. These are the reticulospinal fibers, which are related to motor control. They end mainly on interneurons, which, in turn, can influence motoneurons. The ventral reticulospinal tracts are both crossed and uncrossed, mediating both inhibitory and excitatory effects on spinal motoneurons. The axons of the reticulospinal neurons are characterized by their axonal branching pattern, with collateral offshoots, which are given off at several levels of the spinal cord.
The ascending efferent fibers of the reticular formation end in the intralaminar thalamic nuclei, following a different pattern from the specific afferent tracts, which end in the lateral thalamic nucleus. Some efferent fibers also run all the way to establish connections in the hypothalamus. They are very important for the general level of activity of the cerebral cortex, which, in turn, deals with consciousness and attention.
Below, an schematic picture portraying the efferent connections of the reticular formation in the central nervous system. You can notice here that various subdivisions of this brainstem structure send efferent fibers to higher levels, such as the thalamus, the basal ganglia, and the cerebral cortex, while descending fibers end in the ventral (anterior) portion of the spinal cord.
