The scattered neurons of the tegmentum and their network of processes form the reticular formation. This occupies the centralarea of the tegmentum and expands from the medulla oblongata into the rostral mid-brain. Several areas of different structure can be distinguished (A). In the medial part are magnocellular nuclei from where long ascending and descending fiber tracts originate.The parvocellular lateral part is regarded as an association area.
Many of the neurons have long ascending or descending axons, or axons bifurcating into an ascending and a descending branch. As shown by Golgi impregnation, such a neu-ron (B1) can simultaneously reach caudalcranial nerve nuclei (B2) and diencephalic nuclei (B3). The reticular formation containsa large number of peptidergic neurons (enkephalin, neurotensin, and others).
Afferent connections.The reticular forma-tion is reached by impulses of all sensorymodalities. Sensory spinoreticular fibersterminate in the medial field of medulla ob-longata and pons, and so do secondary fibers of the trigeminal and vestibular nu-clei. Collaterals of the lateral lemniscus bring in acoustic impulses, while fibers of the tectoreticular fasciculus bring in optic impulses. Experimental studies on stimula-tion have shown that reticular neurons are excited more by sensory (pain), acoustic and vestibular stimuli than by optic stimuli. Other afferent fibers originate from the cerebral cortex, the cerebellum, the red nu-cleus, and the pallidum.
Efferent connections.The reticulospinal tract runs from the medialfield of medulla oblongata and pons into the spinal cord. Bundles of the reticulothalamicfasciculus ascend to the intralaminar nucleiof the thalamus (truncothalamus). Fiber bundles from the midbrain termi-nate in the oral hypothalamus and in the septum.
Respiratory and cardiovascular control centers. Groups of neurons regulate respi-ration (C), heart rate, and blood pressure (changes upon physical activity or emotion). The neurons for inspiration are localized in the central field of the lower portion of the medulla oblongata (C4), those for expiration are further dorsal and lateral (C5). The higher relay stations for inhibition and stimulation of respiration lie in the pons (locus ceruleus). The autonomic nuclei of the glossopharyngeal nerve and the vagus nerve are involved in regulating heart rate and blood pressure (D). Electrical stimula-tion in the caudal central field of the medulla oblongata causes a drop in blood pressure (depressor center) (D6), while elec-trical stimulation of the remaining reticular formation in the medulla oblongata (D7) leads to an increase in blood pressure.
Effect on the motor system.The reticularformation has a differential effect on the spinal motor system. In the medial field of the medulla oblongata lies an inhibition cen-ter; upon stimulation, the muscle tonedrops, reflexes fail, and the electric stimula-tion of the motor cortex no longer triggers a reaction. By contrast, the reticular forma-tion in pons and midbrain has an enhancingeffect on the motor system.
Ascending activation systems.The reticu-lar formation has an effect on consciousness via connections to the intralaminar nuclei of the thalamus. When strongly stimulated by sensory or cortical input, the organism sud-denly becomes fully alert, a prerequisite for attention and perception. Upon electrical stimulation of the reticular formation, this wake-up function can be objectively assessedby electroencephalography (EEG).
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