cap.26

• The reticular formation extends from the lower end of the medulla to the upper end of the mesencephalon
• Occupies central parts and fills territories not occupied by cranial nerve nuclei and other distinct nuclei and large fiber tracts
• Raphe nuclei (serotonin) and locus coeruleus (norepinephrine) are often included in the reticular formation
• Neurons in the reticular formation have wide dendritic arborizations and long axons with numerous collaterals
• Efferent connections reach most parts of the central nervous system (CNS) from the cord to the cerebral cortex
• Afferents bring all kinds of sensory information
• The reticular formation is built for integration and attends primarily to tasks involving the nervous system and the organism as a whole
• Subdivisions of the reticular formation form premotor networks that organize behaviors such as control of body posture, orientation of the head and body, eye movements, and coordination of visceral organs
• Parts of the reticular formation send ascending connections to the thalamus and cerebral cortex, forming the activating system of the brain stem
• The activating system is a prerequisite for consciousness and is closely linked to control of awareness and attention
• Parts of the reticular formation, especially in the upper pons and mesencephalon, are concerned with regulation of sleep
• Structure and Subdivisions
• Reticular formation consists of several subdivisions with cells differing in shape, size, and arrangement
• Medial part is afferent and lateral part is efferent
• Medial part consists of large cells, lateral part contains small cells
• Medial part sends out many long, ascending and descending fibers, while the lateral part receives most afferents
• Reticular neurons have long, straight dendrites covering a large volume of tissue
• Reticular formation is organized with long ascending and descending efferent fibers giving off numerous collaterals
• Raphe nuclei and locus coeruleus are often considered part of the reticular formation
• Raphe nuclei contain mainly serotonergic neurons, locus coeruleus neurons contain norepinephrine
• Raphe nuclei and locus coeruleus send fibers directly to the cerebral cortex without synaptic interruption in the thalamus
• The Raphe Nuclei and the Locus Coeruleus: Common Features
• Raphe nuclei together form a narrow, sagittally oriented plate of neurons in the midline of the medulla, pons, and mesencephalon
• Locus coeruleus is a small group of about 15,000 strongly pigmented neurons located under the floor of the fourth ventricle
• Raphe nuclei and locus coeruleus contain only a small number of neurons, but their axons have widespread ramifications reaching all parts of the brain and spinal cord
• Raphe nuclei and locus coeruleus are active during wakefulness and less active during sleep
• Raphe nuclei send efferents to the spinal cord, while rostral nuclei send fibers upstream
• Raphe nuclei contain neuropeptides such as substance P and thyrotropin-releasing hormone
• Raphe nuclei send fibers ending in close relation to ependymal cells, contributing to the regulation of transport processes through the ependyma
• The locus coeruleus contains few neurons but its efferent fibers reach virtually all parts of the central nervous system
• The locus coeruleus sends direct fibers to the cerebral cortex, hypothalamus, basal ganglia, raphe nuclei, spinal cord, cerebellum, striatum, amygdala, hippocampus, frontal lobe, thalamus, septal nuclei, and PAG
• The raphe nuclei send fibers rostrally to the thalamus, cortex, and other cell groups, while caudal nuclei project to the spinal cord
• The raphe nuclei supply large parts of the central nervous system with serotonergic fibers
• The locus coeruleus has dense norepinephric innervation of the reticular formation and motor nuclei
• The locus coeruleus receives direct connections from the cingulate gyrus and the orbitofrontal cortex
• Neurons in the locus coeruleus respond preferentially to novel, "exciting" sensory stimuli
• Norepinephrine from the locus coeruleus is believed to play a role in mediating arousal and shifts of behavior
• The locus coeruleus may increase arousal and attention in response to salient sensory information
• The reticular formation sends fibers to the thalamus, spinal cord, brain stem nuclei, and cerebellum
• The reticular formation forms premotor networks that control and coordinate eye muscle nuclei, rhythmic movements, and reflexes like coughing and vomiting