The reticular formation is required for normal attention, wakefulness, and normal wake-sleep patterns. If the brain stem is sectioned or damaged at the mesen-cephalic level, the experimental animal or patient falls unconscious. This happens even if the major ascending sensory tracts, i.e. the medial lemniscus, spinothalamic tract, and the auditory and visual inputs are still intact. Furthermore, the animal cannot be aroused by any sensory input. It is likely that the direct reticulocortical pathways and the reticulothalamic pathways, which terminate in the intralaminar thalamic nuclei, are essential for arousal and consciousness.
Many of the activational fibers from the reticular formation to the thalamus are cholinergic. The projections from the intralaminar thalamic nuclei to the cerebral cortex are extensive and widespread. It is believed that the reticular formation exerts a tonic control over cerebrocortical activity, which maintains the animal in a state of consciousness, with variable states of attention, depending on the frequency and intensity of sensory inputs.
There are also direct projections from the locus ceruleus and the raphe nuclei to the cerebral cortex. Those from the raphe nucleus are mainly serotonergic, while those from the locus ceruleus are noradrenergic. The serotonergic cell bodies of the raphe nucleus extend from the mesencephalon caudally through the pons into the medulla, while the noradrenergic cell bodies of the locus ceruleus are confined mainly to the pons. The serotonergic neurons of the midbrain ramify extensively in the brain stem, and extend to the cerebral cortex and to the cerebellum, while those in the medulla extend caudally to the spinal cord. At least 90% of the noradrenergic cell bodies of the brain stem are confined to the locus ceruleus, and project not only to the cerebral cortex, but also to the cerebellum and caudally into the spinal cord.
The reticular activating system serves not only to maintain consciousness, but also to highlight attention to certain sensory inputs. This is achieved mainly through its inputs to the thalamus, which in turn activate certain cerebrocortical areas to focus attention on a particular sensory stimulus. This is exemplified by the sudden arousal from a near-soporific state by an alarming sensory input. The reticular activating system directs not only cognitive processes to the source of the stimulus, but alerts the motor systems, which direct the head, gaze, and even the postural orientation of the body towards the stimulus.
The reticular activating system is also a regulator of the degree of activation allowed to reach the cerebral cortex. The diffuse direct reticulocortical inputs and those from the thalamus are in some way gated so that the cortex does not receive too intense a level of stimulation, which could result in inappropriate emotional and cognitive responses to stimuli. This is partly achieved through inputs from the reticular formation to the thalamic reticular nucleus, which monitors and gates the output from the thalamic intralaminar nuclei to the cerebral cortex.
ascending reticular Activating system iscending sensory inputs reticular activating system reticular activating system ascending reticular Activating system iscending sensory inputs connections of the reticular activating system cord
connections of the reticular activating system cord noradrenergic serotonergic cholinergic
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.