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“RETICULAR FORMATION” “LEARNING OBJECTIVES” • Reticular formation, • Location, • Function, • Somatic motor control, • CVS control, • Pain modulation, • Sleep and consciousness, • Habituation, • Sensory control, • Visceral control, • Reticular formation, • Nuclei, • Reticular neurons. “RETICULAR FORMATION” The reticular formation is a part of the brain that is involved in actions such as, • Awaking/sleeping cycle, • Filtering incoming stimuli to discriminate irrelevant background stimuli. • It is essential for governing some of the basic functions of higher organisms. • Is one of the phylogenetically oldest portions of the brain. • The reticular formation is an apparently (but not actually) diffusely organised area that forms the central core of the brainstem. The reticular formation is an apparently (but not actually) diffusely organised area that forms the central core of the brainstem. “LOCATION” • The reticular formation is a poorly- differentiated area of the brain stem. • Centered roughly in the pons. • The reticular formation is the core of the brainstem running through the mid-brain, pons and medulla. • The ascending reticular activating system connects to areas in the thalamus, hypothalamus, and cortex. • the descending reticular activating system connects to the cerebellum and sensory nerves. “FUNCTIONS” • The reticular formation consists of more than 100 small neural networks, • with varied functions including the following: • Some motor neurons send their axons to the reticular formation nuclei. • Giving rise to the reticulospinal tracts of the spinal cord. • These tracts function in maintaining tone, balance, and posture-- especially during body movements. • “SOMATIC MOTOR CONTROL” • The reticular formation also relays eye and ear signals to the cerebellum so that the cerebellum can integrate visual, auditory, and vestibular stimuli in motor coordination. • Other motor nuclei include gaze centers, which enable the eyes to track and fixate objects. • central pattern generators, which produce rhythmic signals to the muscles of breathing and swallowing. “CARDIOVASCULAR CONTROL” • The reticular formation includes the cardiac and vasomotor centers of the medulla oblongata “PAIN MODULATION” • The reticular formation is one means by which pain signals from the lower body reach the cerebral cortex. • It is also the origin of the descending analgesic pathways. • The nerve fibers in these pathways act in the spinal cord to block the transmission of some pain signals to the brain. “SLEEP AND CONSCIOUSNESS” • The reticular formation has projections to the thalamus and cerebral cortex that allow it to exert some control over which sensory signals reach the cerebrum and come to our conscious attention • It plays a central role in states of consciousness like alertness and sleep. • Injury to the reticular formation can result in irreversible coma. “HABITUATION” • This is a process in which the brain learns to ignore repetitive, meaningless stimuli while remaining sensitive to others. • A good example of this is when a person can sleep through loud traffic in a large city, but is awakened promptly due to the sound of an alarm or crying baby. • Reticular formation nuclei that modulate activity of the cerebral cortex are called the reticular activating system or extrathalamic control modulatory system. “SENSORY CONTROL” • Reticular neurons exert some control over activity in spinal reflex arcs. • They can control over the access of sensory information to ascending pathways. • Tonic inhibition of flexor reflexes originates in the reticular formation. • The result that only noxious stimuli can normally evoke such a reflex. • In addition, stimulation of certain regions of the medullary reticular formation causes inhibition of some sensory interneurons and tract cells in the spinal cord. • This seems to be important in the regulation of pai perception. “VISCERAL CONTROL” • Centres controlling inspiration, expiration, and the normal rhythm of breathing have been identified physiologically in the medulla and pons. • Other centres controlling heart rate and blood pressure have been identified in the medullary reticular formation. “RETICULAR FORMATION” The reason it appears to be diffusely organised is twofold: • Its pattern of connectivity is characterised by a great deal of convergence and divergence, so that a single cell may respond to several different sensory modalities or to stimuli applied practically anywhere on the body; • Although it is involved in several quite separate functions, the areas involved in these functions overlap considerably “NUCLEI” • With the reticular formation, it is as though several nuclei had been scrambled together and dispersed along the brainstem. • Their constituent cells retained their original connections. At most levels of the brainstem, the reticular formation can be divided into 3 longitudinal zones arranged in a medial to lateral sequence: • The raphe nuclei • The medial zone • And the lateral zone “THE RAPHE NUCLEI” • These are thin plates of cells in and immediately adjacent to the sagittal plane. • Like the cells of the locus ceruleus, raphe neurons (G. raphe, seam) have exceedingly far-flung connections. • Serotonin is used as the neurotransmitter. “THE MEDIAL ZONE” • This, alongside the midline raphe nuclei, contains a mixture of large and small neurons. • It is the source of most of the long ascending and descending projections from the reticular formation. • Some of the neurons in the medial zone of the rostral medullary reticular formation are so large that this area is referred to as the gigantocellular reticular nucleus. “THE LATERAL ZONE” • This is particularly prominent in the rostral medulla and caudal pons. It is primarily concerned with cranial nerve reflexes and visceral functions “THE RETICULAR NEURONS” • Many of these have extensive and complex axonal projections. • They may innervate multiple levels of the spinal cord or send numerous collaterals to the brainstem and diencephalon. • The generally do not send axons directly to the cerebral cortex, however. “THE RETICULAR NEURONS” • Many of these have extensive and complex axonal projections. • They may innervate multiple levels of the spinal cord or send numerous collaterals to the brainstem and diencephalon. • The generally do not send axons directly to the cerebral cortex, however. • A few may even have bifurcating axons that give rise to both ascending and descending connections. • Reticular neurons have large fields of dendrites. • The sometimes spread out in a plane perpendicular to the long axis of the brainstem. • This allows them to receive synaptic inputs from ascending sensory pathways, descending cortical axons, and a variety of other sources.
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