INTERNAL CAPSULE Reticular Formation. Objectives 1.Describe the structure of the internal capsule 2.Identify different areas of the internal capsule 3.Describe

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INTERNAL CAPSULEReticular FormationObjectives1.Describe the structure of the internal capsule2.Identify different areas of the internal capsule3.Describe the structure and distribution of reticular formation 4. List the afferent and efferent projections5. List the functions of reticular formationINTERNAL CAPSULEIt is a V-shaped band of projection fibresIt is divided into:Anterior limbGenuPosterior limbRetrolenticular partSublentiform


INTERNAL CAPSULEAnterior limb:Site: between head of caudate nucleus & lentiform nucleusContents: Fibres from anterior nuclear group of thalamus to cingulate gyrus (Thalamocortical)Fibres from medial nuclear group of thalamus to prefrontal cortex (Thalamocortical)Frontopontine fibres

INTERNAL CAPSULEGenu:Site: between head of caudate nucleus & thalamusContents:Part of superior thalamic radiationFrontopontineCorticonuclear


Site: between thalamus & lentiform nucleusContents:Corticospinal fibres (Ant. Two 3rds)Fibers from ventral posterior nucleus of thalamus to postcentral gyrus (Thalamocortical)Fibers from ventral anterior & ventral lateral nuclei of thalamus to motor regions of frontal lobes (Thalamocortical)Temporopontine & parietopontine fibres

INTERNAL CAPSULERetrolenticular part:

Site: behind lentiform nucleusContents:Fibers from medial geniculate body of thalamus to auditory cortexFibers from lateral geniculate body of thalamus to visual cortexParieto- temporo- & occipitopontine fibres

AADCBD-Retrolenticular (RL) & Sublenticular (SL) parts contain optic radiations & auditory radiations respectively.

Thalamocortical fibresCorticopontine fibresCorticonuclear &corticospinal fibresANTERIOR LIMBAnterior thalamic radiation Frontopontine

GENUPart of superior thalamic radiationFrontopontineCorticonuclearRETROLENTIFORMPost thalamic radiation - Optic radiationParieto-pontineTemporo-pontine

SUBLENTIFORMInf thalamic radiation - Auditory radiationPOSTERIOR LIMBSuperior thalamic radiationFrontopontineCorticonuclear (corticobulbar)CorticospinalExtrapyrimidal Brain Stem Reticular FormationReticular = netlikeLoosely defined nuclei and tractsExtends through the central part of the medulla, pons, and midbrainIntimately associated withAscending/descending pathwaysCranial nerves/nucleiInput and output to virtually all parts of the CNS12

Reticular FormationRF is formed of 2 types of cells1- Sensory neurons : discharge impulses to motor neurons2- Motor neurons : receive impulses from sensory neurons. The axons of the motor neurons divide into:a- descending branch : ventral and lateral reticulospinal tracts : spinal cordb- ascending branch : reticular activating system (RAS) to cerebral cortex

RETICULAR FORMATION RF receives impulses from:1- All sensory pathways (general or special sensations)2- Cerebral cortex3- cerebellum4- Basal ganglia5- Vestibular nuclei6- Red nuclei The reticular nuclei are divided into two groups:1- Pontine (excitatory) reticular system2- Medullary (inhibitory) reticular system


Reticular FormationConnectivity is extremely complexMany different types of neurons:Innervate multiple levels of the spinal cordNumerous ascending and descending collateralsSome have bifurcating collaterals that do bothMany have large dendritic fields that traverse multiple levels of the brain stem

17Vestibulospinal and reticulospinal tracts descending in the spinal cord to excite (solid lines) or inhibit (dashed lines) the anterior motor neurons that control the bodys axial musculature

Reticular FormationCan be roughly divided into three longitudinal zonesMidline - Raphe NucleiMedial Zone - Long ascending and descending projectionsLateral Zone - Cranial nerve reflexes and visceral functions

19Reticular Formation FunctionsI. Participates in control of movement through connections with both the spinal cord and cerebellumTwo reticulospinal tracts originate in the rostral pontine and medullary reticular formationMajor alternate route by which spinal neurons are controlledRegulate sensitivity of spinal reflex arcsInhibition of flexor reflexesMediates some complex behavioral reflexesYawningStretchingBabies sucklingSome interconnectivity with cerebellar motor control circuitry20Reticular Formation FunctionsII. Modulates transmission of information in pain pathwaysSpinomesencephalic fibers bring information about noxious stimuli to the periaqueductal grey Periaqueductal grey also receives input from the hypothalamus and cortex about behavioral stateEfferents from the periaqueductal grey project to one of the raphe nuclei and medullary reticular formationThese project to the spinal cord and can suppress transmission of pain information in the spinothalamic tract

21Reticular Formation FunctionsPeriaqueductal GreyRapheSpinal Cord LevelCortexHypothalSpinothalamicTractThalamus22Clinical CorrelationPain ManagementPeriaqueductal grey has high concentration of opiate receptorsNatural pain modulation relies on endogenous opiatesExogenous opiates are used for pain management

23III. Autonomic reflex circuitryReticular formation receives diverse input related to environmental changesAlso receives input from hypothalamus related to autonomic regulationOutput to :cranial nerve nucleiIntermediolateral cell column of the spinal cordInvolved in:BreathingHeart rateBlood pressureReticular Formation FunctionsReticular Formation FunctionsIV. Involved in control of arousal and consciousnessInput from multiple modalities (including pain)Ascending pathways from RF project to thalamus, cortex, and other structures.Thalamus is important in maintaining arousal and cortical toneThis system is loosely defined, but referred to as the Ascending Reticular Activating System (ARAS)ARAS is a functional system, not an anatomically distinct structure