INTERNAL CAPSULE Reticular Formation

Objectives 1.Describe the structure of the internal capsule 2.Identify different areas of the internal capsule 3.Describe the structure and distribution of reticular formation 4. List the afferent and efferent projections 5. List the functions of reticular formation

INTERNAL CAPSULE It is a V-shaped band of projection fibres It is divided into: Anterior limb Genu Posterior limb Retrolenticular part Sublentiform

INTERNAL CAPSULE Anterior limb: Site: between head of caudate nucleus & lentiform nucleus Contents: 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 CAPSULE Genu: Site: between head of caudate nucleus & thalamus Contents: Part of superior thalamic radiation Frontopontine Corticonuclear

INTERNAL CAPSULE Site: between thalamus & lentiform nucleus Contents: Posterior limb: Site: between thalamus & lentiform nucleus Contents: 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 CAPSULE Retrolenticular part: Site: behind lentiform nucleus Contents: Fibers from medial geniculate body of thalamus to auditory cortex Fibers from lateral geniculate body of thalamus to visual cortex Parieto- temporo- & occipitopontine fibres

D-Retrolenticular (RL) & Sublenticular (SL) parts contain optic radiations & auditory radiations respectively. A B A C D

ANTERIOR LIMB Anterior thalamic radiation Frontopontine GENU Part of superior thalamic radiation Corticonuclear RETROLENTIFORM Post thalamic radiation - Optic radiation Parieto-pontine Temporo-pontine SUBLENTIFORM Inf thalamic radiation - Auditory radiation POSTERIOR LIMB Superior thalamic radiation Frontopontine Corticonuclear (corticobulbar) Corticospinal Extrapyrimidal Thalamocortical fibres Corticopontine fibres Corticonuclear & corticospinal fibres

Brain Stem Reticular Formation Reticular = “netlike” Loosely defined nuclei and tracts Extends through the central part of the medulla, pons, and midbrain Intimately associated with Ascending/descending pathways Cranial nerves/nuclei Input and output to virtually all parts of the CNS

Reticular Formation RF is formed of 2 types of cells 1- Sensory neurons : discharge impulses to motor neurons 2- Motor neurons : receive impulses from sensory neurons. The axons of the motor neurons divide into: a- descending branch : ventral and lateral reticulospinal tracts : spinal cord b- 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 cortex 3- cerebellum 4- Basal ganglia 5- Vestibular nuclei 6- Red nuclei

RETICULAR FORMATION The reticular nuclei are divided into two groups: 1- Pontine (excitatory) reticular system 2- Medullary (inhibitory) reticular system

Reticular Formation Connectivity is extremely complex Many different types of neurons: Innervate multiple levels of the spinal cord Numerous ascending and descending collaterals Some have bifurcating collaterals that do both Many have large dendritic fields that traverse multiple levels of the brain stem

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

Reticular Formation Can be roughly divided into three longitudinal zones Midline - Raphe Nuclei Medial Zone - Long ascending and descending projections Lateral Zone - Cranial nerve reflexes and visceral functions

Reticular Formation Functions I. Participates in control of movement through connections with both the spinal cord and cerebellum Two reticulospinal tracts originate in the rostral pontine and medullary reticular formation Major alternate route by which spinal neurons are controlled Regulate sensitivity of spinal reflex arcs Inhibition of flexor reflexes Mediates some complex “behavioral” reflexes Yawning Stretching Babies suckling Some interconnectivity with cerebellar motor control circuitry

Reticular Formation Functions II. Modulates transmission of information in pain pathways Spinomesencephalic fibers bring information about noxious stimuli to the periaqueductal grey Periaqueductal grey also receives input from the hypothalamus and cortex about behavioral state Efferents from the periaqueductal grey project to one of the raphe nuclei and medullary reticular formation These project to the spinal cord and can suppress transmission of pain information in the spinothalamic tract

Reticular Formation Functions Cortex Thalamus Hypothal Spinothalamic Tract Periaqueductal Grey Raphe Spinal Cord Level

Clinical Correlation Pain Management Periaqueductal grey has high concentration of opiate receptors Natural pain modulation relies on endogenous opiates Exogenous opiates are used for pain management

Reticular Formation Functions III. Autonomic reflex circuitry Reticular formation receives diverse input related to environmental changes Also receives input from hypothalamus related to autonomic regulation Output to : cranial nerve nuclei Intermediolateral cell column of the spinal cord Involved in: Breathing Heart rate Blood pressure

Reticular Formation Functions IV. Involved in control of arousal and consciousness Input from multiple modalities (including pain) Ascending pathways from RF project to thalamus, cortex, and other structures. Thalamus is important in maintaining arousal and “cortical tone” This system is loosely defined, but referred to as the Ascending Reticular Activating System (ARAS) ARAS is a functional system, not an anatomically distinct structure