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:1. Anterior limb2. Genu3. Posterior limb4. Retrolenticular part5. Sublentiform

INTERNAL CAPSULEAnterior limb:• Site: between head of

caudate nucleus & lentiform nucleus

• Contents: 1. Fibres from anterior

nuclear group of thalamus to cingulate gyrus (Thalamocortical)

2. Fibres from medial nuclear group of thalamus to prefrontal cortex (Thalamocortical)

3. Frontopontine fibres

INTERNAL CAPSULE

Genu:• Site: between head of

caudate nucleus & thalamus

• Contents:1. Part of superior

thalamic radiation2. Frontopontine3. Corticonuclear

INTERNAL CAPSULEPosterior limb:

• Site: between thalamus & lentiform nucleus

• Contents:

1. Corticospinal fibres (Ant. Two 3rds)

2. Fibers from ventral posterior nucleus of thalamus to postcentral gyrus (Thalamocortical)

3. Fibers from ventral anterior & ventral lateral nuclei of thalamus to motor regions of frontal lobes (Thalamocortical)

4. Temporopontine & parietopontine fibres

INTERNAL CAPSULERetrolenticular part:

• Site: behind lentiform nucleus• Contents:1. Fibers from medial geniculate body of

thalamus to auditory cortex2. Fibers from lateral geniculate body of

thalamus to visual cortex3. Parieto- temporo- & occipitopontine

fibres

A

A

D

C

B

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

Thalamocortical fibres

Corticopontine fibres

Corticonuclear &corticospinal fibres

ANTERIOR LIMBAnterior thalamic radiation Frontopontine

GENUPart of superior thalamic radiationFrontopontineCorticonuclear

– RETROLENTIFORM• Post thalamic

radiation - Optic radiation

• Parieto-pontine• Temporo-pontine

– SUBLENTIFORM• Inf thalamic radiation -

Auditory radiation

POSTERIOR LIMBSuperior thalamic radiationFrontopontineCorticonuclear (corticobulbar)CorticospinalExtrapyrimidal

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

The reticular nuclei are divided into two

groups:

1- Pontine (excitatory) reticular system

2- Medullary (inhibitory) reticular system

RETICULAR FORMATION

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

Periaqueductal Grey

Raphe

Spinal Cord Level

Cortex

Hypothal

SpinothalamicTract

Thalamus

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

• III. Autonomic reflex circuitryReticular formation receives diverse input related to environmental changesAlso receives input from hypothalamus related to autonomic regulationOutput to :• cranial nerve nuclei• Intermediolateral cell column of the spinal cordInvolved in:• Breathing• Heart rate• Blood pressure

Reticular Formation Functions

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