1. INTERNAL CAPSULE
Reticular Formation

2. 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

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

5. 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

6. INTERNAL CAPSULE Genu:
Site: between head of caudate nucleus & thalamus
Contents:
Part of superior thalamic radiation
Frontopontine
Corticonuclear

7. 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

9. 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

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

11. 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

12. 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

14. 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

15. 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

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

17. 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

18. 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

19. 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

20. 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

21. 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

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

23. 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

24. 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

25. 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