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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
•
•
1.
2.
3.
4.
5.
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:
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 radiation
2. Frontopontine
3. Corticonuclear
Posterior limb:
•
•
1.
2.
3.
4.
INTERNAL CAPSULE
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:
1. Fibers from medial geniculate body of
thalamus to auditory cortex
2. Fibers from lateral geniculate body of
thalamus to visual cortex
3. 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
Frontopontine
Corticonuclear
POSTERIOR LIMB
Superior thalamic radiation
Frontopontine
Corticonuclear (corticobulbar)
Corticospinal
Extrapyrimidal
Thalamocortical fibres
Corticopontine fibres
Corticonuclear &
corticospinal fibres
– RETROLENTIFORM
• Post thalamic
radiation - Optic
radiation
• Parieto-pontine
• Temporo-pontine
– SUBLENTIFORM
• Inf thalamic radiation
- Auditory radiation
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
Spinothalamic
Tract
Hypothal
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