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Reticular Formation
Dr. G. R. Leichnetz
Reticular Formation
What is it? What does it do?
The reticular formation is the phylogenetically-oldest part
of the CNS that occupies the central core of the brainstem,
surrounding principal sensory & motor nuclei and tracts.
It coordinates involuntary functions essential to life:
states of consciousness (sleep & wakefulness, including EEG
changes, and associated muscle atonia and eye movements),
visceromotor activities (heart rate, respiration) maintained
even when asleep.
It also has descending pathways to the spinal cord that
modulate pain and affect muscle tone.
The reticular formation occupies the core of the
brainstem (midbrain, pons, medulla), surrounding
major sensory and motor nuclei and tracts.
The reticular formation occupies the central core of the
brainstem. It appears to be homogeneous.
How is it divided it into separate nuclei?
Reticular Formation
CRITERIA FOR SUBDIVIDING THE
RETICULAR FORMATION
Subdivided into nuclei on the basis of:
1. Cell size (eg, magnocellular, parvicellular)
2. Brainstem level (eg. nucleus reticularis pontis
caudalis)
3. Neurotransmitter Specificity
“A” cell groups: catecholamines (NE, DA)
“B” cell groups: indolamines (serotonin)
“Ch” cell groups: acetylcholine
The reticular formation can be divided into three zones:
median (raphe), medial (magnocellular), and lateral
(parvicellular) regions
M
ML
L
Barr
The three zones extend rostro-caudally
throughout the brainstem. The large cells
(magnocellular) of the medial region are
found primarily in the pons and rostral
medulla.
Niewenhuys
MEDIAN RETICULAR FORMATION
(Brainstem Raphe)
M
The median region of
the brainstem reticular
formation (brainstem
raphe) contains the cell
bodies of all of the
serotoninergic neurons
of the brain, given “B”
designation according
to the classification by
Dahlstrom & Fuxe.
Midbrain raphe
B7-B8
Pontine raphe
Medullary raphe
B1-B3
A mid-sagittal section thru the brainstem shows the
serotonergic midline raphe nuclei of the median
reticular formation.
B8 B7
B5
Sup. central raphe nucleus
B3
B1
B2
Carpenter
Median Reticular Formation (Raphe)
Nuclei are serotonergic.
Caudal Midbrain/Rostral PonsDorsal Raphe Nucleus (B7)
Superior Central Nucleus (B8)
Mid-Pons- Nucleus Raphe Pontis (B5)
Rostral Medulla- Nucleus Raphe Magnus (B3)
Mid-Medulla- Nucleus Raphe Obscurus (B2)
Nucleus Raphe Pallidus (B1)
Midbrain
raphe
(B7, B8)
The midbrain
raphe (B7- B8)
gives rise to
ascending
serotonergic
projections to
higher levels of
the CNS (sleep).
The pontine
raphe projects to
the cerebellum.
Medullary
raphe (B1,
B2, B3)
Niewenhuys
The medullary
raphe (B1-B3)
gives rise to
descending
serotonergic
projections to
the spinal cord.
Carpenter
B3
Serotonergic raphespinal
projections descend from the
nucleus raphe magnus of the
rostral medullary raphe (B3) to
laminae I and V of the dorsal
horn of the spinal cord, the
laminae that receive primary
nociceptive (pain) afferents.
The NRM is activated by
morphine and modulates pain
at the level of the dorsal horn
(“gate”).
Serotonergic
projections to
lamina I & V
MEDIAL RETICULAR FORMATION
The medial region of
the reticular
formation contains
the largest neurons,
ie. is magnocellular.
Most of the largest
neurons are found in
the pons and medulla.
The nuclei in this
region are contiguous
and named on the
basis of their
brainstem level:
ML
NRPO
Preponderance
of large neurons
in pons and
rostral medulla
NRPC
NRGC
Nucleus reticularis
gigantocellularis (medulla)
Nucleus reticularis pontis
caudalis (caudal pons)
Nucleus reticularis pontis
oralis (rostral pons)
Niewenhuys
Medial Region
Magnocellular
Most of the large cells are in
the medulla and pons. The
nuclei are contiguous and
named on the basis
of brainstem level.
Nucleus reticularis
pontis oralis (NRPO)
Nucleus reticularis
pontis caudalis (NRPC)
Nucleus reticularis
gigantocellularis
Carpenter
Medial Reticular Formation
(Magnocellular)
Medulla- Nuc. reticularis gigantocellularis
Caudal Pons- Nuc. reticularis pontis caudalis
(contains the physiological PPRF,
“horizontal gaze center”)
Rostral Pons- Nucleus reticularis pontis oralis
Cytology
The large cells of the medial region of the
reticular formation have long ascending and
descending collaterals.
The ascending branches project as far
rostrally as the thalamus (reticulothalamics),
and the descending branches project caudally
into the spinal cord (reticulospinals).
Thus these cells can have a broad influence
over the entire CNS neuraxis.
Collaterals to
intralaminar
nuclei of the
thalamus
Collaterals to
brainstem and
spinal cord
Large cell in the
medial magnocellular
reticular formation
NRPC
NRG
Pontine
reticulospinal
tract
Medullary
reticulospinal
tract
Carpenter
Descending branches of
large neurons in the
pontine and medullary
reticular formation
(NRPC, NRG)
constitute the classical
reticulospinal tracts
that project to the
medial part of the
ventral horn to affect
muscle tone.
In general, pontine
reticulospinals are
excitatory, and
medullary
reticulospinals are
inhibitory, on spinal
motoneurons;
particularly those in
the medial ventral
horn.
Ascending reticulothalamics
from large neurons in the
medial pontine and medullary
reticular formation
Ascending branches from
large neurons in the
medial region of the
pontine and medullary
reticular formation
project as far rostrally as
the thalamus
(reticulothalamics),
terminating primarily in
the intralaminar nuclei.
These reticulothalamics were
originally thought to be the
substrate for the reticular
activating system of Moruzzi
and Magoun.
Carpenter
The concept of “reticular activating system” (Moruzzi &
Magoun) held that the medial brainstem reticular
formation activated the cortical EEG (waking state) thru
projections to the intralaminar nuclei of the thalamus,
which projected diffusely to broad areas of the cerebral
cortex.
Gross stimulation
produced awake state;
large lesions of the
medial reticular
formation prevented
the animal from
waking up… leading
to the concept that the
reticular formation
contained centers
essential for cortical
EEG activation.
More recent evidence, however, shows that all three
regions of the reticular formation contain nuclei that
affect the sleep/waking cycle:
midbrain raphe (serotonin; EEG synchronization, slow
wave sleep); locus ceruleus (norepinephrine; EEG
desynchronization, waking state); pedunculopontine
nucleus (acetylcholine; onset of REM sleep).
The medial region of the reticular formation also
contains the premotor centers for control of saccadic
eye movements. The riMLF, INC in the rostral
midbrain- vertical saccades. The PPRF in the caudal
pons (NRPC)- horizontal saccades.
INC
riMLF
PPRF
The rostral interstitial nucleus of the MLF (riMLF),
the center for vertical downward saccades, is located
in the rostralmost midbrain reticular formation
where it merges with the subthalamic region.
Prerubral
fields
Subthalamic
Nucleus
riMLF
The center for vertical
gaze, (riMLF) in the
monkey lies in the most
rostral midbrain
reticular formation in
the prerubral fields.
RN
Parvicellular
red nucleus
The interstitial nucleus of Cajal contains premotor
neurons that project to cell groups in the oculomotor
nuclei (superior rectus and inferior oblique) for vertical
upward eye movements.
INC
Oculomotor
Complex
The physiological area for control of horizontal saccades,
PPRF, is located within the nucleus reticularis pontis
caudalis (NRPC) in the medial pontine reticular
formation at the level of the abducens nucleus.
VI
PPRF
(NRPC)
Large neurons in the PPRF
have ascending branches to
abducens nucleus, and
descending projections to
cervical spinal cord (gaze)
LATERAL RETICULAR FORMATION
L
The lateral region of the
reticular formation contains
predominantly small cells
(parvicellular).
Ch5
A6
The region contains NE and
ACh neurotransmitterspecific cell groups.
A5-A7
Small neurons in
cell groups
identified by
neurotransmitterspecificity
A1-A3
Niewenhuys
NE cell groups A5, A7 have
ascending branches to the
hypothalamus, and (along with A1A3) descending branches to the
spinal cord involved in
visceromotor activities.
The locus ceruleus (A6) projects
throughout CNS, produces waking
state.
Ch5 cell group (pedunculopontine
nucleus) gives rise to cholinergic
trigger for the onset of REM sleep.
At the level of the rostral pons, the cholinergic
pedunculopontine nucleus (Ch5) and noradrenergic
locus ceruleus (A6) are located in the lateral region of the
rostral pontine reticular formation.
Carpenter
The locus ceruleus (A6) is located in the periaqueductal
gray of the rostral pons. It contains neuromelanin pigment
which is the byproduct of catecholamine synthesis (NE).
IVth
ventricle
Its axons have long ascending and
decsending branches which are highly
collateralized throughout the entire
CNS neuraxis; effects activation of the
entire CNS during stress (anxiety).
Locus
ceruleus
A6
Niewenhuys
The locus ceruleus
gives rise to
ascending
projections to all
higher levels of the
CNS, and
descending
projections to the
lower brainstem
and spinal cord.
Its connections
facilitate broad
CNS activation
during “fight or
flight.”
Other NE cell groups (A1-A3, A5, A7) in the lateral
pontine and medullary reticular formation have
connections related to control of visceromotor (autonomic)
activities.
Hypothalamus
A5
A7
A1-A3
Niewenhuys
Cell group A5, located in the lateral region of the
caudal pontine reticular formation, is concerned
with the control of blood pressure.
A5
A5
NE cell group A5 gives rise to
ascending projections to the
hypothalamus, and descending
projections to autonomic centers
(eg. dorsal motor nucleus of
vagus, intermediolateral cell
column (sympathetics) of the
spinal cord) that affect blood
pressure.
(to hypothalamus)
A5
Descending
projections
Carpenter
The lateral medullary reticular formation contains the vital
dorsal and ventral respiratory areas. The DRA concerned
with chemical aspects of respiration (pCO2) and the VRA
the mechanical aspects of respiration (inspiration, expiration).
Solitary nucleus
DMV
DRA
VRA
DRA is in the
reticular
formation
surrounding the
solitary nucleus
VRA is in the
reticular
formation
surrounding the
nucleus
ambiguus;
contains NE cell
groups A1-A3
Lateral Reticular Formation
(parvicellular)
Dorsolateral PonsLocus Ceruleus (NE, A6)
Pedunculopontine Nucleus (ACh, Ch5)
Caudal Pons- Pressor Area (NE, A5)
Medulla- Dorsal Respiratory Area
Ventral Respiratory Area
NE Cell
Groups
A5, A7
NE Cell
Groups
A1-A3
A6
Ch5