The reticular formation is a network of neurons located in the brainstem that performs several important functions. It receives sensory information from the spinal cord and senses arousal levels. The reticular formation contains nuclei that are involved in motor control, sleep-wake cycles, autonomic functions, and modulating pain. It has ascending and descending pathways that connect to the thalamus and cerebral cortex and help regulate states of consciousness like sleep and wakefulness.

1. Reticular Actvating System
Susanth

2. RETICULAR FORMATION
• Extensive fields of intermingled neurones and nerve fibres in
brainstem
• Ascending and descending components that are partly crossed
and uncrossed
• Highly organized and differentiated, consisting of distinct
populations of neurons with specific functions

3. • Subserve somatic and visceral functions
• Receives a summary of much of the sensory information that
enters the spinal cord and brain stem
↓
important in influencing the arousal level of an organism
• Contains interneurons responsible for generating spinal
reflexes and simple motor patterns
RETICULAR FORMATION

4. RAS
Several areas traditionally included are
• Midbrain Reticular Formation
• Mesencephalic Nucleus
• Thalamic Intralaminar nucleus (centromedian nucleus)
• Dorsal Hypothalamus
• Tegmentum
• Basal forebrain structures

5. Anatomical subdivisions of RAS
• Median nuclei
• Medial nuclei
• Lateral nuclei

6. MEDIAN COLUMN OF RETICULAR
NUCLEI
• extends throughout the medulla, pons and midbrain
• bilateral, vertical sheets, blended in the midline and occupying
the paramedian zones
• Collectively called the nuclei of the raphe, or raphe nuclei

7. Serotoninergic and are grouped into nine clusters, B1-9
Medulla
• Raphe pallidus nucleus and raphe obscurus nucleus
Pons
• Raphe magnus nucleus (B3 neurones)
• Pontine raphe nucleus (B5 neurons)
• Central superior raphe nucleus (B6 and B8 )
Midbrain
• Dorsal (rostral) raphe nucleus ( B7)
MEDIAN COLUMN OF RETICULAR
NUCLEI

8. Medial Column Of Reticular Nuclei
Mostly orientated in the transverse plane
Upper medulla
• Nucleus reticularis gigantocellularis of olszewski and baxter
Pons
• Pontine gigantocellular (magnocellular) nucleus
• Caudal and oral pontine tegmental reticular nuclei
Midbrain
• Cuneiform nucleus and subcuneiform nucleus

9. Lateral Column Of Reticular Nuclei
• Ventrolateral reticular area
• Lateral pontine tegmental area
• Parvocellular reticular area

10. Ventrolateral reticular area
Rostrally - Lateral Paragigantocellular Nucleus
Caudally - Nucleus Retroambiguus
Contains
• Noradrenergic cell groups A1, A2, A4-A7 (A3 is absent in
primates)
• Adrenergic cell group C1
Lateral Column Of Reticular Nuclei

11. Lateral pontine tegmental reticular grey matter
• Medial and lateral parabrachial nuclei
• Ventral kölliker-fuse nucleus - pneumotaxic centre.
• Locus coeruleus (noradrenergic cell group A6)
• Area subcoeruleus (Noradrenergic cell group A7)
• Cholinergic group ch5 in the pedunculopontine tegmental
nucleus
• Two micturition centres - dorsomedial and ventrolateral parts
Lateral Column Of Reticular Nuclei

12. Connections of RF
Afferent connections
Efferent connections

13. Afferents to RF
Mainly on medullary reticular formation
• Spinal cord
• Collaterals from second order neurons of sensory cranial
nuclei
• Cerebral cortex
• Cerebellar nuclei

14. Spinal cord
I. Spinoreticular fibres
Caudal half of nucleus reticularis gigantocellularis
II. Collaterals of spinothalamic fibres
Lateral reticular nucleus → cerebellum
Afferents to RF

15. • Collateral fibres from auditory, vestibular, trigeminal, visceral
(solitarius)
Parvicellular reticular nucleus
• Retinotectal and tectoreticular fibres – visual impulses
• Medial forebrain bundle - olfactory impulses
Primary sensory fibres do not terminate on RF (medial
leminiscus)
Afferents to RF

16. Cerebral cortex
From sensorimotor areas – crossed and uncrossed
Nucleus reticularis pontis oralis and caudalis, nucleus
reticularis gigantocellularis
Afferents to RF

17. Cerebellum
Ends on paramedian nuclei
• Fastigeal nucleus via uncinate fasiculus
• Dentate nucleus via crossed descending division of superior
cerebellar peduncle
Afferents to RF

18. Functional Division
Magnocellular zone:
• Large cells restricted
to medial two-thirds
of reticular formation
Parvocellular zone:
• Small cells found in
lateral regions

19. Functional Division
Magnocellular zone:
• Long ascending and descending axons
• Modulate the actions of neurons involved in movement and
posture, pain, autonomic functions, and arousal
Parvocellular zone:
• close to the motor nuclei of the cranial nerves
• coordinate reflexes and simple stereotyped behaviors mediated
by the cranial nerves

20. Parvocellular Reticular Formation
• Extends from the region lateral to the hypoglossal and
ambiguus nuclei through the area surrounding the facial
nucleus up to and adjacent to the trigeminal motor nucleus
• Involved with the coordination of orofacial motor responses
Chewing - neurons adjacent to the trigeminal motor nucleus
Lip movements - neurons near the facial motor nucleus
Movements of the tongue - neurons near the hypoglossal nucleus

21. RF surrounding the facial motor nucleus
• Organizing emotional facial expressions
• Descending input is bilateral and facial expressions can be
triggered by either hemisphere
Pontine paramedian reticular formation
Controls ipsilateral horizontal eye movements and majority of
fibres to MLF
Lateral to the oculomotor nucleus
• Coordinate vertical eye movements as well as the convergence
Parvocellular Reticular Formation

22. Magnocellular Zone
Long ascending/descending pathways:
Terminate in:
Hypothalamus.
Midline and intralaminar thalamic nuclei.
Segments of spinal cord.
Long ascending fibers from:
Lower pons and medulla.

23. Ascending Reticular Activating System - ARAS
 Receives fibers from the
sensory pathways via long
ascending spinal tracts.
 Alertness, maintenance of
attention and wakefulness.
 Emotional reactions,
important in learning
processes.

24. Ascending arousal system
(ARAS)
Monoaminergic cell groups in the brain stem
Ascending cholinergic inputs from the pedunculopontine and
laterodorsal tegmental nuclei
Junction of the midbrain and
diencephalon
Activates and modulates
thalamic relay nuclei as
well as intralaminar and
related nuclei
Extensive diffuse cortical projections
Lateral hypothalamic
area
hypothalamic and basal
forebrain cell groups

25. ARAS
Main function is to modify and potentiate thalamic and cortical
function such that EEG desynchronization ensues
During alert wakefulness
Low-voltage, fast (>12 Hz) electrical activity (desynchronized)
During deep sleep
High-voltage, slow (<3 Hz) electrical activity (synchronized )

26. Specific rhythmic pattern of the
EEG
• Reflects synchronized waves of excitatory synaptic potentials
reaching the cerebral cortex from the thalamus
• Depend on two important states of the thalamic relay neurons
a) Transmission mode
b) Burst mode

27. During sleep
Burst mode
Thalamic Relay Neurons
Voltage-gated
calcium channel
Hyperpolarized
Calcium current
Calcium-activated
potassium current
Reticular
nucleus
GABA-B

28. Rhythmic and synchronous firing of thalamic relay neurons
↓
Waves of excitatory postsynaptic potentials in dendrites of
cortical neurons
↓
Rhythmic slow waves in EEG
Pattern indicating that the thalamus is unable to relay sensory
information to the cortex
During sleep

29. During
wakefulness
Cholinergic input from the pedunculopontine
and laterodorsal tegmental nuclei in the
brain stem and basal forebrain
Desynchronized pattern of the EEG
Sensory stimuli
Transmission mode
Thalamic relay nuclei
Reticular nucleus
Near firing threshold

30. Wakefulness-promoting Neurons
• Noradrenergic neurons in the locus ceruleus
• Serotonergic neurons in the dorsal raphe of the brainstem
• Histaminergic neurons in the tuberomammillary nucleus of the
hypothalamus
• Dopaminergic neurons in the ventral tegmental area, substantia
nigra, and ventral periaqueductal area
• many additional neurons mostly glutamatergic
• Lateral hypothalamic neurons containing orexin
• Cholinergic and noncholinergic neurons in the magnocellular
basal forebrain nuclei

31. Regulating Sleep-Wake Transitions
Reduced activity of hypocretin systems may be partly
responsible for inducing sleepiness
Ventrolateral
preoptic neurons
Ascending
arousal system
GABA and galanin

32. NREM to
REM sleep
REM-off
Ventrolateral
periaqueductal graymatter
Lateral pontine
tegmentum
Rem-on
Sublaterodorsal nucleus
Pre-coeruleus region
Dorsal raphe nucleus
(DRN)
Locus coeruleus (LC)
Cholinergic neurons
Pedunculopontine (PPT)
& Laterodorsal tegmental
nuclei (LDT)
orexin neurons
Ventrolateral preoptic
nucleus
GABA
Glutamate
Brainstem and spinal cord
Motor manifestations of REM
sleep, including atonia
GABA
Basal forebrain
EEG phenomena of REM
sleep

33. Modulation of Posture, Gait, and
Muscle Tone
Two Reticulospinal Tracts
Medial and Lateral reticulospinal tracts

34. Medial Reticulospinal tracts
Origin
• Oral and caudal pontine reticular
nuclei
• Gigantocellular reticular nucleus
in the medulla.

35. Medial Reticulospinal Tract
Pontine fibres
Excite motor neurones of axial and limb muscles
Medullary fibres
Excite, or inhibit motor neurones of cervical muscles and excite
motor neurones of axial muscles.

36. Concerned with
• Posture
• Steering of head and trunk movements in response to external
stimuli
• Crude, stereotyped movements of the limbs ( stepping )
Medial Reticulospinal Tract

37. Lateral reticulospinal tract
• Arise from the medullary reticular formation, mostly from the
gigantocellular nucleus
• Inhibiting excitatory axial extensor muscles
• Produces loss of motor tone, or atonia
• Associated with the atonia that occurs in REM sleep(under the
control of cholinergic neurons in the pedunculopontine
nucleus)

38. Modulation of pain
• Serotoninergic raphe magnus nucleus in the midline of the
rostral medulla
• Noradrenergic cell groups in the pons
Activation of either of these inhibit the transmission of
nociceptive information
Endogenous opiates released from enkephalinergic neurons in the
periaqueductal gray matter activate the descending modulatory
pathway

39. Modulation of autonomic function
Ventrolateral medullary reticular formation
• Gastrointestinal responses (such as swallowing and vomiting)
• Respiratory activities (including the initiation and modulation
of respiratory rhythm, coughing, hiccupping and sneezing)
• Cardiovascular responses (such as baroreceptor reflexes and
responses to cerebral ischemia and hypoxia)

40. Pneumotaxic centre
Project to
• Inspiratory centre in the ventrolateral part of the nucleus
solitarius
• Mixed expiratory-inspiratory centre in the superficial
ventrolateral reticular area
↓
Phrenic nucleus and T1-T3 sympathetic preganglionic neurones
bilaterally
Modulation of autonomic function

41. Hypothalamic, median preoptic and paraventricular nuclei
↓
Lateral parabrachial nucleus and Micturition centres
↓
Bilateral projections in lateral spinal funiculus
↓
• Preganglionic parasympathetic neurones in the sacral cord
(which innervate the detrusor muscle in the urinary bladder)
• Nucleus of onuf (which innervate the musculature of the pelvic
floor and the anal and urethral sphincters)
Modulation of autonomic function

42. Locus Coeruleus
• Seat of the visceral alerting response
• Subserves cardiovascular, baroreceptor, chemoreceptor and
respiratory reflexes.
• Terminate bilaterally on sympathetic preganglionic neurones
in the thoracic spinal cord
Modulation of autonomic function

43. Superficial ventrolateral area
• Supraoptic and paraventricular hypothalamic nuclei
• Release of vasopressin from the neurohypophysis.
Medullary Noradrenergic Cell Groups A1 And A2
• Median eminence
• Control the release of growth hormone, luteinizing hormone
and adrenocorticotrophic hormone (ACTH)
Modulation of endocrine function

44. Reticular
formation and
cerebellum
Augments the dorsal and ventral spinocerebellar, cuneocerebellar,
accessory cuneocerebellar and trigeminocerebellar tracts
Lateral and paramedian
reticular nuclei and the nucleus
of the pontine tegmentum
Ipsilateral vermis,
hemisphere and
flocculonodular lobule
via I/L restiform body
(inferior cerebellar
peduncle)
Contralateral primary motor and
sensory neocortices
Crossed descending
rubrobulbar axons
Spinoreticular pathway

45. Monoaminergic neurons in RF
Distinct chemoarchitectonic nuclear groups
• Three catecholaminergic groups
a) Noradrenergic (group A)
b) Adrenergic (group C)
c) Dopaminergic (group A) neurons
• Serotoninergic neurones (group B cells)
• Cholinergic neurones (group ch cells)

46. Noradrenergic Cell Groups
Two columns, one dorsal and one ventral
Medulla
• Ventral column - associated with the nucleus ambiguus (A1
group)
• Dorsal column - component of the nucleus of the solitary tract
and the dorsal motor vagal nucleus (A2 group)
Both groups project to the hypothalamus
Control cardiovascular and endocrine functions

47. Pons
• Ventral column - A5 and A7
Projections to the spinal cord that modulate autonomic
reflexes and pain sensation
• Dorsal column - Locus ceruleus (A6 cell group)
Maintains vigilance and responsiveness to unexpected
environmental stimuli
Extensive projections to the cerebral cortex and
cerebellum, as well as descending projections to the
brain stem and spinal cord
Noradrenergic Cell Groups

48. Adrenergic Cell Groups
C1
Rostral extension from the A1 column
Project to
• Spinal cord, particularly to the sympathetic preganglionic
column
Provide tonic excitatory input to vasomotor neurons
• Hypothalamus
Modulate cardiovascular and endocrine responses

49. C2 adrenergic neurons
• component of the nucleus of the solitary tract
• contribute to the ascending pathway to the parabrachial
nucleus
• transmit gastrointestinal information
C3 adrenergic group
• near the midline at the rostral end of the medulla
Adrenergic Cell Groups

50. Dopaminergic neurons
Substantia nigra (A9 group)
Adjacent retrorubral field (A8 group)
• Terminates in the striatum
• Involved in initiating motor responses
Ventral tegmental area (A10 group)
• Innervate the frontal and temporal cortices and the limbic
structures of the basal forebrain
• Implicated in emotion, thought, and memory storage

51. Hypothalamic (A11 and A13 ) in the zona incerta
• Long descending pathways to the autonomic areas of the lower
brain stem and the spinal cord
• Regulate sympathetic preganglionic neurons
A12 and A14 groups
• Located along the wall of the third ventricle
• Involved with endocrine control
Also (A15 cells in the olfactory tubercle and A16 in the olfactory
bulb) and in the retina (A17 cells)
Dopaminergic neurons

52. Serotonergic Cell Groups
Along the midline of the brain stem in the raphe nuclei
B1-B3 cell groups
• along the midline of the caudal medulla
• descending projections to the motor and autonomic systems in
the spinal cord
• regulating tone in motor systems

53. B4 (raphe magnus nucleus )
• projects to the spinal dorsal horn
• modulate the perception of pain
B5-B9 ( pons and midbrain )
• pontine, dorsal, and median raphe nuclei
• project to virtually the whole of the forebrain
• regulate wake-sleep cycles, affective behavior, food intake,
thermoregulation, and sexual behavior
Serotonergic Cell Groups

54. Dorsal raphe spinal projections
• Pain-control pathway
Intermediate raphe spinal projection
• Inhibitory, and, in part, modulates central sympathetic control
of cardiovascular function
Ventral raphe spinal system
• Excites ventral horn cells and enhance motor responses to
nociceptive stimuli
• Promote the flight and fight response
Serotonergic Cell Groups

55. Cholinergic neurons
Basal forebrain cholinergic groups
• Medial septum (MS) (Ch1 group)
• Nuclei of the vertical and horizontal limbs of the diagonal
band (DBv and DBh) (Ch2 and Ch3 groups)
• Nucleus basalis of Meynert (BM) (Ch4 group)

56. Pontine cholinergic neurons
A. Laterodorsal (LDT) (ch5 group)
B. Pedunculopontine (PPT) (ch6 group), tegmental nuclei
Provide cholinergic innervation to the brain stem and the
thalamus
Critical for inducing a state of cortical arousal, both during
wakefulness and dreaming
Cholinergic neurons

57. Histaminergic Cell Groups
• Posterior lateral hypothalamus
• Tuberomammillary nucleus
Help maintain arousal in the forebrain

58. RAS & pathological correlates
Schizophrenia
Intractable schizophrenic patients have a significant increase
(> 60%) PPN neurons and dysfunction of NO signaling
involved in modulating cholinergic output of the RAS.
Post-traumatic stress disorder, Parkinson’s Disease, REM
behavior disorder
significant (>50%) decrease in the number of locus coeruleus
(LC) neurons, resulting is increased disinhibition of the PPN

59. RAS & pathological correlates
Narcolepsy
significant down-regulation of PPN output and a loss of orexin
peptides,
Progressive supranuclear palsy (PSP)
Dysfunction of NO signaling
Depression, autism, Alzheimer's disease, attention deficit
disorder

61. Magnocellular Zone Afferents
Spinoreticular fibers.
Sensory cranial nerves.
Cerebellum.
Hypothalamus.
Basal nuclei.
Cerebral cortex:
Esp. premotor cortex.

62. Descending Reticular System
 Inhibitory:
- Smoothness and accuracy
of voluntary movements
- Reflex movements
- Regulates muscle tone
- Maintenance of posture
- Control vegetative
functions
 Facilitatory:
- Mantains the muscle tone
- Facilitates autonomic
functions
- Activates ARAS

63. Other Function
Somatic motor control
Cardiovascular control
Pain modulation - The reticular formation is one means by which
pain signals from the lower body reach the cerebral cortex.
Sleep and consciousness
Habituation
Dream