For medical, nursing, and physiotherapy undergraduates, and postgraduates

1. RETICULAR FORMATION
DR. SUNDIP CHARMODE
ASSISTANT PROFESSOR

2. BACKGROUND
• Diffuse ill-defined mass of intermingled neurons and nerve fibers
occupying the tegmental core throughout the length of brainstem
• Light microscopic appearance of a vague network of nerve cells and
nerve fibers
• Phylogenetically, it represents the old reticular core of brain which
contains within it the vital cardiac and respiratory centers.

3. IMPORTANCE OF RETICULAR FORMATION
• Regulates level of consciousness
and alertness
• Regulates respiration, blood
pressure, heart rate and other
vegetative functions
• Regulates tone of skeletal muscles
• Modulates the impulses in the pain
pathways

4. PHYLOGENETIC ORIGIN
• Considered more primitive in vertebrate phylogeny, upon which the
specific lemniscal system and the pyramidal and extra-pyramidal
pathways of motor system are subsequently superimposed.
• Reticular formation receives data from most of sensory systems and
has efferent (direct and indirect) connections with all the levels of
neuraxis.

5. EXTENT OF RETICULAR FORMATION
• Cranially: Diencephalon
• Subthalamus- Zona Inserta
• Hypothalamus
• Thalamus – Intralaminar and
Ventro-anterior nuclei
• Caudally: Into the neurons of lamina
VII of spinal grey matter with the
associated Spino-reticular and
Reticulo-spinal tracts

6. FEATURES OF RETICULAR FORMATION
• Central tegmental tract (Ascending and descending) forms a
connecting link between the neurons of brainstem and diencephalic
reticular formation.
• Although RF - to be consisting of nerve fibers and scattered neurons,
among them several regions with localized cell groups called Reticular
nuclei

7. PECULARITIES
• Tegmental core of reticular formation is connected to all levels of
neuraxis from the cerebral cortex and limbic system to the spinal cord
by ascending and descending fibers.
• Pathways of reticular system are polysynaptic and consist of crossed
and uncrossed ascending and descending fibers.

8. PECULARITIES
• Such system, conveys non-specific sensations and acts as anatomical
substrate to
1. Adjust the degree of arousal (from sleep to consciousness)
2. Modulation of sensory input
3. Integration of vital visceral functions (respiratory & cardio-vascular)
4. Tonic and phasic motor activities

9. TYPES OF NEURONS
1. Most of the multipolar reticular neurons are Iso-
dendritic - Such Golgi type I neuronal
organization permits profuse polysynaptic
interconnections with other neurons for
convergence and divergence of impulses.
2. A few reticular neurons are Idio-dendritic and
Allo-dendritic – possess profusely branched
short and intermediate dendrites, respectively.
3. The reticular formation contain few, if any, Golgi
type II neurons

10. RETICULAR NUCLEI IN BRAINSTEM
• Reticular nuclei in brainstem are arranged into three bilateral
longitudinal columns/zones
• Median column: lies in midline, consist of intermediate size neurons.
The nuclei of this column are termed as ‘Raphe nuclei’
• Medial column: consist of large size neurons. The nuclei of this
column are termed as ‘Magnocellular column’
• Lateral column: consist of small size neurons. The nuclei of this
column are termed as ‘Parvocellular column’

13. RAPHE NUCLEI (MEDIAN GROUP OF NUCLEI)
• These extend along the entire length of the median and paramedian
plane of brainstem and are named caudo-cranially as:
• Nucleus Raphe Obscurus and Pallidus: Upper 2/3rd of Medulla
• Nucleus Raphe Magnus: Ponto-medullary junction
• Nucleus Raphe Pontis and Superior Central Nucleus: Pons
• Dorsal and Ventral Raphe Nuclei and Nucleus Linearis: Midbrain

14. RAPHE NUCLEI (MEDIAN
GROUP OF NUCLEI)
• Serotonin is the
neurotransmitter of most
of the raphe neurons.

15. MEDIAL GROUP OF NUCLEI
• These nuclei in ascending order are as
follows:
• Giganto-cellular nucleus/Magno-
cellular: Upper medulla
• Caudal pontine, Oral pontine nucleus
and Nucleus Ceruleus: Ponto-
mesencephalic junction
• Cuneiform and sub-cuneiform nuclei:
Midbrain

16. MEDIAL GROUP OF NUCLEI
• Act as effector area and their elongated axons divide into ascending
and descending branches – both crossed and uncrossed.
• Ascending branches: Form central tegmental tract and extend into
midbrain reticular nuclei
• Descending branches: Project to spinal cord as Reticulo-spinal tracts

18. LATERAL GROUP OF NUCLEI
• These include caudo-cranially:
• Central Reticular Nucleus:
Medulla
• Parvo-cellular nucleus: Medulla
and Pons
• Parabrachial and Pedunculo-
pontine nucleus: Midbrain

19. LATERAL GROUP OF NUCLEI
• Parvo-cellular nuclei of lateral zone act as sensory area and receive
afferents from multiple sources.
• Their short axons are mostly projected to the effector neurons of the
medial zone.

21. CONNECTIONS OF RETICULAR FORMATION-
AFFERENTS
• Principles of afferent connections
1. Spinal afferents for cutaneous sensations
2. Brain stem afferents
3. Basal ganglia afferents
4. Limbic system afferents
5. Afferents from cerebral cortex

23. PRINCIPLES OF AFFERENT CONNECTIONS
• Nucleus of lateral zone receives receives:
1. Collaterals from most of the ascending and descending pathways that
traverse brainstem:
1. Somatic or visceral
2. Motor or sensory
3. Purely Inter-nuncials
2. Collaterals from sensory pathways reach the reticular formation from
second order of sensory neurons and not directly from the first order

24. PRINCIPLES OF AFFERENT CONNECTIONS
3. Few afferents bypass the reticular formation which include:
1. Dorsal column pathway through the medial lemniscus
2. Retino-geniculo-calcarine tract
3. Tonotropic fibers of the auditory system

26. SPINAL AFFERENTS OF CUTANEOUS SENSATIONS
1. Collaterals from the Spino-reticular tracts:
1. These neurons are located mainly in contralateral laminae 7 and 8, the
lateral reticulated part of lamina 5, in lamina 1, area 10, and in the lateral
spinal nucleus - mainly cervical and lumbar
2. Spino-reticular tract is involved in the control of descending modulation,
motivational-affective aspects of pain, and in motor and neurovegetative
responses to pain

28. SPINAL AFFERENTS OF CUTANEOUS SENSATIONS
2. Collaterals from the Spino-thalamic tracts: Large number of fibers of
Spino-thalamic tract terminate in Lateral Reticular Nucleus which in
turn project to Cerebellum
• Two subsystems:
• Direct (for direct conscious appreciation of pain)
• Indirect (for affective and arousal impact of pain). Indirect projections
include
• Spino-Reticulo-Thalamo-Cortical (part of the ascending reticular
arousal system, aka ARAS)
• Spino-Mesencephalic-Limbic (for affective impact of pain).

30. BRAINSTEM AFFERENTS
1. Cutaneous: Collaterals from the Trigemino-thalamic tract
2. Visceral: Nucleus Solitarius
3. Visual: Superior Colliculus (Tecto-reticular fibers)
4. Auditory: Cochlear nuclei and Superior Olivary Nuclear complex
(Tecto-reticular fibers)
5. Vestibular: Vestibular nuclei, Nucleus Fastigius of deep cerebellar
nuclei

31. BASAL GANGLIA AFFERENTS
• Globus pallidus – Afferents reach Pedunculo-pontine nucleus of
midbrain

32. LIMBIC SYSTEM AFFERENTS
1. Basal forebrain and hypothalamus via Medial fore brain bundle
2. Mamillary bodies via Mamillo-tegmental tract
3. Habenular nucleus via Fasciculus Retroflexus

33. AFFERENTS FROM CEREBRAL CORTEX
• Through Cortico-reticular fibers and Medial fore-brain bundle
conveying psychic stimuli for attention.

34. OTHER FACTORS INFLUENCING ACTIVITY OF RF
• Hormones, chemical substances like adrenaline, acetylcholine, carbon
dioxide
• Drugs like barbiturates, anesthetics and tranquillizers

35. EFFERENTS FROM RETICULAR FORMATION
1. Ascending fibers
2. Descending fibers
3. Lateral fibers

36. ASCENDING FIBERS
These fibers pass through Central tegmental tract and Medial fore brain
bundle and terminate into:
DORSALLY
• Intralaminar nucleus- collaterals
project to cerebral cortex
• Ventro-anterior nucleus
• Dorso-anterior nucleus of
thalamus
• Such ascending reticular system
is concerned with the state of
alertness or arousal.
VENTRALLY
• Hypothalamus and Septal areas
via Mamillary bundle. These
fibers bypass the thalamus
• Further rostrally, the fibers
project to Orbito-frontal Cortex

37. LATERAL FIBERS
• Cerebellar fibers pass laterally from the Paramedian and Lateral
reticular nuclei to the Roof nuclei and Cortex of cerebellum

38. DESCENDING FIBERS
• These fibers extend to spinal cord through:
1. Medial Reticulo-spinal tract
2. Lateral Reticulo-spinal tract
3. Visceral centers of brain stem through polysynaptic pathways

39. MEDIAL (PONTINE) RETICULO-SPINAL TRACT
• Arises from Oral and Caudal pontine reticular nuclei- descend
ipsilaterally - acts on Gamma motor neurons supplying intra-fusal
fibers of extensor muscles
• Also excite Alpha motor neurons via gamma loop.

40. LATERAL (MEDULLARY) RETICULO-SPINAL TRACT
• Arises from Giganto-cellular nucleus of medulla, descends bilaterally
downwards, supply Alpha and Gamma motor neurons of flexors.
1. Reticulo-spinal tract in collaboration with Vestibulo-spinal tract
maintain postural tone during sitting and standing.
2. Cerebral cortex through Cortico-reticular and Reticulo-spinal tract
influences the activities of antagonistic flexor and extensor group of
muscles.

41. RAPHE-SPINAL TRACT
• Arises from Nucleus of Raphe Magnus of medulla (rich in serotonergic
neurons)- project bilaterally – to neurons of Substantia Gelatinosa-
stimulate the release of enkephalin from SG cells.
• Enkephalin blocks the action of Substance –P (polypeptide neuro-
transmitter for pain sensation)
• Raphe-spinal tract modulates pain transmission in the posterior horn

43. DESCENDING RETICULAR FIBERS REGULATING RESPIRATORY
CENTERS IN BRAINSTEM
• The respiratory centers consist of three groups of paired nuclei of
reticular formation
1. Inspiratory nuclei in medulla
2. Expiratory nuclei in medulla
3. Pneumotaxic center in pons

45. INSPIRATORY CENTRE
• Composed of Dorsal Respiratory Nuclei
• Located ventro-lateral to the nucleus solitarius at the level of Obex
• Consists of excitatory neurons surrounded by inhibitory neurons
• Excitatory neurons: Axons cross middle line, descends, terminate mono-
synaptically via Reticulo-spinal tract on lower motor neurons of Phrenic
nerves (C3-5) and intercostal nerves (T1-12).

46. INSPIRATORY CENTRE
• Factors activating excitatory neurons are:
1. Cortico-reticular fibers
2. Spino-reticular fibers
3. Chemoreceptors from carotid and aortic bodies through IX and X
cranial nerves (when O2 decreases)
4. From the chemoreceptor surface of the medulla (when CO2 rises)
• Excitatory neurons regulate intrinsic rhythmicity and depth of respiration

47. INSPIRATORY CENTRE
• Inhibitory neurons: Factors which activate these neurons reflexly are:
1. Stretch receptors (J receptors) of lung alveoli
2. Afferents from muscle spindles of intercostal and abdominal muscles
during inspiration
3. Pneumotaxic center

49. EXPIRATORY CENTRE
• Composed of Ventral respiratory nuclei in association with Nucleus
Ambiguus of the medulla
• Expiratory center appears to function only in forced expiration
• During quite breathing, the expiratory neurons remain practically
inactive, as normal expiration takes place passively by elastic recoil of
lung.

50. PNEUMOTAXIC CENTRE
• Located on the medial side of superior cerebellar peduncle, hence called
as Parabrachial nuclei
• Influences the rate of breathing
• Inhibits the inspiratory centre and shortens the respiratory cycle

51. FUNCTIONAL DIVISIONS OF RETICULAR SYSTEM
• Functionally, the reticular formation is divided into two systems:
• Ascending reticular activating system (ARAS)
• Descending reticular system (DRS)

52. ASCENDING RETICULAR ACTIVATING SYSTEM (ARAS)
• (Most of the ascending tracts) Spinothalamic tract, Trigeminal
lemniscus, Lateral lemniscus and Central vestibular pathway (while
passing through the brainstem) give collaterals to Lateral part of
reticular system which projects to Intralaminar and Reticular nuclei of
Thalamus which projects to Cerebral cortex
• This pathway is responsible for maintaining state of alertness and
consciousness.

53. DESCENDING RETICULAR ACTIVATING SYSTEM (DRS)
• This consists of descending pathways from reticular formation to the
autonomic centers in the brainstem and to the lateral and anterior
horn cells in spinal cord.

54. FUNCTIONS OF RETICULAR FORMATION
1. Maintains the normal state of consciousness/wakefulness through
the connection with cerebral cortex by way of ‘Ascending Reticular
Activating System’
2. Regulates respiration, heart rate, blood pressure and other
vegetative functions through autonomic reflex centers present
within it in brainstem
3. Controls receptivity of sensory end organs

55. FUNCTIONS OF RETICULAR FORMATION
4. Controls muscular activity, directly through reticulo-spinal
projections to lower motor neurons and indirectly by influencing
the activities of cerebellum, red nucleus, substantia nigra, corpus
striatum and cerebral cortex.
5. Controls threshold of central sensory pathways
6. Regulates endocrine, visceral and emotional functions through its
connections with hypothalamus and limbic system

56. CLINICAL CORRELATION
1. Electrical stimulation of either Dorsal Raphe Nucleus or Nucleus
Raphe Magnus results in the loss of ability to experience pain from
sites of injury or disease
2. The visual and auditory stimuli can stimulate the RAS to maintain
alertness and attention like sudden bright light, sound of alarm
clock.
3. Functions of RAS can be affected by drugs. Anesthetics and
tranquillizers suppress it whereas ammonia and irritants stimulate it

57. CLINICAL CORRELATION
4. A coma is a state of unconsciousness due to inactivity of RAS. Even
most powerful external stimuli cannot cause arousal.
5. Descending fibers from reticular formation constitute one of the
most important motor pathways
6. The fibers from reticular formation to autonomic centers in
brainstem are critical in controlling respiratory and cardiac rhythms
and other vital functions

58. THANK YOU