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RETICULAR FORMATION
DR.NILESH KATE.
M.D.
ASSOCIATE PROFESSOR,
DEPARTMENT OF PHYSIOLOGY,
ESIC MEDICAL COLLEGE, GULBARGA.
OBJECTIVES
Introduction.
Location of Reticular Formation.
Nuclei of Reticular Formation.(neuronal aggregates)
Afferents and Efferent of Reticular Formation.
Functions of Reticular formation.
Ascending Reticular activating system.
Descending Reticular activating system
Role in sleep and wake fullness.
Role in EEG and learning.
Applied physiology.
INTRODUCTION
PHYSIOLOGICAL ANATOMY.
Formed by neurons & processes left over after well defined named nuclei & pathways.
Phylogenetically :- old system.
Poorly defined
Located in the brain stem.
Comprises of medullary, pontine,& midbrain RF.
Poly synaptic
Has both ascending and descending components.
EXTENT OF RETICULAR FORMATION:
The reticular
formation is situated
in brain stem, and
extends downwards
into spinal cord and
upwards up to
thalamus and sub
thalamus.
NEURONAL AGGREGATES.
RETICULAR NUCLEI.
The entire reticular formation is broadly arranged into three columns:
Median
Medial and
Lateral columns
Position:- Lie next to middle line.
Nuclei:- Collectively called nuclei of Raphe.
Extension:- From the medulla up to the midbrain in
vertical sheets bilaterally
Present in the Para median zone.
Serotonergic neurons.
THE MEDIAN COLUMN
Position:- lateral to the nuclei of median column.
Nuclei:- Collectively made up of large cells mainly
1. Giganto cellular nucleus.
2. Pontine tegmental reticular nucleus.
3. Cunei form and sub cuneiform nuclei.
THE MEDIAL COLUMN
Position:- Lateral to the nuclei of medial
column.
Nuclei:- Collectively made up of small cells
mainly
Parvocellular cells:- Control mainly the
visceral functions.
Extension:- From medulla to mid brain
THE LATERAL COLUMN
FUNCTIONAL NEURONAL AGGREGATES.
Cardiac centers
Respiratory centers.
Vasomotor centers.
Salivatory centers
Chemoreceptors neurons.
RETICULAR PATHWAYS
Connections
Afferent connections.
Efferent connections.
Descending projections.
Ascending projections.
AFFERENTS TO RETICULAR FORMATION
1. Spinal cord via the spino reticular tract and via collaterals from all ascending tracts.
2. Brain stem afferents from the cranial nerves . (Including Vestibular)
3. Tectoreticular :- Tectum (Superior and inferior colliculi) conveying visual and auditory impulses
AFFERENTS TO RETICULAR FORMATION
4 Cerebellum (cerebello reticular)
5. Basal ganglia directly and
indirectly
6. Neocortex – Cortico reticular
fibres from the motor, sensory
cortex, orbital, prefrontal, parietal
and temporal lobes, cingulate
gyrus and collaterals from the
corticofugal fibres.
7. Limbic lobe including the
amygdaloid, hippocampus.
EFFERENTS FROM RETICULAR FORMATION
Efferent connections are:
1. To the spinal cord.
The descending reticulospinal
tracts (medial inhibitory and lateral
facilitatory) connect with the
anterior horn cells either directly or
through internuncial neurons.
They also connect to the lateral
horn cells which are the cells of
origin of sympathetic nervous
system.
2. To brain stem.
The reticulo bulbar fibres connect
to the cranial nerve motor nuclei.
EFFERENTS FROM RETICULAR FORMATION
3. To the Cerebellum
4. To the red nucleus,
substantia nigra and
tectum in the midbrain
5. To the thalamus, sub
thalamic nuclei and
hypothalamus
6. To the corpus striatum,
Neocortex and limbic
lobe indirectly through
the thalamus and
hypothalamus.
FUNCTIONAL COMPONENTS OF RETICULAR FORMATION
On the basis of functions,
Two systems namely:
1. Ascending reticular
activating system
2. Descending reticular
activating system.
ASCENDING RETICULAR ACTIVATING SYSTEM
It projects into cerebral cortex in
two ways
1. Through Subthalamus and
2. Through Thalamus
Begins in lower part of brain
stem, extends upwards through
the Pons, midbrain, thalamus and
finally projects throughout the
cerebral cortex.
DESCENDING RETICULAR SYSTEM
Functionally, descending reticular system is divided into twosubdivisions namely
i) Descending inhibitory reticular projection
ii) Descending facilitatory reticular projection
FUNCTIONS OF RETICULAR FORMATION
Role in Sleep and Wakefulness cycle.
Responsible for the alerting responses to emotion and to
muscular work.
Controls muscle tone .
Influences Endocrine Secretion
Role in visceral function .
Influences circadian rhythm.
Influences EEG And LEARNING.
Modulates afferent transmission
Influences Autonomic nervous system.
ROLE IN SLEEP AND WAKEFULNESS
RAS :- Strong facilitatory drive to central neurons
Input through trigeminal
lemniscus and visuoauditory
tracts.
Experimental evidence
A section at the upper border
of the spinal cord separating
the entire brain - electrical
activity of the cortex showed a
desynchronized pattern,
indicating that the animal is
awake.
ROLE IN SLEEP AND WAKE FULLNESS
A section above the superior colliculi separating the entire brainstem. Electrical activity of the cortex showed, synchronizedpattern indicating that the animal is in sleep.
C) Extensive lesion of the ARAS produces Sleep
ROLE IN MUSCLE TONE
Mainly the Descendingreticular formation has a role in regulating the muscle tone and hence maintenance posture and equilibrium
Muscle tone is maintained by facilitatory and inhibitory reticular formation
ROLE IN MUSCLE TONE:
These two divisions act through γ motor neurons and there by modulate the muscle tone. FRFincreases the muscle tone of antigravity muscles. IRFdecreases.
Normally there is a balance between the activities of the FRF and the IRF.
ENDOCRINE CONTROL - THROUGH HYPOTHALAMUS
A) Stress :-
Stimulate the reticular formation, which in turn can active hypothalamus through the CNS.
So, an increased release of CRF, which while acting on the anterior pituitary releases ACTH. This increases cortisol secretion.
B) There is increased secretion of catecholamine and Gastric HCL secretion
C) stimulates TSH secretion through the hypothalamus.
D) Cause release of gonadotrophins.
ROLE IN VISCERAL FUNCTION Visceral function like gastric
secretion, GIT motility, heart rate,
BP, Respiration, Salivation,
Vomiting, etc., are influenced by
various centers located in the RF
of the medulla.
These are VMC, CIC, respiratory
centre, vomiting centre, salivary
nuclei, etc., the effects are
modified mainly through
autonomic nervous system.
CIRCADIAN RHYTHM
Reticular activating
system influences
sleep and
wakefulness
Thereby Regulates
Circadian rhythm
ROLE IN EEG AND LEARNING
Activation of the entire cerebral
cortex .
The EEG pattern obtained in this
state is desynchronized 18 –
30Hz.
The animal is wakeful, alert and
the learning is facilitated
Inactivation of the reticular
formation leads to
Synchronized EEG pattern.
Produce sleep hence animal
cannot learn.
AROUSAL FROM THE CORTEX
The RAS can also be activated from the cortex, the mosteffective parts being the
Orbital part of the frontal lobe, superior temporal gyrus and
The cingulate gyrus (and to some extent the sensory motor cortex).
This may be responsible for the alerting responses to emotion and to muscular work.
MODULATION OF SENSORY INPUT
Impulse modulation:- Impulses in the sensory receptor of their
transmission can be modulated by reticular formation.
It has been shown that stimulation of the bulbar reticular formation inhibits transmission at the first synapse of the ascending sensory tracts.
Selective attention
It is also well known that when one’s attention is intensely fixed on one object or task, other sensory impressions are less effective .
NEUROTRANSMITTERS OF RETICULAR FORMATION
Large cholinergic neuron.:- project to cortex via thalamus.
Small adrenergic neurons:- via Intralaminar nuclei of thalamus.
Noradrenergic neurons.:- To cerebellum.
Dopaminergic neurons :- To Basal ganglia
Serotonergic neurons:- Project to Thalamus, Cerebral cortex, Hypothalamus & Limbic structure.
APPLIED PHYSIOLOGY.
Drugs Excite Reticular Formation:
Alerting and arousal :-Sympathomimetic drugs (eg., adrenaline, nor adrenaline, amphetamine).
Acetylcholine also increases cortical activity.
Increased CO2, rise of BP also increase the excitability of the RAS.
Drugs Inhibits Reticular Formation:
General anesthetics, sedatives eg., barbiturates diminish RAS activity
AT THE END OF THE CLASS YOU CAN FIND REASONS FOR
Many prerequisites for sleep.
Selective attention.
Circadian rhythm.
Sleep and wakefulness cycle.
Action of different sedatives & hypnotics.
Ways of Arousal from sleep.
Thank you