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