RETICULAR FORMATION THE RETICULAR FORMATION - PowerPoint Presentation

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RETICULAR FORMATION

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THE RETICULAR FORMATION

RF is formed of large number of neurons

present through the entire brainstem, it

extends upward to the level of the thalamus

and downward to be continuous with the

interneurons

of the spinal cord.

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RETICULAR FORMATION

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NUCLEI OF RETICULAR FORMATION

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Numerous connections.

It is connected to almost all parts of the nervous system directly or indirectly.

The pathways involved are both:---

Ascending and descending

Crossed and uncrossed

Somatic and visceral.

It is NOT merely a relay station of these pathways.

It has an important REGULATORY role, both

facilitatory

and inhibitory.

CONNECTIONS OF RETICULAR FORMATION

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AFFERENT CONNECTIONS

EFFERENT CONNECTIONS

DESCENDING PROJECTIONS

ASCENDING PROJECTIONS

RETICULAR PATHWAYS

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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

(SUPERIOR AND INFERIOR COLLICULI)

CONVEYING VISUAL AND AUDITORY IMPULSES

AFFERENTS TO RETICULAR FORMATION

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4. CEREBELLUM

cerebelloreticular

5.BASAL ganglia

directly and indirectly

6.NEOCORTEX

corticoreticular

fibres

from the motor, sensory cortex, orbital, parietal and temporal lobes,

cingulate

gyrus and collaterals from the

corticofugal

fibres

.

7.LIMBIC SYSTEM

Including the

amygdaloid

, hippocampus

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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.

EFFERENTS FROM RETICULAR FORMATION

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2.To brain stem

The reticulobulbar

fibres

connect to the cranial nerve motor nuclei.

3.To the cerebellum.

4.To the red

nucleus,substantia

nigra

and

tectum

in the midbrain.5.To the thalamus, subthalamus

and hypothalamus.

6.To the corpus striatum,

neocortex

and limbic system indirectly through the thalamus and hypothalamus

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CORTICO-RETICULAR-SPINAL PATHWAYS

CEREBELLORETICULAR CONNECTIONS

ASCENDING RETICULAR ACTIVATING SYSTEM

CONNECTIONS OF RETICULAR FORMATION

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The Reticular formation receives impulses from the motor and other areas of the cerebral cortex and relays them to the spinal cord through the MEDIAL and LATERAL RETICULOSPINAL TRACTS.

The

cortico

-reticular

fibres

descend along with

cortico

-spinal

fibres

.

They terminate mainly in relation to the oral and caudal reticular nuclei of the

pons

and the giganto-cellular nucleus of the medulla.

Cortico

-

Reticulo

-Spinal pathways

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The Medial

Reticulo-spinal Tract originates from the oral and caudal pontine

reticular nuclei and the

gigantocellular

reticular nucleus of medulla.

Pontine

fibres

descend mainly

ipsilaterally

in the ventral

funiculus

of the cord.

Medullary fibres descend both ipsilaterally

and

contralaterally

in the ventral

funiculus

and the ventral part of the lateral

funiculus

.

These

fibres have many collaterals.

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Two-thirds of these

reticulospinal neurons that reach the cervical cord also descend to lumbosacral

levels.

These

fibres

terminate widely in spinal grey

mater,but

the exact lamina of termination is controversial.

Majority of the terminals of medial

reticulospinal

fibres

are distributed to laminae six yo eight.

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Alpha and Gamma motor neurons are influenced by

reticulospinal fibres through polysynaptic and monosynaptic connections.

Reticulo

-spinal

fibres

from

pontine

sources excite motor neurons of axial and limb muscles.

Medullary

fibres

excite , or inhibit motor neurons of cervical muscles and excite motor neurons of axial muscles.

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Functionally Medial

Reticulospinal tract is concerned with posture, the steering of head and trunk movements in response to external stimuli, and crude, stereotyped movements of the limbs.

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The Lateral

Reticulo-spinal Tract arises from the neurons of the ventrolateral

part of reticular formation of the

pons

(CAUDAL and ORAL

pontine

reticular nuclei).

The

fibres

cross to the opposite side of medulla oblongata and run in the lateral

funiculus

of the spinal cord.

Axons of this tract terminate in laminae one, five and six .

This pathway is involved in the control of pain perception and in motor functions.

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RETICULO-CEREBELLAR FIBRES

The reticular formation receives

fibres

from and sends

fibres

to cerebellum.

Impulses passing from the cerebellum to the reticular formation are relayed to the spinal cord and to cranial nerve nuclei through

reticulospinal

and

reticulonuclear

pathways; and to the thalamus through

reticulothalamic

fibres.

Connections between cerebellum and reticular formation

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The cerebellum receives

fibres mainly from three nuclei in the reticular formation.

1. Lateral reticular nucleus in the medulla

2.

Paramedian

reticular nucleus (lying in lower part of medulla in medial longitudinal fasciculus).

3.Nucleus

reticularis

tegmenti

pontis

.(NRTP)Paramedian reticular nucleus sends

fibres

to the entire

cerebellar

cortex.

The lateral and NRTP give collaterals to

cerebellar

nuclei(

fastigial

nucleus mainly).

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Cerebellar

nuclei project to the lateral reticular nucleus and the NRTP.Fibres

to the lateral reticular nucleus are mainly from

fastigial

nucleus.

Some of these

fibrtes

reach the reticular formation through the descending branch of the superior

cerebellar

peduncle.

Fibres

from the

fastigial nucleus also reach the tegmentum of middle brain (including the dorsal

tegmental

nucleus,the

central grey)the

raphe

nucleus and the locus

coeruleus

.

Fibres

from the

dentate,emboliform

and

globose

nuclei end in the medial reticular formation of the

pons

and medulla and in the NRTP (

mainly from emboliform

nucleus)

CEREBELLO-RETICULAR CONNECTIONS

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Various ascending tracts passing through the brainstem are intimately related to the reticular formation.

Many of the fibres

in these tracts give off collaterals to it.

These come from the

spinothalamic

tracts, from secondary trigeminal pathways and from auditory pathways.

These collaterals terminate predominantly in lateral reticular formation.

ASCENDING RETICULAR ACTIVATING SYSTEM (ARAS)

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Fibres

arising here project to the intralaminar and reticular nucleiof

the thalamus.

These nuclei in turn project to widespread areas of the cerebral cortex.

These pathways form part of the ascending reticular activating system which is believed to be responsible for maintaining a state of alertness.

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ASCENDING RETICULAR ACTIVATION 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.

Tumor or lession – sleeping sickness or coma.

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Major afferents of reticular formation

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Major

efferents

of reticular formation

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Control of somatic and visceral sensations

Control of ANS

Influence the biologic clock

The reticular activating system

Control of endocrine nervous system

Control of skeletal muscles

FUNCTIONS OF RETICULAR FORMATION

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SOMATO MOTOR CONTROL

Reticular formation has an influence on fine control of movements including those involved in postural adjustments, skilled use of the

hands,speech

etc. through its direct connections with the spinal cord and indirectly through the corpus striatum, the cerebral cortex and the cerebellum

FUNCTIONS OF RETICULAR FORMATION

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The reticular formation influences conduction through

somatosensory pathways.Similar effects may also be exerted on visual and auditory pathways.

SOMATOSENSORY CONTROL

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Stimulation of certain areas in the reticular formation of the medulla has great influence on respiratory and cardiovascular function.

The region influencing respiratory activity corresponds approximately to the

gigantocellular

nucleus and

parvocellular

nucleus.

Stimulation of the

gigantocellular

nucleus and the upper part of the ventral reticular nucleus causes depression of vasomotor activity while stimulation of other areas has a

pressor

effect. These effects are mediated through connections between the reticular formation and autonomic

centres

in the brainstem and spinal cord,but

the pathways concerned are not well defined.

VISCERAL CONTROL

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Reticular formation influences activity of the

adenohypophysis and of the neurohypophysis

through its connections with the hypothalamus.

It also influences the pineal body. pineal gland secretes the hormone melatonin which shows a marked circadian rhythm which appears to be strongly influenced by exposure of animal to light.

Activity is greater in darkness.

NEUROENDOCRINE CONTROL

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Reticular formation controls arousal and the state of consciousness through the ARAS.

ARAS is also known as extrathalamic

control

modulatory

system or simply reticular activating system (RAS).

RAS is a collection of different nuclei- more than 20 on each side in the upper

brainstem,the

pons

, medulla and posterior hypothalamus.

The most significant components of the ARAS include;-

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Serotonergic

nuclei-dorsal raphe nucleus and median

raphe

nucleus (RAPHE NUCLEI).

Dopaminergic

nuclei-ventral

tegmental

area

Noradrenergic nuclei-Locus

coruleus

Histaminergic

nuclei-

tuberomammillary nucleus

Cholinergic nuclei-

pontine

tegmental

nuclei

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DESCENDING RETICULAR ACTIVATION SYSTEM - DRAS

INHIBITORY

Smoothness and accuracy of voluntary movements;

Reflex movements;

Regulates muscle tone;

Maintenance of posture;

Control

of

vegetative functions.

FACILITATORY

Ma

i

ntains

the muscle tone;

Facilitates autonomic functions;

Activates ARAS.

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REGULATION OF SLEEP

, thus, the maintenance of the

SLEEPING cycle or CIRCADIAN

rhythm

;

Filtering of incoming stimuli to discriminate irrelevant background stimuli;

It’s crucial to maintain the state of

CONSCIOUSNESS

related to the circadian rhythm –

MELATONIN effects on RAS

;

ANS control

– respiratory rate, heart rate, GIT activity.

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DISORDERS ASSOCIATED WITH RETICULAR FORMATION

NARCOLEPSY

Associated with excessive sleepiness, sleep paralysis, hallucinations and in some cases episodes of cataplexy (loss of muscle control often triggered by strong emotion such as laughter.

SCHIZOPHRENIA

Mental disorder

charecterised

by abnormal social

behaviour

and failure to understand what is real.

There is

overactivity

of reticular formation.

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PARKINSONS DISEASE

It is degenerative disorder characterized by shaking rigidity, slowness of movement and difficulty with walking.

This is because of decrease in dopamine in the area of

substantia

nigra

of midbrain.