RETICULAR
FORMATION
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.
RETICULAR FORMATION
NUCLEI OF RETICULAR
FORMATION
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
AFFERENT CONNECTIONS
EFFERENT CONNECTIONS
DESCENDING PROJECTIONS
ASCENDING PROJECTIONS
RETICULAR PATHWAYS
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
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
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
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
CORTICO-RETICULAR-SPINAL PATHWAYS
CEREBELLORETICULAR CONNECTIONS
ASCENDING RETICULAR ACTIVATING SYSTEM
CONNECTIONS OF RETICULAR FORMATION
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
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.
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.
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.
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.
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.
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
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).
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
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)
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.
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.
Major afferents of reticular
formation
Major efferents of reticular
formation
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
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
The reticular formation influences conduction through somatosensory pathways.
Similar effects may also be exerted on visual and auditory pathways.
SOMATOSENSORY
CONTROL
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
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
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;-
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
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
Maintains the muscle tone;
Facilitates autonomic functions;
Activates ARAS.
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.
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.
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.
