RETICULAR FORMATION THE RETICULAR FORMATION RF is formed

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

CONNECTIONS 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.

RETICULAR PATHWAYS � AFFERENT CONNECTIONS � EFFERENT CONNECTIONS � DESCENDING � 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 (SUPERIOR AND INFERIOR COLLICULI) CONVEYING VISUAL AND AUDITORY IMPULSES

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

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 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

CONNECTIONS OF RETICULAR FORMATION � CORTICO-RETICULAR-SPINAL PATHWAYS � CEREBELLORETICULAR CONNECTIONS � ASCENDING RETICULAR ACTIVATING SYSTEM

Cortico-Reticulo-Spinal pathways � 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.

� 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.

Connections between cerebellum and reticular formation � 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.

� 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).

CEREBELLO-RETICULAR CONNECTIONS � 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)

ASCENDING RETICULAR ACTIVATING SYSTEM (ARAS) � 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.

� 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

FUNCTIONS 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

SOMATOSENSORY CONTROL � The reticular formation influences conduction through somatosensory pathways. � Similar effects may also be exerted on visual and auditory pathways.

VISCERAL 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.

NEUROENDOCRINE 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.

� 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 � Cholinergic nuclei-tuberomammillary nucleus 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, SLEEP thus, the maintenance of the SLEEPING cycle or CIRCADIAN rhythm; 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; 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.