Reticular formation

RETICULAR FORMATION

1. Reticular Formation DR.NILESH KATE. M.D. ASSOCIATE PROFESSOR, DEPARTMENT OF PHYSIOLOGY, ESIC MEDICAL COLLEGE, GULBARGA.

2. 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.

3. Introduction Definition:- Reticular formation is a diffused ill defined mass of nerve cells and fibres forming an ill defined meshwork of reticulum in the central portion of the brain stem.

4. 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.

5. 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.

6. NEURONAL AGGREGATES. RETICULAR NUCLEI. The entire reticular formation is broadly arranged into three columns: Median Medial and Lateral columns

7. 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

8. 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

9. 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

10. Functional neuronal aggregates. Cardiac centers Respiratory centers. Vasomotor centers. Salivatory centers Chemoreceptors neurons.

11. Reticular pathways Connections Afferent connections. Efferent connections. Descending projections. Ascending projections .

12. 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

13. 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.

14. 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 .

15. 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.

16. Functional Components of Reticular Formation On the basis of functions, Two systems namely: 1. Ascending reticular activating system 2. Descending reticular activating system.

17. 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.

18. Descending Reticular System Functionally, descending reticular system is divided into two subdivisions namely i ) Descending inhibitory reticular projection ii) Descending facilitatory reticular projection

19. 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.

20. 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.

21. Role in Sleep and wake fullness A section above the superior colliculi separating the entire brainstem. Electrical activity of the cortex showed, synchronized pattern indicating that the animal is in sleep. C) Extensive lesion of the ARAS produces Sleep

22. Role in Muscle Tone Mainly the Descending reticular 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

23. Role in Muscle Tone : These two divisions act through γ motor neurons and there by modulate the muscle tone. FRF increases the muscle tone of antigravity muscles. IRF decreases. Normally there is a balance between the activities of the FRF and the IRF.

24. 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.

25. 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.

26. Circadian rhythm Reticular activating system influences sleep and wakefulness Thereby Regulates Circadian rhythm

27. 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.

28. Arousal From the Cortex The RAS can also be activated from the cortex, the most effective 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.

29. 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 .

30. 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.

31. APPLIED PHYSIOLOGY. Drugs Excite Reticular Formation: Alerting and arousal :-Sympathomimetic drugs (eg., adrenaline, nor adrenaline, amphetamine). Acetylcholine also increases cortical activity. Increased CO 2 , rise of BP also increase the excitability of the RAS. Drugs Inhibits Reticular Formation: General anesthetics, sedatives eg., barbiturates diminish RAS activity

32. 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.

33. Thank you