Thalamocortical organization


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brief overview about thalamocortical organization and function

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  • the manner in which a thalamic cell passes messages on to cortex, is not constant but dependson the attentive state of the whole animal (waking or sleeping) and probablyon the local salience of a particular stimulus or group of stimuli aswell: are they new, threatening, interesting, or merely a continuation ofprior conditions?
  • Thalamocortical organization

    2. 2. Thalamus • It’s a paired subcortical structure. • Situated lateral to third ventricle. • During development 3 areas differentiate from thalamic plate ,i.e epithalamus,dorsal thalamus & ventral thalamus. • Neurons of dorsal thalamus send axons to cerebral cortex & form thlamocortical radiations.
    3. 3. Position and relative size of thalamus in different mammals Exploring the Thalamus ,S. Murray Sherman R. W Guillery
    4. 4. Dorsal thalamus
    5. 5. Thalamic nuclei & connections NETTER’S CONCISE NEUROANATOMY
    8. 8. Blumenfeld, 2002
    9. 9. Thalamic Reticular nucleus • It belongs to ventral thalamus. • It is a sheath of cells which surrounds the anterior & lateral aspects of dorsal thalamus intercalated b/w external medullary lamina and internal capsule. Reticular nucleus
    10. 10. • Afferents: thalamocortical & corticothalamic axons , tegmental , basal forebrain , brainstem reticular neurons. • Efferents:Mainly GABAergic inhibitory neurons to dorsal thlamic nuclei.
    11. 11. Cerebral Cortex • Convoluted,laminated sheet of neurons. • 2.5-4.0 mm thick, 600cm³ volume & many billion neurons. • Highest region of signal computation. • Most evolved in primates.
    12. 12. Cortical cell types • pyramidal cells: - Triangular in shape with base downward & apex directed toward cortical surface. - Basal & apical dendrites are seen - vary in size from 15 x 10μ to 120 x 90 μ Lubke et al J. Neurosci., July 15, 2000, 20(14):5300– 5311
    13. 13. • Stellate or granule cells: - small cell bodies & dendrites spread in all directions • Fusiform cells: - spindle shaped cell bodies. - dendries arise from both ends of spindle. Lubke et al J. Neurosci., July 15, 2000, 20(14):5300–5311
    14. 14. Layered distribution of cortical cells Principles of neuroscience 4th ed.Kandel et al., 2004
    15. 15. Cortical afferents • Specific thalamocortical projection fibers ascend myelinated in layer 5 & 6 ,divide profusely in layer 4 & lower part of layer 3. • Non specific thalamocortical afferents project to different cortical areas & gives off collaterals to all the layers & end in layer 1. • Association & commissural fibers give collaterals to layer 4 cells & terminate in layer 2 & 3.
    16. 16. Cortical efferents • Pyramidal cells of layer 2&3 gives commissural & ipsilateral corticocortical efferent fibers. • Pyramidal cells of layer 5 are origin of coricofugal projection fibers to basal ganglia,brain stem & spinal cord • Pyramidal & fusiform cells of layer 6 are origin of corticothalamic fibers.
    17. 17. Cortical efferents Principles of neuroscience 4th ed.Kandel et al., 2004
    18. 18. Representation of thalamo-cortical circuitry
    19. 19. Types of Thalamocortical systems • Specific thalamocortical system which includes specific thalamic relay nucleus for particular sensation & their axons projecting to particular cortical area. corticothalamic fibers project back to same relay nucleus. • Generalized thalamocortical system which includes one of the intralaminar nuclei getting its afferents from ARAS & other brainstem nuclei & axons project diffusely to entire cortical area.
    20. 20. Recruiting responses recorded from cerebral cortex of cat Waxing and waning of recruiting responses during train of stimulus Medical physiology – Vernon Mountcastle 14th edition,page no.301
    21. 21. Role of thalamocortical system in generation of EEG • Electrical activity measured from the surface of skull reflects the summated a of synaptic activity of synaptic potentials in the dendrites of cortical neurons. • Thalamic neurons have two physiologic states: -Transmission mode -Burst mode
    22. 22. Burst mode During the burst mode, the neuronal response to such an input consists of brief bursts of action potentials separated by silent periods. Transmission mode During the transmission mode of firing, the neuronal response to a depolarizing input generates stream of action potentials of certain frequency and duration corresponding linearly to stimulus strength and duration. Kandel et al., 2004
    23. 23. Thalamic relay neurons: • Transmission mode • Transition mode • Burst mode
    24. 24. Thalamocortical feedback loop Neuroscience 3rd ed. Dale purves
    25. 25. Ascending reticular activating system Kandel et al., 2004 Kandel et al., 2004
    26. 26. Thalamic relay during sleep & wakefullness Neuroscience 3rd ed. Dale purves
    27. 27. EEG recorded from scalp electrodes
    28. 28. • Thalamocortical dysrhythmia syndrome: set of neurons in the thalamus displays low rhythmicity in an awake brain state. Dysrhythmia is generated by membrane hyperpolarization due to T-type Ca2 channel deinactivation.
    29. 29. References • Medical physiology – Vernon Mountcastle 14th edition. • Neuroscience 3rd edition- Dale purves. • Principles of neuroscience 4th ed.- Eric kandel. • Exploring thalamus – Sherman & Guillery. • Functional Organization of Thalamocortical Relays –S.Murray sherman & R.W.Guillery, Journal of neurophysiology vol 76,1996.
    30. 30. Thank you