Download - Thalamocortical organization
THALAMOCORTICAL ORGANIZATION
VIJAY KUMAR
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.
Position and relative size of thalamus in different mammals
Exploring the Thalamus ,S. Murray ShermanR. W Guillery
Dorsal thalamus
Thalamic nuclei & connections
NETTER’S CONCISE NEUROANATOMY
NETTER’S CONCISE NEUROANATOMY
NETTER’S CONCISE NEUROANATOMY
Blumenfeld, 2002
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
• Afferents: thalamocortical & corticothalamic axons , tegmental , basal forebrain , brainstem reticular neurons.
• Efferents:Mainly GABAergic inhibitory neurons to dorsal thlamic nuclei.
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.
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
• 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
Layered distribution of cortical cells
Principles of neuroscience 4th ed.Kandel et al., 2004
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.
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.
Cortical efferents
Principles of neuroscience 4th ed.Kandel et al., 2004
Representation of thalamo-cortical circuitry
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.
Recruiting responses recorded from cerebral cortex of cat
Medical physiology – Vernon Mountcastle 14th edition,page no.301
Waxing and waning of recruiting responses during train of stimulus
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
Burst mode Transmission mode
Kandel et al., 2004
During the burst mode, the neuronal response to such an input consists of brief bursts of action potentials separated by silent periods.
During the transmission mode of firing, the neuronal response to a depolarizing input generates stream of action potentials ofcertain frequency and duration corresponding linearly tostimulus strength and duration.
Thalamic relay neurons:
• Transmission mode
• Transition mode
• Burst mode
Neuroscience 3rd ed. Dale purves
Thalamocortical feedback loop
Ascending reticular activating system
Kandel et al., 2004
Kandel et al., 2004
Thalamic relay during sleep & wakefullness
Neuroscience 3rd ed. Dale purves
EEG recorded from scalp electrodes
• 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.
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.
Thank you