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