brief overview about thalamocortical organization and function

1. THALAMOCORTICAL
ORGANIZATION
VIJAY KUMAR

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. Position and relative size of thalamus
in different mammals
Exploring the Thalamus ,S. Murray Sherman
R. W Guillery

4. Dorsal thalamus

5. Thalamic nuclei & connections
NETTER’S CONCISE NEUROANATOMY

6. NETTER’S CONCISE NEUROANATOMY

7. NETTER’S CONCISE NEUROANATOMY

8. Blumenfeld, 2002

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. • Afferents: thalamocortical & corticothalamic
axons , tegmental , basal forebrain , brainstem
reticular neurons.
• Efferents:Mainly GABAergic inhibitory neurons
to dorsal thlamic nuclei.

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. 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. • 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. Layered distribution of cortical cells
Principles of neuroscience 4th ed.Kandel et al., 2004

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. 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. Cortical efferents
Principles of neuroscience 4th ed.Kandel et al., 2004

18. Representation of thalamo-cortical
circuitry

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. 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. 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. 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. Thalamic relay neurons:
• Transmission mode
• Transition mode
• Burst mode

24. Thalamocortical feedback loop
Neuroscience 3rd ed. Dale purves

25. Ascending reticular activating system
Kandel et al., 2004
Kandel et al., 2004

26. Thalamic relay during sleep &
wakefullness
Neuroscience 3rd ed. Dale purves

27. EEG recorded from scalp electrodes

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. 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. Thank you