Getting to and from the cerebral cortex

2. Getting to and from the cerebral cortex

3. thalamus

4. THALAMUS • Oval, nuclear mass • Forms 80% 0f diencephalon • Anterior extent- interventricular foramen • Superiorly- transverse cerebral fissure, floor of 3rd ventricle • Inferiorly- hypothalamic sulcus • Posteriorly- overlaps midbrain

13. All sensory pathways relay in thalamus. • Many circuits used by cerebellum, basal nuclei and limbic system involve thalamus. • These utilize more or less separate portions of thalamus, which has been subdivided into a series of nuclei.

14. Thalamic nuclei • Nuclei can be distinguished from each other by topographical locations within thalamus and by input/output patterns. • Thalamus is divided into medial and lateral nuclear groups by a thin curved sheet of myelinated fibres called internal medullary lamina..

15. It splits anteriorly to enclose a group of nuclei, collectively called anterior nucleus, which is close to interventricular foramen • Medial group contains one large nucleus called dosomedial nucleus • Lateral group is subdivided into a dorsal and ventral tier

16. All thalamic nuclei are a mixture of projection neurons, whose axons provide the output of thalamus, and small inhibitory interneurons that use GABA as a neurotransmitter • Projection neurons account for 75% or more of the neurons of the most thalamic nuclei, though the relative proportions of projection neurons and interneurons vary in different nuclei

19. Dorsal tier consists of lateral dorsal, lateral posterior nuclei and pulvinar. • Lateral posterior nucleus and pulvinar have almost similar connections

20. Nuclei of ventral tier • Ventral anterior, ventral lateral- concerned with motor control; are connected to basal nuclei and cerebellum • Ventral posterior is subdivided into ventral posterolateral[ smatosensory input from body] and ventral posteromedial [somatosensory input from head]

21. Lateral and medial geniculate nuclei / bodies are considered as posterior extensions of ventral tier Intralaminarnuclei • Embedded in internal medullary lamina • Largest of this group are centromedian and parafascicular nuclei

23. C.S. anterior thalamus

24. C.S. midthalamus

25. C.S. posterior thalamus

26. Reticular nucleus • Lies between lateral thalamic surface and external medullary lamina • Reticular nucleus is developmentally not a part of thalamus. • It has distinct anatomical and physiological properties. • Considered a part of thalamus because of location and extensive involvement in thalamic function.

27. Midline nuclei • Rostral continuation of periaqueductal gray matter • Form interthalamic adhesion [when present]

28. Role of thalamic nuclei • Pipelines for flow of information to cerebral cortex • Site where decisions are implemented about which information should reach cerebral cortex for processing • Any particular type of information affected by any thalamic nucleus is a function of its input and output connections

29. Inputs • Specific - Regulatory • Specific inputs convey information that a given nucleus may pass to cerebral cortex [and for some nuclei to additional sites]. • Examples; Medial lemniscus specifically to VPL. Optic tract to LGB

30. Regulatory inputs contribute to decisions about whether or in what form information leaves a thalamic nucleus

32. Sources • cortical area to which the nucleus projects • thalamic reticular nucleus • diffuse cholinergic, noradrenergic, serotonergic endings from brainstem reticular formation

33. Categories of nuclei depending on pattern of inputs Relay nuclei • receive well defined specific input fibres and project to specific functional areas of cerebral cortex • deliver information from specific functional systems to appropriate cortical areas Intralaminar and midline nuclei seem to have special role in function of basal nuclei and limbic system

34. Association nuclei • project to association areas of cerebral cortex • receive major inputs from cerebral cortex and subcortical structures • probably important in distribution and gating of information between cortical areas

35. SCHEME OF THALAMIC ORGANIZATION • Every nucleus of the thalamus except the reticular nucleus sends axons to the cerebral cortex, either to a sharply defined area or diffusely to a large area. • Every part of the cortex receives afferent fibers from the thalamus, probably from at least two nuclei.

36. Every thalamocortical projection is faithfully copied by a reciprocal corticothalamic connection. • Thalamic nuclei receive other afferent fibers from subcortical regions. • Probably only one noncortical structure, the striatum , receives afferent fibers from the thalamus. • .

37. The thalamocortical and corticothalamic axons give collateral branches to neurons in the reticular nucleus, whose neurons project to and inhibit the other nuclei of the thalamus • No connections exist between the various nuclei of the main mass of the thalamus, although each individual nucleus contains interneurons

38. The synapses of the interneurons are inhibitory, and most are dendrodendritic. • Other synapses in the thalamus are excitatory, with glutamate as the transmitter, and so are thalamocortical projections

44. Connections and functions of thalamic nuclei

45. RETICULAR NUCLEUS

46. Intralaminar nuclei

47. Ventral group of nuclei

48. Medial geniculate body

49. Lateral geniculate body

50. Ventral posterolateral