this lecture looked at some of the basic ideas involved in the development, structure, and function of the somatosensory cortex. roughly speaking, the somatosensory cortex is a portion of the cerebral cortex that processes sensory information from the body-- touch, pain, temperature, proprioception. the cerebral cortex is the outermost layer of the cerebral hemispheres and is formed from the migration of neural plate cells from the medial and lateral ganglionic eminences to the edges of the cortex via glial cells. the migrating cells self organize into 6 semi-distinct layers which contain each contain different cell types which are specialized for different functions; for example, the stellate cells in layer IV receive information from the thalamus, whereas the pyrimidal cells in layer V project information to the other regions of the CNS. in layer V, there are three types of cells: association cells, which project to the ipsilateral cortex (via the longitudinal, cingulate, uncinate fasciculi) and are involved in integration of information, commisural cells which project to the contralateral cortex, and subcortical cells which project to the basal ganglia, thalamus, brain stem, and spinal cord via the internal capsule.
A few other notes about the structural organization of the cortex: cortical columns are sections of the cortex parallel to the surface which are composed of groupings of neurons of similar modality and receptive field; the phenomenon of somatotopy is the mapping out of the body layout on the surface of the brain. somatotopy is also exhibited in the spinal cord and thalamus, where contiguous portions of these CNS structures represent contiguous portions of the body. however, the mapping of the body on the cortex or thalamus is not necessarily representative of the physical shape of the body; this is demonstrated the unusual proportions of the homunculus, which is the physical representation of the body based on the density of receptor sites in the different areas of the cortex that represent different body parts-- for example, because of the high number of receptor sites in the somatosensory cortex for the lips and hands, these parts are unusually large on the homunculus.
MRI is a tool used to detect brain activity and is useful in pinpointing the brain locations which are specialized to process sensory data from specific body parts. the mechanism works by detecting hydrogen ions: a primary vertical magnetic field aligns the hydrogen ions, and a horizontal pulsed magnetic field causes a vibration in the hydrogen ions, the decay from which can be measured by the fMRI machine. the blood flow to the brain can be measured due to the paramagnetic properties of oxygenated hemoglobin and its effect on the hydrogen ions whose energy is measured.
the primary somatosensory cortex is located in the postcentral gyrus, and primarily receives information from the thalamus, which has nuclei that contain groupings of functionally distinct neurons. specific sensory information is processed in the VPL, VPM, medial and lateral geniculate nuclei, and sent to different areas of the somatosensory cortex based on the particular modality and receptive field of information that they encode. cortical relay nuclei include the VA and VL nuclei, which are involved in the communication between the cerebrum, basal ganglia, and the cerebellum. association nuclei (also called pulvinar nuclei) are involved in integrating information from wide areas of the cortex, and there are also non-specific nuclei which are involved in the reticular system of activation and alertness.
the secondary somatosensory cortex (SII) receives information from the primary and as such is involved in higher somatosensory processing; such as processing of bilateral information or forming 3D representations of single objects from multiple sensory sources. the activity of SII is also interrelated with emotional / motivational state, perhaps because of its relationship with the hippocampus and amygdala.
the last part of the lecture dealt with the dynamic aspects of somatosensory processing. it showed an experiment which mapped out neural activity on the somatosensory cortex of a mouse whisker over time-- showing that stimulation of one whisker caused the receptive field to radiate outward as time passed. another graph showed how the same stimulation can elicit different dynamic responses in different brain areas- some brain areas such as the thalamus and cortex undergoing relatively long periods of oscillation between excitatory and inhibitory states whereas other areas such as the sensory trigeminal nuclei produced a much simpler and shorter response. finally, the concept of neural plasticity was introduced in reference to the ability of the somatotopic map to adapt to experience and learning; areas that correspond to body parts that are more frequently used can expand and take over neighboring areas that are not being used.
questions
development...
1. what part of the neural tube is the cerebral cortex derived from?
2. describe the migration of cells in the formation of the cerebral cortex.
3. describe the formation of distinct layers in the cerebral cortex.
4. which layer are stellate cells in and where do they receive sensory input from?
5. what are the cells in layer V that project information to other CNS regions?
6. describe the role of interneurons in the cerebral cortex.
brodmann's areas...
7. what are brodmann's areas?
8. what are the three cell categories in layer V that project to other CNS regions?
9. describe the association cells in layer V.
10. describe the commisural cells in layer V.
11. describe the subcortical cells in layer V.
neurotransmitters and cortical columns...
12. what are some of the neurotransmitters that cortical cells use?
13. what are the main excitatory and inhibitory neurotransmitters?
14. which neurotransmitters does the reticular formation use?
15. what is a cortical column?
16. multiple columns form...
fMRI...
17. brain imaging is monitored by...
18. MRI is often set to detect...
19. describe the mechanism of detection of an MRI.
20. in an MRI image, the amount of pixels are proportional to...
21. what does fMRI use to measure neurally related blood flow?
22. what is the mechanism for detection of blood oxygen level?
thalamus and primary somatosensory cortex...
23. postcentral gyrus receives...
24. what is the thalamus?
25. what are the different types of nuclei present in the thalamus?
26. what are some examples of sensory nuclei and what do they do?
27. what are some examples of cortical relay nuclei and what do they do?
28. what do association nuclei do? what is another name for them?
29. what do non-specific nuclei of the thalamus do?
30. describe the particular types of somatosensory information encoded in brodmann area 3.
31. sensory input from thalamus to area 3b of SI is processed and...
32. areas 3a and 3b converge onto areas 1 and 2, where neurons...
secondary somatosensory cortex...
33. what type of information does the secondary somatosensory cortex receive?
34. lesion of somatosensory cortex causes...
35. SII response depends on...
36. each level of the somatosensory system generates...
37. describe the movement of the receptive field when the somatosensory cortex of a rodent is stimulated by movement of single whiskers.
integration, dynamic aspects of somatosensory cortex...
38. what is a "homunculus"?
39. what are some areas of high and low densities of sensory receptors?
40. what is "somatotopy"? where else is it exhibited in the body?
41. what does neural plasticity have to do with somatotopy?
answers
1. the rostral part of the neural tube; telencephelon (also derivative of basal ganglia).
2. cells migrate from lateral and medial ganglionic eminences (which becomes the basal ganglia) into the cortical surface, along glial cells.
3. cells organize into 6 relatively distinct layers based upon meaningful spatiotemporal patterns that are projected to other brain structures.
4. layer IV, from the thalamus.
5. pyrimidal cells.
6. interneurons form intricate circuits that generate both excitatory and inhibitory activity.
7. histologically distinct regions of the cerebral cortex.
8. association, commisural, subcortical.
9. axons which project to the ipsilateral cortical regions via the longitudinal, uncinate, cingulate fasciculi. involved in integration of information.
10. axons which project to the contralateral cortical regions.
11. axons which project to the basal ganglia, thalamus, brainstem, and spinal cord via the internal capsule.
12. glutamate, GABA, CCK, VIP, neuropeptide Y, somatostatin, substance P, corticotropin releasing hormone.
13. glutamate is excitatory and GABA is inhibitory.
14. AcH, NE, dopamine, serotonin.
15. a grouping of neurons of the same modality and similar receptive field which form a column on the cerebral cortex perpendicular to the surface of the cortex.
16. an array that will map out different body areas or sensory modalities. (somatotopy)
17. functional magnetic resonance imaging.
18. presence of hydrogen ions.
19. a vertical magnetic field aligns the hydrogen ions, and a briefly applied horizontal magnetic field causes the ions to vibrate. the energy released from the recovery from this vibration is measured in the MRI.
20. the amount of hydrogen ions.
21. BOLD- blood oxygen level detection.
22. oxygenated hemoglobin has more paramagnetic than deoxygenated hemoglobin and thus will affect the hydrogen ions measured by the fMRI differently.
23. touch, proprioceptive, pain, and temperature input via both lemniscal and anterolateral system via the VPL of the thalamus.
24. a pair of oval shaped structures in the diencephelon that contain clusters of nuclei which project axons to the cerebral cortex.
25. specific sensory nuclei, cortical relay nuclei, association nuclei, and non-specific nuclei.
26. VPL, VPM, lateral and medial geniculate nuclei- convey information to different cortical areas based on particular modality of information.
27. VA, VL- they establish the connections between the cortex, basal ganglia, cerebellum.
28. integrate sensory information in wide areas of the cortex. also called pulvinar nuclei.
29. convey activity from reticular formation to wide areas of cortex for attention and wakefulness.
30. brodmann area 3a is proprioception and area 3b is SA and RA cutaneous receptors.
31. projected to adjacent areas of primary somatosensory cortex.
32. respond to more abstract somatosensory aspects such as orientation, motion, spatial arrangement.
33. projections from SI representing multiple sources of contact of a single object, or bilateral input from body.
34. loss of fine localization, texture, pressure.
35. behavioral context, or motivational state, because of their connection with the hippocampus and amygdala.
36. a particular spatiotemporal pattern of neural activity across large populations of neurons.
37. the receptive field starts at a particular point in the somatosensory cortex and radiates outwards over time.
38. the sensory representation of the body based on its representation within the cortex.
39. hands and mouth are high, back is low.
40. the mapping out of body areas on regions of cortical surface. also occurs on the spinal cord and thalamus.
41. neural plasticity is manifested in the dynamic adaptation of somatotopic maps according to different patterns of body movement.
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