Somatosensory Pt. 2

Question 1

1. How many layers are there in neocortex?

Answer 1

Six cell layers

projected to other cortical areas, as well as to the limbic system, brain stem, spinal cord

Question 2

Which layers of the neocortex contain the major input receiving cell and the output cells?

Answer 2

Stellate cells in layer IV receive sensory input from the thalamus
Pyramidal cells in layer V project information to the other regions of the CNS

Question 3

What are those input and output cells of neocortex called?

Answer 3

input : STELLATE (IV)

output: PYRAMIDAL (V)

Question 4

2. What are Brodmann’s areas?

Answer 4

Area of cortex have characteristic cells that are histologically distinguishable

•  Each area has been numbered and is sued for anatomical identification of the functional areas

Question 5

3. What does fMRI measure? No paramagnetic details needed here!

Answer 5

fMRI enables visualization of BRAIN ACTIVITY in response to ongoing mental and physical activity
visualization depends on oxygen usage and blood/ Hb oxygenation levels

Question 6

4. Describe the general organization of the thalamus.

Answer 6

Pair of oval shaped clusters of nuceli in the diencephalon

•   Lie on each side of the third ventricle
•   Project axons to all areas of the cortex

Question 7

Which areas of the thalamus are associated with the somatosensory cortices?

Answer 7

Thalamohcortical connections
•  Thalamus contains several nuclei. Each nucleus projects to axons carrying either sensory, motor or integrated neural info to cerebral cortex

•  VPL/VPM projects to primry SS cortex

Question 8

5. Where and what is the primary somatosensory cortex?

Answer 8

Post central gyrus forms the primary SS cortex (SI)
•  Somatosensory info enters the primary SS cortex via the VPL
(body) and VPM(head) nucelei of the thalamus
• VPL recieves input from the dorsal column lemniscal and anterolateral pathways
•  VPM receives input from the head via the trigeminal n.
•  VPL and VPM are jsut ventro-posterior lateral or medial Primary and secondary SS cortices elaborate and contextualize sensory info into meaningful experiences

Question 9

What are cortical columns?

Answer 9

• Neurons with similar sensory receptive properties, i,e modality
receptive fields cluster into 300-600μm wide columns
perpendicular to surface
•Several columns form arrays that will map different body areas or sensory modalities

Question 10

6. What is somatotopy? The homunculus?

Answer 10

Homunculus: contiguous areas of the body represented within the cortex in proportion to the density of sensory receptors in that area of the body

High density areas(mouth and hand)
have large representations,low density areas(the back) have smaller representation

**Somatotopy occurs in many areas of the CNS

Question 11

7. What is meant by cortical plasticity?

Answer 11

Cortical sensory representation changes with experience and learning
•  Corticalcolumn neurons respond primarily to one specific area of body, but input from neighbor areas of the body surface also project to same columns , but without eliciting activity i.e.their connections are latent

Question 12

8. Give a general description of lateral inhibition. How does it impact incoming somatosensory information?

Answer 12

mechanism in the thalamus and dorsal column nuclei that enhances difference between SS receptive fields (RFs)
stimulus: DCN nuclei stimulated by sensory neurons from CENTER of RF
Inhibitory: interneurons within DCN supress activity
varied response
•  Central stimulation of RF–>positive response

•  Peripheral stimulation–> negative response

Question 13

9. How do the sensory and motor cortices affect lateral inhibition and information processing in the ascending somatosensory system?

Answer 13

Without lateral inhib. by interneurons, activity of the stimulus is disperesed among neighboring neurons and diffused (amplitude declines)
sensory and motor cortices descending pathways modulate afferent input from upper limb for tactileexploration

Question 14

10. What is two-point discrimination?

Answer 14

Ability to discriminate between separate but simultaneous pin pricks to skin

•  Common neuro test measures integrity of dorsal column lemniscal system

Question 15

What part of the somatosensory system is responsible for two-point discrimination?

Answer 15

dorsal column-lemniscal system

Question 16

11. What are the somatosensory “dorsal and ventral streams”?

Answer 16

Primary SS cortex projects posteriorly along dorsal and ventral streams to secondary and associated cortex
Ventral stream:
•  SI projects to secondary somatosensory (SII) cortex bilaterally
•  What is it?
Form processing: SII responds to global features of objects such as size and shape,i.e. tactile object recognition
Dorsal stream:
•  SI projects to posterior parietal cortex (PPC) areas 5,7 for multisensory integration, directing
attention and interacting with the motor cortex in frontal lobe
•  Where? and How? to position hand to grasp an object

Question 17

What happens to somatosensory information as it spreads from primary somatosensory into SII and association cortices along either the dorsal or ventral streams?

Answer 17

ventral: SII receives tactile and proprioceptive projections from SI–> internal 3D pic of body. SII orientation selectivity. SII sends info to nd from hippocampus and amygdala–>memories
dorsal, areas 5,7: information from primary and secondary somatosensory cortex either directly via arcuate fibers or indirectly via thalamus. generating a conscious body image. extends into speech comp.

Question 18

12. What aspects of somatosensory information are associated with the posterior parietal cortex?

Answer 18

body schema/ egocentric space
integrates touch, proprio,etc
some speech, wernickes area
convergence of visual,auditory,and somatosensory input

Question 19

13. What role does the posterior parietal cortex play in how vision and somatosensory information impacts somatosensory awareness?

Answer 19

EGO CENTRIC SPACE for body awareness , unconsciously updated during movement–> primarily for spatial organization of action

PERIPERSONALSPACE,vision dependent ,maps position of object in environmentin terms of own body
•  Extends into speech comp area (Wernicke’s area) eg. somatosensory of reading, writing

Question 20

13. What are the characteristics and cortical issues associated with phantom limb phenomena?

Answer 20

Somatosensation in SI and II leads to:
•  SOMATOPERCEPTION:what body is felt to be like
•  SOMATOREPRESENTATION:abtsract knowledge
and and attitudes about body
•  Phantom limb are due TO THEIR DISSOCIATION
representation and perception at odds
•  Caused by reorg. of SS cortex.

Question 21

14. What is astereognosis? neglect syndrome?

Answer 21

Astereognosis: inability to identify and object by touch

Neglect Syndrome:
•  Deficit in attenttion/awareness to one side of space

Question 22

What part of the brain is usually involved in neglect?

Answer 22

Most neglect due to LESION IN RIGHT INF. PARIETAL (esp.
angular gyrus)
This integrates SS info,

Question 23

What is the difference between illusion and delusion?

Answer 23

delusion is cray