Cortex

Question 0

Allocortex

Answer 0

Primitive functions, in medial temporal lobes

Paleocortex = piriform lobe -> olfaction
Archicortex inc hippocampus (3-layered) -> memory, spatial

vs “isocortex”/”neocortex” - 6-layered, all other areas

Question 1

Global organization of cortex

Answer 1

2-3 mm thick
Gyri and sulci -> surface area

Primary cortices = motor, sensory (easily defined)
Association cortices - rostral frontal, post parietal, ant occipital, temporal lobes

Brodmann’s areas - cytoarchitecture via staining
- correlates with functional areas (ex area 17 = primary visual)

Question 2

Cortical layers

Answer 2

Superficial to deep
Different cell types,

Supragranular -> connectivity (commissural/callosal and associational)
Layer 1 = molecular: synapses, few GABA/inhibitory neurons
Layer 2 = external granular
Layer 3 = external pyramidal
Internal granular = thalamocortical input -> spiny stellate cells
Layer 4 - most developed in primary sensory, absent in motor (“agranular”)
Infragranular - projections to sub-cortical regions (ex motor)
Layer 5 = internal pyramidal
-> basal ganglia, brain stem, spinal cord, cortex (commissural and associational)
Layer 6 = multiform/fusiform -> thalamus

Question 3

Spiny stellate cells

Answer 3

Fast excitatory (glutamate)
Small dendrites with spines (synapses)
Prominent in layer 4
- integrate input from cortex (most input!) and thalamus
- project vertically to layer 2/3 (most) and 5, 6 (less)

Question 4

Pyramidal cells

Answer 4

Fast excitatory (glutamate)
Prominent in layers 2/3, 5, 6
- apical dendrite -> tuft in layer 1 (synapses)
- oblique (from apical), basal (from soma) dendrites - spiny!
- pyramidal soma
- thin axon -> collaterals (nearby), long process (-> cortex, subcortical)
Integrate info from multiple layers -> output
- specific projection/connectivity patterns
- ex layer 6 - dendrites from 4 -> output to thalamus

Question 5

Non-pyramidal neurons

Answer 5

Inhibitory/GABAnergic
- balances out highly excitatory and interconnected cortex
- GABAa (enhanced by benzos) or GABAb
Many cell types - 20% of total, all cell layers
- feed-forward - co-activated -> inhibit pathway downstream
(ex: thalamus -> neuron -> inhibits pyramidal)
- feed-back - activated -> inhibits source of activation
(ex: thalamus -> neuron -> reciprocal inhibition of thalamus)

Question 6

Cortical information flow

Answer 6

Layers:
- thalamic input to Layer 4 (spiny stellate)
-> Layer 2/3 (pyramidal) -> interconnections
-> Layer 5/6 (pyramidal) -> output to subcortical
- inhibition (feed-forward and feedback throughout)
Generalized: input -> primary sensory -> secondary sensory -> association -> secondary motor -> primary motor -> output
(this is simplified, everything is multidirectional)

Question 7

Excitotoxicity

Answer 7

Constant threat due to highly interconnect, excitatory cortex
- vs inhibitory patterns are sufficient for simple motor movements
- too much excitation kills neurons
Triggers: trauma, hypoxia, ischemia, hypoglycemia, genetic

Regulation: 20% of cortical neurons are inhibitory
Tx:
 - block excitation/glutamate
 - enhance GABA (benzos)
 - limit APs (block Na channels)

Question 8

Coincidence detection

Answer 8

Pyramidal neurons = major excitatory output
Input from two sources
- apical dendrite - layer 1 - higher cortical and non-specific thalamic = “top-down” expectations
- peri-somatic (oblique, basal) - specific thalamic input (via Layer 4) = “bottom-up” perception
Coincidence = sensory perception meets expectation
- simultaneous dendritic input -> electrical activity -> output

Question 9

Somatosensory cortices

Answer 9

Primary = post-central gyrus = areas 1,2,3
- input from VPM, VPL of thalamus -> granular layer
- somatotopy - homonculus in four vertical submaps (3a,3b,1,2)
- remapping due to increased use, amputation
- damage -> decreased thresholds, deficits
Secondary - inferior parietal, area 40
- input from bilateral S1, non-specific thalamic
- bilateral, less precision
Association - superior parietal, areas 5,7
- input from primary and secondary
- complex associations

Question 10

Visual cortex

Answer 10

Primary = V1, area 17 = striate = calcarine
- granular, ocular dominance columns (adjacent = homonomous)
- macula/fovea -> posterior, upper visual field -> lower cortex
Association areas - V2, V3, areas 18, 19
- input from primary cortex, thalamus (lateral geniculate)
- color - V4
- movement - V5/middle temporal/MT

Question 11

Auditory cortices

Answer 11

Primary - superior/transverse temporal gyri - A1, area 41
- tonotopic map
- completely bilateral (no focal deficits with damage)
Association - A2, area 42 - interpretation of sound
- Wernicke’s = area 22 - understanding language
(dominant hemisphere only, non-dominant -> tone of voice)

Question 12

Perceptive areas

Answer 12

Vision - occipital
Auditory - temporal
Somatic - parietal/post-central gyrus

Taste - inferior post-central gyrus
Vestibular - superior temporal, inferior parietal

Question 13

Motor cortices

Answer 13

Primary - M1, area 4 - precentral gyrus
- input: VL of thalamus, sensory, premotor
- output: pyramidal/Betts/”upper motor” neurons in layer 5->
-> corticospinal, cortical bulbar
- somatotopy - movements vs specific body parts
- lesion -> spastic contralateral weakness
Premotor - area 6 (anterior to primary)
- input - sensory association, basal ganglia (via VA, VL of thalamus)
- output - primary motor cortex, some to spinal cord, brain stem
- more complex movements
- lesion -> less weakness, more spasticity
Supplementary = M2, superiormedial area 6
- initiation of movement (active prior)
- lesions -> abulia