Cortex

Question 1

Forebrain

Answer 1

• encompasses diencephalon and telencephalon
  o Diencephalon – thalamus, hypothalamus, subthalamus, epithalamus
  o Telencephalon encompasses – basal ganglia, hippocampus, amygdala, cerebral cortex

Question 2

Three Types of Cortex

Answer 2

o Neocortex (isocortex) – 6 layers; primary sensory and motor cortex; “association” cortex
o Allocortex (paleocortex) – 3-5 layers
 Entorhinal cortex (parahippampal gyrus), piriform cortex (uncus)
o Archicortex – 3 layers; hippocampal formation

Question 3

Neocortex

Answer 3

o Thickness: 2-4 mm thickness ; Area: 2,500 cm2
o Location: surrounds the cerebrum (like bark around a tree)
o Structure: laminar; 6 layers; columnar; distinct columns; different cerebral regions have distinct cytoarchitectonics and distinct connectivity
o Functions: large range including, sensory, motor, cognitive, social, & emotional; vary by region

Question 4

Laminar Microscopic Organization: Layer I & II

Answer 4

o Layer I – Molecular – acellular – contains apical dendrites of pyramidal cells that have many afferent connections; near the meninges
o Layer II - External Granule Cell Layer - granule (stellate) cells - small cells w/ local connections

Question 5

Laminar Microscopic Organization: Layer III & IV

Answer 5

o Layer III – External Pyramidal Cell Layer – variety of cells including small pyramidal – project to other cortical regions (either on same side or to other side through corpus collosum)
o Layer IV – Internal Granule Cell Layer – primarily granule (stellate) cells – termination zone of primary thalamic inputs (i.e. thalamus projects to this layer)

Question 6

Laminar Microscopic Organization: Layer V & VI

Answer 6

o Layer V – Internal Pyramidal Cell Layer – mainly pyramidal cells (larger than layer III) – form corticofugal efferents to all regions of brain outside the neocortex, except the thalamus
o Layer VI – Polymorphic (Multiform or Fusiform) Cell Layer – variety of neurons – neurons form reciprocal projections back to the thalamic nuclei that project to same cortical column

Question 7

Columns

Answer 7

– local, highly interconnected cortical neurons that are derived from radial migration
o During development, form layer 6 first then form layers outward to layer 2
o Topographically organized
o Function: role as elementary processing modules

Question 8

Pyramidal Cells

Answer 8

– principal output neurons of cerebral cortex
 Shape – pyramid
 Size – medium-large (Betz cells) (10-100 µm)
 Location – primarily layers III & V; some VI
 Dendrites – apical dendrites ascend vertically; basal dendrites near base
 Axons – typically long axons to other cortex or subcortical sites

Question 9

Interneurons

Answer 9

– stellate cells and others
 Shape – wide assortment
 Size – typically small (< 10 micrometers)
 Location – various layers; stellate cells mostly in layer IV
 Axons – short axons which do NOT leave the cortex

Question 10

Cytoarchitectonics

Answer 10

– organization of cortical regions according to cell type and distribution over cortical layers

Question 11

Homotypic vs. Heterotypic Cortex

Answer 11

oHomotypic Cortex – 6 layers of similar thickness; areas not used for sensory OR motor
oHeterotypic Cortex – does NOT have 6 distinctive layers

Granule Cortex – thin and contains granule cells associated with sensory cortex
• Large (thicker) in layer IV – gets inputs from thalamus
• Reduced (thin) in layers III and V

Agranular Cortex – thick and associated with motor cortex
• Large (thicker) in layers III and V (many pyramidal cells)
• Reduced (thin) in layer IV

Question 12

Brodmann’s Area: 1,2,3,4,6

Answer 12

 1,2,3 – parietal lobe (touch/pain sensation)
 4 – posterior aspect of frontal cortex (primary motor)
 6 – premotor cortex (higher/secondary motor/level processing)

Question 13

Brodmann’s Area: 17,18,22

Answer 13

 17 – primary visual cortex
 18 – secondary (higher order) visual cortex
 22 – posterior portion (Weirneckie’s) -posterior part of super temporal gyrus (language)

Question 14

Brodmann’s Area: 39,40,41

Answer 14

 39 – angular gyrus
 40 – supramarginal gyrus
 41 – primary auditory cortex

Question 15

Brodmann’s Area: 42,44,45

Answer 15

 42 – secondary (higher order) auditory cortex

 44,45 (Brocha’s area) – speech and grammer

Question 16

Efferents

Answer 16

Commissural – bundle of white matter that connects the two hemispheres
 ex: from layer III to contralateral cortex

Associational – from layers II and III to ipsilateral cortex (same side of cortex)
 Short connections: arcuate fibers to adjacent gyrus
 Long connections: arcuate fasciculus and other tracts

Corticofugal (via Internal Capsule)
 From layer VI to thalamus
 From layer V to various non-cortical structures

Question 17

Internal Capsule

Answer 17

– major axonal pathway to and from the cerebral cortex, thalamus, basal ganglia, brainstem and spinal cord
o Primates – passes through neostriatum, separating it into caudate nucleus and putamen
oComposed of 3 parts:
 Anterior limb – separates caudate and putamen
 Genu – near anterior nucleus of thalamus
 Posterior limb – separates lentiform nuclei from thalamus

Question 18

Commissures

Answer 18

Corpus Callosum- carries most fibers between homologous contra-lateral cortex
 Structures (rostral/anterior to caudal): genu, body, splenium

Anterior Commissure – carries fibers between the contra-lateral homologous amygdalae and anterior temporal cortex

Question 19

Methods for Localizing Cortical Functions

Answer 19

o Lesion studies
o Functional Neuroimaging – Positron Emission Tomography (PET); functional Magnetic Resonance Imaging (fMRI)
o Electrophysiology – Event Related Potentials (ERPs); direct brain recording and stimulation
o Magnetoencephalography (MEG)
o Transcranial Magnetic Stimulation (TMS)
o Animal studies

Question 20

Lesion Studies

Answer 20

– limited lesion leading to limited deficit suggests localization of impaired functions
o Patients – focal lesions from stroke or traumatic brain injury; neurodegenerative diseases
o Pros – specific lesions can strike particular brain structures
o Cons – lesions often large, cover many structures; inference of brain function can be problematic
 Plasticity – brain can adapt after a lesion to regain function; making inference difficult

Question 21

Functional Neuroimaging

Answer 21

– measure changes in blood flow or oxygenation levels, which correlate with changes in neuronal activity
o Positron Emission Tomography (PET) – radioactive metabolites allow imaging of cellular metabolism
o Functional Magnetic Resonance Imaging (fMRI) – measures nutrient (usually O2) uptake levels
o Pros – good spatial resolution – ability to distinguish specific location for brain activity
o Cons – bad temporal (time) resolution – can be a second or two lag time between the brain performing a task and blood response which is detected by PET or fMRI
 Electrophysiology and performing tasks occurs on ms scale

Question 22

Electrophysiology

Answer 22

o Event-related Potentials (ERPs) – wear a cap that maps electrical signals from neurons
o Direct brain stimulation and recording – invasive method of activate/de-activating neuronal activity with electrical stimulation
o Pros – good temporal resolution
o Cons – bad spatial resolution (especially ERPs)

Question 23

Magnetoencephalography

Answer 23

– similar to ERPs; non-invasive; records magnetic fields through scalp that are associated with electrical activity of neurons
o Pros – good temporal resolution
o Cons – bad spatial resolution

Question 24

Transcranial Magnetic Stimulation

Answer 24

– activate/de-activate neurons with currents induced by localized pulsed magnetic field from outside the brain
o Pros – multiple, local, temporary activations/de-activations  great experimental control
o Cons – invasive

Question 25

Animal Studies

Answer 25

– all approaches used in humans, as well as more invasive techniques, are used to localize functions in animals; can do more on animals than humans o Pros – invasive studies are possible o Cons – generalization to humans is uncertain

Question 26

Occipital Lobe Anatomy

Answer 26

– BA 17,18,19 o Posterior – V1(primary) (BA 17), V2 (secondary) (BA 18) – low level visual processing  Lesion cause – scotoma (blindspot); hemianopia (loss of visual field L/R); blindsight – reacting to something but “without seeing it coming” o Middle – midlateral BA 19 – motion perception  Lesion cause – motion pereption deficits o Inferior – ventral BA 19 –color and form perception  Lesion cause – color perception, imagery, and naming deficits o Anterior inferior – lingual, fusiform gyri – object recognition  Lesion cause – object recognition (agnosia); face recognition (prosopagnosia)

Question 27

Temporal Lobe Anatomy

Answer 27

– BA 41,42,22,38 o Lateral – long-term memory for words, facts, events  Lesion cause – anomia (deficit in word finding ability); retrograde amnesia (deficit in retrieving long-term memory) o Superior – BA 41,42,22 – auditory perception & recognition  Lesion cause – deficit in auditory perception & processing (hard of hearing) o Inferior – BA 38 – visual object recognition  Lesion cause – object agnosia (deficit in object recognition) o Medial – hippocampus and related structure - learning new words, facts, events  Lesion cause – anterograde amnesia (deficit in learning words, facts, events)

Question 28

Parietal Lobe Anatomy

Answer 28

– BA 1,2,3,39,40 o Anterior (postcentral gyrus) – BA 1,2,3 – somatosensory (touch) perception  Lesion cause – deficit in touch perception and processing o Posterior – touch & visual integration for movement (trying to catch a ball) o Superior Parietal Lobule – tactile and spatial processing  Lesion cause – deficit in spatial attention – where to look/focus eyes o Inferior Parietal Lobule – BA 39 (angular gyrus) ,40 (supramarginal gyrus) – skilled movement and phonology (word sounds)  Lesion cause – hemispatial neglect (only focus on one half L/R side of objects); deficits in skilled movement (ideomotor apraxia), drawing, phonology (word sounds), and phonological dyslexia (reading)

Question 29

Frontal Lobe Anatomy

Answer 29

– BA 4,6,44,45 Motor – BA 4 – low-level motor  Lesion cause – deficit in movement precision, speed, strength Premotor – BA 6 – motor programming (complex, skilled movements)  Lesion cause – deficit in skilled movement Prefrontal  Broca’s area – BA 44,45 – speech, grammar, reading, motor • Lesion cause – deficit in speech, grammar (Broca’s aphasia); reading (phonological dyslexia)  Orbital – smell, sex, social behavior • Lesion cause – deficit in smell, sex, and social behavior  Dorsolateral (DLPF) – short-term (temporary) memory, inhibition • Lesion cause – deficit in keeping things in one’s mind; in inhibiting responses

Question 30

Dorsal vs. Ventral Pathways

Answer 30

o Dorsal – “where” pathway o Ventral – “what” pathway o General Rule: tracts in superior (dorsal) regions of brain help us determine where something is  Inferior (ventral) regions of the brain help us determine what something is