Cortex Flashcards
Forebrain
- encompasses diencephalon and telencephalon
o Diencephalon – thalamus, hypothalamus, subthalamus, epithalamus
o Telencephalon encompasses – basal ganglia, hippocampus, amygdala, cerebral cortex
Three Types of Cortex
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
Neocortex
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
Laminar Microscopic Organization: Layer I & II
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
Laminar Microscopic Organization: Layer III & IV
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)
Laminar Microscopic Organization: Layer V & VI
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
Columns
– 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
Pyramidal Cells
– 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
Interneurons
– 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
Cytoarchitectonics
– organization of cortical regions according to cell type and distribution over cortical layers
Homotypic vs. Heterotypic Cortex
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
Brodmann’s Area: 1,2,3,4,6
1,2,3 – parietal lobe (touch/pain sensation)
4 – posterior aspect of frontal cortex (primary motor)
6 – premotor cortex (higher/secondary motor/level processing)
Brodmann’s Area: 17,18,22
17 – primary visual cortex
18 – secondary (higher order) visual cortex
22 – posterior portion (Weirneckie’s) -posterior part of super temporal gyrus (language)
Brodmann’s Area: 39,40,41
39 – angular gyrus
40 – supramarginal gyrus
41 – primary auditory cortex
Brodmann’s Area: 42,44,45
42 – secondary (higher order) auditory cortex
44,45 (Brocha’s area) – speech and grammer
Efferents
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
Internal Capsule
– 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
Commissures
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
Methods for Localizing Cortical Functions
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
Lesion Studies
– 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
Functional Neuroimaging
– 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
Electrophysiology
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)
Magnetoencephalography
– 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
Transcranial Magnetic Stimulation
– 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
Animal Studies
– 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
Occipital Lobe Anatomy
– 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)
Temporal Lobe Anatomy
– 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)
Parietal Lobe Anatomy
– 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)
Frontal Lobe Anatomy
– 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
Dorsal vs. Ventral Pathways
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
