Download Cerebral Cortex

Cerebral Cortex
Dr. G. R. Leichnetz
Central sulcus
Cerebral Hemisphere
Lateral View
Parietal Lobe
Frontal Lobe
Occipital
Lobe
Temporal Lobe
Lateral sulcus
There are five lobes in the
cerebrum (insular lobe not seen).
Pre-occipital
notch
Archicortex and Paleocortex are in
the ventromedial temporal lobe
The frontal lobe is the “effector” portion of the cortex. It projects
to “motor” areas of brainstem and spinal cord that initiate motor
and other more complex behaviors. The remainder of the cortex is
involved in complex sensory processing and must project to
frontal areas to affect behavior.
Motor
Somatosensory
Effector
Complex
Sensory
Processing
Complex
behaviors
Auditory
Visual
Cytoarchitecture of the
Cerebral Cortex
Cresyl violet-stained
section through the
cerebral cortex, showing
its cytoarchitecture.
The neocortex (isocortex)
has six layers:
I.
II.
III.
IV.
V.
VI.
Molecular layer
External granular layer
External pyramidal layer
Internal granular layer
Internal pyramidal layer
Multiform layer
I
II
III
IV
Layer III Pyramidal neurons
V
VI
Subcortical white matter
The two principal cell types of the cerebral cortex are the:
pyramidal and granule neurons.
Granule (or
stellate) cells are
GABA-ergic;
involved in local
intra-columnar
processing.
Pyramidal cells
are glutamatergic;
give rise to all
cortical efferents:
associational,
commissural, and
projectional
(corticofugal)
fibers.
Pyramidal neurons, which are
“effector” in function, give rise to
cortical efferents, are found in
layers III and V of the cerebral
cortex.
Axons of pyramidal cells give rise
to associational, commissural,
and projectional fibers.
Granule (or stellate) neurons,
which are “receptor” in function,
are found primarily in granular
layers II and IV.
These cells have short axons and
are involved in local connections,
intrinsic connections within a
cortical column (intra-columnar
processing).
Cortex: Cytoarchitecture
Golgi Stain
I
II
Myeloarchitecture
Nissl Stain
Myelin Stain
Molecular layer
Ext. Granular Layer
III
Ext. Pyramidal Layer
IV
Int. Granular Layer
V
VI
Int. Pyramidal Layer
Polymorphic Cell
Layer
The dense fiber lines in layer IV
are thalamic afferents
Summary of Cortical Layers & Their Connections
Corticostriates
Primary sensory areas: area 3, 17, 41/42
Primary motor areas: areas 4, 6
Associational cortex is found in broad regions of the
frontal, parietal, occipital, and temporal lobes
between the principal sensory and motor regions.
Perception occurs in
“multimodal” associational
cortex, not in primary
sensory areas.
Sensory
cortex
Associational
cortex
Motor
cortex
Associational cortex is the
most typical type of
isocortex, having six regular
layers (homotypical).
There is little difference in the
“supragranular” layers I-III
between regions of cortex.
Layer IV
Layer V
Primary sensory and motor
cortex are heterotypical,
ie. granular or pyramidal
layers expanded.
In sensory cortex, layer IV is
thickest.
In motor cortex, layer V is
thickest.
Cytoarchitectural Subdivisions
of the Cortex
(Brodmann’s Areas)
Brodmann’s
Areas
From Wilkinson
Brodmann (1909)
identified about 50
cytoarchitecturally
distinct regions in
the cerebral cortex.
These numbers have
become synonymous
with the regions.
Columnar Organization
of the Cortex
The cortical column (first described by Mountcastle 1955) is the
basic unit of organization in all areas of the cerebral cortex
They are “vertical processing machines.”
Cortical column
The column
actually consists
of many parallel
vertical arrays of
cells, known as
“minicolumns.”
The cortical column is the
basic unit of cortical
processing. It is made up
of many “minicolumns.”
Each minicolumn consists
of a vertical cell array of
perhaps 80-100 neurons,
separated from adjacent
minicolumns by vertical
cell-sparse zones.
Casanova
It is not certain, but
probable, that all the cells
within a minicolumn have
overlapping “receptive
fields” within the same
sector of somatosensory,
visual, or auditory space.
Following Mountcastle’s (1955) description of columns in
cat somatosensory cortex, Hubel & Wiesel identified
columns in the primary visual cortex (orientation columns
or ocular dominance columns) by injecting a labeled
radioisotope in the eye. Received Nobel Prize in 1981.
Internal granular layer IV of the primary visual cortex (area 17)
receives the major thalamic input from the lateral geniculate
nucleus, the ocular dominance columns.
The primary
somatosensory cortex
of the postcentral
gyrus (area 3) also has
columns representing
the digits of the hand.
Neurons within the
columns have
cutaneous (skin)
receptive fields related
to a portion of that
digit.
From Kandel, et al.
Even prefrontal associational cortex is organized in
columns. These were labeled by injecting the tracer
HRP into the parietal cortex.
Functional Areas of the Cortex
(Homunculus)
Electrical stimulation of the cortex (Penfield) showed a
complete representation of the body on the pre- and
post-central gyri (extending into the paracentral lobule),
called the motor and sensory homunculus.
The leg is represented on the
medial aspect of the hemisphere
in the paracentral lobule.
From Purves, et. al, Neuroscience
Neural Plasticity in the
Cerebral cortex
Sensory experience is responsible
for the normal differentiation of the
cortex.
A tangential section across layer IV of the primary
somatosensory (SI) cortex shows “barrel fields”
representing the vibrissae on the snout of a rat.
Removal of a line of vibrissae in the neonatal rat pup results in
failure of corresponding barrel fields to develop in the primary
somatosensory cortex.
The maps seen in sensory cortex are
not fixed, static.
There is neural plasticity in the
adult brain.
Blind subjects
reading braille
(tactile) show
activation of visual
cortical areas in
the occipital lobe.
Illustrates the
brain’s ability to
remodel itself
(neuroplasticity)
Cortical Connections:
Associational
Commissural
Projectional
(Corticopetal, Corticofugal)
Dissection of the subcortical white matter reveals three
types of cortical fibers: associational, commissural, and
projectional fibers.
From Glubekovic
Associational (Intra-hemispheric) Corticocortical
Connections
Short associational fibers (U-fibers) interconnect adjacent gyri.
Long associational bundles interconnect lobes.
From Warwick & Williams
Commissural (Inter-hemispheric) Corticocortical
Connections
There are three major cerebral commissures: corpus callosum,
anterior commissure, and hippocampal commissure. The corpus
callosum interconnects regions above the superior temporal
gyrus. The anterior commissure interconnects middle & inferior
temporal, occipitotemporal and parahippocampal gyri.
The anterior
commissure
interconnects the
temporal lobes.
Genu
Splenium
The genu of the
corpus callosum
interconnects the
frontal lobes.
The body of the
corpus collosum
interconnects
caudal frontal and
parietal lobes.
The splenium
interconnects
occipital lobes.
Projection (Corticopetal) Connections
The principal source of cortical afferents is the thalamus.
Corticopetal projectional fibers (thalamocorticals) traverse the
internal capsule to reach the cortex.
From Noback & Demarest
Thalamocortical afferents
terminate in layer IV
Specific relay nuclei of the
thalamus project to
specific regions of the
cerebral cortex.
VA- premotor area 6
VLprimary motor cortex, area 4 VPLpostcentral gyrus and
paracentral
lobule, area 3 VPM- head area,
postcentral gyrus LGN- primary
visual cortex, area 17 MGN- primary
auditory cortex, areas 41, 42
Other Thalamic Nuclei:
MD- prefrontal cortex
ANT- cingulate gyrus
LD, LP, Pulvinar- parietal
associational cortex
All thalamocortical projections
are glutamatergic, excitatory.
“Extra-thalamic” Cortical Afferents
Other afferents of the cortex are direct, reaching the cortex without
first synapsing in the thalamus. These neurotransmitter-specific
projections affect the background activity of the brain (EEG).
Nucleus basalis of Meynert (Ch4)- cholinergic
Locus ceruleus (A6)- noradrenergic
Midbrain raphe (B7, B8)- serotonergic
Ventral tegmental area of midbrain (A10)- dopaminergic
(these only project to the prefrontal cortex)
Extrathalamic Cortical Afferents
Neurotransmitter-specific cortical
afferents (ACh, NE, 5-HT, DA)
terminate diffusely through all
cortical layers, and affect the
background activity of the cortex.
Projection (Corticofugal) Connections
Cortical Efferents
Corticofugal
projections
originate from
layer V
pyramidal
neurons, including
corticospinals,
corticobulbars,
corticopontines,
corticostriates.
Associational
Corticostriate
Commissural
Corticopontine
Corticospinal
Corticobulbar
Corticothalamic
Only
corticothalamics
originate in
layer VI.
Cortical Efferents
Descending projectional fibers traverse the internal
capsule en route to the brainstem. Frontal projections
traverse the anterior limb, while parietal, temporal
and occipital projections traverse the posterior limb.
From Noback & Demarest
Motor-Related Areas of the
Cerebral Cortex
While corticostriates originate from broad regions of
frontal and parietal cortex, the feedback to motor cortex
from the basal ganglia affects primarily the premotor
cortex (supplementary motor area, M-II)
Corticostriates
SMA
From Kandel, et al.
While corticopontine projections originate from broad
areas of frontal and parietal cortex, the feedback
projections from cerebellum via ventral lateral
thalamic nucleus primarily target the primary motor
cortex (M-I).
Corticopontines
From Kandel, et al.
The superior parietal lobule, a somatosensory associational cortex
involved in perception of the body in space, provides essential
information on body image to the premotor cortex.
Lesions of the SPL result in apraxia (inability to perform learned
movements in the absence of paralysis).
From Kandel, et al.
Language-Related Areas of the
Left (Dominant) Hemisphere
Roger Sperry’s “split
brain” experiments
demonstrated
hemispheric
specialization.
The left hemisphere
processes language.
The right hemisphere
primarily deals with
spatial constructions.
In “split brain,” the subject
cannot verbally identify an
object held in the left hand, and
cannot draw with the right hand
(image must reach left motor
cortex to direct movement of the
right hand)
Language-Related Areas of the Cerebral Cortex
Broca’s Area (Anterior Speech Cortex)- involved in speech
production- lesion results in “expressive aphasia.”
Wernicke’s Area (Posterior Speech Cortex) involved in language
comprehension- lesion results in “receptive aphasia.”
Arcuate
fasciculus
Language areas are in
the left dominant
hemisphere in about
98% of humans.
The posterior speech
cortex (Wernicke’s) is
connected thru the
arcuate fasciculus to
the anterior speech
cortex (Broca’s).
Aphasias: language dysfunctions
Receptive Aphasias: lesions of Wernicke’s area;
difficulties with language comprehension,
recognition of symbols of language (visual, auditory,
somatosensory)
1. Tactile Agnosias (astereognosis)- supramarginal
gyrus
2. Visual Agnosias (word blindness)- angular gyrus
3. Auditory Agnosias (word deafness)- caudal part
of superior temporal gyrus
Expressive Aphasias: lesions of Broca’s area;
disruption of speech production; poor syntax; word
omissions; normal comprehension of language, but
speech is labored.
With MRI, PET functional-imaging techniques, areas
which are active during certain functions can be seen.
Visual Processing in the Cortex:
Attention/ Eye Movement
vs. Visual Discrimination
Visual information flows through the cortex in
sequential multisynaptic associational pathways, the
dorsal and ventral streams. The dorsal stream deals with
“where” the object is, and the ventral stream with
“what” the object is.
VISUAL PROCESSING IN EXTRASTRIATE VISUAL AREAS
The dorsal stream (parietal) begins
with V1, goes through visual area
V2, then to visual area V3, visual
area MT (also known as V5) and to
the inferior parietal lobule. The
dorsal stream, sometimes called the
“Where Pathway” is associated
with representation of object
location, and direction of motion.
The ventral stream (temporal)
begins with V1, goes through visual
area V2, then through visual area
V4, and to the temporal lobe. The
ventral stream, sometimes called the
“What Pathway” is associated
with object recognition.
The dorsal stream affects
attention and eye
movements to look at
“where” an object is,
whereas the ventral
stream (inferotemporal
cortex) is concerned with
visual discrimination,
recognizing objects, faces,
ie “what” the object is.
Dorsal and Ventral
Streams of Visual
Processing
From: Gazzaniga
Hemi-Inattention Syndrome
Attention
When asked to make
drawings, the patient
with a posterior
parietal lesion
(unilateral neglect)
draws all the numbers
on a clock face on the
ipsilateral side, or
only draws the
ipsilateral side of the
body in a stick figure.
Lesion of the
posterior parietal
cortex can
produce
hemi-inattention
syndrome
Prefrontal Cortex and
Complex Behaviors
The prefrontal cortex
(granular frontal cortex)
shows a noteworthy
enlargement as one
ascends the phylogenetic
scale.
The prefrontal cortex is involved in
executive functions, cognition,
working memory, social behavior
Different regions of
prefrontal cortex have
different functions.
Modified from Purves
Prefrontal Cortex
The prefrontal cortex receives
its principal input from the
mediodorsal (MD) nucleus of
the thalamus, which conveys
limbic influence from the
amygdala.
Mediodorsal
nucleus
Dorsolateral Prefrontal Control Systems
Initiating and shifting behavior- damage impairs the
ability of an individual to initiate and change actions,
motor movements, and mental plans.
Inhibiting behavior- suppression of unimportant or
distracting information
Simulating behavioral consequences- mental
rehearsal of possible behaviors
Ventromedial Prefrontal/Orbitofrontal Control Systems
Inhibition of socially inappropriate behavior
Anterior Cingulate Control Systems
Emotion and motivation of behavior
Working memory refers to the process of actively
maintaining relevant information in mind for brief
periods of time.
Lateral prefrontal neurons show stimulus-specific
sustained discharge during the delay period. This
sustained activity has been interpreted to be the neural
correlate of maintenance processes that take place
during the delay, and thus has been taken to be the
neural signature of working memory.
The “delayed response test” has classically been a used
to evaluate prefrontal function, eg. after brief delay
period, food reward changes compartments
Lesions result in perseveration.
Cannot alter behavioral strategy.
Working memory and attention are closely related
cognitive processes, which has led to the idea that they
may share common neural mechanisms.
What we perceive depends critically on where we direct
our attention (voluntary saccades, FEF).
Attention is highly flexible and can be deployed in a
manner that best serves the organism’s momentary
behavioral goals, either to locations, to visual features, or
to objects.
Attention can be based on internal goals (endogenous, eg.
finding a familiar face in a crowd) or can depend on the
external environment (exogenous, eg. when a loud alarm
sounds).
“Emotional salience” of an object is probably conveyed to
the PFC (via MD) from the amygdala.
Working memory and attention are closely related
cognitive processes, which suggests that they may share
common neural mechanisms.
What we perceive depends critically on where we direct our
attention (voluntary saccades, FEF).
Attention
Working
Memory
Eye Movement