8 - Cerebral Cortex Flashcards
A 63 year old male had a stroke while traveling
Deficits:
Left sided weakness
Stopped noticing things on his left
Not visual, he could see items on the left since he would not bump into furniture if it was on his left
More precisely, he did not consciously notice things on his left
E.g. If he had a pile of pens on the left side of his desk he would still ask for a pen if one wasn’t on the right side and he needed one
76 year old male has a stroke
Left side weakness/ paralysis, wheelchair bound, slurred speech
Refused to resign from work
Believed there was nothing wrong
Told people, “he merely stumbled and it wasn’t a stroke”
Claimed stories of him being paralyzed were false rumors and he invited naysayers to go hiking with him
Told them he was kicking 40 yard field goals with his left leg that very morning and some suggested that he try out for the local NFL team
Cerebral cortex
Sheet of neurons 2 ft2 area; 2- 5 mm thick Half the brain’s weight 25-50 billion neurons 100,000 km of axons Receives 1014 synapses
Cerebral cortex most
highly developed in humans: roles in language, abstract thinking, adapt to environment, etc
Neocortex makes up most of
cortex, 6-layers; 95% of total cortex in humans
Paleocortex 3-layers:
uncus, olfaction
Archicortex 3-layers: most of
hippocampus
Allocortex or heterogenetic cortex
Pyramidal cells:
Pyramidal cells:
Apical dendrite:
one/ cell; extend to top layer of cortex
Pyramidal cells:
Basal dendrites:
sever/ cell; extend horizontally in respective layer
Pyramidal cells:
Axons have
recurrent branches, excite neighboring pyramidal cells
Most prevalent type of neuron:
Pyramidal cell
Cortical neuron structure - pyramidal cell
Apical (to top of cortex) & basal dendrites (horizontal)
Long axons to other cortical areas and subcortical sites
Excitatory (glutamate) synapses
pyramidal cell Principle
projection neurons of cortex
Pyramidal cell: Dendritic spines:
preferential site of excitatory synapses
Pyramidal cell: ;Suggested to be sites of
synapses that are selectively modified as a result of learning
Small changes in spine configuration lead to electrical properties and in turn synapse efficacy
Pyramidal cell: Some forms of intellectual disability may be associated with
poor spine development
Autism, Fragile X syndrome, etc
Cortical neuron structure: Nonpyramidal cell: All cortical neurons that are not
pyramidal cells
Cortical neuron structure: Nonpyramidal cell: Tend to have short axons that remain in the
cortex
diverse*
Cortical neuron structure: Nonpyramidal cell: Most make
inhibitory (GABA) synapses
Cortical neuron structure: Nonpyramidal cell: Principal
interneurons of cortex
Nonpyramidal cell types: Smooth stellate cells:
non-spiny dendrites, receive recurrent collateral branches from pyramidal cells, inhibitory (GABAergic synapses with pyramidal cells)
Silence weakly active cell columns in the cortex (similar to focusing action noted in cerebellar cortex by Golgi cells)
Nonpyramidal cell types: Bipolar cells: Located mainly in
outer layers, contain peptides co-released with GABA
Nonpyramidal cell types: Spiny stellate cells:
Spiny dendrites, generally excitatory, glutaminergic synapses with pyramidal cells
Receive most of the afferent input from thalamus, other cortical areas
Cortex has a laminar organization I
I. Molecular: ends of pyramidal cell apical dendrites distal ends of some thalamocortical (intralaminar nuclei) axons
Cortex has a laminar organization : II
II. Outer granular: small pyramidal and stellate cells
Cortex has a laminar organization: III
III. Outer pyramidal: medium-sized pyramidal and stellate cells
Cortex has a laminar organization: IV
IV. Inner granular: stellate cells receiving thalamocortical axons (relay nuclei)
Cortex has a laminar organization: V.
Inner pyramidal: large pyramidal cells to striatum & spinal cord
Cortex has a laminar organization: VI
VI. Fusiform: modified pyramidal cells projecting to thalamus
Afferents to cortex five sources: Association fibers (long and short): from
small & medium sized pyramidal cells in other parts of ipsilateral cortex
Afferents to cortex five sources: Commissural fibers: from
medium sized pyramidal cells via corpus callosum or anterior commissure from corresponding contralateral cortex
Afferents to cortex five sources: Thalamocortical fibers:
From relay or association nuclei
Afferents to cortex five sources: Non-specific thalamocortical fibers: From
intralaminar nuclei
Afferents to cortex five sources: Cholinergic & aminergic: from
basal forebrain, hypothalamus (tuberoinfundibulum), brainstem (midbrain raphe, LC)
Efferents from cortex
All efferents are pyramidal cell axons and all are excitatory
Short association
-e.g. sensory cortex motor cortex
Efferents from cortex: Long association
e.g. prefrontal cortex motor cortex
Efferents from cortex: Commissural fibers: from
contralateral cerebrum via corpus callosum and anterior commissure
Efferents from cortex: Fibers from
primary sensory and motor cortex make up largest input to basal ganglia
Efferents from cortex: Thalamus receives input from
all of the cortex
Corticopontine, corticospinal, corticobulbar
Commissures: Interconnect the
cerebral hemispheres
Corpus callosum
Predominant interconnection between hemispheres
Anterior commissure
Interconnects temporal lobes (inferior), anterior olfactory nuclei
Corpus callosum: All parts of the brain receive
commissural fibers except hand area of somatosensory & motor cortex and parts of primary visual cortex
Disconnection syndromes: Can result from
white matter damage
Alexia without agraphia
Can write but
unable to read
Cannot read words even they wrote
Also have right homonymous hemianopia
Rare, stokes are a frequent cause
Alexia without agraphia: Language areas on left isolated from
all visual input
Alexia without agraphia: Left visual cortex damaged by
stroke
Alexia without agraphia: Right visual cortex intact but
corpus callosum damaged
Alexia without agraphia: Language areas intact, so
speech is unaffected
Association bundles
Interconnect areas of one hemisphere
Short: U-fibers
Long: travel to different lobes
Association bundles
Longer fibers in distinct bundles
Areas that send off long axons have more
pyramidal cells
Primary sensory areas project to
nearby cortex, no need for long axons so fewer pyramidal cells
Granular and agranular cortex is
irregularly distributed
Brodmann characterized
44 areas, with imprecise boundaries
Total cortical volume rather constant, but large variation in
Brodmann area sizes among individuals
Types of cortical regions
Primary motor areas- areas that give rise to much of the corticospinal tract
Primary sensory areas- receive information from thalamic sensory relay nuclei
Association areas
Limbic areas
Primary motor areas- areas that
give rise to much of the corticospinal tract
Primary sensory areas- receive
information from thalamic sensory relay nuclei
Sensory areas: Have a
topographical organization where the body surface, range of frequencies, visual world are mapped on cortical surface
Sensory areas: Map is distorted so that highly sensitive areas (fingers, fovea) have disproportionately
large cortical representations
Primary somatosensory cortex: postcentral gyrus (3, 1, 2)
Initial processing of
Parietal lobe
tactile and proprioceptive information
Inferior parietal lobule of one hemisphere (typically left) involved with
language comprehension
Rest of parietal cortex: complex aspects of
spatial orientation and directing attention
Occipital lobe functions: Primary visual cortex (striate cortex; 17) in banks of
calcarine sulcus
Occipital lobe functions: Visual association cortex, involved in
higher order visual processing
Bilateral injury to:
Inferior occipital lobe: color blindness
Occipital-temporal junction: motion blindness
Occipital lobe: Line of Gennari: thin stripe of
myelin in primary visual cortex, aka striate cortex
Striate cortex parallels calcarine sulcus and extends a bit onto
posterior surface
Visual fields: Fibers from nasal half of retina cross to
contralateral optic tract
Visual fields: Fibers from temporal half of retina enter
ipsilateral optic tract
LGN: Structure
6-layered, precise retinotopic arrangement
Pattern in the same in each layer so any given point in the visual field is represented as a column in all 6 layers
Each layer gets input from one eye
1, 4 & 6 contralateral eye
2, 3 &5 ipsilateral eye
Parvocellular layers
3- 6 (color & form)
Magnocellular layers
1- 2 (movement & contrast)
Fibers representing inferior visual fields most superior in
radiations
Those representing superior visual fields, most inferior in
radiations
Optic radiations end retinotopically in
occipital cortex, above and below the calcarine sulcus (area 17)
Inferior visual fields above calcarine sulcus
Superior visual fields below calcarine sulcus
Macula is represented most
posteriorly, peripheral fields more anteriorly
Primary visual cortex
Breaks visual information down into
component parts: orientation, color, depth, motion, brightness, etc
Distributes this info to
specialized parts of extrastriate cortex
Example of simultaneous, parallel processing
Common in nervous system accounts for speed we can assess a visual field even thou our neurons are slow
Cortex also has a columnar organization: Neurons are functionally arranged in columns that extend
radially thru all 6 horizontal layers
Cortex also has a columnar organization: All of the neurons in each column (several hundred) are sensitive to one
modality, i.e. modality-specific
Cortex also has a columnar organization: One column may respond to movement of a
particular joint, another a patch of skin, the orientation of an object in the visual field
Cortex also has a columnar organization: Striate (primary visual) cortex has an array of
repeated, modular collections of neurons arranged in columns
Columns in one cortical module analyze
all aspects of visual information arriving from discrete areas of visual field
Modules in foveal part analyze
small areas of visual field, so fova has many more modules and therefore better resolution
Process starts in LGN Parvocellular layers (color, form)-
ventral striate cortex
Ventral stream
Magnocellular layers (location, movement)-
dorsal striate cortex
Dorsal stream
Selective damage to extrastriate cortex can lead to
strange visual deficits: selective deficit in distinguishing colors, motion, faces
Primary auditory cortex (41, 42): transverse temporal gyri superior surface of
Temporal lobe functions
superior temporal gyrus
Temporal lobe functions
Auditory association cortex
Temporal lobe functions: Language comprehension,
Wernicke’s area- posterior aspect of one hemisphere (usually left)
Temporal lobe functions: Higher order
visual processing
Gustatory:
frontal lobe (operculum) & insula
Vestibular:
superior temporal gyrus and posterior insula
Primary olfactory cortex is
paleocortical, not neocortical
Olfactory Cortex consists of
Cortex near lateral olfactory tract, a.k.a. piriform cortex
Cortex covering amygdala, periamygdaloid cortex
Small part of parahippocampal gyrus
Frontal lobe: Broca’s area:
inferior frontal gyrus of one hemisphere (usually left), production of spoken and written language
Frontal lobe: Prefrontal cortex: rest of
frontal lobe, executive functions (personality, foresight, insight)
Primary motor cortex in
precentral gyrus, premotor & supplemental motor areas: part of precentral, nearby portions of superior and middle frontal gyri
Origin of descending corticospinal tract, initiate voluntary movements
Association areas
Mediate higher mental functions
Language, art, music, etc
Very little is known about these functions and most of our knowledge stems from case reports of patients with naturally occurring lesions
Advent of functional imaging scans are advancing our understanding
Association Areas: two types: Unimodal association cortex-
adjacent to primary area
Devoted to elaborating on business of primary area
Example: 18, 19 around 17 in occipital lobe
Association areas: two types: Multimodal association cortex-
high level intellectual functions
Inferior parietal lobule, much of frontal and temporal lobes
Dominant hemisphere: Hemisphere that
produces and comprehends language
Left hemisphere is
dominant hemisphere in most people, even those who are left handed
Nonfluent aphasia
Make few written or spoken words, get by with phrases (“OK”, “And how”)
Very difficult to produce words
All detail & meaning in sentences is lost
Can comprehend language
Often due to damage in Broca’s area
Broca’s aphasia
Fluent aphasia: Can write and speak, but words used and
sequences of words used in sentences is incorrect
Really little or no linguistic content Substitute one letter or word for another (paraphasia) Make up new words (neologisms) Difficulty in language comprehension Often due to damage in Wernicke’s area Wernicke’s aphasia
Damage Broca’s
Deprive motor areas of ability to generate language
Muscle function normally for other activities
Comprehension of language unaffected
Damage Wernicke’s
Broca’s area is unchecked
Words are generated but no meaning
Cortical language areas near
lateral sulcus
Left lateral sulcus extends further posteriorly than the
right as planum temporale is larger on the left
Planum temporale: part of
superior temporal gyrus posterior to primary auditory cortex (transverse temporal gyri)
Language areas border (left) lateral sulcus: Stimulate motor cortex near mouth produce
involuntary grunts, vocalization
Language areas border (left) lateral sulcus: Stimulate other areas on
dominant side:
Subject ceases to speak, but can still move mouth
Yet other areas subject makes linguistic errors, fails “to find” appropriate words
Perisylvian language areas: Broca’s area in
inferior frontal gyrus (opercular and triangular parts)
Perisylvian language areas: Wernicke’s area in
posterior part of superior temporal gyrus, continuing into planum temporale and inferior parietal lobule
Aphasia: Inability to use
language, lose the use of or access to symbols humans use as concepts, i.e. words
Aphasia: Broca’s and Wernicke’s areas provide
framework for two broad types of aphasia classified depending on how easily words are produced:
Nonfluent
Fluent
Language in the right hemisphere: Language more than selecting
correct word and using grammatical rules to put it in the correct part of a sentence
Language in the right hemisphere: Language has
emotional content and some linguistic elements are conveyed rhythmically
Language in the right hemisphere: So-called musical aspects of speech called
prosody; produced in right hemisphere
Language in the right hemisphere: Right inferior frontal gyrus produces
prosody
Language in the right hemisphere: Motor aprosody: can’t convey
authority, anger, etc. in speech
Right posterior temporoparietal region comprehends
prosody
Sensory aprosody difficulty comprehending
the emotional content of speech from others
Parietal cortex: Association areas posterior to
primary somatosensory cortex
Parietal cortex: Unimodal areas:
Visual association cortex, auditory association areas, somatosensory
Parietal cortex: Damage to these areas can cause
sensory specific agnosias
Parietal cortex: Inability to recognize
faces, perceive movement (visual agnosias)
Parietal cortex
Multimodal areas:
Centered on intraparietal sulcus
Monitor relationships of body with outside world
Right parietal lobe damage
Patient has trouble with left half of body
May deny something is wrong with (left) limb and can be convinced the (left) limb is someone else’s
Ignore left half of body, called contralateral neglect
Left parietal lobe damage
Important for taking sensory information needed to plan a movement accurately
Apraxias (“lack of action”)
Patients unable to perform some actions, many different types
Ask patient to imitate examiner who touches finger to face and they can’t do it,
But they can scratch an itch on their face
Prefrontal cortex
Frontal lobe anterior to primary motor (4) and supplemental motor (6) cortices
Prefrontal cortex: Controls activities of
other cortical areas; underlies executive functions
Interconnected with dorsomedial nucleus of thalamus
Prefrontal cortex: two broad types: Dorsolateral, over
lateral convexity
Interconnected with parietal association areas
Important role in working memory “keep in mind”, problems planning, solving problems, maintaining attention
Prefrontal cortex: two broad types: Ventromedial, extends to
orbitofrontal and anterior cingulate areas
Damage makes people impulsive, can’t suppress inappropriate responses/ emotions
