Lecture 17: The Cerebral Cortex

Cerebral Cortex

- Outermost layer of the brain but accounts for nearly half its weight

- ~25 million neurons and 10^14 synapses

- 95% is neocortex

Neocortex

- 6 layers

- Includes primary sensory areas, motor areas, and association areas

- Integrates sensory information, emotional context (limbic system), drives hypothalamus, and motor planning for behavior

Human Cortex is

- Multimodal association cortex that supports: abstract thought, language, decision making, and personality

Development of Neocortex

- Expands outward in C shape during development

- Older cortices (paleo and archicortex) get pushed into inferior/medial brain regions

- Many structures follow C shape pattern

Neocortical Neurons

- Pyramidal cells (80%, glutamatergic, long range projection)

- Non pyramidal cells: stellate (granule) that are local and glutamatergic; GABA interneurons participate in local inhibition

Neocortical Layers (superficial to deep)

- Molecular (I)

- External granule (II)

- External pyramidal (III)

- Internal granule (IV)

- Internal pyramidal (V)

- Polymorphic layer (VI)

Agranular Cortex

Few granule cells, large pyramidal cells (motor cortex)

Granular Cortex

Many small pyramidal and stellate cells (sensory cortex)

Korbinian Brodmann

- Divided cortex into 44 areas by cytoarchitecture

- Anatomical zones match functional regions very well

- BA4, BA17, BA44

BA4

primary motor cortex

BA17

primary visual cortex

BA44

Brocas area

Neurons are

- Arranged in vertical columns, not just layers

- EX: V1 hypercolumns -- each about 2mm wide and encodes orientation, color, etc.

Columns contain

- Vertically aligned apical dendrites (from pyramidal neurons)

- Vertically bundled afferent/efferent axons

Cortical Connectivity -- Afferents

- From thalamus to layers III/IV: main sensory relay

- Neuromodulatory systems (RF): serotonin, Ach, NE, histamine

- Association fibers: within same hemisphere --> Arcuate fasciculus: language processing link

- Commissural fibers: across hemispheres: corpus callosum (main), anterior/posterior commissures

Cortical Connectivity -- Efferents

- To subcortical targets via internal capsule = corticospinal, corticobulbar, corticorecticular

- To cortex (association/commissural) from layer III

Function Classes of Cortex

- Sensory

- Motor

- Association

- Limbic

Sensory (topographically organized)

- Primary areas: S1(BA1-3), V1(BA17), A1(BA41/42), Olfactory, gustatory vestibular

- Secondary = unimodal association cortex

Motor

- M1 (BA4): movement initiation (Betz cells)

- PMA/SMA (BA6): planning and decision to move

Association

- Unimodal: V4=color, V5=motion

- Multimodal: integrates across sessions --> Prefrontal cortex for reasoning, decision, planning and POT cortex for perception, language, and spatial reasoning

Limbic

- Paleocortex with <6 layers

- Primary olfactory/gustatory cortex included

PFC and Executive Function

- PFC handles personality, foresight, planning, abstract thought

- Input from limbic system, hypothalamus, basal ganglia shapes decsion

- Case: Phineas Gage: PFC damage --> impulsivity, loss of judgment

Hemispheric Specialization and Language

- Left hemisphere

- Arcuate fasciculus links them

- Aphasia types

Left Hemisphere

- Dominant for language (95% of right handers)

- Brocas area (BA44): language production

- Wernicke's area (BA22): language comprehension

Aphasia Types

- Broas (Nonfluent): effortful speech, intact comprehension

- Wernicke's (fluent): fluent nonsense, poor comprehension

Case Studies in Aphasia

- MN: Broca's -- few content words, long pauses, anomia

- JS: Wernicke's -- fluent but confusing speech

Posterior Parietal Cortex (PPC)

- Integrates visual, auditory, somatosensory input

- Involved in spatial awareness, attention

Right PPC Damage

- Left contralateral hemineglect

- Ignoring left visual field, even left side of body (makeup on right eye, but none on the left)

Consciousness

- Not in one place, its a network

Network of Consciousness

- Midbrain RF/ARAS: arousal

- Thalamus: gates sensory input, modulates alertness

- Cortex: regulates thalamus; needed for awareness

- Hypothalamus: lateral preoptic = sleep promoting; posterior = wake promoting (orexin, histamine)

Brodmann Area (BA) 1,2,3

- Primary somatosensory cortex (S1)

- Postcentral gyrus (parietal lobe)

Brodmann Area (BA) 4

- Primary motor cortex (M1)

- Voluntary movement, Betz cells

- Precentral gyrus (frontal lobe)

Brodmann Area (BA) 6

- Premotor cortex (PMA) + Supplementary motor area (SMA)

- Motor planning

- Anterior to area 4 (frontal lobe)

Brodmann Area (BA) 17

- Primary visual cortex (V1)

- Calcarine sulcus, occipital lobe

Brodmann Area (BA) 18,19

- Visual association cortex (unimodal)

- Surrounding V1

Brodmann Area (BA) 22

- Wernicke's area

- Language comprehension

- Posterior superior temporal gyrus

Brodmann Area (BA) 41, 42

- Primary auditory cortex (A1)

- Transverse temporal gyri, superior temporal gyrus

Brodmann Area (BA) 44

- Broca's area

- Language production

- Inferior frontal gyrus, opercular/triangular parts

Areas Linked to Functional Zones

- V4

- V5

- Inferotemporal cortex (IT)

V4

- Visual association (color)

- Part of unimodal cortex

V5

- Visual association (motion)

- Part of the unimodal cortex

Inferotemporal cortex (IT)

- Face recognition

- Damage causes prosopagnosia