Cerebral Cortex Dr G R Leichnetz Central sulcus

Cerebral Cortex Dr. G. R. Leichnetz

Central sulcus Cerebral Hemisphere Lateral View Parietal Lobe Frontal Lobe Occipital Lobe Lateral sulcus Temporal Lobe 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. III. IV. V. VI. Molecular layer External granular layer External pyramidal layer Internal granular layer Internal pyramidal layer Multiform layer I II 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. ” Casanova Each minicolumn consists of a vertical cell array of perhaps 80 -100 neurons, separated from adjacent minicolumns by vertical cell-sparse zones. 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: prefrontal cortex cingulate gyrus LD, LP, Pulvinar- parietal associational cortex MDANT- 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 (Ch 4)- cholinergic Locus ceruleus (A 6)- noradrenergic Midbrain raphe (B 7, B 8)- serotonergic Ventral tegmental area of midbrain (A 10)- 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 V 1, goes through visual area V 2, then to visual area V 3, visual area MT (also known as V 5) 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 V 1, goes through visual area V 2, then through visual area V 4, 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 hemiinattention 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