L70. Neuroscience - Cerebral Cortex

• The cerebral cortex originates from the telencephalon.
• Gyri (ridges) and sulci (valleys) form to conserve space within the cerebral cortex.
• The cortex is thicker at the crests of gyri and thinner in the sulci.

Types of Cortex

• Defined by their layers of cells:
• Archicortex: contains 3 layers and includes the hippocampus and dentate gyrus.
• Paleocortex: has 3-5 layers and includes the olfactory cortex.
• Neocortex: contains 6 layers and constitutes most of the cortex.

Neocortical Cell Classification

• Pyramidal cells (cone-shaped cell bodies):
• These have an apical spiny dendrite going toward the pia and basal spiny dendrites extending laterally.
• They are projecting neurons whose axons enter the white matter to project to cortical or subcortical areas.
• Common in layers III (small) and V (large) & VI.
• Stellate (granular) cells (rounded cell bodies):
• These have radiating dendrites, often without spines (aspinous).
• They are often intrinsic neurons that project from one layer to another.
• Common in layers II and IV.
• Fusiform cells (spindle-shaped cells):
• Found in the outermost layer I and innermost layer VI.

Brodmann's Areas

• Based on cytoarchitecture, are widely used and correspond well with function.
• Understanding of brain area functions comes from studies of patients with deficits, recordings from awake patients, and imaging studies.

Connections of the Neocortex

Major Afferent Fibers (entering the cortex, input)

• Corticocortical afferents:
• Originate from ipsilateral (association fibers) or contralateral (commissural) neocortex.
• They are typically excitatory and end mainly in layers 1, 2, and 3.
• Thalamocortical afferents:
• Originate from specific thalamic nuclei (e.g., VPL, VPM, VL, LGN, MGN).
• They are excitatory and end mainly in layer 4.
• Subcortical afferents:
• Originate from modulatory centers, including the nucleus basalis (ACh), locus ceruleus (NE), raphé nucleus (5-HT), and midbrain (dopamine, ventral tegmental area).
• They end in all layers.
• Antidepressants:
• Frequently prescribed with major types like selective serotonin reuptake inhibitors (SSRIs)

Major Efferent Fibers (from the cortex, excitatory, glutamate/aspartate)

• Corticocortical axons:
• Originate in layers 2/3 from small pyramidal cells project mainly to other cortical neurons.
• Corticofugal axons:
• Originate in layer 5 from large pyramidal cells project mainly to subcortical targets (pons, olive, red nucleus, striatum).
• Corticothalamic axons:
• Originate in layer 6 neurons project to specific thalamic nuclei.

Types of Axonal Fiber Bundles

• Association fibers (corticocortical axons): divided into short and long association fibers which connect specific cortical regions to perform particular functions.
• Primary cortex areas: sensory (postcentral gyrus, post-Rolandic), area 17 (visual), area 41 (auditory), or motor (precentral gyrus, pre-Rolandic) project to association areas for higher information processing.
• Diffusion tensor imaging (DTI): sensitive to the direction of water movement (diffusion).
• Short association fibers: connect gyrus to adjacent gyrus, these pathways are called "U" fibers.
• Long association fiber bundles: interconnect various areas of cortex
• Superior longitudinal fasciculus: connects the frontal lobe with the occipital and parietal lobe and is related to eye movements.
• Arcuate fasciculus: connects the frontal lobe with the posterior superior temporal lobe important for language function. Lesions result in conduction aphasia.
• Conduction aphasia: poor repetition of words, difficulty understanding prepositional phrases.
• Uncinate fasciculus associates the anterior temporal lobe with orbital part of frontal lobe which assists in limbic function.
• Cingulum connects the cingulate gyrus and parahippocampal gyrus and septal areas.
• Commissural fibers:
• Type of corticocortical fibers that connect the left and right hemispheres.
• Axons project to homologous areas of the contralateral cerebral cortex by utilizing commissural fibers.
• Corpus callosum: largest bundle (250 million) of commissural fibers that connects most areas of the cerebral cortex with homologous areas of the contralateral hemisphere.
• Anterior commissure: interconnects the rostral parts of the temporal lobes and connects olfactory bulbs.
• Projection fibers: fibers passing to and from the brainstem.

Columnar Organization

• Information flows radially through the cortical layers.
• Inputs to layer 4 are relayed outwardly to layers 2, then inwardly to layers 5, 6.
• This radial pattern of information flow creates a functional columnar organization within the cortex.
• Adjacent columns of cells can process information differently and relatively independently.
• Example: striate cortex with ocular dominance columns (each eye input), orientation columns (cells respond to a particular orientation), and blobs (process color information).

Sensory & Motor Homunculus

• The figures right are made using the proportion of primary sensory or motor cortex devoted to a particular region.
• Cortical mapping may change with injury.
• Complete spinal cord injury or amputation of a limb: adjacent areas' representation spreads to the areas with lost motor or sensory modalities.
• Damage to a particular area of cortex can lead to other areas of cortex contributing to performing the function which is sometimes seen in recovery (plasticity of the brain).
• Reversible damage to the body causes the representation to shrink then with return of function it regains its normal size.

Functions and Damage to Different Lobes of the Brain

Frontal Lobe

• Divided into: primary motor cortex, premotor area, and prefrontal cortex.
• On the medial surface, anterior to the central sulcus, the paracentral lobule contains the motor representation of the lower limb.
• Prefrontal Cortex:
• Anterior to the precentral gyrus is involved in executive function (judgment, evaluation, goal setting, motivation, and planning).
• Has reciprocal connections with DM thalamus, influencing affective behavior and perception.
• Orbitofrontal prefrontal cortex lateral to the olfactory sulcus may be important with decision-making and ventral anterior cingulate with mood.
• It is evaluated using the Gambling task and Wisconsin card sort.
• Motor speech area (Broca's area): found within the pars triangularis (area 45) and pars opercularis (area 44) of the inferior frontal gyrus on the language dominant (usually left) side.
• Frontal eye field stimulation: stimulation of the posterior part of the middle frontal gyrus causes both eyes to move toward the opposite side.
• Lesions:
• Lesions of Areas 4 and 6: upper motor neuron symptoms (spasticity, etc.).
• Lesion of primary motor alone: hypotonia.
• Area 6 alone lesion: apraxia.
• Frontal eye fields stimulation: irritation causes both eyes to move to the opposite side
• Destruction causes transient deviation to the side of the lesion.

Broca's expressive aphasia

• Damage to pars opercularis (44) and pars triangularis (45) of the inferior frontal gyrus of language dominant hemisphere.
• Patients have difficulty expressing words but retain understanding of words.

Frontal lobotomy

• Once used to quiet uncontrollable patients.
• Involved damage to decision-making processes

Parietal Lobe

• Integrates sensory information with the post-central gyrus being the primary somatosensory cortex (anterior parietal cortex).
• The posterior part of the paracentral lobule contains the sensory representation for the lower limb, and is part of areas 3, 1, 2.
• Posterior parietal lobe is posterior to the post-central gyrus and divided into superior and inferior lobes, directing “where" and "how” and being the dorsal stream visual processing.
• Lesions:
• Primary sensory cortex (areas 3,1,2) lesions: causes loss of sensation on the opposite side of the body, with patients possibly experiencing pins and needles sensation.
• Superior Parietal lobe lesions: lead to astereognosia (inability to recognize objects in hand by shape, texture, etc.), apraxia, loss of hand-eye coordination, and optic ataxia (problem in visually guided hand movements).
• Inferior parietal lobe lesions:
• Non-dominant hemisphere lesions: contralateral neglect.
• Dominant hemisphere lesions: Gerstsmann's syndrome with alexia, agraphia, acalculia, left-right problems, and sometimes anomia.

Temporal Lobe

• Parahippocampal gyrus and the hippocampal formation: important for the formation of new memories (declarative memory).
• The cortex associated with the medial and rostral part of the parahippocampal is the uncus, which is olfactory cortex.
• Temporal lobe epilepsy: often produces olfactory hallucinations during its onset.
• Inferior temporal lobe (ventral stream visual processing): is directed at the “what” in higher processing of visual information.
• Wernicke's area (posterior part, superior temporal gyrus, area 22 in dominate hemisphere):
  • Lesions: lack of understanding of spoken and written word.
  • Posterior part, superior temporal gyrus, area 22 in non-dominant hemisphere: difficulty in perceiving rhythm or emotion of speech.

Occipital Lobe

• Processes visual information with area 17 (primary visual cortex, striate cortex, V1) found along the calcarine sulcus and occipital pole.
• Visual association cortex (areas 18, V2 and area 19, V3) surrounds area 17.
• Lesions:
• Lesion of part of area 17: a small change in vision (scotoma).
• Complete lesion of striate cortex: blindness, but the person might still avoid objects in path (blindsight) or identify an object more than just chance.
• Fusiform gyrus: part of both the occipital and temporal lobe, associated with processing color vision and face recognition.
• Lesion of the occipito-temporal gyrus (V4): loss of color vision or loss of face recognition (prosopagnosia). Synesthesia: stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. Always see certain letters in a particular color.

Types of Brain Injury

• Sequelae may include epilepsy and Alzheimer's disease
• Stroke: 3rd most common cause of death (risk factors: age, hypertension, heart disease) in adults:
• Ischemic (83%): atherosclerosis, thrombosis, and embolism.
• Hemorrhagic (17%): Location: intracerebral, subarachnoid.
• Causes: aneurysm, arteriovenous malformations, in children especially < 2 years old hemorrhagic stroke more common.
• Traumatic brain injury (TBI): commonly evaluated using Glasgow Coma Score.
  • Contusive: bruise involving contusion at the site of impact and opposite side of brain, 20-30% head injuries.
  • Penetrating is most often caused by bullets.
• Diffuse axonal injury (DAI): acceleration/deceleration injury with rotation, damages axons due to stretching or shearing forces
  • Abusive Head Trauma (Shaken baby syndrome): ~DAI, violent shaking of head of babies.
  • Blast injuries: often caused by improvised explosive devices that create a blast wave.
• Tumor: may be cancerous or benign with metastatic forms being 10X more common.
• Glioblastoma: most common malignant brain tumors
• Surgery for medical reasons: used in severe epilepsy, involves either removal (resection) or cutting pathways.
• Brain abscess: unless related to penetrating brain injury, infection 2ndary to infection elsewhere in the body.
• Anoxia: lack of oxygen to brain, may occur with cardiac arrest, choking, strangulation.
• Deficits-
  • Apraxia: difficulty carrying out skilled tasks.
  • Agnosia: difficulty in recognizing objects, sounds, shape (astereognosia) etc.
  • Aphasia: difficulty in using or understanding language.
  • Anomia (word finding problem), conduction aphasia (ability to repeat words); Global aphasia – can't talk or understand of speech due to emboli or thrombus in middle cerebral artery.
  • Apulia: difficulty initiating movement.
  • Disconnection syndrome: based on centres for information processing, disconnecting centres may have similar symptoms as destroying these centres.
  • Alexia: loss of ability to read or understand written words.
  • Acalculi: acquired difficulty in math.
  • Agraphia: loss of ability to write.
  • Anomia: difficulty coming up with the name of everyday objects.

Brain Dominance

• Roger Sperry: Nobel Prize for discoveries concerning the specialization of the cerebral hemispheres based on "split brain" studies (surgical section of the corpus callosum).
• Dominant hemisphere (usually the left) is typically involved in language comprehension and expression, as well as logical thinking
• Non-dominant hemisphere (usually the right) is important in facial recognition, dressing, making block designs, drawing three dimensional cubes, creativity.
• Brain dominance is related to handedness (right-handed people have 5% & left-handed people have 20%). Never assume left brain dominance