Brain Organization, Functional Dissociation, and Stroke

Phrenology (1808)

  • focused on measurements of the human skull

  • based on the concept that the brain is the organ of the mind

  • certain brain areas have localized, specific functions or modules

  • brain composed of 21 personality organs with a specific mental function, each found at a specific location in the cerebral convolutions of the brain

  • conclusions based on “extremes of society” such as criminals or clergyman

Wilder Penfield (1951/54)

  • treated epilepsy patients by destroying nerve cells in the brain where the seizures originated

  • stimulated the brain with electric probes while patients were conscious and observed their responses

  • accurately targeted areas of the brain responsible, reducing side effects of the surgery

  • technique allowed for creation of sensory and motor homunculus

Brain Anatomy (Cortex)

Frontal Lobe

  • located in the front of the brain, the most rostral

  • posterior border is the central sulcus

  • largest of the four lobes

  • responsible for higher level executive functions

    • attention, critical thinking, impulse control

  • influences personality

  • last brain region to fully develop

  • location of the primary motor cortex

    • responsible for planning and executing movement

    • specifically located in the precentral gyrus

  • Broca’s Area

    • sections 44 and 45

    • speech production

    • articulation, language comprehension, grammar, sound sequencing, semantics

Parietal Lobe

  • divided from frontal lobe by central sulcus

  • sense different tactile properties of things in the world around us with our body

  • proprioception: the ability to identify where parts of your body are located

  • higher level visual processing

  • sensory cortex

    • interprets sensry input like touch, temperature, pain, vision, sound, taste, and smell

  • primary somatosensory cortex

    • in postcentral gyrus

    • sections 1, 2, 3

    • responsible for the perception of touch and pain

    • represented by sensory homunculus

      • neural spatial representation of sensors for body parts, disproportionate 

Temporal Lobes

  • side of the brain, separated from frontal and parietal by lateral fissue

  • sensory processing (hearing, smell, taste and higher level visual processing)

  • hippocampus

    • buried medially and ventrally in temporal lobe

    • ability to remember important facts

    • memory-related processes

  • primary auditory cortex

    • section 41: initial processing of sound, detecting frequencies

    • section 42: basic processing of auditory information (pitch rhythm and volume

  • Wernicke’s Area

    • comprehension and interpretation of speech and complex sounds

Occipital Lobes

  • back of the brain

  • processing of visual stimuli (light, objects in motion, object orientation, color)

  • primary visual cortex

    • section 17

    • receives raw visual data from the retina

    • initial processing of visual input

  • visual association cortex

    • section 18

    • interpretation and recognition of visual images

    • complex processing

Anatomical Planes

  • frontal/coronal plane

    • vertical plane

    • runs parallel to eyes or ears

    • divides into front and back regions

  • sagittal plane

    • vertical plane

    • runs perpendicular to the eyes or ears

    • divides into left and right regions

  • horizontal plane

    • runs parallel to the ground

    • divide into top and bottom regions

Nervous System

  • divided into CNS and PNS

  • CNS includes brain and spinal cord

  • PNS includes cranial and spinal nerves

    • info travelling towards the brain is afferent

    • info travelling from the brain is efferent

  • multiple sclerosis affects afferent and efferent

Sensory Afferent Pathways

  • sensory info can be external or internal (skin, gut, heart, etc.)

  • dorsal column lemniscal pathways

    • touch, pressure

  • spinothalamic pathways

    • pain, temperature

Motor Efferent Pathways

  • carries from CNS (primary motor cortex) down into brainstem then to muscles/glands

Vascular Innervation

  • brain requires oxygen and nutrients to function

    • accomplished by blood pumped around ithe body using a network of blood vessels (circulatory system)

  • brain has a high demand for oxygen and nutrients

    • 2% of total body weight but receives 15% of total cardiac output

Blood Supply to Brain

  • from two major arteries

  • common carotid arteries

    • major blood vessels in the neck that supply oxygenated blood to the head, neck, and brain

    • branches into internal and external carotid arteries

    • anterior cerebral artery (ACA)

      • supplies the medial surface of the frontal and parietal lobes, the frontobasal cortex, most of the corpus callosum

    • middle cerebral artery (MCA)

      • supplies the largest area, including the lateral surfaces of the frontal, parietal, and temporal lobes, as well as the insular cortex

      • supplies blood to broca’s area, when blocked can cause broca’s aphasia (difficulty saying words)

      • supplies to wernicke’s area, when blocked can cause wernicke’s aphasia (difficulty understanding language)

      • deep branches supply the basal ganglia and internal capsule

        • lenticulostriate arteries supply basal ganglia (caudate nucleus, etc. ) and internal capsule

          • common site for small strokes from bloackage, cause motor deficits

    • anterior choroidal artery

      • supplies parts of the hippocampus, amygdala, posterior limb of the internal capsule and the thalamus

    • ophthalmic artery

      • supplies the structures of the eye/orbit

  • vertebral arteries

    • provide 20-30% of the blood supply to the brain

    • supply the brainstem, cerebellum, and posterior cerebral hemispheres

    • posterior inferior cerebellar artery

      • supplies the inferior cerebellum and parts of the brainstem

    • basilar artery

      • formed by the joining of vertebral arteries, supplies the pons and cerebellum

    • superior cerebellar artery

      • supplies the superior cerebellum and parts of the midbrain

    • posterior cerebral arteries

      • supply the occipital lobes (visual cortex) and the inferior part of the temporal lobes

  • particular blood flow to specific regions

    • blockage can cause specific deficits

Stroke

  • the 3d leading cause of death in canada (after cancer and heart disease)

  • caused by disruption of blood flow to part of the brain

  • deprives neurons and other brain cells of glucose and oxygen leading to cell death

  • results in impaired or lost function

  • hemorrhagic stroke = hemorrhage/blood leaks into brain tissue

    • 20% of strokes

    • caused by uncontrolled bleeding in the brain

      • weakened/diseased blood vessels rupture

    • subarachnoid hemorrhage = uncontrolled on the surface of the brain in the area between the brain and the skull (direct impact to head)

    • intracerebral hemorrhage = when an artery deep within the brain ruptures

    • aneurysm = a weakened area in the blood vessel wall of the blood vessel that fills with blood and bulges

      • high blood pressure or trauma can cause the bulge to rupture, resulting in uncontrolled bleeding into the brain

  • ischemic stroke

    • 80% of strokes

    • interruption of blood flow due to a blood clot

    • plaque (fatty materials, calcium, and scar tissue) contributes to ost ischemic strokes by narrowing the arteries that supply blood to the brain (atherosclerosis sclerosis)

    • thrombotic strokes = blood clot forms in an artery directly leading to the brain

    • embolic strokes = clot develops somewhere else in the body and travels to the brain

    • core is where there are dying neurons (necrosis) and quick cell death

      • cells have no energy, loss of oxygen and glucose

      • blood flow below 10-25%

    • penumbra is surrounding core

      • blood flow is ~25-50%

      • cells may be viable for hours, reversible damage

      • can eventually die by aptosis - recognize low oxygen and glucose levels and start cell destruction pathways

      • can be brought back with the return of blood flow

    • transient ischemic attack = mini stroke

      • caused by temporary interruption of blood flow to the brain

      • many people can have a TIA without even knowing it

      • can put a person at an increased risk of a full blown stroke, vascular dementia or neurodegeneration

      • clot breaks off and travels to smaller arteries

      • watershed areas = areas highly susceptible to stroke due to low perfusion pressure

  • stroke kills neurons, neurons are responsible for behaviour

    • decreased blood flow causes metabolic dysfunction (no oxygen, glucose, ATP), disrupts neural activity in area

Neuropathology of a Stroke

  • reduced cerebral blood flow (can be ischemic (thrombotic or embolic) or hemorrhagic) causes ATP depletion (lack of oxygen and glucose)

  • ATP depletion leads to:

    • neurochemical injury

      • lactic acidosis

      • edema

      • oxidative stress

    • neuroinflammation injury

      • reactive astrocytes

      • activated microglia

      • neutrophil infiltration

  • neurochemical injury and neuroinflammation injury lead to cell damage and neuronal death

Pathophysiology of Stroke

  • “neural shock”

  • areas distant from the damage are functionally depressed (penumbra)

    • both pathogenic and self-protective mechanisms occur in penumbra

    • ischemic penumbra is functionally compromised but potentially “salvageable”

  • areas related to the damaged region suffer sudden withdrawal of excitation or inhibition (core)

    • irreversible bran damage in ischemic core

  • may be followed by changes in the metabolism of the injured hemisphere, glucose utilization, or both, which may persist for days

Ischemic Cascade

  • neurons and glial cells (astrocytes, oligodendrocytes) lose their ability to produce energy in the absence of glucose due to interrupted blood flow

    • astrocytes bring glucose to neurons

  • when astrocytes react, blood brain barrier breaks down

  • in ischemic stroke: reduced blood flow to brain —> reduced glucose to astrocytes —> glucose cannot go to neurons —> cannot make ATP —> cell death