NE3501 Revision Notes: Mechanisms of Brain Disorders

NE3501 Revision Notes: Mechanisms of Brain Disorders

How to Use These Notes

• Each mechanism section opens with a viva-ready one-liner, then gives deep mechanistic notes disease by disease.

• Bullet points in BOLD are the highest-yield exam facts — make sure these are in your long-term memory.

• Exam Insight boxes = critical evaluation points that distinguish first-class answers.

• Viva Opening boxes = how to start your answer fluently.

• Everything links: synapse loss connects to protein aggregation, neuroinflammation, mitochondrial dysfunction, and oxidative stress — always think cascades.

• FIRST-CLASS RULE: Never just describe a phenomenon. Explain the molecular mechanism, name the proteins/receptors/pathways involved, state the functional consequence, and critically evaluate the evidence or therapeutic implications.

Mechanism 1: Synapse Loss 💬 Viva Opening

• Key Concept: 'Synapse loss is the single most therapeutically important shared feature of neurological disease. It is the earliest detectable pathological change — preceding neuronal death by years — and unlike neuronal loss, it is potentially reversible. In Alzheimer's disease, synapse loss is the strongest correlate of cognitive decline, stronger than amyloid plaque burden or neurofibrillary tangle count. Across schizophrenia, depression, epilepsy, MS, TBI, stroke, and PD, different initiating mechanisms all converge on the loss of functional synaptic connections, producing impaired circuit function and — eventually — irreversible neurodegeneration.'

Anatomy and Physiology of a Synapse

• **Presynaptic Terminal:
    - Active zone, synaptic vesicles (glutamate, GABA, dopamine (DA), serotonin (5-HT), acetylcholine (ACh) packaged by VMAT2/VAChT)   - Mitochondria (providing ATP for vesicle fusion)   - Calcium channels (Cav2.1/2.2)   - SNARE proteins (synaptobrevin, syntaxin, SNAP-25)

• **Postsynaptic Density (PSD):
    - NMDA receptors (ionotropic, Ca²⁺-permeable, require glutamate + glycine + depolarization)   - AMPA receptors (fast excitatory current)   - Scaffolding proteins (PSD-95, SHANK, Homer, drebrin)   - Signaling enzymes (CaMKII, calcineurin, PKA)

• Astrocytic Ensheathment:
    - GLT-1 (EAAT2) removes excess glutamate; K⁺ buffering; metabolic support; tri-partite synapse.

• Synaptic Plasticity: LTP vs LTD
    - Long-Term Potentiation (LTP): ↑ AMPA receptor expression/conductance → memory encoding.
    - Long-Term Depression (LTD): ↓ AMPA receptor expression → forgetting.

Consequences of Synapse Loss

• No vesicle release → no postsynaptic depolarization → no signal propagation → no LTP → no memory formation/motor control/sensory processing depending on circuit.

Timeline of Synapse Loss

1. Stage 1 — Presynaptic Dysfunction:
     - Decreased vesicle cycling, decreased neurotransmitter release — neurons still alive, synapse structurally intact.

2. Stage 2 — Postsynaptic Receptor Loss:
     - NMDA/AMPA receptor endocytosis, PSD-95 downregulation, spine morphology changes — partially reversible.

3. Stage 3 — Structural Synapse Loss:
     - Dendritic spine retraction, axonal terminal withdrawal — harder to reverse but possible.

4. Stage 4 — Axonal Dying-Back:
     - Neuron withdraws its axon before cell body dies — still alive but disconnected.

5. Stage 5 — Neuronal Death:
     - Apoptosis (caspase-mediated) or necrosis — IRREVERSIBLE.

Therapeutic Implications

• Intervene at Stage 1-3 = possible recovery.

• Intervene at Stage 5 = impossible.

• Most patients are diagnosed at Stage 4-5. This is why treatments fail.

Exam Insight

• Evidence from Prion Mouse Models:   - Trazodone + DBM administered at week 7 post-RML infection (signifying synaptic dysfunction but pre-neuronal-loss) preserved hippocampal architecture and extended survival. Treatment given post-neuronal-loss had zero effect. This demonstrates that early intervention is essential for real therapeutic benefit.

Biomarkers of Synaptic Loss

• Elevated Biomarkers in Alzheimer’s Disease (AD):   - Neurogranin (NRGN): Postsynaptic Ca²⁺ signaling protein; elevated in CSF of AD patients before symptom onset; best current biomarker of synaptic loss.
    - Synaptotagmin-1: Presynaptic vesicle fusion sensor; elevated in CSF in AD.
    - SNAP-25: Presynaptic SNARE protein; elevated in CSF in AD and schizophrenia.
    - SV2A: Synaptic vesicle protein 2A; ligand for levetiracetam; imaged via [¹¹C]UCB-J PET — shows reduced synaptic density in AD, depression, epilepsy, and schizophrenia.

1.1 Synapse Loss in Alzheimer’s Disease and Dementia

• Key Fact: Synapse loss is the STRONGEST correlate of cognitive decline in AD — stronger than amyloid plaque burden OR neurofibrillary tangle count.   - This conclusion has been replicated in multiple post-mortem studies.   - Synapse density in frontal and temporal cortex correlates directly with MMSE scores.   - Synapse loss begins in the entorhinal cortex and hippocampus — explaining why episodic memory is the first symptom.

Mechanisms of Synapse Loss in Alzheimer’s Disease

• Amyloid-Driven Synapse Loss:
    - Aβ42 oligomers (NOT mature plaques) are the primary synaptotoxic species. They directly bind to PSD-95 and NMDA receptors at synaptic membranes, triggering receptor endocytosis via calcineurin activation → ↓ surface NMDA receptors → impaired LTP.
    - Aβ oligomers also activate calcineurin → dephosphorylate GluA1 (AMPA receptor subunit) → endocytosis of AMPA receptors → ↓ synaptic strength.
    - Aβ disrupts Ca²⁺ homeostasis: oligomers form membrane pores OR sensitize NMDA receptors → excess Ca²⁺ → activates calpain (protease) → degrades PSD-95, SHANK → disassemble postsynaptic scaffold.

• Tau-Driven Synapse Loss:
    - Hyperphosphorylated tau (pTau) detaches from microtubules → microtubule collapse → axonal transport fails → synaptic terminals starved of mitochondria, neurotrophins (BDNF), and vesicle proteins → presynaptic terminals die.   - pTau also mislocalizes into dendritic spines, disrupting postsynaptic signaling.   - Tau tangles correlate MORE strongly with cognitive symptoms than amyloid — tau pathology directly drives synaptic dysfunction and neuronal death.
    - Downstream Effects:     - Synapse loss → ↓ BDNF release (activity-dependent) → ↓ TrkB signaling → ↓ neuronal survival → further synapse loss.

• Neuroinflammation Amplifies the Process:
    - A1 astrocytes (induced by microglial TNF-α/IL-1α/C1q) lose GLT-1 expression → excess extracellular glutamate → excitotoxic synapse damage.

Therapeutics Targeting Synaptic Loss in AD

• Memantine: NMDA antagonist → reduces excitotoxic Ca²⁺ influx → protects synapses; approved for moderate-severe AD.

• Donepezil: ↑ ACh at remaining synapses via AChE inhibition → symptomatic; does not halt synapse loss.

• BDNF-Based Strategies: TrkB agonists, exercise (↑ BDNF), SSRI co-treatment — in trials.

• Anti-Oligomer Antibodies: (e.g., lecanemab targets Aβ protofibrils): showing some evidence of slowing decline — directly addresses toxic species.

Exam Insight

• Why Removing Amyloid Plaques Doesn’t Restore Cognition:   - Plaques are DOWNSTREAM of the toxic process — soluble oligomers, not plaques, drive synapse loss. By the time plaques are visible on PET, synaptic damage is already extensive. This forms the core argument against the simple amyloid hypothesis and explains why aducanumab (which clears plaques) had minimal clinical benefit.

1.2 Synapse Loss in Parkinson’s Disease

• Key Concept: Nigrostriatal synapse loss PRECEDES Substantia Nigra compacta (SNc) neuronal loss; compensation occurs until ~60-80% of dopaminergic terminals are lost, creating a long presymptomatic period.

Mechanisms of Synapse Loss in Parkinson’s Disease

• α-Synuclein Dysfunction:   - At presynaptic terminals: regulates SNARE complex assembly and vesicle recycling via VMAT2. Aggregated α-syn impair these processes → ↓ dopamine packaging → ↓ quantal DA release.   - α-syn oligomers embed in synaptic vesicle membranes → ↓ vesicle fusion efficiency → further ↓ DA release.

• Circuit Imbalance:   - Striatal dopamine loss shifts D1:D2 Medium Spiny Neuron (MSN) balance → Basal Ganglia circuit imbalance → hypokinesia.

• Cholinergic Synapse Loss in Basal Forebrain: Cognitive symptoms in PD dementia arise (similar mechanism to AD).

• Lewy Body Spread (Braak Staging): Synaptic loss follows α-syn propagation — olfactory → autonomic → SNc → limbic → cortex.

Treatment Implications

• L-DOPA and Dopamine Agonists: Restore synaptic dopamine levels but do NOT halt ongoing loss of dopaminergic synapses — purely symptomatic.

• DBS of STN: Corrects circuit imbalance downstream of synaptic loss — also purely symptomatic.

1.3 Synapse Loss in Depression

• Key Concept: Chronic psychosocial stress → HPA axis hyperactivation → ↑ cortisol → glucocorticoid receptor activation in hippocampus and prefrontal cortex (PFC).

Mechanisms of Synapse Loss in Depression

• BDNF Deprivation:   - ↑ glucocorticoids → ↓ BDNF transcription → ↓ TrkB signaling → ↓ dendritic spine density and complexity in PFC layer 2/3 and hippocampal CA3.

• Synapse Loss Consequences:   - ↓ synaptogenesis + ↑ synapse elimination → net synapse loss → reduced connectivity between PFC and limbic structures → loss of cognitive control over emotional responses.

Morphological Evidence

• Post-mortem studies show ↓ dendritic spine density in PFC and hippocampus; ↓ synaptophysin, ↓ PSD-95; hippocampal volume reduction partly due to synapse/dendritic loss and partly due to ↓ neurogenesis in dentate gyrus (stress → ↓ BrdU+ cells).

Treatment Strategies

• Ketamine:
    - Rapid reversal mechanism: blocks NMDA receptors at rest → disinhibits mTORC1 → activates protein synthesis → rapid translation of synaptic proteins (PSD-95, GluA1, synapsin) → ↑ AMPA:NMDA receptor ratio at synapses → synaptogenesis within 2 hours.

• SSRIs (Slow Mechanism):
    - ↑ extracellular 5-HT → gradual desensitization of 5-HT1A autoreceptors → sustained ↑ postsynaptic 5-HT → slow ↑ BDNF → gradual synaptogenesis over weeks.

• BDNF Hypothesis:
    - SSRIs, exercise, ECT all work by restoring BDNF — the synaptic maintenance factor — in hippocampus and PFC.

1.4 Synapse Loss in Schizophrenia

• Key Concept: Synaptic pruning hypothesis — during adolescent brain maturation, complement proteins (C1q, C3) tag synapses for microglial phagocytosis — normal pruning refines circuits.

Mechanism Leading to Excessive Pruning

• Dysregulation of Pruning:   - In schizophrenia: EXCESSIVE pruning, especially in PFC → ↓ PFC connectivity → cognitive symptoms (working memory, executive function) and negative symptoms.

• C4A Gene Insight:
    - GWAS studies show C4A copy number strongly associated with schizophrenia risk — more C4A → more complement-mediated synapse elimination.

Consequences of NMDA Receptor Hypofunction

• Hypofunction at GABAergic Parvalbumin Interneurons:   - Loss of synaptic drive → ↓ GABAergic inhibition of pyramidal neurons → disrupted gamma oscillations → impaired working memory.   

• Post-Mortem Findings:   - Reduced dendritic spine density in PFC post-mortem in schizophrenia patients.

Glutamate Hypothesis

• Role of NMDA Hypofunction:
    - ↓ drive on GABAergic cells → ↓ inhibition of mesolimbic DA neurons → ↑ mesolimbic DA → positive symptoms; + ↓ mesocortical DA → negative symptoms.

• Support from NMDA Antagonists:   - Ketamine and PCP produce full schizophrenia-like syndrome including negative symptoms — supporting this model.

1.5 Synapse Loss in Epilepsy

• Key Concept: Epilepsy involves a paradox: seizures CAUSE synapse loss, and synapse loss CAUSES seizures — bidirectional relationship.

Mechanisms of Seizure-Induced Synapse Loss

• Prolonged Seizures:
    - → excitotoxicity → Ca²⁺ overload → calpain activation → synaptic protein degradation → interneuron loss.

• Consequences of GABAergic Interneuron Loss:
    - Loss of inhibitory synapses → ↑ excitatory/inhibitory (E/I) ratio → more seizures → more synapse loss — vicious cycle.

Vulnerabilities in Epilepsy

• Hilar Mossy Cells and Basket Cells:
    - Especially vulnerable in hippocampus; their loss contributes to mesial temporal lobe epilepsy (MTLE).

• Mossy Fibre Sprouting:
    - Surviving granule cells in dentate gyrus sprout new axon collaterals that form aberrant excitatory synapses on other granule cells → pathological recurrent excitation.

Homeostatic Synaptic Plasticity

• Synaptic Scaling:
    - After prolonged reduced activity, neurons upscale excitatory synapses — if excessive, this creates a hyperexcitable network → seizure susceptibility.

• Status Epilepticus:
    - Prolonged seizure → massive glutamate release → NMDA → Ca²⁺ → excitotoxic synapse/neuron loss → cognitive consequences.

1.6 Synapse Loss in Traumatic Brain Injury (TBI)

• Key Concept: Within seconds of impact, glutamate surge (~50× normal) → NMDA receptor overactivation → massive Ca²⁺ influx → immediate proteolysis of synaptic proteins.

Mechanisms of Synapse Loss Following TBI

• Primary Synapse Loss:
    - Immediate synaptic destruction due to Ca²⁺ overload and proteolytic activity.

• Traumatic Axonal Injury (TAI/DAI):
    - Abnormal influx of Ca²⁺ → impaired axonal transport → loss of presynaptic terminals.

• Secondary Synapse Loss:   - Acute neuroinflammation → excitotoxicity → continued synapse loss extending far beyond initial injury site.

Vulnerable Cells after TBI

• GABAergic Parvalbumin Interneurons:
    - Particularly vulnerable to TBI; their loss leads to E/I imbalance and increases risk for post-traumatic epilepsy.

• Chronic Synaptic Dysfunction:
    - Increased GABA-A-mediated inhibition after sub-acute TBI leads to impaired recovery.

Long-Term Implications

• Chronic Traumatic Encephalopathy (CTE):
    - Result of repetitive mild TBI leads to cumulative synapse loss + tau accumulation → progressive cognitive decline even without severe single TBI.

• Biomarkers:
    - ↑ serum/CSF GFAP (astrocyte damage), NfL (axonal damage), synaptotagmin-1 after TBI.

1.7 Synapse Loss in Multiple Sclerosis (MS)

• Key Concept: White matter lesions disrupt axonal connectivity — demyelinated axons lose saltatory conduction → chronic conduction block → axonal retraction → synapse loss at axon terminals.

Mechanisms of Synapse Loss in MS

• Chronic Active Lesions:
    - Smouldering microglial inflammation causes ongoing axonal transection — leading to progressive synapse loss.

• Complement-Mediated Synapse Elimination:
    - Activated complement tags synapses for microglial phagocytosis; similar to schizophrenia pruning hypothesis.

• Cognitive Impairment in MS:
    - Correlates with grey matter atrophy (synapse/neuron loss), not white matter lesion load; grey matter damage is the primary driver.

Distinct Lesion Types in MS

1. Gel lesions (GELs):
     - Acute relapse phase — respond well to treatment.

2. Chronically Active Lesions (CALs):
     - Closed BBB, persistent smouldering inflammation, ongoing axonal transection.

3. Slowly Expanding Lesions (SELs):
     - Markers of ongoing inflammation behind an intact BBB — progressive synapse and axon loss.

4. Paramagnetic Rim Lesions (PRLs):
     - Indicator of chronic inflammation with predictive value for disability.

1.8 Synapse Loss in Stroke

• Key Concept: Core infarct leads to complete ischemia and ATP depletion within minutes; reperfusion can save surrounding penumbral tissue unless delayed.

Mechanisms of Synapse Loss in Stroke

• Core Infarct:
    - Complete ischaemia → ATP depletion → massive glutamate release → excitotoxic synapse and neuron death.

• Penumbra:
    - Hypoxic but viable zone; neurons still alive but synapses dysfunctional; critical time for reperfusion treatments.

Consequences of Stroke

• Diaschisis:
    - Remote brain areas connected to infarcted area lose input → transsynaptic synaptic loss results in broader cognitive/motor deficits.

• Neurorehabilitation:
    - Motor cortex plasticity (use-dependent plasticity) can rewire surviving circuits to compensate for lost synaptic connections.

• Proportional Recovery Rule:
    - Most biological synapse recovery occurs in the first 0-3 months; intensive rehabilitation is necessary within this acute phase.

Mechanism 2: Dopamine Dysfunction 💬 Viva Opening

• Key Concept: 'Dopamine dysfunction is a unifying feature across disorders that may appear unrelated: Parkinson's disease, Huntington's disease, schizophrenia, ADHD, Tourette's syndrome, and depression all involve abnormalities in dopaminergic signaling — but critically in DIFFERENT circuits and in DIFFERENT DIRECTIONS. Understanding this circuit-specificity distinguishes a first-class answer.'

Dopamine Synthesis, Packaging, and Release

• Synthesis:
    - Tyrosine → L-DOPA (tyrosine hydroxylase, TH — rate-limiting step) → Dopamine (DOPA decarboxylase/AADC).

• Packaging:
    - Dopamine packaged into vesicles by VMAT2 — target of tetrabenazine in HD/Tourette's.

• Release:
    - Action potential → Cav2.1/2.2 opens → Ca²⁺ → SNARE complex → vesicle fusion → DA into synapse.

• Receptors:
    - D1-like (D1, D5 — Gs/Golf coupled → ↑ adenylyl cyclase → ↑ cAMP → ↑ PKA → ↑ AMPA receptor phosphorylation) and D2-like (Gi coupled → ↓ cAMP).

• Reuptake:
    - Dopamine transporter (DAT) — target of methylphenidate, cocaine, amphetamine — recycles DA back into presynaptic terminal.

• Degradation:
    - MAO-A and MAO-B (in mitochondria) → DOPAC; COMT → 3-MT → HVA (homovanillic acid in urine — marker of DA turnover).

The Basal Ganglia Circuit

• Cortex (Glutamatergic) → Striatum
    - Input nucleus containing GABAergic medium spiny neurons (MSNs) expressing D1 (direct pathway) or D2 (indirect pathway) receptors.

• Direct Pathway (D1 MSNs + substance P/dynorphin):
    - Striatum → suppresses GPi/SNr (GABAergic) → ultimately disinhibits the thalamus → enhances thalamocortical excitation → MOVEMENT PROMOTED.

• Indirect Pathway (D2 MSNs + enkephalin):
    - Striatum → inhibits GPe → disinhibits STN → STN excites GPi/SNr → GPi/SNr inhibits the thalamus → suppresses thalamocortical excitation → MOVEMENT SUPPRESSED.

Net Effects of Dopamine

• Dopamine promotes movement by strengthening the direct pathway (D1) and weakening the indirect pathway (D2).

• GPI/SNr is the final common output receiving signals from both pathways and providing the tonic GABAergic brake on the thalamus.

• Dopamine = the GAS PEDAL of the motor system: When dopamine is removed (PD), bradykinesia occurs as acceleration is impaired. When indirect pathway control is lost (HD), excessive movement occurs.

Primary Dopaminergic Nuclei

• Substantia Nigra Pars Compacta (SNc):
      - Nigrostriatal pathway → caudate + putamen → motor control → primary site of degeneration in PD leading to bradykinesia, rigidity, tremor.

• Ventral Tegmental Area (VTA):
      - Mesolimbic pathway → nucleus accumbens, amygdala, hippocampus → reward, motivation, emotion → excess mesolimbic dopamine leads to positive psychosis symptoms.

2.1 Dopamine in Parkinson’s Disease

• Primary Lesion:
      - Progressive degeneration of SNc dopaminergic neurons → decreased nigrostriatal dopamine.

• Pathology:
      - Lewy bodies (α-synuclein aggregates) disrupt mitochondrial function → cell death.

• Circuit Consequence:
      - Decreased DA → decreased D1 activation + increased D2 inhibition → leads to hypokinesia.

Treatments

• L-DOPA:
      - Dopamine precursor that crosses BBB; combined with carbidopa to prevent peripheral side effects.

• DA Agonists:
      - Stimulate D2/D3 directly but have impulse control disorders as side effects.

• Deep Brain Stimulation (DBS):
      - Reduces GPi/SNr activation → improves hypokinesia; symptom control only.

2.2 Dopamine in Huntington's Disease

• Primary Pathology:
      - GAG CAG repeat expansion → leads to striatal MSN degeneration, especially D2 MSNs → circuit consequences that mimic excess dopamine leading to chorea.

Treatments for Huntington's Disease

• VMAT2 Inhibitors:
      - Block DA packaging and release, reducing chorea.

• Antipsychotics:
      - Block D2 receptors → reduce chorea and treat psychosis, but worsen bradykinesia; this emphasizes that HD is not a pure dopamine disorder.

Mechanism 3: Protein Aggregation and Misfolding 💬 Viva Opening

• Key Concept: 'Protein misfolding and aggregation is the central pathological event across neurodegenerative diseases. Despite different causative proteins — Aβ and tau in Alzheimer's, α-synuclein in Parkinson's and Lewy body dementia, PrPSc in prion diseases, polyQ huntingtin in Huntington's, TDP-43 in frontotemporal lobar degeneration (FTLD) — the shared principle is a conformational change from a normally soluble, functional protein to a β-sheet-rich, insoluble, toxic form that overwhelms the cell's protein homeostasis machinery. The most toxic species in virtually all cases are soluble intermediate oligomers — not the large visible aggregates that were historically the focus. These oligomers impair synaptic function, trigger neuroinflammation, and propagate through the brain in a prion-like fashion along neural circuits, explaining the stereotyped anatomical progression seen across diseases.'

Protein Homeostasis (Proteostasis)

• Defense Mechanisms:
    1. Chaperones: (e.g., HSP70, HSP90) — attempt to refold misfolded proteins; if unsuccessful, proteins are flagged for degradation.   2. Ubiquitin-Proteasome System (UPS):
      - Tags misfolded proteins for degradation by adding polyubiquitin chains.   3. Autophagy:
      - Engulfs large misfolded proteins or organelles for lysosomal degradation.

• Decline Mechanism with Aging:
    - Aging leads to a decline in all three lines of defense—contributing to neurodegeneration.

β-Sheet Structure and Pathology

• Normal vs Amyloid:
    - Normal proteins contain α-helices; amyloid aggregates have cross-β structures which are highly stable and resistant to degradation.

• Most Toxic Oligomers:
    - Exposed hydrophobic surfaces can interact with membranes, receptors, and organelles, contributing to toxicity.

3.1 Protein Aggregation in Alzheimer’s Disease — Aβ and Tau

• Amyloid-β (Aβ) Processing:
    - APP processing leads to Aβ generation, with Aβ42 being more toxic due to higher propensity for aggregation.

Mechanism of Aβ's Toxicity

• Non-Amyloidogenic vs Amyloidogenic Pathway:
    - Switch in proteolytic processing leads to different dominant species affecting synaptic health.

Aβ Clearance Mechanisms

• Normal mechanisms include CSF flow and microglial phagocytosis, but are impaired in aging and AD.

Tau Pathology in Alzheimer’s Disease

• Pathophysiological Role of Tau:
    - Hyperphosphorylated tau detaches from microtubules, disrupting axonal transport and worsening cell health.

Mechanism 4: Neuroinflammation 💬 Viva Opening

• Key Concept: Neuroinflammation can transition from a protective to a detrimental role in brain disorders. The effects depend on the contextual initiation (primary vs secondary inflammation).

Microglia and Inflammation

• Homeostatic vs Disease-Associated Microglia (DAM):
    - Transition involves a change in gene expression profiles from maintenance of cell health to promoting inflammation.   

Cytokine Mediation

• Role of Cytokines in Neuroinflammation:
    - Pro-inflammatory cytokines can lead to neurodegeneration while anti-inflammatory cytokines can promote recovery.

Neuroinflammation in Multiple Sclerosis (MS)

• In MS, inflammation can drive neurodegeneration directly, suggesting it is both the disease initiation and the exacerbating factor.

Mechanism 5: Oxidative Stress 💬 Viva Opening

• Key Concept: Oxidative stress results from an imbalance of reactive oxygen species (ROS) and antioxidant defenses. It is a shared mechanism across various neurological diseases, particularly affecting the vulnerable brain tissues.

Component Interactions

• What ROS Damage:
    - Leading to oxidative damage to proteins, lipids, and DNA, which contributes to neurodegeneration and links with neuroinflammation and mitochondrial dysfunction.

Antioxidant Defense Systems

• Failure in Disease States:
    - Both enzymatic (e.g., SOD, catalase) and non-enzymatic (e.g., GSH) defenses are compromised, exacerbating disease progression.

Examination Insights

• Critical Evaluation of Treatments:
    - Recognize that targeting single pathways often does not yield significant improvement due to the complexity and multi-factorial nature of diseases.   - Encourage the exploration of combinatorial therapeutic strategies that can address multiple implicated pathways simultaneously, guided by biomarkers for early intervention.