The enteric nervous system (ENS), often called the "second brain," is a complex network of neurons embedded in the lining of the gastrointestinal tract. Multiple system atrophy (MSA) is a fatal neurodegenerative disease characterized by autonomic failure, parkinsonism, and cerebellar ataxia. The ENS is prominently affected in MSA, with alpha-synuclein pathology occurring in enteric neurons years before central nervous system symptoms manifest. This page covers the role of enteric neurons in MSA pathogenesis, clinical implications, and diagnostic significance.
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| Taxonomy |
ID |
Name / Label |
| Cell Ontology (CL) |
CL:0007011 |
enteric neuron |
- Morphology: enteric neuron (source: Cell Ontology)
- Morphology can be inferred from Cell Ontology classification
| Database | ID | Name | Confidence |
|----------|----|------|------------|
| Cell Ontology | CL:0007011 | enteric neuron | Medium |
| Cell Ontology | CL:4042028 | immature neuron | Medium |
¶ Structure and Organization
The ENS is the largest component of the peripheral nervous system, containing approximately 200-600 million neurons organized into two major ganglionated plexuses:
The ENS contains multiple distinct neuronal populations:
- Intrinsic primary afferent neurons (IPANs): Detect stretch, chemical stimuli, and luminal content
- Vagal afferents: Transduce information to the central nervous system
- Excitatory motor neurons: Release acetylcholine, promote contraction
- Inhibitory motor neurons: Release nitric oxide (NO) and vasoactive intestinal peptide (VIP)
- Ascending interneurons: Propagate signals orally
- Descending interneurons: Propagate signals anally
Enteric neurons express various neurotransmitters:
- Acetylcholine (ACh): Primary excitatory neurotransmitter
- Nitric oxide (NO): Primary inhibitory neurotransmitter
- Vasoactive intestinal peptide (VIP): Inhibitory, promotes relaxation
- Substance P: Excitatory, involved in pain transmission
- Calcitonin gene-related peptide (CGRP): Sensory signaling
- Serotonin (5-HT): Motility regulation
In MSA, enteric neurons accumulate abnormal alpha-synuclein aggregates:
- Lewy bodies: Intracytoplasmic inclusions composed of phosphorylated, ubiquitylated alpha-synuclein
- Lewy neurites: Abnormal neuritic processes with alpha-synuclein deposits
- Phosphorylated alpha-synuclein (pS129): Pathological form detected in enteric neurons
The "body-first" propagation model suggests:
- Peripheral initiation: Alpha-synuclein pathology begins in the ENS
- Retrograde transport: Pathological proteins travel via vagal nerve to the dorsal motor nucleus of the vagus (DMV)
- Centripetal spread: Progresses to central autonomic structures
- Brainstem involvement: Eventually affects the substantia nigra, cerebellum, and cortex
This model is supported by:
- Detection of phosphorylated alpha-synuclein in enteric neurons of pre-motor MSA patients
- Experimental models showing vagal nerve-mediated spread
- Braak staging in synucleinopathies
Enteric neurons in MSA show selective vulnerability:
- Oxidative stress: High metabolic activity, mitochondrial dysfunction
- Neuroinflammation: Local immune activation
- Environmental exposures: Gut microbiome alterations
- Genetic susceptibility: GBA mutations, SNCA polymorphisms
- Prevalence: Up to 90% of MSA patients
- Onset: Often precedes motor symptoms by years
- Mechanism: Loss of inhibitory enteric neurons, impaired peristalsis
- Severity: Progressive, often refractory to laxatives
- Delayed gastric emptying: Nausea, early satiety
- Small intestinal bacterial overgrowth (SIBO): Bloating, malabsorption
- Dysphagia: Oropharyngeal phase involvement
- Nocturia: Frequent nighttime urination
- Urgency: Sudden urge to urinate
- Frequency: Increased daytime urination
- Urinary retention: Incomplete bladder emptying
- Overflow incontinence: Dribbling urination
- Neurogenic bladder: Detrusor overactivity or underactivity
- Definition: ≥20 mm Hg systolic or ≥10 mm Hg diastolic drop within 3 minutes of standing
- Mechanism: Sympathetic neurocirculatory failure
- Symptoms: Lightheadedness, dizziness, syncope
- Prevalence: Common in MSA with orthostatic hypotension
- Complication: Complicates treatment of orthostatic hypotension
- Erectile dysfunction: Often an early symptom in males
- Abnormal sweating: Anhidrosis or hyperhidrosis
- Temperature dysregulation: Intolerance to heat or cold
The ENS offers accessible tissue for diagnosis:
- Rectal biopsy: Detection of phosphorylated alpha-synuclein
- Colon biopsy: Higher sensitivity in early disease
- Submandibular gland biopsy: Higher yield than gastrointestinal sites
¶ Sensitivity and Specificity
- Sensitivity: 50-80% in MSA
- Specificity: Challenging due to overlap with Parkinson's disease
Enteric involvement in MSA vs. other synucleinopathies:
| Feature |
MSA |
Parkinson's Disease |
Dementia with Lewy Bodies |
| GI onset |
Often precedes motor |
Can precede motor |
Variable |
| Constipation severity |
Severe |
Moderate |
Moderate |
| Alpha-syn spread |
Oligodendrocyte-first |
Neuron-first |
Variable |
- Prokinetic agents: Metoclopramide, domperidone
- Laxatives: Osmotic (polyethylene glycol), stimulant
- Botulinum toxin: For spasticity, achalasia
- Dietary modification: Fiber, hydration
- Orthostatic hypotension: Fludrocortisone, midodrine, droxidopa
- Urinary dysfunction: Anticholinergics, alpha-blockers, intermittent catheterization
- Erectile dysfunction: PDE5 inhibitors
- Antioxidants: Targeting oxidative stress
- Anti-inflammatory agents: Modulating neuroinflammation
- Alpha-synuclein targeting: Immunotherapies, aggregation inhibitors
- Gene therapy: Viral vector delivery of neurotrophic factors
- Cell replacement: Stem cell-derived neurons
- Microbiome modulation: Probiotics, fecal microbiota transplantation
The gut-brain axis involves multiple pathways:
- Neural: Vagus nerve, enteric nervous system
- Endocrine: HPA axis, cortisol signaling
- Immune: Cytokines, immune cell trafficking
- Metabolic: Short-chain fatty acids (SCFAs), bile acids
- Microbiome: Bacterial metabolites, dysbiosis
MSA patients show altered gut microbiota:
- Reduced diversity: Fewer bacterial species
- Proinflammatory shift: Increased Firmicutes/Bacteroidetes ratio
- SCFA producers: Decreased butyrate-producing bacteria
- Functional implications: May influence alpha-synuclein pathology
Modulating the gut-brain axis:
- Probiotics: Restore microbial balance
- Prebiotics: Promote beneficial bacteria
- Fecal microbiota transplantation (FMT): Experimental approach
- Dietary intervention: Mediterranean diet, fiber supplementation
Current research areas include:
- Early detection: Biomarkers in enteric neurons before CNS symptoms
- Propagation mechanisms: Understanding alpha-synuclein spread
- Microbiome studies: Correlation with disease progression
- Neuroprotective strategies: Targeting enteric neurons
- Personalized medicine: Genetic subtypes and enteric involvement
Enteric nervous system involvement is a hallmark of Multiple System Atrophy, with alpha-synuclein pathology detectable in enteric neurons often years before central nervous system manifestations. Gastrointestinal dysautonomia, particularly constipation, is among the earliest and most prevalent symptoms. The ENS provides accessible tissue for diagnostic biopsy and offers potential therapeutic targets. Understanding the role of enteric neurons in MSA pathogenesis may lead to earlier diagnosis and disease-modifying interventions.