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# Reading a muscle biopsy: neurogenic vs myopathic

- Topic

Muscle looks different when its nerve is lost (small angular fibres in groups) than when the muscle itself is sick (necrosis, regeneration, fibrosis).

Updated Nov 12, 2026

## Key points

- Type I fibres: slow twitch, many mitochondria, myoglobin, lipid; dark on NADH-TR and ATPase pH 4.3 (soleus). Type II: fast, glycolytic, glycogen (pectoral muscles).
- Neurogenic: small angular fibres, fibre-type grouping (reinnervation), grouped atrophy, target fibres.
- Myopathic: rounded fibres of varied size, internalized nuclei, necrotic fibres with myophagocytosis, basophilic regenerating fibres, endomysial fibrosis and fat.
- Type II atrophy: disuse, glucocorticoids, cachexia; also MG.
- Serum CK is very high in necrotizing myopathies and dystrophies, normal or mildly high in neurogenic disease.
- EMG: neurogenic gives large, long motor unit potentials with fibrillations; myopathic gives small, short, polyphasic potentials.

## What it is

Skeletal muscle has two main fibre types arranged in a checkerboard: type I (slow, oxidative, red) and type II (fast, glycolytic). The type of a fibre is set by its motor neuron. When motor neurons or axons are lost, the denervated fibres shrink into small angular fibres; surviving neurons sprout and adopt them, so fibres of one type come to lie in groups (fibre-type grouping), and if those units are lost too, whole groups atrophy together. Primary muscle disease instead produces variation in fibre size, rounded fibres, central nuclei, necrosis, regeneration, inflammation and replacement by fat and fibrous tissue. Type II atrophy alone is a non-specific finding of disuse, glucocorticoids and cachexia.

## Structure

- Normal: polygonal fibres of similar size, nuclei at the periphery, checkerboard of fibre types.
- Denervation: scattered small angular fibres that stain dark with nonspecific esterase.
- Reinnervation: large groups of the same fibre type; later grouped atrophy when the reinnervating neuron dies.
- Target fibres: central zone of disrupted myofibrils, typical of denervation and reinnervation.
- Myopathy: necrotic pale fibres invaded by macrophages, regenerating fibres with blue cytoplasm and large nuclei, splitting, central nuclei and fibrosis.

## Step by step

- Each motor neuron sets the type of all the fibres it supplies, which are scattered among other units.
- When a neuron dies, its fibres lose trophic input and shrink into angular forms.
- Neighbouring neurons sprout and adopt orphan fibres, converting them to their own type: grouping.
- If the adopting neuron dies, the whole group atrophies at once: grouped atrophy.
- In myopathy the fibres themselves necrose; satellite cells regenerate them until repair fails and fat and fibrous tissue replace muscle.

## Neurogenic vs myopathic change

| Feature | Neurogenic | Myopathic |
| --- | --- | --- |
| Atrophic fibres | Small, angular, in groups | Rounded, scattered, varied size |
| Fibre types | Type grouping | Checkerboard kept |
| Necrosis and regeneration | Absent | Present |
| Fibrosis and fat | Late | Endomysial, early in dystrophies |
| CK | Normal or mildly raised | Often high |
| Examples | ALS, spinal muscular atrophy, neuropathies | Dystrophies, inflammatory and toxic myopathies |

## Clinical notes

- Weakness with fasciculations, atrophy and low reflexes suggests a lower motor neuron problem; proximal symmetric weakness with high CK suggests myopathy.
- Biopsy is taken from a moderately affected muscle, not an end-stage one.
- Sarcopenia of aging shows loss of type II fibres and motor units.

## Memory aid

Nerve loss shrinks fibres in groups like a neighbourhood losing its streetlights; muscle disease breaks individual houses.

## Test yourself

Think of the answer, then tap the question to see it.

## What is fibre-type grouping?

Clusters of fibres of one type, created when surviving neurons reinnervate denervated fibres.

## What does type II fibre atrophy alone suggest?

Disuse, glucocorticoid excess or cachexia; it is non-specific.

## What are three myopathic features on biopsy?

Fibre size variation with rounded fibres, central nuclei, necrosis with regeneration, and endomysial fibrosis.

## Which fibre type is rich in mitochondria and myoglobin?

Type I (slow-twitch, oxidative).

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## Elsewhere in DurgiPedia

- [Skeletal muscle: sarcomere and contraction](https://durgical.com/en/histology/skeletal-muscle-sarcomere-and-contraction/)
- [Muscle injury, repair and satellite cells](https://durgical.com/en/histology/muscle-injury-repair-and-satellite-cells/)
- [Muscle hypertrophy, atrophy and remodeling](https://durgical.com/en/physiology/muscle-hypertrophy-atrophy-and-remodeling/)

## Connections

## Related 4

- [Atrophy](https://durgical.com/en/general-pathology/atrophy/)
- [Duchenne and Becker muscular dystrophy](https://durgical.com/en/general-pathology/duchenne-and-becker-muscular-dystrophy/)
- [Dermatomyositis and antisynthetase syndrome](https://durgical.com/en/general-pathology/dermatomyositis-and-antisynthetase-syndrome/)
- [Motor neuron diseases](https://durgical.com/en/general-pathology/motor-neuron-diseases/)

## Related conditions 6

- [Spinal muscular atrophy](https://durgical.com/en/conditions/spinal-muscular-atrophy/)
- [Amyotrophic lateral sclerosis](https://durgical.com/en/conditions/amyotrophic-lateral-sclerosis/)
- [Sarcopenia](https://durgical.com/en/conditions/sarcopenia/)
- [Immobility syndrome and hospital-associated deconditioning](https://durgical.com/en/conditions/immobility-syndrome-and-hospital-associated-deconditioning/)
- [Inflammatory myopathies (dermatomyositis, polymyositis)](https://durgical.com/en/conditions/inflammatory-myopathies/)
- [Duchenne muscular dystrophy](https://durgical.com/en/conditions/duchenne-muscular-dystrophy/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Spinal muscular atrophy: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/spinal-muscular-atrophy/) (MedlinePlus, US National Library of Medicine, 2025); [Muscular Dystrophy | MD | MedlinePlus](https://medlineplus.gov/musculardystrophy.html) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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