Neurogenic Changes

Slide 1 · Neurogenic Changes

Denervation • Reinnervation • Morphologic interpretation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Relate a motor unit to the histochemical fiber-type pattern.

Explain denervation atrophy, reinnervation and motor-unit enlargement.

Differentiate fiber-type grouping from grouped atrophy.

Interpret these changes in acquired neurogenic disease and spinal muscular atrophy.

Slide 3 · Two routes to skeletal muscle weakness

Neurogenic injury: Loss or disruption of the innervating motor neuron/axon.

Neurogenic

Motor neuron / axon

Loss of effective innervation

Myopathic

Muscle fiber

Primary myofiber injury

Two injury compartments; Morphology provides localization clues

Slide 4 · Adult skeletal muscle: Organizational baseline

Multinucleated myofibers assemble into fascicles.

Fiber phenotype differs among myofibers within one fascicle.

Recognition of abnormal patterns requires a normal reference.

Type I

Type II

Schematic — not histologic evidence

Slide 5 · A motor unit is a functional ensemble

One motor neuron and its axon form the neural component.

All skeletal myofibers supplied by that neuron belong to its motor unit.

Motor-unit composition explains the spatial effects of axonal loss.

Motor neuron

Motor axon

Myofibers of one type

Schematic causal sequence

Slide 6 · Fiber type is specified by motor innervation

Myofibers of a single motor unit share the same fiber type.

Different motor units supply intermingled fibers in normal muscle.

Changing innervation may change the phenotype of a rescued fiber.

Type I

Type II

Schematic — not histologic evidence

Slide 7 · Normal muscle displays a checkerboard pattern

Type I and type II fibers are admixed rather than segregated.

Alternating histochemical staining reflects mixed motor-unit territories.

Large same-type territories suggest prior reinnervation.

Type I

Type II

Schematic — not histologic evidence

Slide 8 · Type I fibers: Endurance-oriented phenotype

Type I fibers support sustained force and aerobic activity.

Type I

Sustained force / aerobic

High oxidative capacity

Type II

Fast movement

High glycolytic capacity

Higher lipid content and myoglobin are associated with a red appearance.

Slide 9 · Type II fibers: Rapid force phenotype

Type II fibers support fast movement and high power.

Type II

High power / fast activity

Relatively fatigable

Type I

Sustained force

More fatigue resistant

Relative mitochondrial and lipid content are lower than in type I.

Slide 10 · Functional differences between fiber types

Feature

Type I

Type II

Action

Sustained force

Fast movement

Power & fatigue

Low power; fatigue resistant

High power; fatigues earlier

Metabolism

Oxidative dominant

Glycolytic dominant

Substrate tendency

Lipid rich; glycogen lower

Lipid lower; glycogen rich

Editable comparison; Textbook-derived categories

Slide 11 · Muscle histochemistry: Enzyme contrasts

Feature

Type I

Type II

NADH-TR

NADH-TR: dark

NADH-TR: light

ATPase (pH 4.3)

ATPase pH 4.3: dark

ATPase pH 4.3: light

ATPase (pH 9.4)

ATPase pH 9.4: light

ATPase pH 9.4: dark

Editable comparison; Textbook-derived categories

Slide 12 · ATPase pH 9.4: The polarity of staining

Type I fibers stain light.

Type II fibers stain dark.

In a normal cross-section, the two signals form a mosaic.

Type I: Light

Type II: Dark

Schematic — not histologic evidence

Slide 13 · ATPase pH 4.3: Reverse staining contrast

Type I fibers stain dark at pH 4.3.

Type II fibers stain light at pH 4.3.

A change in stain polarity does not change biological fiber identity.

Type I: Dark

Type II: Light

Schematic — not histologic evidence

Slide 14 · NADH-TR: Oxidative enzyme readout

Type I fibers have dark NADH-TR staining.

NADH-TR

Type I: Dark

High oxidative activity

NADH-TR

Type II: Light

Lower oxidative activity

Read the reaction in the context of fiber-type biology.

Slide 15 · Fiber type and biopsy pattern: An interpretive chain

Motor neuron identity regulates fiber phenotype.

Motor units ordinarily form a dispersed mosaic.

Loss and replacement of innervation reorganize that mosaic.

Neural identity

Fiber type

Spatial mosaic

Schematic causal sequence

Slide 16 · Denervation begins with interruption of input

An axon or its motor neuron is damaged or lost.

Its dependent muscle fibers lose normal innervation.

Affected fibers subsequently diminish in size.

Axonal loss

Denervation

Fiber atrophy

Slide 17 · The immediate morphologic consequence is atrophy

Denervated fibers become smaller in cross-section.

The surrounding unaffected fibers may remain relatively preserved.

Distribution of the atrophic cells is more informative than size alone.

Denervated / atrophic

Schematic — not histologic evidence

Slide 18 · Denervated fibers may become angulated

The outline often becomes flattened or sharply angulated.

Small angular profiles contrast with normal polygonal/rounded profiles.

This shape is characteristic, not an exclusive diagnostic criterion.

Denervated / atrophic

Schematic — not histologic evidence

Slide 19 · Why grouped atrophy forms

A single lost motor axon can denervate multiple myofibers.

These fibers may appear as clusters of small, angular profiles.

The grouping is a clue to a shared neural dependence.

Denervated / atrophic

Schematic — not histologic evidence

Slide 20 · Scattered atrophy versus grouped atrophy

A smaller isolated fiber is a nonspecific observation.

Isolated small profiles

Atrophy alone

Requires context

Clustered small profiles

Group atrophy

Consider denervation

Pattern recognition should integrate shape, distribution and context.

Slide 21 · The denervation sequence

Motor neuron/axon damage precedes loss of innervation.

Loss of input precedes reduced myofiber caliber.

Small angular fibers may eventually appear in groups.

Axonal loss

Denervation

Fiber atrophy

Slide 22 · Atrophy is a shared endpoint, not a diagnosis

Denervation is one cause of decreased muscle mass.

Neurogenic pattern

Fiber groups

Denervation-associated

Other atrophy

Several etiologies

Disuse / steroids / cachexia

Two injury compartments; Morphology provides localization clues

Slide 23 · Surviving motor axons can compensate

Denervated myofibers may receive innervation from residual motor axons.

The rescued fiber becomes incorporated into a different motor unit.

This response partially restores the lost neural input.

Axons survive

Reinnervation

Rescue of fibers

Schematic causal sequence

Slide 24 · Reinnervation can restore myofiber caliber

A previously denervated fiber may regain size and shape.

An apparently preserved fiber can therefore have a history of denervation.

Fiber-type architecture helps identify that history.

Denervated fiber

New innervation

Caliber restored

Slide 25 · Reinnervation can change fiber identity

The new motor neuron may differ from the original motor neuron.

The reinnervated fiber may switch fiber type.

Nearby myofibers supplied by one surviving axon then share the same type.

Type I

Type II

Schematic — not histologic evidence

Slide 26 · Fiber-type grouping marks remodeling

Contiguous fibers become predominantly one histochemical type.

The normal checkerboard appearance is diminished locally.

This pattern points to repeated denervation and reinnervation.

Type I

Type II

Schematic — not histologic evidence

Slide 27 · Normal mosaic versus type grouping

The same total number of fibers can have altered spatial organization.

A change in neighborhood, not necessarily loss of all fibers, is central.

Compare an admixed pattern with a single-type territory.

Type I

Type II

Schematic — not histologic evidence

Normal

Regrouped

Slide 28 · Motor units enlarge with ongoing axonal dropout

Residual axons acquire increasing numbers of muscle fibers.

Each enlarged motor unit maintains a common fiber type.

Large single-type territories can accumulate in the biopsy.

Axonal dropout

Surviving units expand

Larger unit territory

Schematic causal sequence

Slide 29 · Motor-unit enlargement: Benefit and liability

Benefit: Rescued myofibers regain innervation and caliber.

Compensation

More fibers rescued

Improved muscle innervation

Liability

More fibers per axon

Larger affected territory if lost

Another axonal injury can affect an unusually large territory.

Slide 30 · A remodeled unit can undergo grouped atrophy

A surviving enlarged motor unit is lost later.

The now-denervated same-type territory atrophies together.

Type grouping and grouped atrophy may thus appear sequentially.

Denervated / atrophic

Schematic — not histologic evidence

Slide 31 · Repeated denervation and reinnervation

Axonal loss induces atrophy.

Reinnervation may restore size while producing fiber-type grouping.

Subsequent axonal loss can generate grouped atrophy in an enlarged unit.

Loss of input

Rescue + type grouping

New loss + group atrophy

Schematic causal sequence

Slide 32 · Two hallmark patterns encode different stages

Grouped atrophy emphasizes current or unresolved denervation.

Type grouping

Fiber phenotype

Historical reinnervation

Grouped atrophy

Fiber size

Loss of innervation

Both can coexist in an evolving neurogenic process.

Slide 33 · Reading H&E and fiber-type stains together

Feature

Biopsy finding

Reading

Angulated fibers

Small flattened, angulated fibers

H&E: atrophy / altered shape

Clustered small fibers

Clustered small profiles

H&E: group atrophy

Same-type territories

Contiguous same-type profiles

ATPase: fiber-type grouping

Checkerboard mosaic

Near-alternating fiber types

ATPase: normal-type mosaic

Editable comparison; Textbook-derived categories

Slide 34 · The pattern of atrophy helps assign etiology

Feature

Distribution

Association in Robbins

Clusters

Grouped atrophic fibers

Neurogenic process

Fascicle periphery

Perifascicular atrophy

Dermatomyositis

Type II selective

Selective type II fiber atrophy

Steroid exposure or disuse

Editable comparison; Textbook-derived categories

Slide 35 · Grouped atrophy is spatial clustering

Atrophic fibers are concentrated in a contiguous region.

They may be flattened and angulated.

The group distribution suggests a common motor-unit mechanism.

Denervated / atrophic

Schematic — not histologic evidence

Slide 36 · Perifascicular atrophy is a different geometry

Small fibers preferentially occupy the fascicle edge.

This is the pattern with dermatomyositis.

Do not mistake a peripheral ring for random motor-unit group loss.

Denervated / atrophic

Schematic — not histologic evidence

Slide 37 · Selective type II atrophy is different again

Type II fibers are preferentially reduced in size.

Type I fibers are relatively spared.

This with prolonged corticosteroids or disuse.

Type I

Type II

Schematic — not histologic evidence

Slide 38 · Three distinct muscle-atrophy patterns

Feature

Distribution

Association in Robbins

Clusters

Grouped clusters

Neurogenic disease

Fascicle periphery

Perifascicular distribution

Dermatomyositis

Type II selective

Preferential type II small fibers

Disuse or prolonged corticosteroids

Editable comparison; Textbook-derived categories

Slide 39 · Neurogenic and myopathic injury differ in origin

Neurogenic: Loss of innervation drives secondary fiber atrophy.

Neurogenic

Motor neuron / axon

Loss of effective innervation

Myopathic

Muscle fiber

Primary myofiber injury

Two injury compartments; Morphology provides localization clues

Slide 40 · Primary myopathic injury has a different pathway

Segmental myofiber degeneration may release creatine kinase.

Macrophages clear damaged segments by myophagocytosis.

Activated satellite cells support regeneration of muscle components.

Fiber degeneration

Macrophage clearance

Satellite-cell regeneration

Slide 41 · Neurogenic versus myopathic morphologic emphasis

Neurogenic: Angulated/grouped atrophic fibers and type grouping.

Neurogenic

Angulation / groups

Motor-unit mechanism

Myopathic

Degeneration / repair

Primary fiber injury

Some atrophy may occur in both—interpret combined evidence.

Slide 42 · Neurogenic atrophy in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is an example of a motor-neuron disease producing neurogenic muscle atrophy.

The muscle pathology reflects loss of effective motor innervation.

Muscle biopsy morphology indicates mechanism; ALS requires broader clinical evaluation.

Slide 43 · Radiculopathy as a neurogenic cause

Degenerative spine disease can injure a motor nerve root.

Downstream muscle fibers may show denervation-associated atrophy.

The biopsy pattern must be interpreted with anatomic and clinical data.

Slide 44 · Peripheral neuropathy as a neurogenic cause

Peripheral axonal injury disrupts motor input to muscle.

Denervation and compensatory reinnervation can reshape muscle architecture.

Muscle biopsy cannot by itself specify which neuropathy is present.

Slide 45 · Neurogenic changes span multiple lesion sites

Feature

Lesion site

Illustrative cause

Motor neuron

Motor neuron

Amyotrophic lateral sclerosis

Nerve root

Motor nerve root

Degenerative radiculopathy

Peripheral axon

Peripheral motor axon

Peripheral neuropathy

Editable comparison; Textbook-derived categories

Slide 46 · Spinal muscular atrophy: Inherited motor-neuron disease

Spinal muscular atrophy involves loss of motor neurons.

Autosomal recessive disease caused by loss-of-function variants in SMN1.

Loss of motor innervation produces weakness and muscle atrophy.

Slide 47 · SMN1 function and motor-neuron maintenance

SMN1 encodes survival motor neuron protein.

The protein participates in spliceosomal functions and ribonucleoprotein biogenesis.

SMN1 deficiency perturbs processes required for motor-neuron homeostasis.

SMN1 function

RNA / RNP processes

Motor-neuron maintenance

Slide 48 · SMN1 deficiency leads to denervation

Deficiency can cause loss of motor neurons.

Loss may begin before birth.

Affected skeletal myofibers may never receive normal initial innervation or become denervated.

SMN1 deficiency

Motor-neuron loss

Reduced muscle innervation

Schematic causal sequence

Slide 49 · Severe infantile SMA: The characteristic biopsy picture

Broad zones contain markedly atrophic, rounded fibers.

Scattered fibers are normal-sized or hypertrophied.

The contrast reflects severe motor-neuron loss with residual innervation.

Small rounded fiber

Schematic — not histologic evidence

Slide 50 · Rounded atrophic SMA fibers and angular fibers

Severe infantile SMA: Many extremely small, rounded myofibers.

Severe infantile SMA

Small round fibers

Scattered enlarged fibers

Other denervation

Small angulated fibers

Group atrophy possible

Morphologic pattern varies with disease setting and timing.

Slide 51 · Why some SMA fibers remain large

Remaining motor neurons innervate a subset of fibers.

These preserved fibers may develop compensatory hypertrophy.

Thus an SMA biopsy can show striking fiber-size disparity.

Small rounded fiber

Schematic — not histologic evidence

Slide 52 · SMA illustrates developmental denervation

SMN1 deficiency may impair initial innervation in utero.

Early SMA

May start in utero

Initial innervation impaired

Acquired axonal injury

Input lost later

Previously innervated fibers

Both processes ultimately yield neurogenic muscle changes.

Slide 53 · Interpretation exercise: Clustered angular fibers

An adult with focal limb weakness undergoes muscle biopsy.

Multiple adjacent myofibers are small, flattened and angulated.

What underlying pathophysiologic compartment should be investigated first?

Clinical-pathologic scenario (hypothetical)

Grouped angular fibers

Slide 54 · Case interpretation: Favor a neurogenic process

The grouped, angular atrophy points to impaired innervation.

Clinical testing should localize the abnormality to motor neuron, nerve root or peripheral nerve.

Histology alone does not specify the particular cause.

Interpretation

Anatomic localization remains essential

Slide 55 · Interpretation exercise: Altered ATPase mosaic

A biopsy contains broad adjoining territories with one ATPase-defined fiber type.

Fiber caliber is mostly preserved within those territories.

What preceding biological event best explains this organization?

Clinical-pathologic scenario (hypothetical)

Large same-type territories

Slide 56 · Case interpretation: Reinnervation and switching

The finding represents fiber-type grouping.

Formerly denervated fibers can be reinnervated by a residual axon.

New motor-unit identity can alter the fiber type and restore caliber.

Interpretation

A fiber-type map preserves the reinnervation history

Slide 57 · Interpretation exercise: Severe infantile weakness

An infant with severe generalized weakness has numerous small rounded fibers.

Rare larger fibers are normal-sized or hypertrophied.

Which source-described inherited motor-neuron disorder fits the morphology?

Clinical-pathologic scenario (hypothetical)

Numerous small rounded fibers

Slide 58 · Case interpretation: Spinal muscular atrophy

The pattern matches Robbins’ description of severe infantile SMA.

SMN1 deficiency causes motor-neuron loss and impaired innervation.

Residual innervated fibers can remain large or become hypertrophic.

Interpretation

Residual innervation produces size disparity

Slide 59 · A practical morphology-first reading sequence

Identify where the small fibers lie: Grouped, peripheral or type-selective.

Assess fiber shape and whether an enzyme-stain mosaic is preserved.

Combine morphology with clinical localization and cause.

Atrophy geometry

Fiber type / stain

Clinical localization

Slide 60 · Interpretive pitfalls to avoid

Do not treat every small fiber as proof of denervation.

Do not confuse same-type grouping with grouped atrophy; The former is phenotypic, the latter size-based.

Do not infer ALS, radiculopathy or SMA from a single histologic sign.

Slide 61 · Integrated pathology conclusions

Denervation commonly yields small, angulated myofibers.

Compensatory reinnervation can restore caliber and produce fiber-type grouping.

Subsequent loss of enlarged motor units produces grouped atrophy.

Severe infantile SMA can instead show many rounded atrophic fibers with a few hypertrophied survivors.

Slide 62 · Primary reading

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th ed. (2026).

Chapter 27: Peripheral Nerves and Skeletal Muscles, pp. 1116–1117.

Sections: Skeletal Muscle Atrophy; Neurogenic Changes; Myopathic Changes.

Supporting items: Table 27.2; Figs. 27.7 and 27.8.