This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g., "corticospinal_tract_axonopathy#Distal Length-Dependent Degeneration of Long CNS Axons").
THIS IS THE CNS COUNTERPART OF peripheral_axonal_degeneration, NOT A DUPLICATE OF IT. Both modules model length-dependent dying-back axonopathy, and the distinction is the compartment and the glial partner. peripheral_axonal_degeneration models the peripheral nerve - a lower motor or sensory neuron whose axon is myelinated by Schwann cells, producing distal weakness, sensory loss, and areflexia. This module models the central compartment - an upper motor neuron whose axon runs in the corticospinal tract and is myelinated by oligodendrocytes, producing spasticity, hyperreflexia, and extensor plantar responses. The signs are opposite at the bedside for the same underlying cell-biological failure. A disorder with both a central and a peripheral axonopathy (as many complicated HSPs have) conforms to both modules at different nodes; do not collapse the two arms onto one node.
Key disorder-specific substitutions for the trigger node follow the functional categories of the SPG proteins: microtubule severing and axonal transport (SPG4/spastin, SPG10/KIF5A, SPG30/KIF1A); endoplasmic reticulum morphology (SPG3A/atlastin-1, SPG31/REEP1, SPG12/reticulon 2); mitochondrial function (SPG7/paraplegin, SPG13/HSPD1); endosomal membrane traffic and vesicle formation (the AP-4 complex disorders SPG47/SPG50/SPG51/SPG52, and SPG48/AP5Z1 of the AP-5 complex); protein folding and the ER stress response (SPG6/NIPA1, SPG8/strumpellin, SPG17/BSCL2); and fatty acid and phospholipid metabolism (SPG28/DDHD1, SPG35/FA2H, SPG54/DDHD2, SPG56/CYP2U1). Two further SPG categories are deliberately NOT modelled here because their primary lesion is elsewhere: myelin formation (SPG2/PLP1, SPG44/GJC2), which belongs to cns_myelin_failure, and corticospinal tract neurodevelopment (SPG1/L1CAM, SPG22/MCT8), which is a failure to build the tract rather than a failure to maintain it. A PLP1 or L1CAM disorder that also shows corticospinal degeneration may still conform at the degeneration node, but not at the trigger.
THE MODULE IS ABOUT MAINTENANCE, NOT DEVELOPMENT. The chain presupposes an anatomically formed corticospinal tract that subsequently degenerates. A disorder in which the tract never decussates or never forms does not conform, however severe its spasticity.
Long-Axon Maintenance Machinery Defect
trigger
A heritable lesion disables one of the cell-biological systems a very long axon depends on to stay supplied: microtubule severing and organisation, the tubular endoplasmic reticulum network that extends the length of the axon, mitochondrial quality control, endolysosomal and vesicular membrane traffic, or the lipid metabolism that maintains axonal membranes. None of these systems is specific to the corticospinal tract, and none is dispensable in shorter neurons; what makes the lesion selective is the downstream length-dependence, not the gene's expression pattern.
Downstream
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Impaired Axonal Transport and Organelle Distribution
Loss of the maintenance machinery degrades the delivery of mitochondria and membrane-bound cargo along the axon.
Show evidence (3 references)
"In general, proteins encoded by HSP genes have diverse functions including (1) axon transport (e.g. SPG30/KIF1A, SPG10/KIF5A and possibly SPG4/Spastin); (2) endoplasmic reticulum morphology (e.g. SPG3A/Atlastin, SPG4/Spastin, SPG12/reticulon 2, and SPG31/REEP1, all of which interact); (3)..."
Enumerates the housekeeping systems whose failure constitutes this trigger node and names the SPG proteins conforming entries substitute into it. Evidence source is OTHER because this is a clinico-pathological review synthesizing more than 50 genetic types of HSP.
"Hereditary spastic paraplegia (HSP) is a syndrome designation describing inherited disorders in which lower extremity weakness and spasticity are the predominant symptoms. There are more than 50 genetic types of HSP."
Establishes that a large and genetically heterogeneous set of lesions converges on one clinical syndrome, which is the convergence this module exists to model.
"(7) fatty acid and phospholipid metabolism (e.g. SPG28/DDHD1, SPG35/FA2H, SPG39/NTE, SPG54/DDHD2, and SPG56/CYP2U1); and (8) endosome membrane trafficking and vesicle formation"
Covers the two remaining housekeeping systems named in this node's description - lipid metabolism and endolysosomal membrane traffic - which the first evidence item's quoted span does not reach. Together the two snippets support all five systems the node asserts.
Impaired Axonal Transport and Organelle Distribution
amplifier
Cargo delivery along the axon fails. Because microtubules are the track on which axonal transport runs, and because the tubular ER and the mitochondrial population are themselves cargo that must be distributed along the axon, all of the trigger lesions converge here. The measurable consequence is reduced anterograde movement of mitochondria and membrane-bound organelles, and the morphological consequence is focal axonal swellings packed with cytoskeletal proteins, mitochondria, and amyloid precursor protein - seen both in mutant mice and in human HSP post-mortem material. The swellings are themselves amplifying: transport distal to a swelling is far worse than transport through an unaffected segment, so the lesion propagates down the axon.
Downstream
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Distal Length-Dependent Degeneration of Long CNS Axons
Sustained supply failure is lethal first to the most distal segment of the longest axons.
Show evidence (2 references)
"We found that mutant spastin perturbs anterograde transport of both cargoes. In neurons with axonal swellings we found that the mitochondrial axonal transport defects were exacerbated; distal to axonal swellings both anterograde and retrograde transport were severely reduced."
Direct quantitative demonstration of the transport failure this node asserts, including the distal exacerbation that makes it an amplifier rather than a static defect. Evidence source is MODEL_ORGANISM because the transport measurements are time-lapse imaging in mutant mouse neurons.
"Pathological analyses of human HSP cases caused by spastin mutations revealed the presence of similar axonal swellings."
Confirms in human post-mortem material the morphological correlate observed in the mouse model, so this node is not a mouse-only claim. Evidence source is HUMAN_CLINICAL because it reports neuropathology of human HSP cases.
Distal Length-Dependent Degeneration of Long CNS Axons
central effector
The rate-limiting, disorder-agnostic convergence point of the module and its key conformance target. The distal ends of the longest central axons degenerate in a dying-back pattern. Post-mortem studies of HSP consistently find degeneration of corticospinal tract axons maximal in the thoracic spinal cord - the furthest point from the motor cortical cell bodies - together with degeneration of fasciculus gracilis fibres maximal in the cervico-medullary region, which is the furthest point from the dorsal root ganglion cell bodies of the ascending sensory arm. The two tracts are affected at opposite ends of the cord for the same reason: distance from the soma. Conforming entries should preserve this length-dependence explicitly rather than describing undifferentiated "corticospinal degeneration".
Downstream
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Loss of Supraspinal Inhibitory Control of the Stretch Reflex
Degeneration of the descending tract removes the supraspinal input that normally restrains the spinal stretch reflex.
Show evidence (3 references)
"Postmortem studies consistently identify degeneration of corticospinal tract axons (maximal in the thoracic spinal cord) and degeneration of fasciculus gracilis fibers (maximal in the cervico-medullary region)."
The neuropathological statement of this node, including the tract-specific maxima that establish length-dependence. Evidence source is HUMAN_CLINICAL because it summarizes human post-mortem findings.
"HSP syndromes thus appear to involve motor-sensory axon degeneration affecting predominantly (but not exclusively) the distal ends of long central nervous system (CNS) axons."
States the dying-back, length-dependent character of the degeneration and the qualification "predominantly but not exclusively", which is why this module does not claim the corticospinal tract is the only tract involved.
"The most common neuropathological sign is the axonal degeneration involving the lateral corticospinal tracts in both the cervical and thoracic spinal cord."
Independent confirmation from a second review that corticospinal axonal degeneration is the defining neuropathological finding. Evidence source is OTHER because this is a review of HSP classification and neuropathology.
Loss of Supraspinal Inhibitory Control of the Stretch Reflex
effector
Degeneration of the descending corticospinal axons disrupts the balance of supraspinal inhibitory and excitatory inputs converging on the spinal cord, leaving the stretch reflex disinhibited and producing the velocity-dependent hypertonia that defines spasticity. The release is not the whole story: plastic changes within the spinal cord - notably progressive reduction of post-activation depression with limb immobilization - and secondary soft tissue changes in the paretic limbs both add non-reflex hypertonia on top of the reflex component. This distinction between reflex-mediated and intrinsic hypertonia is why physiotherapy and limb mobilization are curated as treatments in conforming entries alongside antispastic pharmacotherapy.
Downstream
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Progressive Lower-Limb Spasticity and Weakness
Disinhibited stretch reflexes plus loss of descending motor drive produce the spastic paraparesis.
Show evidence (2 references)
"The central lesion causing the UMNS disrupts the balance of supraspinal inhibitory and excitatory inputs directed to the spinal cord, leading to a state of disinhibition of the stretch reflex."
States the mechanism of this node - a central lesion unbalances descending control and disinhibits the spinal stretch reflex. Evidence source is OTHER because this is a pathophysiology review.
"Therefore, in patients with UMNS, hypertonia can be divided into two components: hypertonia mediated by the stretch reflex, which corresponds to spasticity, and hypertonia due to soft tissue changes, which is often referred as nonreflex hypertonia or intrinsic hypertonia."
Supports the qualification in this node that the hypertonia is not purely a release phenomenon, which is the mechanistic basis for the rehabilitation arm of treatment in conforming disorders.
Progressive Lower-Limb Spasticity and Weakness
consequence
The clinical endpoint of the module: a spastic paraparesis in which lower extremity weakness and spasticity predominate. Symptoms may begin at any age, and tempo depends on age at onset - gait impairment beginning after childhood usually worsens very slowly over many years, whereas impairment beginning in infancy and early childhood may not worsen significantly. In pure HSP this is the whole phenotype; in complicated HSP it is accompanied by the additional features the disorder-specific SPG protein produces outside the corticospinal tract, which conforming entries curate on their own nodes rather than here.
Show evidence (2 references)
"Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative disorders which involve the corticospinal tracts and present with distinct spasticity and weakness of the lower extremities."
States the clinical endpoint and ties it explicitly to corticospinal tract involvement, closing the module's chain. Evidence source is OTHER because this is a review.
"Symptoms may begin at any age. Gait impairment that begins after childhood usually worsens very slowly over many years. Gait impairment that begins in infancy and early childhood may not worsen significantly."
Supports the qualified statement of tempo in this node and is why the module does not assert a uniform progressive course across conformers.