Axonopathies

Wallerian degeneration as an active programme

When an axon is transected or its transport is catastrophically interrupted, the distal stump does not passively wither. It executes a genetically encoded self-destruct programme whose master switch is the NAD⁺-cleaving enzyme SARM1. Under normal conditions the labile axonal-survival factor NMNAT2, delivered continuously by fast anterograde transport, maintains a metabolite milieu (a low NMN-to-NAD⁺ ratio) that holds SARM1 inactive. Severance halts NMNAT2 delivery; NMNAT2 has a half-life of minutes to a few hours, so it is depleted quickly, NMN accumulates, and SARM1 is allosterically disinhibited. Activated SARM1 then consumes axonal NAD⁺ catastrophically, collapsing the energy charge, defeating the ATP-dependent ion pumps, and triggering calcium influx that activates calpains. The cytoskeleton undergoes granular disintegration— neurofilaments and microtubules are proteolysed into debris — and the axolemma fragments. Schwann cells dedifferentiate to a repair phenotype, downregulate myelin genes, and, with recruited macrophages, phagocytose myelin and axonal debris while laying down the Bands of Büngner that will later guide regrowth.

The decisive clinical fact is that conduction persists distally for roughly 3 to 9 days after the axon is functionally severed, because the disconnected stump retains enough membrane integrity and ionic gradient to propagate an impulse until the SARM1-driven NAD⁺ collapse defeats it. Motor fibres typically fail before sensory fibres. This latency is the mechanistic reason a nerve stimulated distal to an acute complete lesion still yields a normal CMAP in the first days — the distal segment has not yet degenerated — whereas by roughly day 7 to 9 the motor response is lost and the sensory nerve action potential falls out a few days later still.

Mechanisms of axonal loss: transport failure and dying-back

Most toxic, metabolic, and hereditary axonopathies do not transect the axon; they starve it. Because the distal axon is the territory most remote from the soma's synthetic machinery and most reliant on the integrity of axonal transport — kinesin-driven anterograde delivery of mitochondria, membrane precursors, and survival factors; dynein-driven retrograde signalling — it fails first when transport, mitochondrial ATP production, or cytoskeletal track integrity is impaired. The result is a length-dependent, dying-back neuropathy: the longest and largest-calibre axons degenerate from their terminals proximally, which is the cellular basis of the stocking-glove distribution and of the rule that the feet are affected before the hands. Electrically this manifests as the earliest and most severe abnormalities in distal muscles and in the longest sensory nerves (sural before median), with relative preservation of proximal segments.

On nerve conduction studies, axon loss produces a characteristic signature: low amplitudes with relatively preserved velocities and latencies. Conduction velocity falls only modestly, and only because the largest, fastest-conducting axons are preferentially lost, lowering the ceiling of the velocity distribution; it does not approach the demyelinating range. This is the quantitative discriminator — amplitude collapses while velocity holds — and mistaking the mild velocity drop of severe axon loss for primary demyelination is a classic interpretive error addressed below.

Regeneration and the growth cone

If the cell body survives and the endoneurial tubes remain patent, the proximal stump regenerates. A growth cone— a motile, actin-rich tip studded with filopodia — advances along the Bands of Büngner, guided by Schwann-cell-derived neurotrophins and adhesion molecules, at a rate of approximately 1 to 3 mm per day (faster proximally, slower distally), the cellular ceiling that governs the months-long timetable of clinical recovery. Regenerating units are initially thinly remyelinated with short internodes, so the earliest recovered responses are low in amplitude, dispersed, and slowed. Misdirection is the inherent imperfection of this process: a regrowing motor axon may enter a sensory tube or reinnervate the wrong muscle fascicle, producing synkinesis and incomplete functional recovery even when reinnervation is robust.

Collateral sprouting and motor-unit enlargement

When axon loss is partial — the usual case — the dominant repair mechanism is not slow axonal regrowth but rapid collateral sprouting. Surviving intramuscular axons extend terminal and nodal sprouts to adopt the denervated muscle fibres in their vicinity, capturing them into the surviving motor unit within days to weeks. A single neuron may thereby come to innervate two, three, or many times its normal complement of fibres. Because MUAP amplitude and duration scale with the number and packing density of fibres in the unit, reinnervated units become large in amplitude and long in duration; while sprouts are still immature and transmission across nascent endplates is insecure, they are also polyphasic and unstable (jiggling moment to moment). The disappearance of fibrillations as orphaned fibres are recaptured, together with the emergence of these enlarged units, is the EDX signature of successful chronic reinnervation — and the reason collateral sprouting, not regeneration, accounts for most functional recovery in incomplete axonopathy.