Neurological localisation is the one part of clinical reasoning where a careful examination still routinely outperforms imaging as the first step. Before you decide which scan to order, and before it reaches the radiologist, you need to know roughly where the lesion is, and the first fork in that decision tree is almost always the same question: upper motor neurone or lower motor neurone?
Get this right and the differential collapses from hundreds of conditions to a manageable handful. Get it wrong and you scan the wrong part of the nervous system.
Key takeaways
- The entire distinction rests on one boundary: the anterior horn cell. Everything above it is upper motor neurone, everything from it outwards is lower motor neurone.
- Every sign follows from that anatomy. Spasticity is released inhibition, and fasciculations require a living but irritable lower motor neurone.
- The pattern of weakness is more reliable than the reflexes. Pyramidal distribution points upper, territorial distribution points lower.
- Three situations mislead: spinal shock in the first 48 hours, combined upper and lower signs, and lesions at the conus and cauda equina junction.
The anatomical dividing line
The whole distinction rests on one boundary: the anterior horn cell.
The upper motor neurone begins in the primary motor cortex, descends through the corona radiata and internal capsule, passes through the cerebral peduncle, pons and medulla, decussates at the pyramids and continues in the lateral corticospinal tract of the spinal cord. It ends by synapsing on the anterior horn cell.
The lower motor neurone starts at the anterior horn cell and runs out through the ventral root, plexus, peripheral nerve and neuromuscular junction to the muscle.
So an upper motor neurone lesion is anything from cortex down to, but not including, the anterior horn cell. A lower motor neurone lesion is the anterior horn cell and everything distal to it.
Every sign you elicit follows from this single anatomical fact.
The signs, and why they occur
| Feature | Upper motor neurone | Lower motor neurone |
|---|---|---|
| Tone | Increased, spastic, clasp-knife | Reduced, flaccid |
| Reflexes | Exaggerated, brisk | Reduced or absent |
| Plantar response | Extensor (Babinski positive) | Flexor or absent |
| Clonus | Often present | Absent |
| Muscle bulk | Preserved early, disuse atrophy late | Marked, early wasting |
| Fasciculations | Absent | Characteristic |
| Weakness pattern | Pyramidal distribution | Distribution of nerve, root or plexus |
| Distribution | Regional, affects movements | Focal, affects individual muscles |
Two entries in that table deserve explanation, because they are the ones examiners probe.
Why tone increases in UMN lesions. The descending tracts exert net inhibitory control over spinal reflex arcs. Remove that inhibition and the stretch reflex becomes disinhibited, producing spasticity, hyperreflexia and clonus. The lesion does not create anything new. It releases something that was always being suppressed.
Why fasciculations occur only in LMN lesions. A fasciculation is the spontaneous discharge of a single motor unit. That requires an intact but irritable lower motor neurone. If the anterior horn cell is dead or the axon is severed, there is no motor unit left to fire.
The pyramidal pattern of weakness
This is the most useful and most under-taught sign in the list.
In an upper motor neurone lesion, weakness is not uniform. In the upper limb, extensors are weaker than flexors. In the lower limb, flexors are weaker than extensors. This is why a hemiplegic patient holds the arm flexed and adducted and the leg extended, and why they circumduct when walking.
If a patient has weakness that does not respect this pattern, and instead maps neatly onto a single nerve or root territory, you are looking at a lower motor neurone problem regardless of what the reflexes are doing.
Localising further once you have decided
If the lesion is upper motor neurone
- Cortical: contralateral weakness, often with cortical signs such as aphasia, neglect, seizures or a focal pattern following the homunculus. Face and arm affected disproportionately in middle cerebral artery territory.
- Internal capsule: dense contralateral hemiplegia affecting face, arm and leg roughly equally, with few cortical signs. Fibres are tightly packed here, so a small lesion produces a large deficit.
- Brainstem: crossed signs. Ipsilateral cranial nerve palsy with contralateral hemiparesis is nearly pathognomonic for a brainstem lesion, and the cranial nerve involved tells you the level.
- Spinal cord: bilateral involvement below a sensory level, sphincter disturbance, no cranial nerve signs. The sensory level is the single most important thing to establish, because it determines where you image.
If the lesion is lower motor neurone
- Anterior horn cell: pure motor, wasting and fasciculations, no sensory loss. Think motor neurone disease, poliomyelitis, spinal muscular atrophy.
- Root: myotomal weakness with dermatomal sensory loss, often with radicular pain.
- Plexus: deficits crossing multiple root and nerve territories in one limb.
- Peripheral nerve: deficit confined to the territory of a named nerve, motor and sensory together.
- Neuromuscular junction: fatigable weakness, normal reflexes early, no sensory loss, ocular and bulbar predominance.
- Muscle: proximal symmetrical weakness, no sensory loss, reflexes preserved until late.
The facial nerve exception, explained properly
An upper motor neurone facial palsy spares the forehead. A lower motor neurone facial palsy does not.
The reason is that the part of the facial nucleus supplying the upper face receives bilateral corticobulbar input, while the part supplying the lower face receives predominantly contralateral input. A unilateral upper motor neurone lesion therefore leaves the forehead with an intact ipsilateral supply.
This is the difference between a stroke and a Bell’s palsy at the bedside, and it is asked in every clinical examination for a reason.
Mixed and misleading pictures
Three situations will make a confident student look foolish.
Spinal shock. In the acute phase after a cord injury, reflexes are absent and tone is flaccid, despite the lesion being unambiguously upper motor neurone. Spasticity and hyperreflexia develop over days to weeks. Do not localise on tone alone in the first 48 hours.
Combined UMN and LMN signs. Wasting and fasciculations alongside brisk reflexes and extensor plantars is a striking combination with a short differential: motor neurone disease, cervical myeloradiculopathy, subacute combined degeneration of the cord and, in the appropriate setting, syphilitic taboparesis.
Conus medullaris and cauda equina. The conus contains upper motor neurone tissue, the cauda equina is composed of nerve roots. Lesions at the junction produce mixed signs, with the additional and urgent feature of sphincter and saddle involvement.
Absent reflexes with an extensor plantar should always prompt you to think about a peripheral neuropathy coexisting with a central lesion.
A workable bedside sequence
- Inspect for wasting and fasciculations before you touch the patient.
- Assess tone, including for clonus.
- Test power and ask whether the pattern is pyramidal or territorial.
- Elicit reflexes and the plantar response.
- Establish a sensory level if the cord is in question.
- Only then decide what to image, and where.
Where structured practice helps
Localisation is a skill built by repetition against feedback, not by rereading a table. The doctors who become fluent at it are the ones who commit to a localisation out loud before the imaging comes back, then check themselves against the result. Being wrong and knowing why is how the pattern recognition forms.
For clinicians who want that practice in structured form, accredited short courses in clinical reasoning and neurological assessment offer case-based repetition without needing a neurology attachment. They are not a substitute for bedside teaching, but they fill the gap for doctors whose current post does not put enough neurology in front of them.
Frequently asked questions
Can a single lesion cause both UMN and LMN signs?
Yes. A cervical cord lesion can damage anterior horn cells at that level, producing LMN signs in the arms, while interrupting descending tracts and producing UMN signs in the legs.
Is a positive Babinski sign always pathological?
In adults, yes. In infants under approximately one year it is a normal finding, before myelination of the corticospinal tracts is complete.
What does clasp-knife rigidity mean?
Resistance that is high initially and then suddenly gives way. It is characteristic of pyramidal spasticity and distinguishes it from the uniform lead-pipe rigidity of extrapyramidal disease.
Why are reflexes brisk in UMN lesions but reduced in LMN lesions?
The reflex arc itself is intact in a UMN lesion, merely disinhibited. In an LMN lesion the efferent limb of the arc is damaged, so the reflex cannot be completed.
This article is intended for medical education and revision. It is not clinical guidance for the management of individual patients and should not replace local protocols, senior review or specialist referral.
