Upper vs lower motor neuron lesions: signs and localisation

Same weakness, opposite signs

Two patients come to clinic with the same complaint: a weak right foot. In the first, the leg is stiff, the knee jerk is brisk, and stroking the sole sends the big toe upward. In the second, the leg is floppy, the ankle jerk is gone, and the calf is thin and twitching.

Same weakness, opposite signs. This lesson is about upper vs lower motor neuron lesions: how each one changes tone, reflexes, the plantar response, fasciculations and wasting, and how the signs help you place the damage.^[2][8][9]

The chain of two neurons

Voluntary movement needs a chain of two neurons. The upper motor neuron starts in the primary motor cortex of the frontal lobe, and its axon runs down the corticospinal tract.^[1][2]

Most of its fibres cross to the opposite side at the pyramidal decussation, at the bottom of the medulla, then descend in the lateral column of the cord.^[1][2]

The lower motor neuron has its cell body in the ventral horn of the cord, or in a cranial nerve motor nucleus, and its axon runs in a peripheral nerve to the muscle.^[1][2]

So the upper neuron lies wholly inside the central nervous system, while the lower neuron sends its axon out to touch the muscle. The lower neuron keeps muscle tone and runs the reflex arc, and the upper neuron's descending input modifies it.^[2][7]

Lower motor neuron lesion signs: the muscle loses its wire

Cut the lower motor neuron and the muscle loses its wire. The result is flaccid paralysis.^[2][4][8]

• Low tone
• Reflexes reduced or absent
• Fasciculations (and fibrillations on EMG)
• Then wasting of the muscle

The weakness follows the damaged segment or nerve, so it is local. Think of poliovirus in ventral horn cells, or a crushed nerve.^[4][8]

That is the second patient from the opening: floppy leg, absent ankle jerk, thin and twitching calf.

Upper motor neuron lesion signs: spasticity and the Babinski sign

Cut the upper motor neuron instead. The lower neuron and its reflex arc are still intact, but have lost descending control. The result is weakness with spasticity, brisk reflexes and a Babinski sign.^[2][3][9]

Spasticity is raised tone, worse with a fast stretch, that can give way suddenly like a pocket knife closing: the clasp-knife response.^[2][7][9]

The Babinski sign is the big toe extending, with the other toes fanning, when the sole is stroked. It is normal in infants, but in an adult it points to damage of the corticospinal tract.^[1][2][9]

The lower neuron still feeds the muscle, so there are no fasciculations and no marked wasting. That is the first patient.^[2][3]

Place the lesion: capsule, cord and spinal shock

The signs also help you place the lesion. Because most corticospinal fibres cross at the pyramidal decussation, the side of the upper motor neuron signs depends on whether the damage is above or below it. Lower motor neuron signs are usually on the same side as the lesion.^{[1][2][9][10]}

• A stroke in the left internal capsule hits fibres that have not yet crossed, so the signs, with spasticity once the acute phase passes, appear in the right arm and leg.
• A lesion in the cord is below the crossing, so the signs stay on the same side.

Some diseases damage both neurons, giving upper and lower signs together. Amyotrophic lateral sclerosis is diagnosed clinically from both upper and lower motor neuron signs in a patient with progressive muscle weakness.^[3][5]

Three things to keep

1. Upper vs lowerUpper means weak, spastic, brisk, Babinski, no wasting. Lower means weak, floppy, reduced reflexes, fasciculations, wasting.
2. Which sideAbove the pyramidal decussation, upper motor neuron signs are on the opposite side; in the cord, the same side.
3. Both togetherUpper and lower signs together mean both neurons are involved.