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Upper and lower motor neurones

What are they?

The upper motor neurones are the motor component of the central nervous system, taking impulses from the relevant part of the brain down to the synapses with the lower motor neurones.

The lower motor neurones are those that actually connect with the target muscle. So they run between the UMN synapse and the final action point.

This is kinda messed up with cranial nerves, but let's not worry about that too much.

Why do they have different signs?

Killing nerve signals to a muscle has an effect on that muscle. The difference comes from whether just the impulse is missing (upper motor neurone lesion), or whether the actual nerve attached to the muscle is malfunctioning (lower motor neurone lesion).

What are the signs?

Feature Upper Lower
Reflexes

Hyperactive

Diminished or absent

Babinski sign Present Absent
Fasciculations

Absent

Present

Tone

Increased

Decreased or absent

Atrophy Absent
Although after years, this will occur also.

Present