An upper motor neuron lesion of facial nerve will cause: March and September 2007

Okay, let's tackle this question about an upper motor neuron lesion of the facial nerve. Hmm, first, I need to remember the difference between upper and lower motor neuron lesions. Upper motor neurons are the ones coming from the brain to the lower motor neurons, right? So the facial nerve is cranial nerve VII, but wait, the upper motor neuron pathway for facial muscles is a bit tricky. I think the facial nerve itself is the lower motor neuron, so an upper motor neuron lesion would affect the pathway from the motor cortex to the facial nerve nucleus.

Wait, the facial nerve nucleus is in the pons. The upper motor neurons for the facial muscles project from the cerebral cortex to this nucleus. So if there's a lesion in the upper motor neuron (like in the cerebral cortex or the internal capsule), it would affect the contralateral side because the motor pathways cross. But the facial nerve nucleus has some bilateral input. Which part of the face is affected? The upper face muscles (like the forehead) receive bilateral input, so a unilateral upper motor neuron lesion wouldn't affect them. The lower face muscles (like the mouth) are controlled unilaterally. So a UMN lesion would cause weakness on the lower part of the face on the opposite side of the lesion. That means the patient can still raise their eyebrows on the affected side but can't close their eye or smile on the lower part. So the classic sign is inability to smile on the contralateral side but preserved forehead movement.

Looking at the options, the correct answer should be that the lower face is affected contralaterally. The other options would be about lower motor neuron lesions, which cause ipsilateral facial weakness (total palsy) or other features. Also, if the lesion is in the lower motor neuron (like the facial nerve itself), it would cause ipsilateral weakness of all facial muscles. So the key is to differentiate between UMN and LMN lesions. The UMN lesion spares the upper face because of bilateral input, while LMN affects all facial muscles on the same side.

**Core Concept**
Upper motor neuron (UMN) lesions affecting the facial nerve pathway result from damage to the corticobulbar tracts originating in the motor cortex. These tracts decussate in the medulla and synapse on the contralateral facial nerve nucleus, which innervates the lower facial muscles unilaterally. The upper facial muscles receive bilateral UMN input, preserving their function in UMN lesions.

**Why the Correct Answer is Right**
In UMN lesions, the corticobulbar tracts to the facial nerve nucleus are damaged, leading to contralateral weakness of the lower facial muscles (e.g., inability to smile or pucker lips). The upper facial muscles (e.g., frontalis, orbicularis oculi) remain functional due to bilateral cortical control. This results in a "central" or "supranuclear" facial palsy, where the patient can raise the eyebrows on the affected side but cannot fully close the eye or move the lower face on the contralateral side.

**Why Each Wrong Option is Incorrect**
**Option A:** *Ipsilateral lower facial weakness* – Incorrect. UMN lesions cause **contralateral** lower facial weakness due to crossed pathways.
**Option B:** *Complete ipsilateral facial palsy* – Incorrect. Complete facial palsy is caused by

✓ Correct Answer: B. Paralysis of the lower half of face on the opposite side