Bell’s palsy affects all the muscles of facial expression on one side of the face. The facial nerve (cranial nerve VII) controls more than 20 muscles responsible for everything from raising your eyebrows to closing your eyes to smiling, and when the nerve becomes inflamed or compressed, every one of those muscles can lose function. The condition also impacts a tiny muscle inside the ear and can disrupt taste sensation on the front two-thirds of the tongue.
How the Facial Nerve Reaches Your Muscles
The facial nerve exits the skull and passes through the parotid gland (the salivary gland in front of your ear), where it splits into two main trunks. Those trunks branch into five named divisions: temporal, zygomatic, buccal, mandibular, and cervical. Each division controls a different zone of the face. In Bell’s palsy, inflammation typically occurs where the nerve passes through a narrow bony canal inside the skull, so all five branches lose signal at once. That’s why the paralysis hits the entire half of the face rather than just one region.
Forehead and Eye Muscles
The temporal branch controls the frontalis muscle, which wrinkles your forehead and raises your eyebrows. When this muscle is paralyzed, the affected side of the forehead stays completely smooth even when you try to look surprised. This is actually one of the most important clinical details about Bell’s palsy: the forehead is involved. In a stroke, the forehead is typically spared because it receives nerve signals from both sides of the brain. If your forehead still moves normally, the cause of facial weakness may not be Bell’s palsy at all.
The temporal and zygomatic branches together control the orbicularis oculi, the ring-shaped muscle that closes your eyelid. Losing function here means you can’t fully shut your eye on the affected side, a problem called lagophthalmos. The lower eyelid may also droop and pull away from the eyeball. Without a proper blink, the eye dries out quickly, and if your eye doesn’t roll upward when you try to close it (a normal protective reflex), the cornea is especially vulnerable to damage. The corrugator muscle, which draws your eyebrows together when you frown, is also controlled by the temporal branch and goes slack.
Cheek and Mouth Muscles
The buccal branch supplies the muscles you use most during conversation and eating. The zygomaticus major, the primary muscle that pulls the corner of your mouth upward when you smile, loses its pull on the affected side, creating the classic asymmetric smile. The buccinator, which presses your cheek against your teeth and helps keep food in position while chewing, also fails. People with Bell’s palsy often bite the inside of their cheek or find food collecting between the cheek and gum.
The orbicularis oris, the circular muscle around the lips, is controlled by both the buccal and mandibular branches. Its paralysis causes what doctors call oral incompetence: difficulty sealing the lips, which leads to drooling, trouble drinking from a glass, and an inability to whistle or pronounce certain sounds clearly. The muscles that lift the upper lip and depress the lower lip on the affected side also weaken, further distorting speech and facial expressions.
Jaw and Neck Muscles
The mandibular branch controls the muscles that pull the lower lip and corner of the mouth downward. The cervical branch supplies the platysma, a broad, thin sheet of muscle that stretches from the collarbone up across the neck and jaw. Platysma involvement is often overlooked, but it becomes especially noticeable during recovery. In one study of 69 patients treated for post-recovery complications, 65% had platysma synkinesis, involuntary neck tightening triggered by other facial movements, and the vast majority reported it was bothersome.
Before the facial nerve even reaches the face, it gives off a branch to the posterior belly of the digastric muscle (which helps open the jaw) and the stylohyoid muscle (which assists swallowing). These are deep muscles you’d never notice in a mirror, but their temporary weakness can contribute to subtle difficulty with chewing and swallowing in the early days of Bell’s palsy.
The Stapedius: A Tiny Muscle With a Big Effect
One easily missed muscle sits inside the middle ear. The stapedius is the smallest skeletal muscle in the human body, and the facial nerve controls it directly. Its job is to contract reflexively in response to loud sounds, stiffening the chain of tiny ear bones to dampen noise before it reaches the inner ear. When Bell’s palsy paralyzes the stapedius, that protective reflex disappears, and ordinary sounds like clattering dishes, traffic, or your own voice can feel uncomfortably loud. This heightened sound sensitivity, called hyperacusis, affects many Bell’s palsy patients and usually resolves as the nerve recovers. Hearing itself stays normal because the stapedius plays no role in basic sound detection.
Taste and Tear Production
The facial nerve isn’t purely a motor nerve. A branch called the chorda tympani carries taste signals from the front two-thirds of the tongue. When Bell’s palsy inflames the nerve upstream of where this branch splits off, taste can become dulled or distorted on the affected side. Sweet, salty, sour, and bitter foods may all register differently, and some people describe a persistent metallic taste. The nerve also carries fibers that stimulate tear production, which is why the affected eye often produces fewer tears, compounding the dryness problem caused by incomplete eyelid closure.
How Severity Is Measured
Not every case of Bell’s palsy knocks out all these muscles equally. Doctors use the House-Brackmann scale, a six-point grading system, to describe how much function remains. At Grade II (mild), you can still close your eye with gentle effort and have only slight asymmetry when smiling. At Grade IV (moderate), the forehead doesn’t move, the eye won’t close completely, and the mouth is clearly asymmetric even with maximum effort. Grade VI means complete paralysis with no movement on the affected side.
About 80% of people with Bell’s palsy reach a favorable recovery. Most improvement happens within the first three weeks, though full recovery can take several months. The muscles that return to function first are generally the forehead and eye muscles, with the mouth and lower face taking longer.
Synkinesis: When Muscles Cross-Wire During Recovery
As the facial nerve regrows, some fibers find their way to the wrong muscles. The result is synkinesis, involuntary movement in one part of the face triggered by intentional movement in another. The most common pattern is the eye narrowing or closing when you smile, or the cheek tightening when you blink. Synkinesis can occur in any of the facial muscles, including the platysma in the neck. It develops weeks to months after the initial paralysis and, for some people, becomes more functionally bothersome than the original weakness. Physical therapy focused on slow, isolated facial movements is the primary approach to retraining these misfired connections.