Oral-Palatal Deviations & The Gag Reflex

Oral-Palatal Deviations & The Gag Reflex: CN 5, 10, and 12 Motor Deviations

Normal motor innervation pattern

Left side of brain activates the right side of brainstem.
Right side of brainstem activates right motor system.

Same on opposite side:

Right brain activates left brainstem.
Left brainstem activates left motor system.

Minor innervation from the brain to the ipsilateral brainstem exists, because cortical innervation is actually bilateral with contralateral predominance.

Physiology of the CN motor systems.

CN 5 trigeminal motor system provides mastication (chewing).

This system innervates the medial and lateral pterygoids, masseter, and temporalis muscles.

Atrophy to these muscles is an important potential clue of trigeminal neuronal degeneration, such as from ALS.

Here, we focus on the lateral pterygoids, which provide mandible protrusion. The lateral pterygoids protrude at an angle to one another:

Eg, the right motor system produces left jaw deviation.
Together, they push the mandible forward.

Open your jaw and extend your mandible forward to feel this activation.*

CN 10 mediates palate elevation via nucleus ambiguus innervation.

Both sides of the palate cause ipsilateral palate elevation.

The right causes the right side to elevate.
The left, the left side.

Say "Ahhh" to feel your palate elevate.

See this video of palatal weakness.

CN 12 mediates tongue protrusion via hypoglossal nuclear innervation and genioglossus activation.

The genioglossi protrude at an angle to one another.

For instance, specify that the right motor system produces left jaw deviation.

The tongue protrudes forward via genioglossus activation; it attaches the tongue to the anterior mandible to provide tongue protrusion.

Genioglossus is tested clinically because whereas the other extrinsic tongue muscles receive bilateral cortical innervation, it receives predominately contralateral cortical innervation, as already described, and thus is helpful in localization.

Upper and Lower motor neuron patterns of injury

Upper motor neuron injury (eg, a stroke).

There is normal innervation from the left brain to the right brainstem to the right motor system.
There is a loss of innervation to the left brainstem and, thus, a loss of innervation to the left motor system.

Lower motor neuron lesion.

There is normal innervation from the left brain to the right brainstem, and from the brainstem to the right motor system.
There is normal innervation from the right brain to the left brainstem BUT there is injury within the left brainstem, so there's a loss of innervation to the left motor system.

The upper motor neuron lesion caused a contralateral motor system deficit whereas the lower motor neuron lesion caused an ipsilateral deficit.

CN 5: With the loss of left motor system activation, the right motor system produces jaw deviation to the left.

CN 10: With the loss of left motor system activation, left side of the palate drops – it's weak; whereas the right side is strong.

When we evaluate for palatal weakness, the side of the palate that hangs lower is the side that is weak; although many authors address the directionality of the uvula, this only adds an unnecessary layer of complexity to our assessment.* Paralysis of the muscles of the soft palate results in failure of closure of the nasopharyngeal aperture; as a consequence, air escapes through the nose during speech and liquids are regurgitated into the nasal cavity during swallowing.*

In amyotrophic lateral sclerosis (ALS), there is often loss of corticonuclear innervation to the nucleus ambiguus, with resultant characteristic nasal pattern speech.

CN 12: With the loss of left motor system activation, the right motor system activation now causes tongue deviation to the left. The left motor system can't counterbalance it.

Bedside Demonstration for Easy Reference.

CNs 5 and 12: Point your index fingers towards one another to represent the lateral pterygoids or genioglossus.* Drop one of your fingers to represent a weakened lateral pterygoid or genioglossus.*

The intact finger points toward the weak side.
In a lower motor neuron lesion, the jaw deviates toward the injured side.
Cortical innervation is bilateral with contralateral predominance; thus, in cortical injury, if deviation occurs, it is directed away from the injured side.
For instance, in a right hemispheric stroke, the left trigeminal nucleus is unable to activate the left lateral pterygoid and so the jaw may deviate to the left (away from the right cerebral hemispheric injury).

The gag reflex

Often tested to assess the integrity of the brainstem and cranial nerves.

* We brush the patient's soft palate and observe for the characteristic gag response.

The gag reflex relies on the glossopharyngeal nerve for its afferent loop and the vagus nerve for its efferent loop.
Soft palate stimulation sends a volley of afferent impulse along the glossopharyngeal nerve to the spinal trigeminal nucleus and solitary tract nucleus; it is unclear which predominates in the gag reflex.
These nuclei excite motor neurons in nucleus ambiguus and indicate that nucleus ambiguus sends efferent impulse via the vagus nerve for pharyngeal constriction.
The gag reflex additionally involves jaw opening and tongue thrust:

Nucleus ambiguus stimulates the trigeminal motor nucleus in the pons, which is largely responsible for jaw opening via cranial nerve 5.
The hypoglossal nucleus in the medulla provides tongue thrust via cranial nerve 12.

Oral-Palatal Deviations & The Gag Reflex

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