ch 3/4 Motor Speech Disorders: UMN vs LMN Patterns, Facial Nerve Innervation, and Therapy Implications
Context and course structure
Instructor emphasizes that basic information is foundational: missing one or two items isn’t catastrophic, but missing three or more raises questions about understanding foundational material from week 1.
Quizzes are designed to cover evolving material; they aren’t tied to a single week’s content but to the broader motor speech base.
First abstract on the article was discussed; complex questions may arise and be addressed over time.
Recovery: unilateral upper motor neuron (UMN) dysarthria is a key topic because many people recover and it’s often transient. If choosing a dysarthria to have, this is preferable to a stroke with more severe complications.
If a stroke occurs, left vs right hemisphere involvement correlates with aphasia, cognitive issues, and swallowing challenges; audiovisual aspects involve how standardized assessments are scored (what the clinician perceives and hears).
Limitations of standardized assessments: most scoring is perceptual, not purely objective; some computerized methods provide objective measures, but outside of devices, judgments are perceptual. Despite limitations, motor speech chapters are foundational for this course.
Core concepts: motor speech disorders and neuroanatomy
Motor speech disorders come in lower motor neuron (LMN) and upper motor neuron (UMN) varieties; distinctions guide assessment and therapy.
LMN dysarthria (flaccid) vs UMN dysarthria (spastic) differ in pattern, not just location.
The course builds from cranial nerve anatomy to patterns of facial weakness and their speech implications.
Lower motor neuron (LMN) anatomy basics
LMN lesions produce flaccidity and ipsilateral weakness of the body region innervated by the damaged nerve.
Recurrent laryngeal nerve (branch of the vagus) innervates the vocal folds (laryngeal adductors/abductors).
Damage to the right recurrent laryngeal nerve leads to right vocal fold weakness; this affects voice and phonation symmetry.
If LMN damage is present in the lower motor neurons, the outcome is on the same side (ipsilateral) of the body for the muscles involved.
The pharyngeal plexus (branch of the vagus) innervates velum and pharyngeal structures; damage leads to asymmetries in velum function and hypernasality on the affected side.
The trigeminal nerve involvement (e.g., right-side nerve damage) can cause flaccid weakness of jaw/mastication muscles on the affected side.
The hypoglossal nerve (CN XII) when damaged on one side produces ipsilateral tongue weakness (e.g., right tongue weakness when right hypoglossal is damaged).
In LMN pathology, you assess the specific nerve pathways to determine paralyses in facial muscles, tongue, velum, and laryngeal control.
Upper motor neuron (UMN) anatomy basics
UMN damage results in spasticity and contralateral involvement of body regions, due to crossed pathways.
Pattern varies for facial muscles due to bilateral vs contralateral innervation, producing characteristic clinical signs.
Forehead and upper facial muscles often receive bilateral UMN input, creating a “protective redundancy” that can spare the upper face even with unilateral UMN damage.
In contrast, lower facial muscles and many speech-related muscles (e.g., velum, tongue, pharynx) often rely more on contralateral UMN input, so unilateral UMN lesions produce more pronounced contralateral deficits in those regions.
For example, a unilateral UMN lesion affecting the right hemisphere typically causes spasticity on the left side of the body, including the lower face and tongue regions, while the upper face may be relatively preserved due to bilateral innervation.
The diaphragm and some respiratory muscles also have bilateral or redundant innervation; this redundancy can mitigate catastrophic outcomes from unilateral UMN damage, but bilateral UMN lesions can result in severe weakness or loss of function.
Clinically, bilateral UMN involvement can yield bilateral spasticity and symmetric effects, whereas unilateral UMN lesions produce a unilateral pattern with a contralateral predominance.
Facial nerve patterns and how to interpret them
The facial nerve has both upper and lower quadrants on each side; each quadrant’s innervation depends on the combination of cranial nerve input from both hemispheres.
Lower quadrants (LQ) of the face receive predominantly contralateral UMN input.
Upper quadrants (UQ) receive bilateral UMN input (protective redundancy).
When the entire ipsilateral side of the face is weak, this points to LMN (facial nerve) damage because the nerve itself is disrupted.
When only the lower quadrant is weak with sparing of the forehead (upper quadrant appears relatively normal), this points to UMN involvement because the upper quadrant benefits from bilateral innervation.
The green/blue color-coding in the classroom schematic represents left and right UMN inputs; the pattern shows how the same UMN lesions can produce selective weakness in the contralateral lower quadrant due to contralateral innervation, with the upper quadrant preserved due to bilateral input.
A right facial nerve (CN VII) lesion results in a right facial weakness affecting both upper and lower quadrants on the right; this can be observed as asymmetry: reduced wrinkles around the right eye, smooth right forehead, and mouth drift to the right when smiling or speaking.
An upper motor neuron lesion (e.g., right UMN) yields a pattern where the right upper quadrant may be preserved while the left lower quadrant shows weakness, reflecting contralateral lower face involvement.
Diagnostic takeaways: pattern of facial weakness helps distinguish LMN facial nerve damage from UMN corticobulbar involvement; this, in turn, guides therapy decisions (strengthening vs relaxation).
Practical examples and clinical reasoning
Example: A patient with a right facial weakness showing right-sided mouth droop, diminished wrinkles around the right eye, and a smooth right forehead supports a right CN VII (facial nerve) LMN pattern rather than a pure UMN pattern.
If speech is affected, the dysarthria is likely a dysarthria with a predominant pattern linked to LMN (flaccid) involvement.
Example: A unilateral UMN lesion (e.g., right hemisphere stroke) produces spasticity on the left side; the left lower quadrant of the face is weak, while the upper quadrant may be preserved due to bilateral input.
The presence of spasticity in a unilateral UMN syndrome can extend beyond the face to the left arm, left leg, and even left tongue, pharynx, and vocal tract structures, yielding a comprehensive left-sided spastic dysarthria if speech is involved.
Bilateral UMN input to the upper face creates a protective redundancy, so an isolated UMN lesion may spare forehead movement but still affect other regions (tongue, velum, pharynx) that rely on contralateral input.
The takeaway for clinicians: facial paralysis patterns are diagnostic clues that guide whether therapy should emphasize relaxation (for spastic patterns) or strengthening (for flaccid patterns).
Therapy implications based on dysarthria type
If the velum and/or pharynx show flaccid weakness (e.g., left velum flaccidity):
Therapy would focus on strengthening and facilitation to improve closure and reduce hypernasality where appropriate.
If the velum/pharynx show spastic weakness (unilateral UMN damage with contralateral pattern):
Therapy would focus on relaxation and strategies to reduce excessive tone, not on strengthening.
The same logic applies to the lips, tongue, and larynx:
Flaccid patterns: strengthen the weak muscles.
Spastic patterns: relax and coordinate; avoid over-activating already hypertonic muscles.
The clinician must be precise: two patients with similar speech symptoms can have very different underlying mechanisms; mis-applying therapy (e.g., strengthening a spastic velum) can worsen the condition.
Measurements, assessments, and methodological notes
Assessments discussed include perceptual judgments of speech and non-speech observations (e.g., facial symmetry, jaw movement, lip rounding).
Standardized assessments have limitations because clinicians rely on perceptual judgments of what is heard and seen; even when standardized, much of the scoring is perceptual rather than purely objective.
Objective measurements can be generated through computerized analyses (e.g., acoustic analysis, phoneme/voice metrics), but outside of such devices, clinicians rely on perceptual judgments.
This is why the motor speech chapters are foundational: they provide the framework for interpreting what is observed in patients and deciding on appropriate clinical interventions.
Key anatomical connections and conceptual links
Cranial nerves involved in speech and swallowing that were reviewed: CN V (trigeminal), CN VII (facial), CN IX/X (glossopharyngeal/vagus via pharyngeal plexus), CN XII (hypoglossal).
The pharyngeal plexus (a vagal branch) innervates velum, pharynx, and related structures; damage affects velopharyngeal closure and nasality.
The recurrent laryngeal nerve (branch of the vagus) innervates the intrinsic muscles of the vocal folds; damage alters phonation quality and voice strength.
The pattern of innervation for facial muscles is unique: the lower quadrant muscles are largely contralaterally innervated, whereas the upper quadrant muscles receive bilateral innervation, creating a protective redundancy for unilateral UMN lesions.
The diaphragm and some respiratory muscles show considerations of bilateral innervation; unilateral UMN damage may not collapse respiration due to redundancy, but bilateral UMN lesions pose a greater risk.
Quick practical takeaways for differential diagnosis and therapy planning
A unilateral lower facial weakness with forehead involvement suggests a LMN facial nerve lesion (CN VII) rather than an UMN lesion.
A pattern where the forehead is spared but the lower face shows weakness points to a unilateral UMN lesion.
For speech therapy planning, identify whether the dysarthria is flaccid (LMN) or spastic (UMN); treat accordingly (strengthen flaccid muscles; relax spastic muscles).
Always integrate: pattern of facial weakness, voice quality, velopharyngeal function, tongue movement, and respiration to determine underlying neurophysiology and appropriate therapy.
Synthesis and study-oriented reminders
Foundational motor speech material requires understanding of: LMN vs UMN, facial nerve patterns, cranial nerve innervation maps, and the clinical implications for speech and resonance.
Differential diagnosis is essential for selecting the correct therapeutic approach.
When preparing for exams, be ready to interpret facial weakness patterns, predict the underlying neural lesion (LMN vs UMN, left vs right), and describe the corresponding therapy strategy.
Expressions to remember (LaTeX notation)
General UMN-to-body relation with unilateral lesion:
Facial innervation principles:
If UMN damage is on the right, then:
Recurrent laryngeal nerve function:
Pharyngeal plexus function:
Velum deficit patterns and hypernasality can reflect unilateral pharyngeal plexus damage: