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104 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
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Trigeminal nerve
Trigeminal ganglia
Mandibular branch:
•Temporalis
•Tensor veli palatini
Foramen ovale
• Masseter
•Mylohyoid
• Geniohyoid
Pons
Motor cortex
Motor (masticator) track
Medulla oblongata
Ophthalmic branch
Maxillary branch
Uvula
FIGURE 4–2. The trigeminal nerve has three main branches: ophthalmic,
maxillary, and mandibular. The mandibular branch provides lower motor neuron innervation to the jaw muscles.
face, it divides into two major branches. For the most part, the cervicofacial branch innervates the muscles of the lower face through
its buccal, lingual, and mandibular subbranches. The temporofacial
branch innervates the muscles of the upper face through its temporal and zygomatic subbranches. Damage to the facial nerve can
affect the muscles of the entire face on the same (ipsilateral) side
as the lesion if it occurs above the point where the facial nerve
divides into its cervicofacial and temporofacial branches. In such
cases, all the muscles on the same side of the face as the damage
will demonstrate some degree of weakness or paralysis. The result
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Facial nerve
Temporalis
Orbicularis oculi
Stulo mastoid foramen
Cervicofacial branch:
• Buccal branch
• Cervical branch
• Marginal mandibular branch
Depressor anguli oris
Depressor labii inferior
Pons
Buccinator
Platsyma
Risorius
Mentalis
Motor cortex
Levator labii superior
oblique nasi
Nasalis
Levator labii superior
Mandibular branch
•Temporal branch
•Zygomatic branch
Zygomatis major and minor
Obicularis oris
FIGURE 4–3. The major branches of the facial nerve.
of this facial nerve damage most likely will be drooping of the eyelid, mouth, cheek, and other structures on the affected side of the
face. If the facial nerve damage occurs to just one of these branches,
only the muscles innervated by that branch will be affected. For
example, if the damage affects only the cervicofacial branch, the
muscles of the lips will be affected, and the production of bilabial
or labiodental sounds could be distorted.
Upper Motor Neuron Innervation of the Facial Nerve There is
a difference in how the upper motor neurons of the corticobulbar
tract innervate the lower motor neurons in the two branches of the
106 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
Lesion of VII
Lesion of VII
RIGHT HEMISPHERE LEFT HEMISPHERE
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Lower motor
neuron (LMN) lesion
Upper motor
neuron (UMN) lesion
Precentral
gyrus
Nucleus of
facial nerve
BILATERAL
INNERVATION
UNILATERAL
INNERVAION
FIGURE 4–4. A unilateral lesion to the upper motor neurons that inner-
vate the facial nerve will just affect the muscles of the lower face because
that branch of the facial nerve only receives unilateral upper motor neuron
innervation. In contrast, the branch of the facial nerve serving the upper face
receives bilateral upper motor neuron innervation. Therefore, lower motor
neuron lesions will affect all muscles below the point of damage.
facial nerve (Figure 4–4). The branch of the facial nerve that serves
the upper face receives bilateral upper motor neuron innervation
from both the right and left corticobulbar tracts. This means, for
example, that unilateral damage to the right corticobulbar tract will
not result in weakness or paralysis in the upper part of the left
side of the face because the left corticobulbar tract also innervates
this branch of the facial nerve. As a result, the upper face will still
receive some upper motor neuron innervation despite the damage
UMN
UMNLMN
LMN
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to the right corticobulbar tract. Consequently, there will be fairly
normal contractions of all the muscles in the upper part of the face
on both sides.
However, upper motor neuron innervation is different for the
branch of the facial nerve serving the muscles of the lower face.
This branch of the facial nerve receives only unilateral upper motor
neuron innervation from the opposite (contralateral) side of the
brain. Because of this unilateral innervation, right corticobulbar
tract damage will result in weakness or paralysis on the left side
of the lower face. Conversely, left corticobulbar tract damage will
result in weakness or paralysis on the right side of the lower face.
In summary, unilateral upper motor neuron damage in one
cerebral hemisphere will result in nearly normal upper face movements of the eyebrow, forehead, and eyelids on both sides of the
face. However, movements of the cheek and mouth on the side of
the face opposite to the site of the lesion will be notably weak, and
these two parts of the lower face will probably have reduced range
of motion. The type of dysarthria that can result from unilateral
upper motor neuron damage is known as unilateral upper motor
neuron dysarthria, discussed in Chapter 6.
Glossopharyngeal Nerve (IX)
This cranial nerve originates in the brainstem at the medulla (Figure 4–5) and courses out to the pharynx, where it innervates the
stylopharyngeus and superior pharyngeal constrictor muscles.
These muscles assist in the elevation and opening of the upper
pharynx. Eliciting the gag reex is one way to assess the function
of this cranial nerve. The full importance of the glossopharyngeal
nerve for speech is difcult to determine because damage to it also
usually will affect the vagus nerve, a cranial nerve that denitely
makes signicant contributions to speech production. Nevertheless,
the glossopharyngeal nerve probably plays a role in speech resonance and phonation by shaping the pharynx into the appropriate
positions needed to produce various phonemes correctly.
Vagus Nerve (X)
The vagus nerve is one of the most important cranial nerves for
speech production (Figure 4–6). Its origin is in the brainstem at
the medulla, just below the glossopharyngeal cranial nerve, and it
courses out from the medulla in close proximity to the glossopharyngeal and accessory cranial nerves. The vagus nerve is very long
and has many branches serving such varied parts of the body as
the larynx, intestines, heart, and velum, to name but a few. Three
108 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
Nucleus ambiguus
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Glossopharyngeal
nerve
Jugular foramen
Motor cortex
Pons
Medulla oblongata
Styloid process
Stylopharyngeus muscle
Superior pharyngeal constrictor
FIGURE 4–5. The glossopharyngeal nerve branches off from the brainstem
just above the vagus nerve and courses out to the pharynx and tongue.
branches of the vagus nerve have special importance for motor
speech production: the pharyngeal branch, the external superior
laryngeal nerve branch, and the recurrent nerve branch.
Uvula
Tonsil
Tongue
Hyoid bone
Pharyngeal Branch The pharyngeal branch of the vagus nerve
provides motor innervation for many muscles of the pharynx,
of vocal folds
Vagus nerve
Jugular foramen
Nucleus ambiguus
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Pharyngeal branch:
• Levator veli palatini
•Salpingopharyngeus
• Superior pharyngeal
constrictor
• Muscularis uvulae
•Palatopharyngeus
• Inferior pharyngeal
constrictor
Motor cortex
Pons
Medulla oblongata
Uvula
Hyoid bone
laryngeal nerve branch
FIGURE 4–6. The many branches of the vagus nerve are amazingly com-
plex. The recurrent nerve and the superior laryngeal nerve branches innervate
the intrinsic muscles of the larynx. The pharyngeal branch (the pharyngeal
plexus) innervates many muscles of the pharynx and velum.
External superior
Common carotid artery
Subclavian artery
Recurrent nerve branch
All intrinsic muscles of larynx
(except cricoid muscles)
Adductor and abductor muscles
Cricoid muscle
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110 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
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including the musculus uvulae, levator veli palatini, salpingopharyngeus, palatopharyngeus, and the superior and middle pharyngeal constrictor muscles. Damage to the pharyngeal branch of the
vagus nerve can affect the movement of the velum. For instance,
unilateral damage to this branch can result in the affected side of
the velum hanging visibly lower than the other side. In most cases,
unilateral damage usually does not result in hypernasal speech
because the velar muscles on the unaffected side usually will be
able to raise the velum sufciently to ensure adequate velopharyngeal closure on nonnasal phonemes.
However, bilateral damage to the pharyngeal branch of the
vagus nerve can have a very signicant effect on resonance. (Remember, bilateral damage means that both the left and right branches
of a nerve have been injured in some manner.) When the damage
is bilateral, nearly all the muscles of the velum will demonstrate
weakness or paralysis. The result will be speech with moderate to
severe hypernasality. In addition, the pressure consonants (stops,
fricatives, and affricates) might be weak and distorted because of
the nasal emission of air through the unsealed velopharyngeal port.
External Superior Laryngeal Nerve Branch The external supe-
rior laryngeal branch of the vagus nerve innervates the cricothyroid muscle of the larynx. This muscle helps to stretch and tense
the vocal folds during speech. As a consequence, this muscle is
essential in controlling vocal pitch. Unilateral damage to this nerve
branch usually results in only modest difculty in varying pitch.
However, bilateral damage can cause signicant problems. When
the damage is bilateral, the cricothyroid muscle’s ability to stretch
and tense the vocal folds is greatly reduced. In such cases, an
affected individual’s voice might exhibit decreased loudness and
increased breathiness, and the individual could have notable difculty in changing vocal pitch.
Recurrent Nerve Branch This branch of the vagus nerve gets
its name from the “double-back” route it travels from the brainstem to the larynx. The recurrent nerve branches from the vagus
nerve after it leaves the cranium and then courses down near the
heart before turning upward, traveling up along the trachea until it
nally reaches the larynx. The recurrent nerve supplies the motor
innervation to all the intrinsic muscles of the larynx except the
cricothyroid muscle, which is innervated by the external superior
laryngeal nerve. The recurrent nerve is a vital contributor to phonation because it supplies the motor innervation for all the adductor
and abductor muscles of the vocal folds.
Unilateral damage to the recurrent nerve will cause the vocal
fold on the affected side to be xed in the paramedian position,
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which means that the fold is halfway between being fully adducted
or fully abducted. An individual with unilateral vocal-fold paralysis
will have breathy phonation and decreased vocal loudness. Bilateral damage to the recurrent nerve can x both vocal folds in the
paramedian position. When both vocal folds are in this position,
they probably will still be close enough together to permit phonation on exhalation. However, this phonation will be very breathy
and hoarse. Phonation on inhalation (inhalatory stridor) also might
be evident because the vocal folds are xed in this position during
inhalation as well as exhalation.
Accessory Nerve (XI)
The accessory nerve is unique in that it is not a “pure” cranial nerve.
It also contains neurons that branch out from the spinal cord. The
cranial neurons of this nerve originate in the medulla just below
the vagus nerve (Figure 4–7). In fact, many of its motor neuron
axons merge with the vagus nerve shortly after they leave the
medulla. These cranial motor neurons from the accessory nerve,
working in conjunction with the vagus nerve, help innervate the
intrinsic muscles of the velum, pharynx, and larynx. The spinal
components of the accessory nerve supply motor innervation for
the sternocleidomastoid and trapezius muscles. Because the neurons of this cranial nerve are so closely integrated with those of
the vagus nerve, it is practically impossible to separate the functions
of the two. In nearly all instances, damage to the cranial components of the accessory nerve will affect the vagus nerve as well and
vice versa.
Hypoglossal Nerve (XII)
The hypoglossal cranial nerve originates in the medulla and courses
to the tongue (Figure 4–8). This cranial nerve provides the motor
innervation for all the intrinsic and most of the extrinsic muscles
of the tongue. Unilateral damage to the hypoglossal cranial nerve
results in weakness or paralysis in the half of the tongue that is on
the same side as the nerve damage. If the damage is severe enough,
the tongue muscles on the damaged side will eventually atrophy,
leaving that half of the tongue shrunken. Furthermore, when the
tongue is protruded, it will deviate toward the affected side because
only half of the posterior genioglossus muscle is being contracted.
Bilateral damage to the hypoglossal nerve will result in overall
weakness of the tongue, reduction in the range of tongue movement, and muscle atrophy on both sides of the tongue.
face, the hypoglossal cranial nerve primarily receives unilateral
As with the branch of the facial nerve that innervates the lower
112 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
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XI cranial root
Foramen
magnum
XI spinal root
X vagus
Inferior
ganglion of X
FIGURE 4–7. The cranial root and spinal roots of the accessory nerve.
innervation from the upper motor neurons. This means, for example, that most of the innervation for the right hypoglossal nerve
comes only from the upper motor neurons of the corticobulbar
tract that are in the left hemisphere of the brain. Damage to those
left upper motor neurons will result in weakness in the right side of
the tongue. Conversely, damage to the right upper motor neurons
will result in weakness in the left side of the tongue.
Imprecise articulation is the primary characteristic of an indi vidual with hypoglossal nerve damage. In cases of unilateral damage,
the articulatory distortion will probably be mild because the unaffected side of the tongue can usually compensate for the weakened
movements of the impaired side. Bilateral damage, however, can
Intrinsic muscles of tongue
Hypoglossus nucleus
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Hypoglossal
nerve
Hypoglossal canal
Styloid process
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Motor cortex
Pons
Preolivary sulcus of
Medulla oblongata
Extrinsic muscles of tongue
•Styloglossus muscle
•Hyoglossus muscle
•Genioglossus muscle
FIGURE 4–8. The hypoglossal nerve innervates all the intrinsic and most
of the extrinsic tongue muscles.
have a much more signicant effect on articulation. In these cases,
phonemes requiring elevation of the tip or back of the tongue will
be notably distorted (Duffy, 2020). Slow lingual movements also will
be evident.
Spinal Nerves
The spinal nerves originate along the length of the spinal cord,
from the cervical and thoracic sections down to the lumbar region.
Many of the spinal nerves serve an important role in motor speech
• Superior longitudinal
•Transverse and Vertical
• Inferior longitudinal
Mandible
(cut)
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