228 Neuroanatomy: Draw It to Know It
The Gag Refl ex
In neurolog y, the gag refl ex is oft en 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. Th e gag refl ex relies on the glossopharyn-
geal nerve for its aff erent loop and the vagus nerve for its
eff erent loop, and it also involves the motor division of
the trigeminal nerve and the hypoglossal nerve for addi-
tional motor eff ects. Although certain details of the gag
refl ex remain poorly understood, enough of its neuro-
anatomy has been elucidated that we are able create a
reliable diagram for it.
First, show that soft palate stimulation sends a volley
of aff erent impulse along the glossopharyngeal nerve; it
is unclear whether this aff erent impulse is somatic or vis-
ceral. Next, draw the glossopharyngeal sensory nuclei;
include both the spinal trigeminal nucleus for the
GSA fi bers and the solitary tract nucleus for the GVA
fi bers (as either may be involved). Th en, show that these
nuclei excite motor neurons in nucleus ambiguus and
indicate that nucleus ambiguus sends eff erent impulse
via the vagus nerve for pharyngeal constriction. Th us,
the glossopharyngeal nerve provides the aff erent limb of
the gag refl ex and the vagus nerve provides the eff erent
limb. However, the gag refl ex additionally involves jaw
opening and tongue thrust, so also show that nucleus
ambiguus stimulates the trigeminal motor nucleus in the
pons, which is largely responsible for jaw opening via cra-
nial nerve 5, and the hypoglossal nucleus in the medulla,
which provides tongue thrust via cranial nerve 12.
Next, let’s illustrate the supranuclear innervation of the
palate in order to demonstrate the cortical innervation to
the special visceral eff erent component of the glossopha-
ryngeal and vagus nerves. First, draw the bilateral nucleus
ambiguus nuclei. Th en, show that each nucleus ambiguus
innervates the ipsilateral side of the palate. Next, show
that each cerebral hemisphere projects corticonuclear
fi bers to each nucleus ambiguus, but show that the pre-
dominance of fi bers go to the contralateral nucleus.
Now, we will use prototypical injury patterns to show
what happens in both upper motor neuron and lower
motor neuron lesions to solidify our understanding of
this arrangement. Redraw the hemispheres and bilateral
nucleus ambiguus nuclei. Show that in an upper motor
neuron lesion (such as a cortical stroke), the side of
the palate contralateral to the stroke is weak and the
side contralateral to the intact hemisphere is strong :
the majority of fi bers from the intact hemisphere go to
the contralateral nucleus ambiguus and the minority go
to the ipsilateral nucleus ambiguus. Note, however, that
many texts deny that there is any diff erence in the inner-
vation to the ipsilateral and the contralateral nucleus
ambiguus nuclei and simply state that cortical innerva-
tion to the nucleus ambiguus nuclei is bilateral without
ipsilateral or contralateral predominance. When that is
the case, a unilateral upper motor neuron lesion (eg ,
a cortical stroke) has no eff ect on the gag refl ex.
Next, again draw the hemispheres and bilateral
nucleus ambiguus nuclei. Show that in a lower motor
neuron injury (such as a direct lesion to the nucleus
ambiguus, itself, or its exiting fi bers), the corticonuclear
innervation is preserved but there is complete peripheral
fi ber loss on the side of the lesion and, therefore, there is
unilateral palatal paralysis on the side of the lower motor
neuron lesion.^11
When we evaluate for palatal weakness, the side of
the palate that hangs lower is the side that is weak;
dismiss trying to determine the directionality of the
uvula — this only adds an unnecessary layer of complex-
ity 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 oft en loss of corticonuclear innervation
to the nucleus ambiguus, with resultant characteristic
nasal pattern speech.