Anatomy Answers-7

(E) The arytenoid cartilages arise as swellings
from the sixth pharyngeal (branchial) arch at
32 days of gestation. They are located between
the caudal end of the hypobranchial eminence
and the cranial end of the laryngotracheal tube.
The swellings will grow cranially to form the
arytenoids and corniculate cartilages along
with the primitive aryepiglottic folds. In laryngomalacia,
the bulky arytenoid cartilages prolapse
anteromedially on inspiration, resulting
in stridor. The first (choice A), second (choice B),
and third (choice C) pharyngeal arches do not
participate in the development of laryngeal cartilages.
The fifth (choice D) pharyngeal arch
remains rudimentary and does not give rise to
any adult structure.

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Anatomy Answers-5

(B) The abducens (sixth cranial) nerve normally
innervates the lateral rectus muscle. The inferior
oblique (choice A), medial rectus (choice C),
and superior rectus (choice E) are all innervated
by the oculomotor (third cranial) nerve. The
superior oblique (choice D) is innervated by the
trochlear (fourth cranial) nerve. In Duane syndrome,
the absence of lateral rectus innervation
by the abducens (sixth cranial) nerve is replaced
by an aberrant innervation of the oculomotor
(third cranial) nerve. Thus when the patient is
asked to adduct the affected eye, there is cocontraction
of the lateral and medial recti with
globe retraction.

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Anatomy Answers-3

(C) The posterior digastric muscle is derived
from the second branchial arch and thus is innervated
by its nerve, the facial (Seventh cranial)
nerve. The anterior digastric muscle (choice A) is
derived from the first branchial arch and is innervated
by its nerve, the mandibular division of the
trigeminal (fifth cranial) nerve. The posterior
cricoarytenoideus muscle (choice B) and the
superior constrictor muscle (choice E), derived
from a combination of the fourth and sixth
branchial arches, are innervated by the vagus
(tenth cranial) nerve. The stylopharyngeus
(choice D) is derived from the third branchial
arch and is the only voluntary muscle innervated
by the glossopharyngeal (ninth cranial) nerve.

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Anatomy Answers-2

(D) The nucleus dorsalis (Clarke’s column,
Rexed’s lamina VII of the spinal cord) contains
the cell bodies of the second order neurons of
the dorsal (posterior) spinocerebellar tract.
Axons from these neurons ascend ipsilaterally
in the lateral funiculus of the spinal cord, join
the restiform body of the inferior cerebellar
peduncle, and terminate in the vermis of the
cerebellum as mossy fibers. The dorsal (posterior)
spinocerebellar tract conveys proprioception
from muscle spindles and Golgi tendon
organs. Collateral branches of this tract also
terminate in the deep cerebellar nuclei (choice A).
The dorsal root ganglion (choice B) contains the
cell bodies of sensory neurons, including the
first-order neurons of the dorsal (posterior)
spinocerebellar tract. The nucleus cuneatus
(choice C) contains the second order neurons of
the dorsal column pathways, responsible for
conveying sensations of fine touch, pressure,
and vibration sense. Rexed’s lamina IX of the
spinal cord (choice E) contains the spinal cord
motorneurons responsible for the innervation
of voluntary muscles.

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Anatomy Answers-1

Answer is A Dystrophin anchors actin to the sarcolemma,
reinforcing and stabilizing the latter
during muscle contraction. Titin is a large protein
which associates with myosin filaments
and endows them with elastic recoil properties
(choice B). Desmin filaments form a supportive
network extending from Z disk to Zdisk
(choice C). Troponin I inhibits the binding of
myosin to actin (choice D). AlphaB-crystallin
protects desmin filaments from stress-induced
damage (choice E).

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