QUESTION: 69

Desmin is a 53-kDa protein found in skeletal and smooth muscle cells. It plays a role in coordinating muscle cell contraction. Desmin belongs to which type of intermediate filaments?

type I
type III
type IV
type V
type VI

Answer(s): B

Explanation:

Desmin belongs to the type III intermediate filaments. Intermediate filament proteins have three domains: a head domain (N-terminal), an alpha-helical rod domain, and a tail domain (C-terminal). The six types of intermediate filament proteins are differentiated by the sequence in the rod domain. Type I (choice A) intermediate filaments include the cytokeratins. Type IV (choice C) intermediate filaments are found in the nervous system and include the neurofilaments and alpha-internexin. Type V (choice D) intermediate filaments are the nuclear lamins. Nestin, expressed in stem cells of the central nervous system, is a type VI (choice E) intermediate filament.

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QUESTION: 70

Which of the following normally occupy the dark spaces indicated by the arrows in following figure?

blood vessels
differentiating blood cell precursors
osteoblasts
osteoclasts
osteocytes

Answer(s): E

Explanation:

The black areas in this section of dried, compact bone are empty spaces normally occupied by cells and soft tissues. The arrows indicate lacunae interspersed among lamellae of osteons (haversian systems). In life, these are occupied by osteocytes. The fine lines radiating from the lacunae are canaliculi, which contain processes of the osteocytes. Blood vessels (choice A) of compact bone course through Volkmann's canals (not shown) and haversian canals (seen here at the center of each osteon).
Hematopoietic cells (choice B) occur in red marrow located within medullary canals of long bones and the cavities of cancellous bone. Osteoblasts (choice C) are restricted to surfaces of bone at sites of bone apposition. When osteoblasts become entrapped in lacunae as a result of their synthetic activity, they become osteocytes. Osteoclasts (choice D) are large, multinucleated cells found at surfaces of bone at sites that are undergoing absorption.

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QUESTION: 71

A 19-year-old camper fell asleep with the right side of her face against the cold earth on an autumn night. In the morning, she woke up and found that she could not move the right side of the face. She went to the local emergency room in a nearby town and was diagnosed with Bell's palsy. Which of the following muscles is not affected in her condition?

buccinators
levator labii superioris
levator labii superioris alaeque nasi
levator palpebrae superioris
orbicularis oculi

Answer(s): D

Explanation:

The levator palpebrae superioris, which opens the upper eyelid and is located in the orbit, is innervated by the oculomotor (third cranial) nerve. In this case of Bell's palsy, longterm exposure to the cold has affected the peripheral portion of the facial (seventh cranial) nerve. The buccinator (choice A), levator labii superioris (choice B), levator labii superioris alaeque nasi (choice C), and orbicularis oculi (choice E) are all innervated by the facial nerve and will all be impaired.

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QUESTION: 72

In examining a 30-year-old male patient with multiple sclerosis, you notice that when you ask the patient to look to the left, the right eye fails to adduct and when the patient looks to the right, the left eye fails to adduct. However, both eyes in the patient can adduct during convergence. There is no noticeable strabismus when the eyes are focused on a far point. During abduction, both eyes display nystagmus. You record on the patient's chart a diagnosis of bilateral internuclear opthalmoplegia.
Which of the following neural structures is affected?

fourth cranial (trochlear) nerve
medial longitudinal fasciculus (MLF)
second cranial (optic) nerve
sixth cranial (abducens) nerve
third cranial (oculomotor) nerve

Answer(s): B

Explanation:

Conjugate movement of the eyes depends on the integrity of the MLF, which links the ipsilateral lateral gaze center in the pons with the contralateral oculomotor nucleus in the midbrain. Lesion of the MLF disconnects these brain nuclei resulting in internuclear ophthalmoplegia and the bilateral condition can be seen in patients with multiple sclerosis. A patient with a lesion of the fourth cranial nerve (choice A) cannot look downward and inward; there is a slight convergent strabismus when the patient attempts to do so.
Lesion of the second cranial nerve (choice C) will result in blindness. Weakness of eye abduction characterizes lesion of the sixth cranial nerve (choice D), which also displays s a convergent strabismus. In a patient with a third cranial nerve (choice E) lesion, the eye is deviated downward and outward (divergent strabismus); ptosis would also be noticeable.

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