QUESTION: 133

Lymph nodes are populated by lymphocytes that exit the vascular compartment to gain access to the parenchyma of the node by passing through the walls of which of the following?

afferent lymphatic vessels
arterioles
efferent lymphatic vessels
high endothelial postcapillary venules
medullary sinuses

Answer(s): D

Explanation:

High endothelial venules (HEV), located primarily in the deep cortex, are specialized to recruit circulating lymphocytes from the blood. Lymphocytes in the circulating blood adhere to the lining endothelial cells of HEV by way of an integrin-based recognition. Lymphocytes then gain access to the lymph node tissue by actively migrating (a process called diapedesis) between or through endothelial cells. Afferent lymphatic vessels (choice A) conduct lymph, not blood, into the lymph node. The source of the lymph is either upstream lymph nodes or tissue fluid from the region supplied by the node. This component of the system serves as a filter and as a mechanism for antigen-presenting cells to enter the node. Arterioles (choice B) are a component of the circulation of the lymph node, but they are not permeable to cell traffic. Efferent lymphatic vessels (choice C) conduct lymph and cells from the lymph node to either the blood circulation or downstream lymph nodes. Lymph in efferent lymphatic vessels conveys immunoglobulins and recirculating lymphocytes to the bloodstream. Medullary sinuses (choice E) are part of a system of passages that filter lymph and direct it from the afferent lymphatic vessels to the efferent lymphatic vessels. Medullary sinuses occupy spaces between medullary cords, which are occupied by large numbers of plasma cells, the cells that secrete immunoglobulins.

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QUESTION: 134

A 32-year-old female professional gardener complains of increasing muscle weakness and fatigue during the day, requiring her to take frequent rests. She also reports that she cannot enjoy her meals any more because her muscles of mastication quickly weaken and she has to stop chewing. When she watches television at night for a long period of time, her vision becomes blurry and she sees double. Her neurologist makes a preliminary diagnosis of myasthenia gravis. Which of the following is the cause of myasthenia gravis?

Acetylcholine synthesis in motor neurons is impaired.
Acetylcholinesterase synthesis is inhibited.
Autoantibodies destroy cholinergic receptors at the postsynaptic membrane preventing binding of acetylcholine.
Neurotransmitter release is impaired at the presynaptic membrane of the neuromuscular junction.
Signal transduction within the muscle is impaired.

Answer(s): C

Explanation:

Myasthenia gravis is an autoimmune disorder where autoantibodies target the postsynaptic cholinergic receptors and destroy them. Acetylcholine released from motor neurons is thus unable to bind and the muscle contraction weakens due to decreased neurotransmitter communication. Acetylcholine synthesis in motor neurons (choice A) remains normal. Acetylcholinesterase (choice B) is the enzyme which degrades acetylcholine and its synthesis is not affected in myasthenia gravis. The cholinergic neurotransmitter release mechanism (choice D) at the presynaptic membrane, as well as the signal transduction mechanism (choice E) within the muscle, remain normal.

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QUESTION: 135

In Fig. following figure which labeled bracket spans a sarcomere?

Answer(s): A

Explanation:

An individual sarcomere, the unit of contraction in striated muscle, spans the interval between successive Z lines. Each sarcomere encompasses an A-band (choice B) and half of each of two I bands (choice D).
Each myofibril (choice C) of a striated muscle fiber is composed of a tandem series of sarcomeres.
Coupling of excitation and contraction is a critical function of the triad (choice E), which is composed of a T tubule interposed between two cisternae of the sarcoplasmic reticulum.

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QUESTION: 136

The great cerebral vein of Galen indicated by arrow 4 in following figure is formed by the union of two internal cerebral veins and drains into which of the following?

confluence of sinuses
frontal sinus
sphenoid sinus
straight sinus
superior sagittal sinus

Answer(s): D

Explanation:

The great cerebral vein of Galen (arrow 4) drains posteriorly into the straight sinus (arrow 5). The union of the superior sagittal sinus (choice E; arrow 3) and the straight sinus forms the confluence of sinuses (choice A). The straight sinus and superior sagittal sinus are dural venous sinuses: they contain venous blood draining form the brain, skull, and scalp. The frontal sinus (choice B) and the sphenoid sinus (choice C) are bony sinuses: they are hollow, airfilled structures and do not drain venous blood.

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QUESTION: 133

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QUESTION: 135

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QUESTION: 136

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