Cardiomyocytes, the

cardiac muscle cells, work throughout our lives. If we think about this

information, how will these cells differ from skeletal muscle fibers?

A

35-year-old woman comes to a neurologist because she is suspected of having

multiple sclerosis. Unfortunately, further examination will confirm this

diagnosis. Multiple sclerosis is an autoimmune disease affecting the central

nervous system. This disease leads to loss of myelin in inflammatory areas and

diffuse loss of axons. Which type of cell is responsible for the production of

myelin and in which part of the central nervous system is it produced?

An investigator is studying the microstructure

of skeletal muscle fibers to identify new targets for skeletal muscle

relaxation. He obtains a biopsy from the biceps muscle of a test subject and

observes it under a transmission electron microscope. A photomicrograph of the

structures seen during microscopy is shown. Which of the following parts

contains the greatest proportion of actin filaments? 

A 43-year-old female patient developed spasmodic torticollis, also

called "cervical dystonia", about five months ago. Dystonia is a form

of unusual movement that you can't control. Spasmodic torticollis affects the

muscles of the neck. Initially, this patient experienced a twisting of the head

towards the right shoulder during speech or driving. Over time, her head began

to turn virtually only toward that side and now her head is permanently tilted

toward her right shoulder. As the condition is slightly worsening and the cause

has not been identified, she has decided to undergo treatment with botulinum

toxin. Botulinum toxin is injected directly into the affected muscle. The main

mechanism of action is to block the release of acetylcholine from the nerve

endings into the synaptic cleft, thereby temporarily inhibiting muscle work in

the muscle. Where does the acetylcholine in these nerve endings come from?

A 3-year-old boy is

brought to the pediatrician. The parents are worried that the boy has started

falling unexpectedly when walking. Physical examination reveals muscle weakness

especially in the lower extremities. As muscular dystrophy is suspected here

based on history and other investigations, a biopsy of his skeletal muscle is

taken and sent to a pathologist. However, the pathologist's report states that

the sample contains only healthy muscle tissue. Which of the following

characteristics can be used to describe the specimen?