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?

An investigator is studying the cross-bridge cycle of muscle

contraction. Tissue from the biceps brachii muscle is obtained during the

autopsy. Investigation of the muscle tissue shows myosin heads attached to

actin filaments. Binding of myosin heads to which of the following elements

would most likely cause detachment of myosin from actin filaments?

A 13-year-old boy is injured during a football

match and is taken to the hospital for stitches. During the game he fell and cut

himself on a piece of broken glass. The shard cut through the skin on the front

of his thigh and cut into the outer sheath surrounding the quadriceps femoris

muscle. What's this superficial covering of the muscle called?

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 34-year-old woman

presents with complaints of progressive weakness. She says that her symptoms get

worse toward the end of the day, she has difficulty swallowing and smiling, and

she sometimes gets double vision. She is diagnosed with myasthenia gravis, an

autoimmune disorder in which muscle cells cannot respond appropriately to

neural stimulation. Autoantibodies bind to and destroy acetylcholine receptors

on the postsynaptic membrane of the muscle fiber. What does this disease in

muscle fibers lead to?

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?

An investigator is studying muscle contraction in tissue obtained from

the thigh muscle of an experimental animal. After injection

of radiolabeled ATP, the tissue is stimulated with electrical impulses.

ATP binds to myosin heads during contraction. In which part of the sarcomere

does this reaction occur?