Question

question 24
a drug that interferes with the active transport of ca²⁺ from the sarcoplasm back into the sarcoplasmic reticulum would result in
relaxation of the muscle fiber.
contraction with no relaxation.
muscle hypertrophy.
fibrosis of the muscle.
an imbalance of blood calcium.

question 25
if na⁺ cannot enter a muscle fiber in response to a stimulus,
contraction cannot occur.
relaxation cannot occur.
na⁺ will be released from the sarcoplasmic reticulum instead.
the active sites are left exposed.
the action potential travels into the muscle anyway.

question 26
which of the following is not a property of the myosin head?
they form cross - bridges with the active sites of actin.
they have a hinge region to bend and straighten.
they bind to troponin.
they have atpase activity.
they bind to atp.

Explanation:

For Question 24: Calcium re - uptake into the sarcoplasmic reticulum is needed for muscle relaxation. If this is blocked, the muscle remains contracted.
For Question 25: Sodium ion influx is crucial for the generation of an action potential in muscle fibers which is necessary for contraction. Without it, contraction cannot occur.
For Question 26: Myosin heads form cross - bridges with actin, have a hinge region, possess ATPase activity and bind to ATP. They do not bind to troponin; troponin is associated with the thin filament (actin) and is involved in regulating the interaction between actin and myosin.

Answer:

Question 24: contraction with no relaxation.
Question 25: contraction cannot occur.
Question 26: They bind to troponin.