Question

question 12
which of the following situations occurs in electrically excitable cells?
when na+ ion channels open, k+ ion channels close.
the sodium - potassium pump moves sodium into the cell.
depolarization causes voltage - gated sodium ion channels to open.
sodium ion channels are opened by high extracellular calcium levels.
the opening and closing of ligand - gated channels cause depolarization.
question 13
substance a is found to be a competitor with acetylcholine. which step of muscle contra
exposure of the active sites on g actin
release of acetylcholine from the presynaptic terminal
generation of an action potential at the sarcolemma
release of the myosin head following the power stroke
question 14
the length of the resting sarcomere is
longer than the length of a contracted sarcomere.
shorter than the length of a contracted sarcomere.
the same length as a contracted sarcomere.
the same length as the muscle fiber.
the same length as the myofibril.

Explanation:

• Question 12: In electrically excitable cells, depolarization changes the membrane - potential, which causes voltage - gated sodium ion channels to open. The sodium - potassium pump moves sodium out of the cell, not into it. There is no strict rule that when $Na^+$ ion channels open, $K^+$ ion channels close. Sodium ion channels are not opened by high extracellular calcium levels, and the opening and closing of ligand - gated channels may or may not cause depolarization depending on the ligand and ion involved.
• Question 13: Acetylcholine binds to receptors on the sarcolemma to generate an action potential. A competitor of acetylcholine would interfere with this binding and thus the generation of an action potential at the sarcolemma. It does not directly affect the exposure of active sites on G - actin, the release of acetylcholine from the presynaptic terminal, or the release of the myosin head following the power stroke.
• Question 14: During muscle contraction, the sarcomere shortens. So, the length of the resting sarcomere is longer than the length of a contracted sarcomere.

Answer:

• Question 12: C. Depolarization causes voltage - gated sodium ion channels to open.
• Question 13: C. Generation of an action potential at the sarcolemma.
• Question 14: A. longer than the length of a contracted sarcomere.