Which of the following statements is INCORRECT concerning contractile fibers? a. When a contractile fiber reaches threshold, voltage – gated fast sodium channels open allowing a rapid influx of sodium which produces a rapid depolarization which ends after several milliseconds. b. During plateau, depolarization is maintained because potassium inflow balances sodium outflow. c. Repolarization involves outflow of potassium restores the resting membrane potential to -90mV. d. During the refractory period a second contraction cannot be triggered.

B is INCORRECT

Explanation:

Action potential in contractile cells can be divided into 3 phases

1. Rapid depolarization; caused by fast influx of sodium ions into the cells. Last a few milliseconds.

2. Plateau phase: This is a sustained positive membrane potential, after rapid depolarization, caused by a balance between calcium ion influx and slow potassium ion efflux.

3. Rapid repolarization: Eventually there's a closure of all other ion channels except the potassium ion channels causing rapid efflux of potassium ions, returning the membrane potential back to negative.

A refractory period usually occurs after undergoing an action potential. During this time no action potential can occur.

B.During plateau, depolarization is maintained because potassium inflow balances sodium outflow.

Explanation:

The  depolarization of plateau  of  a contractile fibres; is due to   combined effects of  opening of calcium ion channels  and entry of Calcium ions into the sarcolemma, and  restriction of  potassium ions  from leaking out of the of the cell. These two effects extended the period of depolarization of the contractile fibres. Thus  when the graph of  membrane potential produced is plotted against time to determine the course of action potential    the   plateau indicated extended depolarisation

Thus the option B is out of the sequence of event needed for contractile fibers plateau.

i think it is d. objects resist changes in their state of motion.