An increase in sodium conductance is associated with:
A. The plateau phase of the ventricular muscle action potentialB. The downstroke of the skeletal muscle action potential
C. The upstroke of the smooth muscle action potential
D. The end-plate potential of the skeletal muscle fiber
The Answer is D.
The channel opened by ACh when it binds to
receptors on the end plates of skeletal muscle fibers is equally permeable to
potassium and sodium. The increase in sodium permeability allows sodium to flow
into the cell and produces the end-plate potential. The plateau phase of
ventricular muscle action potentials and the upstroke of smooth muscle action
potentials are produced by an increase in calcium conductance. An increase in
potassium conductance is responsible for the downstroke of the action
potential. The refractory period is caused by an increase in potassium
conductance and a decrease in the number of sodium channels available to
produce an action potential (i.e., sodium channel inactivation).
