Question

question 18
which of the following changes would have the most dramatic effect on the membrane potential?
increased permeability of the membrane to na+
increased extracellular concentration of na+
decreased permeability of the membrane to ca+2
increased permeability of the membrane to ca+2
increased permeability of the membrane to k+
increased extracellular concentration of ca+2

question 19
one atp molecule is required for
formation of the cross - bridge.
movement of the cross - bridge.
release of the cross - bridge.
formation of the cross - bridge and for movement of the cross - bridge.
formation of the cross - bridge, for movement of the cross - bridge, and for release of the cross - bridge.

question 20
which of the following structures contains the other four items listed?
postsynaptic membrane
presynaptic terminal
synaptic cleft
neuromuscular junction
receptors on postsynaptic membrane

Explanation:

• Question 18: The membrane potential is highly dependent on the permeability of the membrane to sodium ions ($Na^+$). An increase in $Na^+$ permeability can cause rapid depolarization and have a major impact on the membrane potential. Changes in extracellular $Na^+$ concentration have a less immediate and dramatic effect compared to permeability changes. Changes in $Ca^{2 +}$ and $K^+$ permeability are not as directly linked to the basic resting - and action - potential generation as $Na^+$ permeability.
• Question 19: One ATP molecule is required for the release of the cross - bridge in muscle contraction. The cross - bridge formation is due to the binding of myosin to actin, and its movement is powered by the hydrolysis of ATP, but the specific step that requires one ATP is the release of the cross - bridge.
• Question 20: The neuromuscular junction is a complex structure that includes the presynaptic terminal (where neurotransmitters are released), the synaptic cleft (the space between the pre - and post - synaptic membranes), the postsynaptic membrane (with its receptors), and the receptors on the postsynaptic membrane are part of the postsynaptic membrane structure.

Answer:

• Question 18: Increased permeability of the membrane to $Na^+$
• Question 19: release of the cross - bridge
• Question 20: Neuromuscular junction