Question Changes in membrane potential during nerve transmission along an axon involve the movement of sodium and potassium ions across the axon membrane.
▶️Answer/Explanation
Question 21
I. Acetylcholine is released into the synaptic cleft by exocytosis when calcium ions enter the presynaptic neuron.
II. Acetylcholine diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
III. Acetylcholine is broken down by cholinesterase and resorbed into the presynaptic neuron once an impulse has been initiated on the postsynaptic membrane.
▶️Answer/Explanation
Question
Doctors frequently use medications to restore normal action potentials. The oscilloscope trace illustrates the effect of one such drug, which targets potassium (K) ion channels in cell membranes.
From the graph, what can be concluded about the effect of this drug on membrane potentials?
A. It delays repolarization by preventing the influx of K ions towards the cytoplasm.
B. It delays depolarization due to a build-up of K ions outside the neuron.
C. It lengthens the resting potential as membrane permeability to K ions is decreased.
D. It lengthens the action potential by reducing the rate of K ions released from the cytoplasm.
▶️Answer/Explanation
Ans. D
A drug can lengthen the action potential by reducing the rate of potassium (K+) ions released from the cytoplasm by blocking a potassium channel:
Potassium channels
When voltage-gated potassium channels (Kv) open, potassium ions flow out of the cell, which decreases the membrane potential. The slower kinetics of Kv channels compared to sodium (Na) channels means that Kv channels open after about 1 millisecond.
Blocking potassium channels
Blocking a potassium channel with a drug can prolong the action potential. The degree of prolongation depends on which channel is blocked and how other channels affect the repolarization reserve.
Action potential duration
Prolonging the action potential duration can cause an LQTS phenotype. This can be caused by a decrease in the outward repolarizing current or an increase in the inward depolarizing membrane current.
Question
The diagram shows the synaptic transmission of nerve impulses by the neurotransmitter acetylcholine.
What is the fate of acetylcholine immediately after binding to the receptor?
A. It is pumped into the postsynaptic neuron.
B. It diffuses into the presynaptic neuron.
C. It is broken down in the synaptic cleft.
D. It binds to another receptor in the postsynaptic neuron.
Answer/Explanation
Ans: C
Question
The image shows a neuron.
What is the function of X?
A. Increases the speed of transmission along the axon
B. Increases the rate of exchange of sodium and potassium ions
C. Holds bundles of neurons together to form a nerve
D. Determines the direction of the action potential
Answer/Explanation
Ans: A