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
Explanation:
During repolarization of the neuron after an action potential, potassium (K⁺) channels open, and K⁺ ions flow out of the axon. This causes the inside of the membrane to become more negative again, restoring the resting membrane potential.
Let’s analyze the options:
A. Correct – Potassium channels open and allow potassium ions to move to the outside of the axon membrane. This causes repolarization, which explains the downward phase of the trace.
B. Incorrect – Potassium ions moving into the axon would cause depolarization, not repolarization.
C. Incorrect – The sodium-potassium pump does help restore ion balance after an action potential, but not during the immediate change at point Y.
D. Incorrect – Potassium channels are open at Y; this is the phase where ion movement is happening, not a resting state.
Question
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
Synaptic transmission involves a chain of events:
- Calcium ions enter the presynaptic neuron.
- Vesicles release acetylcholine by exocytosis.
- Acetylcholine binds to receptors on the postsynaptic membrane.
- After triggering a response, acetylcholine is broken down by cholinesterase and recycled.
I. Correct – Calcium triggers exocytosis of acetylcholine.
II. Correct – Acetylcholine binds to specific receptors on the postsynaptic membrane.
III. Correct – Acetylcholine is broken down by cholinesterase and its components are taken back into the presynaptic neuron.
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
Answer: D. It lengthens the action potential by reducing the rate of K ions released from the cytoplasm.
Explanation:
Potassium ion channels are crucial for repolarization. A drug that blocks or slows K⁺ efflux will delay repolarization, leading to a longer action potential.
Let’s analyze the options:
A. Incorrect – K⁺ leaves the cytoplasm, not enters it. Influx of K⁺ is not typical during repolarization.
B. Incorrect – The delay is due to slower efflux of K⁺, not a build-up outside the neuron.
C. Incorrect – The resting potential isn’t significantly lengthened by this drug; it affects the action potential.
D. Correct – By reducing the rate at which K⁺ leaves the cell, the drug extends the repolarization phase, thus lengthening the action potential.
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
Answer: C. It is broken down in the synaptic cleft.
Explanation:
After acetylcholine binds and activates the receptor, it is quickly broken down by the enzyme acetylcholinesterase in the synaptic cleft. This stops the signal to prevent overstimulation.
Let’s analyze the options:
A. Incorrect – Acetylcholine is not pumped into the postsynaptic neuron.
B. Incorrect – It does not diffuse back into the presynaptic neuron.
C. Correct – Acetylcholine is broken down in the synaptic cleft into acetate and choline.
D. Incorrect – Acetylcholine does not rebind to other receptors once used.
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
Answer: A. Increases the speed of transmission along the axon
Explanation:
The myelin sheath is a fatty layer around the axon that helps insulate the nerve fiber and speed up the transmission of electrical impulses by allowing them to “jump” between gaps (nodes of Ranvier) this is called saltatory conduction.
Let’s analyze the options:
A. Correct – Myelin increases the speed of signal transmission.
B. Incorrect – The ion exchange happens at the nodes, not due to myelin.
C. Incorrect – Myelin wraps individual axons, not bundles of neurons.
D. Incorrect – The axon hillock and other regions determine action potential direction, not myelin.