Signals at the axon initial segment

Science · The Human Body · ages 17-18

Name ______________________ Date ____________

At a chemical synapse, transmitter reaches the receiving cell but cannot bind its receptors. Which step is directly prevented?
- Calcium entering the sending terminal through its channels
- Receptor activation that changes the receiving cell's ion flow
- Vesicle fusion that releases transmitter from the sending cell
At a chemical synapse, which sequence follows an arriving action potential?
- Transmitter diffuses, calcium enters, vesicles fuse
- Calcium enters, vesicles fuse, transmitter diffuses
- Vesicles fuse, transmitter diffuses, calcium enters
An inhibitory response opens potassium channels and potassium leaves the postsynaptic cell. How can this reduce firing?
- Positive charge leaves, making the membrane more negative
- Negative charge leaves, moving the membrane toward threshold
- Positive charge enters, moving the membrane toward threshold
A substance blocks acetylcholinesterase at a synapse using acetylcholine. Release is unchanged. What direct effect is expected?
- Acetylcholine enters calcium channels and stops vesicle fusion
- Acetylcholine is broken down faster and leaves receptors sooner
- Acetylcholine remains available longer to stimulate receptors
Case P has overlapping responses from repeated inputs at one synapse. Case Q has overlapping responses from different synapses. Which classification fits?
- P is temporal summation; Q is spatial summation
- P is spatial summation; Q is temporal summation
- P is temporal summation; Q cannot involve summation
Where does a typical neurone begin a new action potential when its combined inputs reach threshold?
- In the synaptic vesicles inside its axon terminal
- In the axon initial segment beside the hillock
- In the myelin sheath wrapped around the axon
An inhibitory synapse must make the membrane more negative to reduce firing. This claim is true.
Circle one: True False
In a simplified model, a neurone starts at -70 mV. Three overlapping EPSPs each add +4 mV and one IPSP adds -3 mV at its initial segment. Threshold is -55 mV. What is the result?
- -55 mV; the inputs reach threshold and start a new spike
- -58 mV; the IPSP leaves the summed voltage unchanged
- -61 mV; below threshold, so no new spike starts

Answer key

For grown-ups. Fold this page away before handing over the rest.

Signals at the axon initial segment W1-mt_0uM8xRV92B-s1

Receptor activation that changes the receiving cell's ion flow · Transmitter binding activates suitable postsynaptic receptors. Without binding, those receptors cannot produce their usual effect on ion flow.
Calcium enters, vesicles fuse, transmitter diffuses · Calcium entry triggers vesicle fusion. Fusion releases transmitter, which then diffuses across the cleft.
Positive charge leaves, making the membrane more negative · Potassium ions carry positive charge. Their outward flow can hyperpolarise the membrane and move it away from threshold.
Acetylcholine remains available longer to stimulate receptors · Acetylcholinesterase breaks down acetylcholine. Blocking it slows that removal route, leaving transmitter available for longer.
P is temporal summation; Q is spatial summation · Temporal summation combines repeated responses at one synapse. Spatial summation combines responses from different synapses. Both cases state that responses overlap.
In the axon initial segment beside the hillock · The axon initial segment beside the hillock is the usual initiation site. Reaching threshold there starts a new action potential.
False · False. Shunting inhibition can reduce the effect of excitation without making the membrane more negative.
-61 mV; below threshold, so no new spike starts · The EPSPs add +12 mV and the IPSP adds -3 mV. The net change is +9 mV, giving -61 mV. This is below the -55 mV threshold.