Biology HL
Action Potentials and Synaptic Integration
- 1.
A hypothetical axon trace rises from -70 mV to +30 mV, falls to -80 mV, then returns to rest. Explain the channel events during depolarization and repolarization and explain why the sodium-potassium pump is not the immediate cause of the rapid falling phase.
[4 marks] · no calculator - 2.
Compare propagation in a myelinated axon with propagation in an unmyelinated axon of similar diameter. Explain the roles of myelin, local currents and nodes of Ranvier, and predict a consequence of loss of myelin.
[4 marks] · no calculator - 3.
In a simplified hypothetical integration model, a neurone rests at -70 mV and fires at -55 mV. At the integration site two simultaneous excitatory inputs each depolarize it by 9 mV, while one inhibitory input hyperpolarizes it by 7 mV. Assume linear summation for this task. Calculate the resulting potential with all three inputs, decide whether it fires, and explain what happens if the inhibitory input is removed. If excitation is increased further once threshold is exceeded, explain whether the amplitude of each action potential increases proportionally.
[4 marks] · no calculator - 4.
During the absolute refractory period after a neuronal action potential, many voltage-gated sodium channels are inactivated. Explain why a second action potential cannot immediately be triggered at that membrane region and how this contributes to forward propagation along an axon.
[3 marks] · no calculator - 5.
A hypothetical loss of myelin leaves most axonal ion-channel distribution initially unchanged. Explain why conduction may slow or fail and why saltatory conduction does not mean sodium ions physically jump from one node of Ranvier to the next.
[3 marks] · no calculator