| Date | May 2018 | Marks available | 2 | Reference code | 18M.3.HL.TZ2.7 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Explain | Question number | 7 | Adapted from | N/A |
Question
An organism’s diminished response to a repeated stimulus is referred to as habituation. The diagrams show the experiments performed on habituation to a stimulus in a sea slug (Aplysia californica). The sea slug is a mollusc that has an external gill for gas exchange. If the siphon of the sea slug is stimulated mechanically, it withdraws the gill in a simple reflex action.
State the effector in this reflex arc.
With reference to the experiments on habituation, state the effect of repeated stimulation on the level of neurotransmitter in the synapses.
With reference to the withdrawal of the siphon in Aplysia californica, explain the concept of summation.
Amphetamines act on synapses that use dopamine as a neurotransmitter by increasing the release of dopamine into the synaptic cleft. Outline the effect this would have on human synaptic transmission.
Markscheme
muscle «that retracts gill»
less neurotransmitters/dopamine are released «to the synaptic cleft by presynaptic interneurons»
a. summation is the sum of all depolarization impulses/neurotransmitters released in the synapse/synaptic cleft of the effector/motor neuron
b. in control more excitatory than inhibitory so effect would be excitatory
c. less of each in habituated so not enough to produce action potential in the effector/motor neuron
a. amphetamines are stimulants
b. increases the release of dopamine in the presynaptic neuron
c. blocks reuptake by presynaptic neuron
d. more dopamine in the synaptic cleft
e. increased binding on / transmission by the postsynaptic neuron
