A student investigates the maximum frequency of impulse transmission in two preparations using isolated tissue from a mammal.
Preparation 1: A single sensory neurone from a taste bud is stimulated directly. The maximum frequency of action potentials recorded is 500 Hz, limited by the refractory period.
Preparation 2: A pair of neurones connected by a cholinergic synapse is used. The presynaptic neurone is stimulated, and the frequency of postsynaptic potentials in the postsynaptic neurone is recorded. The maximum successful transmission frequency is 100 Hz, limited by factors at the synapse.
Compare the mechanisms that limit the maximum frequency of impulses in the sensory neurone (Preparation 1) and the maximum frequency of successful synaptic transmission across the cholinergic synapse (Preparation 2). In your answer, relate these limitations to the events occurring during the refractory period and during synaptic transmission, respectively.