 According to Craver (2007b), we should distinguish between two kinds of relations that make up mechanisms: (1) causal and (2) constitutive relations. The former may hold between different parts of a mechanism. For example, in the basic mechanism of synaptic transmission at the terminal part of a neuron, the influx of calcium causes the release of neurotransmitter in the space between the synaptic terminal and the postsynaptic cell membrane. This causal link can be understood pretty much as discussed in the last two sections. The other kind of relation, mechanistic constitutive relevance (or just mechanistic constitution) holds between the parts and of a mechanism and the phenomenon the mechanism is for. For example, the influx of calcium into an axon terminal, together with other events, constitutes the phenomenon of synaptic transmission. Craver (2007b) contends that this is not a causal relation because the relata cannot be viewed as distinct and non-overlapping.