If substance P fails to fulfil the criteria as a neurotransmitter in somatosensory afferents, what might be its function?

In the first part of this paper, a series of examples are described where substance P content of afferent C fibres and responses which have been attributed to C fibre excitation vary independently. Thirteen sets of data fail to support the proposal that SP is an excitatory transmitter for noxious inputs. In the second part an alternative hypothesis is proposed in which C fibres would have functions in addition to impulse propagation. It is shown that C fibres are involved in establishing and maintaining the connection between afferent fibres and central cells and that transport mechanisms play a role in this connectivity control. Therefore it is proposed that the chemicals in C fibres should be examined for their possible action on regulatory mechanisms as well as for neurotransmitter action.