Mirror neurons have important implications for the evolution of language, suggesting pre-existing brain structure which could provide a basis for human language. Mirror neurons very plausibly suggest exploitable representations of both sounds (or gestures) and meanings. But they cannot, by their very nature, provide a basis for the central, essential structural relation in human language, the arbitrary Saussurean sign. E.g.
Concept of a tree <==> Representation of the sound sequence `tree'.The `<==>' relation is ARBITRARY; there is nothing in the pronunciation of the word that in any way resembles the denoted concept.
Both ends of the sign relation are internal representations. The `meaning' end of the relation is a mental entity (a concept or `sense'), not a referent object in the real world. And the `sound' end of the relation is not an actual utterance, or articulatory/acoustic event located in space-time, but a schematic representation of a class of such events.
Neurally, the following view is plausible. Assume the motor theory of speech perception. Then the `sound' end of the sign relation is the conjunction (or intersection?) of a motor schema and a sensory (auditory) schema. So the motor theory of speech perception implies mirror neurons in the phonetic/phonological representations of words.
Likewise the `sense' or meaning end of the sign relation is neurally a schema summarizing the range of input sensory conditions which would activate (= `bring to mind') a particular concept. Attending to a tree in an appropriate way, or imagining a tree, brings to mind the concept of a tree. Here too, in the `meaning' domain, there are probably aspects of mirror neuron organization. The mental representations of tools involve areas of motor cortex appropriate for handling them, beside sensory information about what the tools look like. It is hard to disassociate the `passive' manual feel of an object from active knowledge of what to do with it.
This rough characterization in neural terms of the two ends of the Saussurean sign relation already suggests mirror neuron organization within the two separate arguments, the sound and the meaning, before even considering the bidirectional relation between them. What, in neural terms, might the bidirectional `<==>' be? The most obvious possibility is that (some of) the neurons constituting the phonetic/phonological sensory/motor sound schema of a word are also part of the conceptual/semantic sensory/motor schema of the word. The well known `arbitrariness of the sign' presents a problem here. The pronunciation of the word `apple' bears no resemblance whatsoever, in sensory or motor affordances, to apples. This holds generally for all words except for marginal onomatopeic words. So the arbitrariness of the sign entails that mirror neurons CANNOT BE involved in the central link between meanings and sounds, if we take `motor' and `sensory' literally. The essential arbitrariness of the linguistic Sign is in effect a principle entailing that the organism's experience of the signifier (e.g. a word) does not overlap with its experience of the signified (e.g. a concept).