Masseter muscle spindles contain small-calibre afferents that express nociceptor markers.

<p>Left and right photomicrographs have identical frames with different sets of fluorescence filters. In each case, portions of the photomicrographs were digitally merged in boxed areas. A: Nerve fibres immunoreactive for PGP9.5 and containing CGRP (A'). B: A small green CGRP-positive fibre (B', thin arrow) runs across three VGLUT1-positive loops of an annulospiral ending. None of the VGLUT1-positive fibres in B corresponded to the CGRP positive fibres in B'. In the merged image, the fibre to the right appears yellow for most of its length (arrowheads) because it passes over red VGLUT1-positive fibres. C and C': Fibres immunoreactive for both PGP9.5 and SP. D and D': Fibres immunoreactive for PGP9.5 and the capsaicin receptor, TRPV1. c: spindle capsule wall. All scale bars = 10 µm.