The number and the distribution of fiber size in the medial (MAN) and poste
rior (PAN) articular nerves of the mouse knee joint were studied by electro
n microscopy. The MAN contained 75 +/- 28 nerve fibers consisting of 63 +/-
24 unmyelinated and 12 +/- 6 myelinated fibers. The PAN was composed of 19
5 +/- 50 nerve fibers, namely 129 +/- 28 unmyelinated and 66 +/- 24 myelina
ted fibers. A skewed unimodal distribution of the unmyelinated nerve fiber
diameters was seen in both nerves ranging from 0.1 to 1.2 mum with a maximu
m between 0.3 and 0.6 mum. The myelinated nerve fibers in the MAN ranged fr
om 1 to 8 mum with a peak between 2 and 5 mum. In the PAN, their diameters
ranged from 1 to 12 mum with a clearly visible peak at 4-5 mum and a platea
u at 8-9 mum that may represent a second maximum. These data show that the
knee joint innervation of the mouse is comparable to those of the cat and r
at concerning the types of nerve fibers and the composition of the two nerv
es. However, in relation to the much smaller area of tissue to be innervate
d the total number of primary afferents is considerable smaller in the mous
e.