MITOCHONDRIA IN FINE AFFERENT NERVE-FIBERS OF THE KNEE-JOINT IN THE CAT - A QUANTITATIVE ELECTRON-MICROSCOPIC EXAMINATION

The distribution of mitochondria, their content and concentration (exp ressed as the ratio of the mean volume of mitochondria and the surface of the sensory axon) were determined in group-III and -IV nerve fibre s innervating the knee joint capsule in the cat. Mitochondria mainly a ccumulated in axonal swellings (''beads'') and end bulbs of the termin al branches. Between single nerve fibres, marked differences in the co ntent and the concentration of mitochondria were obtained in proximal portions (inside of the perineurium) and in distal portions (unmyelina ted sensory endings). Tn group-III nerve fibres, the mitochondrial con centration ranged from 0.005 to 0.030 mu m(3)/mu(3) (proximal portion) and from 0.016 to 0.080 mu m(3)/mu m(2) (distal portion). In unmyelin ated group-IV nerve fibres, the values also showed a broad variation r anging from 0.001 to 0.011 mu m(3)/mu m(2) (proximal portion) and from 0.003 to 0.019 mu m(3)/mu m(2) (distal portion). The wide range of mi tochondrial concentrations may reflect different energy consumption du ring receptive processes: nerve fibres with a low mechanical threshold and a high probability of excitatory events may be rich in mitochondr ia, whereas fibres with a high mechanical threshold and a low probabil ity of excitatory events may be poor in mitochondria.