Neuromuscular organization of avian flight muscle: architecture of single muscle fibres in muscle units of the pectoralis (pars thoracicus) of pigeon(Columba livia)

The M. pectoralis (pars thoracicus) of pigeons (Columba livia) is comprised of short muscle fibres that do not extend from muscle origin to insertion but overlap 'in-series': Individual pectoralis motor units are limited in t erritory to a portion of muscle length and are comprised of either fast twi tch, oxidative and glycolytic fibres (FOG) or fast twitch and glycolytic fi bres (FG). FOG fibres make up 88-90% of the total muscle population and hav e a mean diameter one-half of that of the relatively large FG fibres. Here we report on the organization of individual fibres identified in six m uscle units depleted of glycogen, three comprised of FOG fibres and three c omprised of FG fibres. For each motor unit, fibre counts revealed unequal n umbers of depleted fibres in different unit cross-sections. We traced indiv idual fibres in one unit comprised of FOG fibres and a second comprised of FG fibres. Six fibres from a FOG unit (total length 15.45mm) ranged from 10 .11-11.82mm in length and averaged (+/-s.d) 10.74 +/- 0.79mm. All originate d bluntly ten mass from a fascicle near the proximal end of the muscle unit and all terminated intramuscularly. Five of these ended in a taper and one ended bluntly. Fibres coursed on average for 70% of the muscle unit length . Six fibres from an FG unit (total length 34.76 mm) ranged from 8.97-18.38 mm in length and averaged 15.32 +/- 3.75mm. All originated bluntly and term inated intramuscularly; one of these ended in a taper and five ended bluntl y Fibres coursed on average for 44% of the muscle unit length. Because fibr es of individual muscle units do not extend the whole muscle unit territory , the effective cross-sectional area changes along the motor unit length. T hese non-uniformities in the distribution of fibres within a muscle unit em phasize that the functional interactions within and between motor units are complex.