Summary

Abstract

Mammals have fast and slow contracting motor units often mixed within the same muscle. The differentiation of muscle fibres is thought to be determined by the discharge frequency of motoneurones, which is lower in slow than in fast motor units. Usually muscle fibres of slow contracting units contain many mitochondria and have a high oxidative metabolism, whereas fibres of fast contracting units are poor in mitochondria and their energy supply is mainly glycolytic. Histochemically with respect to enzymes or substrates of metabolism all fibres of a motor unit are of the same type. As far as is known, in mammals the rough correlation of the type of metabolism with contraction time is valid for skeletal muscles but not for laryngeal muscles. In mammals neither the colour (red-white) nor the light microscopical appearance in cross sections (“Fibrillenstruktur” — “Felderstruktur ”) is strictly related to fast and slow muscles or fibres.