Excitability following antidromic activation in spinal motoneurones supplying red muscles

It is well known that red muscles may be distinguished from pale muscles by the slowness of their contraction (Fulton, 1926; Denny-Brown, 1929b). In addition, Denny-Brown (1929a) found that red muscle has a lower threshold for postural reflexes, including the stretch reflex, than fast pale muscle. A histological study of fibre diameter has also revealed that the average diameter of motor fibres to the slow red extensors is smaller than for those to the pale extensors, although there is a considerable overlap (Eccles & Sherrington, 1930). While these findings might suggest some specific differences in properties between motoneurones supplying red and pale muscles, no systematic approach has been made toward this problem until recently. By studying stretch reflexes in the state of post-tetanic potentiation it has recently been suggested that spinal motoneurones innervating extrafusal muscle fibres may be classified into two subgroups (Granit, Henatsch & Steg, 1956; Granit, Phillips, Skoglund & Steg, 1957). The tonic type is characterized by a long-lasting reflex discharge to sustained muscle stretch and may also be distinguished from the other, phasic, type of motoneurones by its slower frequency of discharge. Intracellular recording from spinal motoneurones has shown that the motoneurones of slow red muscles have after-hyperpolarization of greater duration than those of fast pale muscles (Eccles, Eccles & Lundberg, 1957 a). In the latter study it was postulated that the discharge frequency of motoneurones under continuous synaptic excitation is controlled by the duration of after-hyperpolarization. It appears that tonic motoneurones in general tend to activate red muscles. As would be expected from the diameter spectrum of motor fibres to the ankle extensors (Eccles & Sherrington, 1930), soleus motoneurones have generally shown a slower axonal conduction velocity than gastrocnemius motoneurones (Eccles et al. 1957 a). The observation that the phasic axonal spike is large and the tonic small when recorded in the same