No abstract

Ever since the discovery by Ranvier (1874) and Kronecker & Stirling (1878) that red striated muscle has a slower contraction than pale, and consequently gives a fused tetanus at a lower frequency of stimulation, there has been much speculation in regard to the possibility that the slow-red muscles might be specially concerned in the tonic postural movements of limbs, while the fastpale muscles would give the quick phasic movements. Cobb and Fulton (cf. Fulton, 1926) were the first to record with an isometric myograph the independent contractions of the slow soleus and the fast gastrocnemius of the cat. It was shown by Denny-Brown (1929 a, b) that with these ankle extensors there is a more striking discrimination into the slow-red and the fast-pale types than with any other synergic groups of muscles. In addition, DennyBrown systematically investigated the participation of slow and fast muscles in the various types of postural reflexes, including the tonic labyrinthine and neck reflexes of Magnus, as well as in the stretch reflex and the crossed extensor reflex. In all the synergic groups of extensor muscles the slow muscles were activated in the lowest threshold range of the postural reflexes, while the fast muscles were in the middle and upper range of threshold (Denny-Brown, 1929a). On the other hand, the fast muscles were at times more readily excitable in brief reflex actions, such as, for example, were evoked on rapidly moving the head.

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During walking in unrestrained cats the electromyographic activity in many hindlimb muscles has been correlated with the angular movement in the hip, knee and ankle joints. The activity is rather uniform in the extensors but individual in different functional groups of flexor muscles. Observations on the precise timing of the onset of the main extensor activity suggest that it is not a reflex effect produced by stimulation of receptors (from muscle or skin) in the limb. It is assumed that the basic activity is a centrally programmed alternating activation of extensors and flexors. A possible reflex regulation mainly from la afferents of this basic activity is discussed with special attention given to factors that may elucidate the difference in movement at the hip on one side and the knee and ankle on the other.

In unanaesthetized spinal cats, injected with L‐DOPA, volleys in the flexor reflex afferents (FRA) evoke a long‐latency, longlasting discharge in ipsilateral flexor and contralateral extensor motoneurones. It is postulated that this discharge is transmitted by a neuronal pathway which is inhibited in the normal acute spinal cat, presumably from the pathway, which in this state transmits the shortlatency effect from the FRA to motoneurones. The organization of the pathway released by an injection of DOPA has been analyzed by recording the discharges in efferents to flexors and extensors and with intracellular records from motoneurones. Combined stimulation of ipsilateral and contralateral FRA reveals a reciprocal organization in that either flexor or extensor motoneurones are activated. Transmission from the ipsilateral FRA to flexor motoneurones can be inhibited by volleys in the contralateral FRA, and transmissiqn from the contralateral FRA to extensor motoneurones by volleys in the ipsilateral FRA. These inhibitory effects are neither produced postsynaptically in the motoneurones nor presynaptically by depolarization of primary afferents and are hence exerted at an interneuronal level. The organization of reciprocal innervation at an interneuronal level is discussed in relation to the primary afferent depolarization evoked in Ia afferent terminals after DOPA and to rhythmic alternating movements.