Post-tetanic potentiation of myotatic reflexes in man.

Hoffmann originally described how an electric shock to the tibial nerve in man evokes not only a motor response in the calf muscles but also a wellsynchronized reflex response with a latency of about 30 msec. This late response, which he identified with the stretch reflex, was shown to have the following characteristics (Hoffmann, 1918, 1922; Hoffmann and Keller, 1928): (a) It appears only in muscles supplied by the stimulated nerve; (b) it can be evoked by shocks which are subliminal for motor axons and it tends to be blocked at stronger stimulus strengths (by antidromic impulses in motor axons); (c) it is conducted in afferent fibres of high velocity and its central delay is short enough to indicate a monosynaptic path; (d) it is facilitated by a voluntary contraction of the muscle involved and obliterated by a contraction of its antagonist. Hoffmann also claimed that reflexes with these characteristics are not specific for the calf muscles; they do not appear in other muscles of the extremities, however, unless facilitated by voluntary contraction. Many of Hoffmann's observations have been confirmed by other workers (Magladery and McDougal, 1950; Magladery, Porter, Park, and Teasdall, 1951; Schenck, 1951; Paillard, 1955) but Magladery and McDougal (1950) made a distinction between the reflexes appearing in the calf muscles (which they termed H-reflexes) and the smaller and more variable late deflections appearing in response to motor nerve shocks in the peroneal muscles of the leg and in the small muscles of the hand and foot (F-waves). They claimed that the F-wave, contrary to the Hreflex, cannot be depressed by increasing the intensity of the stimulus and they deduced that it is a reflex conducted in afferent fibres which are slower than the motor axons. Teasdall, Park, Languth, and Magladery (1952) made the additional observation that the F-waves in distal muscles were substituted by typical H-reflexes in a number of patients with upper motor neurone signs due to lesions involving the lower part of the brain-stem or the spinal cord. The reflex nature of the F-wave was doubted by Dawson and Merton (1956) who studied these responses in ulnar hand muscles and concluded that 1 they represent recurrent discharges from motor neurones. Johns, Grob, and Harvey (1957), however, found typical H-reflexes instead of F-waves in hypothenar muscles of normal adult subjects and Thomas and Lambert (1960) reported the regular occurrence of H-reflexes in hypothenar muscles of newborn infants. Most authors (Magladery et al., 1951; Hoffmann, 1952; Paillard, 1955; but cf. also Magladery and Teasdall, 1958) emphasize the analogy between the H-reflex in man and the monosynaptic stretch reflex so thoroughly analysed in animal experiments (Lloyd, 1943, 1946; Granit, 1955). The monosynaptic reflex can be easily potentiated by tetanization of the muscle nerve (Lloyd, 1949, 1959; Eccles and Rall, 1951) and, according to F. A. Hoffmann (1952), the phenomenon can also be demonstrated in man, even though the degree of change produced is quite limited. The present study started as a search for such potentiation and the experiments showed that the H-reflex, the simultaneous reciprocal inhibition, and also the mechanically evoked stretch reflex (Achilles and patellar reflexes) can be potentiated. It was also found that the F-waves in the small hand muscles and in the peroneal muscles were often substituted by typical H-reflexes during the period of post-tetanic potentiation.