Post-Tetanic Potentiation of Response in Monosynaptic Reflex Pathways of the Spinal Cord

Abstract

Following tetanic afferent stimulation of a monosynaptic reflex pathway, the transmission through that pathway of isolated reflex volleys is enhanced for some minutes. Post-tetanic potentiation is comparable in the monosynaptic reflex arcs of flexor and extensor muscles. The facilitator and inhibitor actions of monosynaptic reflex afferent fibers, as well as the transmitter action, are potentiated following tetanization. Little post-tetanic change attends reflex transmission through plurisynaptic reflex arcs. Various tests for excitability change made independently of the tetanized afferent fibers reveal none or a slight depression. Hence the potentiating influence of a tetanus is limited to subsequent action on the part of the recently tetanized fibers themselves. Increase in the size of the individual impulses comprising an afferent volley such as might occur during positive after-potential, would accommodate the requirement for a limited process and provide for increased synaptic action. The proposed association between post-tetanic potentiation and positive after-potential (i.e. hyperpolarization) is supported by the following lines of evidence:— 1. Changes in intensity and duration of potentiation with change in frequency and duration of tetanic stimulation are characteristic of, and parallel to, the changes of positive after-potential in similar circumstances. 2. Afferent impulses are increased following a tetanus, and in a fashion that parallels the course of monosynaptic reflex potentiation. Post-tetanic potentiation, as here described, and after-discharge, whatever may be its mechanism, are unrelated phenomena.

Tonic Vibration Reflex in Human and Monkey Subjects

“…However, the data available at present are still not enough to fully explain the phenomenon of motoneuronal firing interval shortening corresponding to the varied frequencies of vibratory stimulation. Forced stimulation of group Ia muscle afferent might well cause facilitation of the stretch reflex through posttetanic potentiation (LLOYD, 1949;GRANIT, 1956;HOMMA and KANO, 1962). Therefore, motoneuronal PTP which is caused by strong afferent bombardment during vibration can be considered as a probable support for the explanation of"vibratory facilitation.…”

A prelude to long-term potentiation

“…3 h long form and a later process, many hours long, the latter requiring new protein synthesis. Lloyd (1949) reported that PTP of monosynaptic spinal reflexes could last up to 7 min. Eccles (1953) proposed that spinal cord synapses could be 'capable of "learning" to operate more effectively' through PTP, in particular if they were in a disused state before the activation.…”

Post‐tetanic and drug‐induced repetitive firing in the soleus muscle of the cat

“…The increased amplitude of the soleus nerve action potential, which was accompanied by an increase in the threshold stimulus necessary for excitation, may have been the result of hyperpolarization of the nerve fibre membranes. Hyperpolarization and an increase in the presynaptic spike amplitude have been demonstrated following repetitive activity in several pathways in the spinal cord (Lloyd, 1949;Eccles & Rall, 1951 ;Wall & Johnson, 1958;Eccles & Krnjevic, 1959a, b;Curtis & Eccles, 1960), and in slow-conducting non-myelinated fibres (Brown & Holmes, 1956). Eccles, Eccles & Lundberg (1958) showed that the hyperpolarization occurring after a single impulse in soleus motoneurones is long-lasting (more than 140 msec), and summation of the positive after potentials after a prolonged high frequency tetanus would therefore probably give rise to a prolonged hyperpolarization of the nerve endings.…”