Relative strength of synaptic input from short-latency pathways to motor units of defined type in cat medial gastrocnemius

Abstract

1. Intracellular recording and stimulation techniques were used in anesthetized cats to study the interrelations between amplitudes of PSPs produced by electrical stimulation of several short-latency pathways to MG alpha motoneurons and the mechanical properties of muscle units innervated by the same cells. Motor-unit types were identified by muscle-unit properties.2. The maximum amplitudes of monosynaptic EPSPs produced in MG motoneurons by activation of homonymous (MG) and heteronymous (LGS) group Ia afferents were clearly related to motor-unit type, being, on the average, largest in type S units, somewhat smaller in type FR and F(int) units, and smallest in type FF units. Correspondingly, group Ia EPSP amplitudes were inversely correlated with muscle-unit tension production and directly correlated with resistance to fatigue. The same input distribution was true for disynaptic IPSPs produced by group Ia afferents from antagonist ankle flexors.3. The amplitudes of monosynaptic EPSPs produced by fibers descending in the ipsilateral ventral funiculi of the low thoracic cord were not clearly related to MG motor-unit type or (therefore) to muscle-unit properties.4. A quantitative input-output model of the MG motor-unit pool, based in part on the present results, suggests that overall characteristics of MG motor units, and their relative numbers in the MG pool, reflect functional specializations determined by specific mechanical demands placed on the MG muscle by the usual motor behavior of the animal.

Enhanced Transmission at a Spinal Synapse TriggeredIn Vivoby an Injury Signal Independent of Altered Synaptic Activity

“…2, diagram). For normal rats, the plot in Figure 2 verifies previous demonstrations (Burke et al, 1978;Manabe et al, 1989) that IA EPSPs produced at LGS3 MG (heteronymous) synapses were smaller than those generated at MG3 MG (homonymous) synapses; the average heteronymous/homonymous amplitude ratio was 0.43 Ϯ 0.03, and no data points were above the line of identity in Figure 2 (0 of 38 open circles). Three days after nerve crush, IA EPSPs produced by crushed LGS afferents were larger than those evoked by normal MG afferents, expressing a significantly larger average amplitude ratio (1.02 Ϯ 0.09; p Ͻ 0.001) and having 29 of 47 data points (filled circles) exceeding the line of identity.…”

Disuse enhances synaptic efficacy in spinal mononeurones.

“…could be revealed by this procedure (upward arrow in Fig. 1 A; also, see Eccles, Eccles & Lundberg, 1957;Burke, 1968;Burke, Rymer & Walsh, 1976). The falling phase of the M-spike (downward arrow in Fig.…”

“…As a more sensitive test, the amplitudes of homonymous and heteronymous e.p.s.p.s in each motoneurone may be compared since the ratio of these e.p.s.p. amplitudes is independent of the type of motoneurone (Burke et al 1976). In both neurones, the e.p.s.p.s evoked by stimulation of the m.g.…”

“…4B, m.g., stippled column) were not significantly enhanced. The maximum amplitudes of monosynaptic e.p.s.p.s from the homonymous and heteronymous nerves may vary widely from neurone to neurone, depending upon the motoneurone (motor unit) type (Burke et al 1976). Thus, the possible increase in the mean e.p.s.p.…”

A morphological study of the axons and recurrent axon collaterals of cat alpha‐motoneurones supplying different functional types of muscle unit.

“…The size of the collateral systems of gastrocnemius ac-motoneurones seems to be inversely related not only to the amplitudes of the homonymous Ia excitatory synaptic potentials of the motoneurones but also to the size of the inhibitory synaptic potentials produced disynaptically by the antagonist primary afferents (Burke et al 1976 (Burke et al 1976).…”