Synaptic organization of defined motor-unit types in cat tibialis anterior

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Abstract
Synaptic potentials were recorded intracellularly in tibialis anterior (TA) motoneurons following stimulation of a descending brain stem pathway, the medial longitudinal fasciculus (MLF) and 3 segmental inputs, the homonymous and heteronymous group Ia afferents, the group I afferents from the antagonist, and the cutaneous and muscle afferents. Intracellular stimulation of the motoneurons was used to classify them, based on the properties of the innervated muscle units, into types FF, F(int), FR and S. The sum of the monosynaptic EPSP [excitatory postsynaptic potential] amplitudes resulting from stimulation of homonymous and heteronymous group Ia afferents (summed group Ia EPSP) was inversely related to motoneuron size, as assessed by motoneuron input resistance, and was inversely related to motor-unit tetanic tension. The amplitudes of disynaptic IPSP [inhibitory postsynaptic potential] resulting from stimulation of group I afferents in the antagonist muscle also showed an inverse relationship to motoneuron size. The amplitudes of the monosynaptic EPSP evoked in TA motoneurons by stimulation of the MLF were distributed rather randomly among all types of TA motoneurons. The polysynaptic effects from cutaneous and muscle afferents in sural and gastrocnemius-soleus nerves were frequently excitatory on type-FF motoneurons, but were primarily inhibitory on type-FR and -S motoneurons. The synaptic organization of the TA nucleus is similar to that of its extensor antagonist, medial gastrocnemius, when analogous neural circuits are compared. A commonality of motor-control systems for flexor and extensor muscles was suggested.

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