Afferent projections to human tibialis anterior motor units active at various levels of muscle contraction.

The synaptic efficacy of muscle and cutaneous afferents on single tibialis anterior motoneurones in man was derived from changes in the firing probability of single, voluntarily activated, motor units in response to electrical stimulation of peripheral nerves or skin. The motor units were recorded with a Macro EMG electrode. The Macro motor unit potential (Macro MUP) recorded with this electrode reflects the electrical activity of all of the muscle fibres in a single motor unit. The amplitude of the Macro MUP is positively correlated with the recruitment threshold of the unit. Motor units with different Macro MUP amplitudes were examined at approximately the same level of voluntary contraction (less than 20% of maximum). The synaptic efficacy of muscle and cutaneous afferents was similar for units with small and with large Macro MUP amplitudes. Single motor units were examined at several different levels of muscle contraction. There was no consistent change in the facilitation from muscle afferents but there was less facilitation from cutaneous afferents during stronger contractions. This was not simply a consequence of the units faster firing rate. It is concluded that, with increasing voluntary drive to tibialis anterior motoneurones in man, there is a reduction in transmission in the pathways from cutaneous afferents to tibialis anterior motoneurones. There is no evidence that low and high threshold units (judging from their Macro MUP amplitudes) have different afferent connections.