Localization and contractile properties of intrinsic longitudinal motor units of the rat tongue

Tongue dysfunction is a hallmark of many human clinical disorders, yet we l ack even a rudimentary understanding of tongue neural control. Here, the lo cation and contractile properties of intrinsic longitudinal motor units (MU s) of the rat tongue body are described to provide a foundation for develop ing and testing theories of tongue motor control. One hundred and sixty-fiv e MUs were studied by microelectrode penetration and stimulation of individ ual motor axons coursing in the terminal portion of the lateral (retrusor) branch of the hypoglossal nerve in the rat. Uniaxial MU force was recorded by a transducer attached to the protruded tongue tip, and MU location was e stimated by electromyographic (EMG) electrodes implanted into the anterior, middle, and posterior portions of the tongue body. All MUs produced retrus ive force. MU twitch force ranged from 2-129 mg (mean = 35 mg) and tetanic force ranged from 9-394 mg (mean = 95 mg). MUs reached maximal twitch force in 8-33 ms (mean = 15 ms) and were resistant to fatigue; following 2 min o f stimulation, MUs (n = 11) produced 78-131% of initial force. EMG data wer e collected for 105 MUs. For 65 of these MUs, the EMG response was confined to a single electrode location: for 26 MUs to the anterior, 21 MUs to the middle, and 18 MUs to the posterior portion of the tongue. Of the remaining MUs, EMG responses were observed in two (38/40) or all three (2/40) tongue regions. These data provide the first contractile measures of identified i ntrinsic tongue body MUs and the first evidence that intrinsic longitudinal MUs are restricted to a portion of tongue length. Localization of MU terri tory suggests a role for intrinsic MU in the regional control of the mammal ian tongue observed during feeding and speech.