A method for the study of single jaw adductor muscle motor units in the rat

Single motor units of the superficial masseter muscle were stimulated by impaling axons in the third division of the trigeminal nerve. Glass micropipette electrodes were driven into the nerve as it runs along the floor of the cranial cavity while passing small current pulses through the electrode to activate axons. Stable penetrations were maintained for as long as 45 min and twitch tensions were recorded for motor units that generated as little as 159 mg (1.6 mN) of force within a superficial masseter muscle capable of producing 20 g of twitch force. These are the first measurements of single motor-unit forces measured directly from the tendon of a whole jaw adductor muscle.