Functional Significance of Facilitation Between the Pronator Teres and Extensor Carpi Radialis in Humans: Studies with Electromyography and Electrical Neuromuscular Stimulation

Abstract

Excitatory (facilitation) or inhibitory spinal reflex arcs (inhibition) mediated by group I afferent fibers from the muscle spindles (group Ia afferents) and Golgi tendon organs (group Ib afferents) among muscles in the human upper limb have been studied (Fig. 1) (Aymard et al., 1995; Baldissea et al., 1983; Cavallari & Katz, 1989; Cavallari et al., 1992; Creange et al., 1992; Day et al., 1984; Fujii et al., 2001; Katz et al., 1991; Kobayashi et al., 2000; Lourenco et al., 2007; Marchand-Pauvert et al., 2000; Miyasaka et al., 1995, 1996, 1998, 2007; Naito et al., 1996, 1998a, 2001; Naito, 2003, 2004; Nakano et al., 2005, 2006; Ogawa et al., 2005; PierrotDeseilligny & Mazevet, 2000; Rossi et al., 1995; Sato et al., 2002; Shinozaki et al., 2001; Suzuki et al., 2005, 2007; Wargon et al., 2006). These reflex arcs modulate motoneuron excitabilities to coordinate smooth muscular movements (Naito, 2003, 2004; Pierrot-Deseilligny et al., 1981; Rothwell, 1994; Tanaka, 1989). The facilitation must function for co-contraction of muscles and the inhibition for alternating or reciprocal contraction among muscles (Naito, 2003, 2004). Musculus (m.) pronator teres (PT) arises from the medial epicondyle of the humerus (humeral head) and the coronoid process of the ulna (ulnar head) and attaches on the lateral surface of the shaft of the radius (Basmajian, 1982; Jenkins, 2008; Standring et al., 2005). It is innervated by the median nerve. M. extensor carpi radialis longus (ECRL) arises mainly from the distal third of the supracondylar ridge of the humerus and attaches to the radial side of the dorsal aspect of the base of the second metacarpal. M. extensor carpi radialis brevis (ECRB) arises mainly from the lateral epicondyle of the humerus and attaches on the radial side of the base of the third metacarpal. Both muscles (ECR) are innervated by the radial nerve. Most textbooks of anatomy describe that PT acts as a forearm pronator and ECR as a wrist extensor and abductor. Our previous studies have demonstrated facilitation between PT and ECR (ECRL, ECRB) in humans (Fig. 1) (Nakano et al., 2005, 2006). The facilitation seems to be mediated by group Ia afferents through a monosynaptic path. It is known that monosynaptic facilitation mediated by group Ia afferents is usually observed