Differential control of the scapulothoracic muscles in humans

Abstract

The control of the scapulothoracic muscles trapezius (Tr) and serratus anterior (SA) has been examined in normal human subjects. Electromyographic recordings were made from the SA and Tr muscles (upper trapezius UTr, lower trapezius LTr) using surface electrodes placed bilaterally. Magnetic stimulation of the motor cortex and electrical stimulation of peripheral nerves were used to examine their descending and reflex control. The average optimal site of cortical stimulation was found to be the same for SA, UTr and LTr (an approximate centre of gravity of -0.6 cm, 3.7 cm where the centre of gravity is expressed as the mean anterio-posterior position, the mean medio-lateral position). Some asymmetry in the cortical representation of UTr was found in each individual tested. Magnetic stimulation evoked bilateral MEPs in Tr (latency contralateral (c) UTr 8.5 +/- 1.6 ms, ipsilateral (i) UTr 19.0 +/- 2.7 ms) but only contralateral responses were evoked in SA (11.2 +/- 2.6 ms). Electrical stimulation of the long thoracic nerve at two sites was used to examine homonymous and heteronymous reflexes of SA, while electrical stimulation of cervical nerve of C3/4 was used to examine the heteronymous reflexes of Tr. Ipsilateral SA H reflexes were evoked at a latency of 9.9 +/- 0.8 ms (proximal site) and 10.8 +/- 1.2 ms (distal site). No group I reflexes were evoked from SA to its contralateral homologue. No group I reflexes were evoked between Tr and SA. Finally, cross-correlation of activity from the Tr muscle pairs and the SA muscle pair revealed that the motoneurones of the Tr muscles share some common presynaptic input whereas there was no detectable common presynaptic input to the SA muscle pair. This study extends and consolidates knowledge regarding the neural control of trapezius and for the first time explores the neural control of SA. The study demonstrates a contrasting bilateral control of Tr and SA. These patterns of connections are discussed in relation to the contrasting bilateral functional roles of these muscles.

Figure 1. MEPs evoked in serratus anterior and trapezius

A, average contralateral MEPs from serratus anterior (upper trace), lower trapezius (middle trace) and upper trapezius (lower trace). These MEPs are the average of 10 sweeps. The artifact is marked with the arrow. The vertical dotted line marks the onset of the MEP. B, averaged rectified MEPs of upper trapezius (top) and serratus anterior (bottom). The contralateral response is on the left with the ipsilateral response on the right. These MEPs are the average of 20 sweeps. The arrows mark the stimulus artifact.

Figure 2. Cortical mapping

A, typical cortical maps for serratus anterior, upper trapezius and lower trapezius for three subjects. Each point on the grid is representative of the average amplitude of the MEP after 10 stimuli. The maps have been normalized as a percentage of the greatest average amplitude response for each individual. B, the positions are marked for the centre of gravity for serratus anterior, upper and lower trapezius muscles for 6 subjects. Each centre of gravity was constructed from averaged MEPs constructed after 10 stimuli. The average centre of gravity for each muscle is also marked.

Figure 3. Asymetry of cortical map for contralateral upper trapezius MEPs

A Illustrates a cortical map for contralateral upper trapezius MEPs evoked from the left and right hemispheres from one individual. The asymmetry of the map is also illustrated by the population results, B. Here the position of the largest amplitude average contralateral MEP for each individual is marked. It is linked by a line to that evoked from the opposite side.

Figure 4. Serratus anterior H reflexes

A, typical example of the ipsilateral serratus anterior H reflex evoked from the proximal site of stimulation. This reflex is the average of 10 sweeps. The vertical dashed line marks the stimulus onset. The upward pointing arrow marks the onset of the M response, and the downward pointing arrow marks the onset of the H reflex. B, two examples of the average ipsilateral serratus anterior H reflex evoked from the distal site (top). No contralateral response is seen (bottom). The stimulus artifacts are marked by the arrows. The responses are an average of 10 sweeps.

Figure 5. Selective action of serratus anterior afferents

A, two examples of the average ipsilateral serratus anterior H reflex evoked from the distal site. B, no ipsilateral trapezius response is seen. The stimulus artifacts are marked by the arrows. The responses are an average of 10 sweeps.

Figure 6. Cross-correlation of the trapezius and serratus anterior muscles

The cross-correlograms were constructed from two subjects during abduction of the arms between multiunit spike trains generated from the bilateral upper fibres of trapezius (A), the bilateral lower fibres of trapezius (B), and the bilateral serratus anterior muscles (C)