Neuromechanical matching of drive in the scalene muscle of the anesthetized rabbit

Abstract

The scalene is a primary respiratory muscle in humans; however, in dogs, EMG activity recorded from this muscle during inspiration was reported to derive from underlying muscles. In the present studies, origin of the activity in the medial scalene was tested in rabbits, and its distribution was compared with the muscle mechanical advantage. We assessed in anesthetized rabbits the presence of EMG activity in the scalene, sternomastoid, and parasternal intercostal muscles during quiet breathing and under resistive loading, before and after denervation of the scalene and after its additional insulation. At rest, activity was always recorded in the parasternal muscle and in the scalene bundle inserting on the third rib (medial scalene). The majority of this activity disappeared after denervation. In the bundle inserting on the fifth rib (lateral scalene), the activity was inconsistent, and a high percentage of this activity persisted after denervation but disappeared after insulation from underlying muscle layers. The sternomastoid was always silent. The fractional change in muscle length during passive inflation was then measured. The mean shortening obtained for medial and lateral scalene and parasternal intercostal was 8.0 +/- 0.7%, 5.5 +/- 0.5%, and 9.6 +/- 0.1%, respectively, of the length at functional residual capacity. Sternomastoid muscle length did not change significantly with lung inflation. We conclude that, similar to that shown in humans, respiratory activity arises from scalene muscles in rabbits. This activity is however not uniformly distributed, and a neuromechanical matching of drive is observed, so that the most effective part is also the most active.

Figures

Fig. 1.

Direction of ribs at functional residual capacity. Mean ± SE data obtained from 6 animals in supine posture. Angle α is formed by an individual rib and the perpendicular to the spine. It represents the slope in ventral direction (known as the pump-handle angle). The minus sign indicates the ribs are slanted caudally.

Fig. 2.

Traces of EMG activity (in arbitrary units) recorded from parasternal and scalene muscles in a representative animal. Each trace represents a respiratory cycle. A: records were obtained during graded inspiratory resistance (resistors R0 to R3 with an orifice of 4, 2, 1, and 0.8 mm in diameter, respectively). B: records were obtained in an intact animal (left), after denervation of the scalene muscle (middle), and after additional insulation (right). Bottom traces correspond to changes in airway pressure.

Fig. 3.

Inspiratory activity recorded from the parasternal intercostal and the medial and lateral scalene after denervation of scalene muscle and after its additional insulation. EMG activities are reported for unimpeded breathing (R0) and for the highest inspiratory resistance (R3). Data are expressed in percentage of the values obtained in the intact animals with the same resistance. Values are means ± SE of 8 animals, except R0 values in the lateral scalene (mean of the 4 active muscles, see text). *P < 0.05 compared with intact animals; #P < 0.05 compared with denervation.

Fig. 4.

Fractional change in muscle length during passive inflation of the chest. Values are means ± SE data of 8 animals. The changes are expressed as percent changes relative to the muscle length at functional residual capacity (LFRC) and are reported for the parasternal intercostal (▪) and sternomastoid (□) muscles, as well as for the medial (○), middle (▴), and lateral (▾) scalene. The minus sign indicates a shortening.

Fig. 5.

Schematic model illustrating the action of scalene muscle. The shortening of the distance (ac) rotates the lower bar (bc) upward (dotted lines), increasing the angle ϕ between the left vertical bar and oblique bar. The distance d between the 2 vertical bars is equal to [(bc) sin ϕ] and increases by Δd during rotation. For a given change in ϕ, Δd becomes smaller when the bar is oriented in a direction closer to horizontal (top bar). Angle α, see text.