J. Mead

Reinforcement versus Fluidization in Cytoskeletal Mechanoresponsiveness

Every adherent eukaryotic cell exerts appreciable traction forces upon its substrate. Moreover, every resident cell within the heart, great vessels, bladder, gut or lung routinely experiences large periodic stretches. As an acute response to such stretches the cytoskeleton can stiffen, increase traction forces and reinforce, as reported by some, or can soften and fluidize, as reported more recently by our laboratory, but in any given circumstance it remains unknown which response might prevail or why. Using a novel nanotechnology, we show here that in loading conditions expected in most physiological circumstances the localized reinforcement response fails to scale up to the level of homogeneous cell stretch; fluidization trumps reinforcement. Whereas the reinforcement response is known to be mediated by upstream mechanosensing and downstream signaling, results presented here show the fluidization response to be altogether novel: it is a direct physical effect of mechanical force acting upon a structural lattice that is soft and fragile. Cytoskeletal softness and fragility, we argue, is consistent with early evolutionary adaptations of the eukaryotic cell to material properties of a soft inert microenvironment.

Muscle activity during chest wall restriction and positive pressure breathing in man.

The effects of sustained constriction of the rib cage (RCC), constriction of the abdomen (AC) and of breathing against a positive pressure of 10 cms of water (PPB) were studied in four normal subjects with moderate constant hypercapnia. Intercostal electrical activity (Eic) was measured by implanted wire electrodes and diaphragmatic electrical activity (Edia) by oesophageal electrodes. There was no fixed relation between Edia and VT. VT was unaltered during AC and RCC: Edia was unaltered during AC but increased during RCC. The response to PPB without constriction varied: three subjects increased end-expiratory VL with increase in Edia and inspiratory Eic. One subject initially, and one subject after training, maintained end-expiratory VL constant with no change in Edia and an increase in expiratory Eic. When PPB was applied during AC and RCC there was an increase in Edia proportional to end-expiratory lung volume. The overall response to distortion was determined by voluntary choice, but muscle electrical activity reflected chest wall configuration: when the diaphragm was shorter and at a mechanical disadvantage its electrical activity increased. This was compatible with a reflex with afferent information from diaphragm tendon organ and muscle spindle receptors.

PRESSURE‐FLOW EVENTS DURING SINGING

The singer must coordinate the movements of the vocal cords and the vocal tract (which determine pitch, loudness, and quality of the tones) with the movements of the respiratory bellows (which produce the pressure and flow of air needed to drive the larynx). During phonation, both subglottic pressure and the flow rate of air through the larynx must remain constant if one wants to produce a tone of constant pitch and loudness. During singing of a song, on the other hand, these pressures and flow rates are subject to changes related to the requirements of the individual tones. We will briefly discuss breathing patterns during singing of songs, but will concentrate on the pressure and flow events during production of single constant tones, when a “steady state” of pressure and flow rate prevails. The mechanical analy3is of these events will be summarized only; its details have been reported elsewhere, as have the technical details of the methods used (Bouhuys et al., 1966).

STUDIES OF ARTICULATORY ACTIVITY AND AIRFLOW DURING SPEECH

One of the principal objectives of research in speech is to understand the mechanism underlying the control of the speech-generating system. Several kinds of experimental observations can be made in order to investigate the nature of this process. The manifestation of speech that is most available is the sound itself, and various procedures have been devised to examine the attributes of the acoustic signal. Other types of experimental evidence come from cineradiographic studies and from observations of electromyographic activity in the tongue, lips, and other structures. Airflow and pressure events during speech can also provide useful information concerning the activities of the various articulatory structures. The purpose of this paper is to present some measurements of airflow during speech production and to indicate that these data lead to certain conclusions concerning the manner in which the speech-generating mechanism is controlled.

Determinants of breathing frequency during walking

Breathing frequency has been shown to depend on metabolic demand, and also to be affected by stepping frequency during walking. To assess the influence of stepping frequency on breathing frequency, we recorded the timing of breathing and stepping in ten naive subjects walking on a treadmill. Five of the subjects showed periods of breathing in rhythm with their stepping (rhythmical entrainment); five subjects never showed rhythmical entrainment. In all subjects, respiratory frequency tended to increase with increases in estimated metabolic rate (EMR) produced by increases in walking speed or treadmill inclination. Breathing frequency was also affected by stepping frequency independent of EMR, both during rhythmical entrainment and in the absence of rhythmical entrainment. The dependence of breathing frequency on stepping frequency may reflect an excitatory influence of voluntary movement on the respiratory center. We conclude that stepping importantly influences breathing frequency during walking whether or not the two are rhythmically linked. Such lability of respiratory frequency probably reflects the small metabolic cost of departure from mechanically optimal frequencies.

MECHANICS OF THE CHEST WALL

Publisher Summary This chapter discusses the mechanics of the chest wall. Diaphragmatic displacements can be estimated from abdominal volume displacements. The wall of the abdomen includes a portion that is ordinarily not regarded as abdominal at all. This is a part of the rib cage beginning at its costal margin and extending cephelad to the diaphragmatic reflection. In this region, the diaphragm is directly apposed to the rib-cage interior, and beneath lies not lung but abdominal contents. The study described in the chapter refers to this region as the area of apposition. At residual volume, it covers approximately half of the internal surface of the rib cage, decreasing to become a narrow ring at total lung capacity—as the diaphragm shortens, the area of apposition decreases, and, finally, virtually disappears. As it decreases, it expands outward and moves upward, and both of these motions need to be taken into account in estimating abdominal displacements. The total volumetric expansion of the rib cage can be estimated by the method of Konno and Mead. The fraction of its surface taken up by the area of apposition times this displacement is an estimate of the outward displacement of the area of apposition.

Effects of Paralysis with Pancuronium on Chest Wall Statics in Awake Humans

The influence of tonic inspiratory muscle activity on the relaxation characteristics of the chest wall, rib cage (RC), and abdominal wall (ABW) has been investigated in four highly trained subjects. Chest wall shape and volume were estimated with magnetometers. Pleural pressure (Pes) and abdominal pressure were measured with esophageal and gastric balloons, respectively. Subjects were seated reclining 30 degrees from upright, and respiratory muscle weakness was produced by pancuronium bromide until RC inspiratory capacity was decreased to 60% of control. Only minor changes were observed for Konno-Mead relaxation characteristics (RC vs. ABW) between control and paralysis. Similarly, although RC relaxation curves (RC vs. Pes) during paralysis were significantly different from control (P less than 0.05), the changes were small and not consistent. The differences between paralysis-induced changes in resting end-expiratory position of the chest wall and helium-dilution functional residual capacity (FRC) suggested changes in volume of blood within the chest wall. We conclude that 1) although tonic inspiratory activity of chest wall muscles exists, it does not significantly affect the chest wall relaxation characteristics in trained subjects; 2) submaximal paralysis produced by pancuronium bromide is likely to modify either spinal attitude or the distribution of blood between extremities and the thorax; these effects may account for the changes in FRC in other studies.

FUNCTIONAL NEURAL COUPLING BETWEEN DIAPHRAGM AND PARASTERNAL INTERCOSTAL MUSCLES IN HUMANS

Publisher Summary This chapter explores functional neural coupling between diaphragm and parasternal intercostal muscles in humans. In a study described in the chapter, electrical activity of intercostal muscles was measured from pairs of fine silver wires, uninsulated over four to eight mm at each end, inserted directly into the muscle by means of a 26 gauge needle. The wires were bent back over the tip of the needle. After insertion into the appropriate muscle, the needle was withdrawn leaving the wires, with hooks at their ends, remaining in the muscle. In this way, the wire electrodes were in a stable position, and records could be obtained for a period of one to two hours without apparent charge because of electrode movement. Nine observations in six adult male subjects yielded acceptable records, apparently uncontaminated by adjacent musculature. It was found that in all subjects, the parasternal intercostal muscles were active during quiet inspiration and quiet during expiration. This activity was invariably seen clearly on the tracings from the wire electrodes inserted into the muscle, but only infrequently discernable from the parasternal surface EMG records. In contrast, no phasic activity was seen during quiet breathing from the wire electrodes in the seventh interspaces laterally or posteriorly.

Preservation of soluble proteins for histological staining in paraffin sections.

Human serum at full strength and in dilutions with physiological saline (0.85%) ranging from 1:1 to 1:72 was allowed to permeate rectangular masses of fibrin foam in small pieces (maximum diameters 0.2 × 0.4 × 1.0 cm), and then placed in 10% neutral formalin, Zenkers solution and Bouins solution. After fixation for 4-12 hr, the fibrin foam and occluded serum proteins were imbedded, sections cut and stained with eosin bluish (CI. 771), 0.25% alcoholic solution, and by the McManus periodic acid-Schiff technique, using basic fuchsin (CI. 677). Undiluted serum (6.4 gin 100 ml) was not stainable after fixation in 10% formalin. With Zenkers solution stainable serum proteins are recognizable at 0.22 gm/100 ml and with Bouins solution at 0.08 gm/100 ml. Dried aliquots (0.2 ml) of the same dilutions, spread over an area of 1.0 cm2, fixed and stained similarly, gave almost identical results.