Neural Mechanical Control of Breathing PDF Notes

WSUSOM Medical Physiology – Respiratory Physiology Page | 1 of 6 Neural Mechanical Control of Breathing ion NEURAL MECHANICAL CONTROL O...

WSUSOM Medical Physiology – Respiratory Physiology Page | 1 of 6 Neural Mechanical Control of Breathing ion NEURAL MECHANICAL CONTROL OF BREATHING Learning Objectives - Upon completion of this session, the student will be able to: 1. Describe where the respiratory rhythm is generated and how the rhythm leads to activation of muscles involved in inspiration and expiration. 2. Identify the muscles involved in respiration and how contraction of these muscles lead to airflow. 3. Identify how contraction of the respiratory muscles leads to generate of negative intrapleural pressure when the lung is at functional residual capacity, and predict the direction that the lung and chest wall will move if air is introduced into the pleural cavity. on 4. Diagram how pleural pressure, alveolar pressure, airflow, and lung volume change during a normal quiet breathing cycle. Identify on the figure the onset of inspiration, cessation of inspiration, and cessation of expiration. Describe how differences in pressure between the atmosphere and alveoli cause air to move in and out of the lungs. Lecture Outline I. Respiratory rhythm generation A. Location, axonal projections and nerves that innervate the respiratory muscles II. Functional contribution of chest wall to inspiration and expiration A. Role of chest wall muscles in generating inspiratory and expiratory flow  Diaphragm and external intercostal muscles  Accessory muscles of inspiration III. Mechanics of Breathing A. Resting positons of the lung and chest wall when separated and coupled V. Lung pressures and pressure gradients  Alveolar pressure  Intrapleural pressure  Atmospheric pressure  Transpulmonary pressure  Recoil pressure of the chest wall  Recoil pressure of the respiratory system  Pressure gradients and inspiratory and expiratory airflow WSUSOM Medical Physiology – Respiratory Physiology Page | 2 of 6 Neural Mechanical Control of Breathing Neural Innervation of Respiratory Muscles  Cyclical neural input to the respiratory muscles establishes the rhythmic pattern of breathing. The respiratory rhythm is generated at the level of the medulla by both pacemaker cells and groups of neurons. These neurons have axonal projections to the spinal cord where they synapse onto phrenic motoneurons (C3 – C5) and intercostal motoneurons (T1 – T12). These motoneurons give raise to nerves that exited the spinal cord and innervate the respiratory muscles.  Pulmonary ventilation is accomplished by altering intrapulmonary pressure which occurs in response to contraction of the inspiratory muscles (inspiration), and the passive recoil of the lungs (expiration) (see mechanics of breathing for further details).  The inspiratory muscles are comprised of the diaphragm and the external intercostal muscles. The diaphragm is innervated by the phrenic nerve which is formed by axons that originate from motoneurons located within the ventral horn of the C3-C5 segment of the spinal column.  The external intercostal muscles are innervated by the external intercostal nerves, which are formed by axons that originate from motoneurons located within the ventral horn of the T1- T12 segment of the spinal column.  The expiratory muscles are comprised of the rectus abdominus and the internal intercostal muscles. These expiratory muscles are activated at increased levels of minute ventilation. These muscles respond to efferent input from motoneurons in the upper lumbar segments of the spinal column, and from motoneurons in T1-T12 of the spinal column. WSUSOM Medical Physiology – Respiratory Physiology Page | 3 of 6 Neural Mechanical Control of Breathing Respiratory Muscle Activity during Inspiration and Expiration  During inspiration the diaphragm pulls downward, the external intercostal muscles pull the ribs upward and outward, and the accessory muscles in the neck stabilize the ribs. During passive expiration, the inspiratory muscles relax and the compliance of the chest wall returns the ribs to their relaxed position expelling air.  During forced expiration the internal intercostal muscles pull the ribs downward and inward and the rectus abdominis and the external oblique muscles pull the chest wall down, compressing the chest gas volume. Coupling of the Lung and Chest Wall  Pressure differences are dependent on the coupling of the lung and chest wall.  The lung and chest wall are coupled by visceral and parietal pleura.  The pleurae form a thin, double- layered serosa. The parietal pleura covers the thoracic wall, superior face of the diaphragm, and continues around the heart between the lungs. The visceral pleura covers the external lung surface, following its contours and fissures. WSUSOM Medical Physiology – Respiratory Physiology Page | 4 of 6 Neural Mechanical Control of Breathing Mechanics of the Lungs and Chest Wall P=0  The lung and the chest wall can be modelled as two springs coupled together (left hand figure). When the two springs are coupled, they come to a resting position (see left figure - lungs and thorax). The resting position when these structures are coupled together is known as functional residual capacity (FRC). This is the positon of the lung and chest wall at the end of expiration/beginning of inspiration.  When the springs are uncoupled, one spring contracts (i.e. lungs alone) and the other spring expands (i.e. thorax alone). The lungs contract to a volume below residual volume (RV). Residual volume is the lowest lung volume that can be achieved when the lung and chest wall are coupled to each other. The other spring (thorax alone) expands to a point that is close to total lung capacity (TLC). Thus, in “real” life if the lung and chest wall are uncoupled (e.g. knife wound) the lungs will tend to collapse and the chest wall will expand (see middle figure above).  At FRC the tendency of the chest wall to expand and the lungs to contract leads to an increase in volume in the space that exists between these two structures. This space is referred to as the intrapleural space.  The end result of an increase in volume is a decrease in pressure, as shown in the right-hand figure above. The relationship between volume and pressure is described by Boyle’s law which states that pressure is inversely related to volume.  The typical intrapleural pressure at FRC is – 5 cmH2O. This pressure is a relative pressure. More specifically, atmospheric pressure is typically 760 mmHg. The intrapleural pressure of – 5 cmH2O is relative to the atmospheric pressure, which means the absolute pressure in the intrapleural space in the example given is 755 mmHg. WSUSOM Medical Physiology – Respiratory Physiology Page | 5 of 6 Neural Mechanical Control of Breathing Lung Pressures and Pressure Gradients  By convention the pressure difference across a structure (i.e. the lung or the chest wall) is measured from the inside to the outside.  The pressure across the chest wall (Pcw) is the difference between the intrapleural pressure and atmospheric pressure. In this example the calculation would be 756 - 760 mmHg = - 4 mmHg if absolute values were used or -4 - 0 mmHg = - 4 mmHg if relative values were used for the calculation.  The pressure across the lungs (i.e. the transpulmonary pressure) is the difference between the alveolar pressure (also referred to as the intrapulmonary pressure as shown in the figure above) and the intrapleural pressure. In this example, the calculation is 760 - 756 mmHg = 4 mmHg.  The alveolar pressure is 760 mmHg because when there is no airflow the alveolar pressure and the pressure measured at the mouth (i.e. atmospheric pressure) must be equal.  The pressure gradient across the entire respiratory system is the difference between the alveolar pressure and atmospheric pressure. The pressure across the respiratory system can also be calculated by determining the sum of the pressure of its two components (i.e. the pressure across the lung plus the pressure across the chest wall). WSUSOM Medical Physiology – Respiratory Physiology Page | 6 of 6 Neural Mechanical Control of Breathing Lung Pressures and Pressure Gradients  During inspiration, the diaphragm and external intercostal muscle contract. As a result, the chest wall moves upward and outward and the volume in the intrapleural space increases.  As stated on the previous page, when volume increases pressure will decrease. Thus, as shown in the example above (left hand side) an increase in volume results in a more negative intrapleural pressure. In this example, the pressure changes from -5 to -7 mmHg.  As a result of the increase in volume in the intrapleural space the pressure across the lung becomes more positive [+ 7 mmHg = 0 mmHg (alveolar pressure) - (-7 mmHg) (intrapleural pressure)]. The increase in transpulmonary pressure leads to expansion of the lungs.  As a result, gas in the lungs is decompressed, and the alveolar pressure (PA) drops below atmospheric pressure. The created negative pressure gradient between the airways and atmosphere generates airflow to the lungs.  As inspiration proceeds, the lungs fill up with air, and the pressure gradient and air flow gradually decrease. At the end of inspiration airflow stops because PA is equal to atmospheric pressure (no pressure gradient).  At the onset of expiration, the diaphragm relaxes, elastic recoil of the respiratory system compresses the gas in the lungs, thereby increasing PA. The positive pressure gradient between the lungs and the atmosphere is reversed and air from the lungs is pushed out to the atmosphere.  As lung volume decreases, the pleural pressure slowly returns to its resting level. At the end of expiration (i.e. at FRC) airflow and PA are 0 (ml/sec and cmH2O, respectively), and Ppl is about -5 cmH20.