Intercostal muscles

Breathing consists of the cyclic action of the lungs to inspire and expire atmospheric gases. Inspiration occurs when the diaphragm and intercostal muscles contract, generating a negative pressure in the pleura surrounding... 

Intercostal muscles Muscles connecting the ribs that aid the diaphragm in... 

Assisting the diaphragm with inspiration are the external intercostal muscles, which connect adjacent ribs. When the external intercostal muscles contract, the ribs are lifted upward and outward (much like a handle on a bucket). Therefore, contraction of these muscles causes an increase in the horizontal dimension of the thoracic cavity and a further increase in thoracic volume. The external intercostal muscles are supplied by the intercostal nerves. 

Deeper inspirations are achieved by more forceful contraction of the diaphragm and external intercostal muscles. Furthermore, accessory inspiratory muscles, including the scalenus and sternocleidomastoid muscles, contribute to this process. Located mainly in the neck, these muscles raise the sternum and elevate the first two ribs. As a result, the upper portion of the thoracic cavity is enlarged. 

FIGURE 43-1 Motor endplates from rabbit intercostal muscle stained with Lowit s gold chloride method, a, terminal axonal arborization b, nucleus d, region where the myelin sheath ends n, neural branch. (From Ramon y Cajal, S. Textura del sistema nervioso del hombre y los vertebrados. Madrid N. Moya, 1899.)... 

Breathing requires sustained activity of diaphragm and intercostal muscles. 

Loss of myofibrillar protein from the diaphragm and intercostal muscles limits the ability to cough, reducing the efficiency of fluid removal from the lungs and bronchioles and so increasing risk of infection. Pneumonia is a likely cause of death. 

Musculoskeletal Effects. Mild pathologic changes in the intercostal muscle tissue, including muscle fibers with subsarcolemmal grouped granular basophilic inclusions and scattered areas of necrosis were reported in the autopsy of a 51-year-old man who died from high acute-duration exposure, via inhalation, to a commercial insecticide spray containing diazinon and malathion. Neuromuscular acetylcholinesterase activity was one-half that of muscle from unexposed persons (Wecker et al. 1985). 

The primary cause of death in acute diazinon poisoning is a depression of the neurons in the brainstem (medulla), collectively known as the respiratory center, resulting in loss of respiratory drive or, in the case of managed treatment, cardiac failure due to electrical impulse or beat conduction abnormalities in cardiac muscles (fatal arrhythmias). Other effects, such as bronchoconstriction, excessive bronchial secretions, and paralysis of the respiratory muscles (intercostal muscles and diaphragm) may also contribute to respiratory insufficiency and death. Thus, death results from loss of respiratory drive and paralysis of the respiratory muscles, or cardiac failure, or both, with attendant asphyxia or cardiac arrest (Klaassen et al. 1986 Shankar 1967, 1978 Williams and Burson 1985). 

Wecker L, Mrak RE, Dettbam WD. 1985. Evidence of necrosis in human intercostal muscle following inhalation of an organophosphate insecticide. J Environ Pathol Toxicol Oncol 6(2) 171-175. 

Actions Not all muscles are equally sensitive to blockade by competitive blockers. Small, rapidly contracting muscles of the face and eye are most susceptible and are paralyzed first, followed by the fingers. Thereafter the limbs, neck, and trunk muscles are paralyzed, then the intercostal muscles are affected, and lastly, the diaphragm muscles are paralyzed. 

The lung is then inflated and the intercostal muscles are sutured closed. The ECG lead is then fixed close to the sternum in the sixth intercostal space. The muscle... 

Q3 Expiration in quiet breathing is passive. When inspiration ceases and the intercostal muscles and diaphragm relax, the volume of the thorax diminishes and the elastic tissues of the lung recoil. This recoil is sufficient to move the normal expiratory volume of air out of the lung. 

Inspiration is an active process involving the diaphragm and external intercostal muscles. Expiration is normally passive, because of relaxation of these muscles and recoil of lung tissue. 

Q4 The expiratory muscles are not used in normal expiration at rest since no muscular effort is required expiration is a passive process because of recoil of lung structures stretched in inspiration. When there is increased outflow resistance, patients use their thoracic and abdominal muscles to increase intrathoracic pressure and push air from the lung. The muscles used include the internal intercostal muscles of the chest wall and the oblique and transversus muscles of the abdomen. 

The authors of this chapter have studied the effect of C and D toxin serotypes, as well as A, B, and E, on human intercostal muscle (Hihnas, unpublished data). All serotypes showed a similar ability to produce complete muscular paralysis in ex vivo human intercostal muscle. Intercostal muscle was excised from patients receiving a thoracotomy and intercostal muscle flap procedure. The muscle was removed tendon to tendon by surgical excision without electrocautery and dissected into multiple bundles with their associated intercostal nerves. The nerve-muscle units were placed in a vertical twitch bath and stimulated at 0.03 Hz (0.2 ms pulses of supramaximal strength) using a novel nerve clamp electrode to illicit an indirect muscle twitch. Potent toxins (1 nM) from various serotypes were added to the bath after confirming the stability of control muscle responses. In each case, twitch tensions declined to negligible amplitudes by 1 h after direct toxin application to the tissue bath. 

Bryant SH. Cable properties of external intercostal muscle fibres from myotonic and nonmyotonic goats. J. Physiol. 1969 204 539-550. 

The significance of these observations for humans exposed Co OP agents is unknown, but it seems likely from animal experiments that myopathy does not develop In the absence of muscle hyperactivity Induced by anti-ChEs. There Is some evidence from human autopsy material of focal necrosis of diaphragmatic and Intercostal muscles after accidental exposure to a single large dose of OP Insecticide (137). 

After ACh has been released into (he synaptic cleft, ih concentration decreases rapidly. It is generally accepted tha there is enough AChE at nerve endings to hydrolyze ink choline and acetate any ACh (hat has been liberated. F example, (here is sufficient AChE in the nerve junction ol rat intercostal muscle to hydrolyze about 2.7 X 10" AD molecules in I millisecond (his far exceeds (he 3 x l(f molecules released by one nerve impulse. ... 

---