Neuro-anatomy of Respiratory Muscles BY AHMAD YOUNES PROFESSOR OF THORACIC MEDICINE

1. Neuro-anatomy of Respiratory Muscles BYAHMAD YOUNES PROFESSOR OF THORACIC MEDICINE Mansoura faculty of medicine

2. Neuro-anatomy of respiratory Muscles • The principal function of the lung is to ventilate the blood. The alternating air flow to and from the alveolar surface is driven by pressure gradients generated by the respiratory muscles. In spite of their specific task which does not allow them to rest during their entire life, the respiratory muscles have the same structure and function as all other limb and trunk muscles. • The specialization of the respiratory muscles derives directly from the characteristics of the fibers of which they are composed. • Respiratory muscle fibers, however, are not only highly specialized for their functional tasks but are also able to modify their properties to adapt to new requirements which might arise from physiological conditions such as physical exercise or from lung or respiratory diseases .

3. Structural properties of the respiratory muscles • Skeletal muscles are composed of several motor units, each with hundreds of muscle fibers. • Three types of muscle fibers are usually present. They are classified according to which isoform of myosin heavy chain (MHC) is expressed. • Slow-twitch (ST or Type I) fibers are identified by a slow contraction time and a high resistance to fatigue. Structurally, they have a small motor neuron and fiber diameter, a high mitochondrial and capillary density, and a high myoglobin content, they contain few of the enzymes involved in glycolysis, but contain many of the enzymes involved in the oxidative path ways. Functionally, ST fibers are used for aerobic activities requiring low-level force production .

4. Structural properties of the respiratory muscles • Fast-twitch (FT or Type II) fibers are identified by a quick contraction time and a low resistance to fatigue. Fast-twitch fibers are further divided into fast-twitch A (FT-A or Type IIA) and fast- twitch B (FT-B or Type IIB) fibers. • FT-A fibers have a moderate resistance to fatigue and represent a transition between the two extremes of the ST and FT-B fibers. Structurally, FT-A fibers have a large motor neuron and fiber diameter, a high mitochondrial density, a medium capillary density, and a medium myoglobin content. They have both a high glycolytic and oxidative enzyme activity. Functionally, they are used for prolonged anaerobic activities with a relatively high-force output . • Fast-twitch B fibers, are very sensitive to fatigue and are used for short anaerobic, high force production activities, Like the FT-A fibers, FT-B fibers have a large motor neuron and fiber diameter, but a low mitochondrial and capillary density and myoglobin content. They also are high in creatine phosphate and glycogen, but low in triglycerides. They contain many glycolytic enzymes but few oxidative enzymes

5. Structural properties of the respiratory muscles The respiratory muscles are mixed muscles containing both fast-twitch and slow-twitch fibers. In diaphragm; type I fiber represents approximately 50% of the muscle fibers, type IIA about 20%, and type IIB about 30%. In intercostal muscles, the proportion of slow fibers is above 60% (that is, slightly higher than in the diaphragm) in both the internal and the external intercostal muscles . The respiratory muscles thus seem to be generally well equipped to sustain continuous rhythmic contraction. The density of mitochondria in each of the three fiber types tends to be greater than in the same fiber types in limb muscles. The volume density of mitochondria in the diaphragm is twofold greater than in the limb muscles. Therefore, the oxygen uptake capacity of the diaphragm is considerably greater than that of limb muscles.

6. Functional Properties of the respiratory muscles • The maximal blood flow also considerably exceeds that of limb muscles because of the greater capillary density, which is about twice the capillary density in the limb muscle . • The length of the muscle prior to the contraction affect the strength of contraction as it determines how much overlap there will be between actin and myocin and, thus, how many cross-bridges can be formed . • Maximal tension is generated at the optimal length. So, with acute hyperinflation, the diaphragm shortens and its capacity to generate force is concomitantly reduced . • With fibers at the optimal length, the force developed by a skeletal muscle is the function of the frequency of stimulation. The frequency-force relationship results from the summation of twitch force during repeated stimulation. Slow muscles will show summation at lower frequency of stimulation than fast muscles.

7. Functional Properties of the respiratory muscles • The velocity of contraction is a direct function of myosin ATPase activity, and, hence, the force-velocity curve is primarily determined by the muscle fiber composition. The respiratory muscles normally function at a low afterload, but with increasing loads; as in case of increased resistance to airflow, the velocity of contraction is reduced. • The production of airflow into the lungs requires power output by the muscles of respiration; consequently, the ability to develop and sustain power is the most important characteristic of respiratory muscle function. Power may be calculated as the product of the values of velocity and force according to the force-velocity relationship. Instantaneous peak power occurs at 30 percent of maximal force and at 30 percent of maximal velocity. The frequency-power relationship shows a similar dependency of force and power upon frequency of stimulation.

8. Anatomy and action of the respiratory muscles • The group of inspiratory muscles includes the diaphragm, external intercostals, parasternal, sternomastoid and scalene muscles. • The group of expiratory musclesincludes the internal intercostal, rectus abdominis, external and internal oblique and transverse abdominis muscles . • During low breathing effort (i.e. at rest) only the inspiratory muscles are active. During high breathing effort (i.e. exercise) the expiratory muscles become active as well

11. The diaphragm • The diaphragm muscle is composed of two domains . The costal diaphragm is a thin domed sheet of muscle composed of a radial array of myofibers extending laterally from the ribs and medially to a central tendon. It arises from the inner surfaces and upper margins of the lower six ribs and sternum. • The crural diaphragm is thicker and located more posteriorly, where it attaches to the first three lumbar vertebrae and the medial and lateral arcuate ligaments . Medially, the myofibers of both the costal and crural muscles insert into the central tendon. The central tendon is located at the apex of the domed diaphragm, holding the diaphragm muscle domains together.

12. Mechanism of CSA

13. The diaphragm • Under normal circumstances, a zone of apposition exists around the outside of the diaphragm where it is in direct contact with the internal aspect of the rib-cage, with fibers arranged in a cranial-caudal direction, with no lung in between, but the parietal pleura still allowing free movement of the diaphragm . • At upright functional residual capacity (FRC) in humans, approximately 55% of the diaphragm surface area is in the zone of apposition .

15. The diaphragm • The diaphragm receives its entire motor supply from the phrenic nerve from cervical segments 3, 4, and 5. • The sensory nerve fibers from the central part of the diaphragm also run in the phrenic nerve, while the peripheral part, including the crura, receives sensory fibers from the lower intercostal nerves . • The diaphragm has an abundant blood supply derived from the phrenic and intercostal arteries and from branches of the internal thoracic (mammary) arteries. Flow can increase to approximately 250 mL/min/100 g of muscle during maximal activation (about half of the maximal blood flow to the heart).In comparison with other skeletal muscles, the diaphragm is extremely active. • Muscle fibers within the diaphragm can reduce their length up to 40% between RV and TLC .

16. Diaphragmatic contraction increases chest wall dimensions because of three distinct reasons. • First, diaphragmatic descent increases the craniocaudal dimensions of the thorax. This may be considered using a "piston in a cylinder" analogy, the trunk representing the cylinder and the diaphragm the piston . • The first possible mechanism is involving downward movement of the diaphragm simply by shortening the zone of apposition around the whole cylinder and leaving the dome shape unchanged. • This is a pure "piston-like" action and has the advantage of being the most energy efficient way of converting diaphragm contraction into lung expansion.

17. “Piston in a cylinder” analogy of the mechanisms of diaphragm actions on the lung volume. (A) Resting end-expiratory position. (B) Inspiration with pure piston-like behavior. (C) Inspiration with pure non-piston-like behavior. (D) Combination of piston-like and non-piston-like behavior in an expanding cylinder, which equates most closely with inspiration in vivo. ZA, zone of apposition.

18. Non-piston-like" behavior • "Non-piston-like" behavior is the second possible mechanism in which zone of apposition remains unchanged but an increase in tension of the diaphragm dome reduces its curvature, so expanding the lung . This is likely to be less efficient than piston-like behavior because much of muscle tension developed simply opposes the opposite side of the diaphragm rather than moving the diaphragm downward, such that in theory, when the diaphragm becomes flat, further contraction will have no effect on lung volume . • Finally, both types of behavior already described but also includes expansion of the lower ribcage (known as "piston in an expanding cylinder") that occurs with diaphragmatic contraction particularly in supine position .

19. Third, because the muscle fibers of the costal diaphragm insert onto the upper margins of the lower six ribs, they also apply a force on these ribs when they contract, and the cranial orientation of these fibers is such that this force is directed cranially. It therefore has the effect of lifting the ribs and rotating them outward . This is the insertional component of diaphragmatic contraction. • During inspiration, as the fibers of the costal diaphragm contract, they exert a force on the lower ribs (arrow). If the abdominal visceral mass effectively opposes the descent of the diaphragmatic dome (open arrow), this force is oriented cranially. Consequently, the lower ribs are lifted and rotate outward .

20. Intercostal Muscles • The external intercostal muscleforms the most superficial layer. Its fibers are directed downward and forward from the inferior border of the rib above to the superior border of the rib below . The lower insertion of the external intercostals muscles is more distant from the ribs axis of rotation than the upper one, and as a result, contraction of this muscle exerts a larger torque acting on the lower rib which raises of the lower rib with respect to the upper one. The net effect of the contraction of these muscles raises the rib cage. • The internal intercostal muscleforms the intermediate layer. Its fibers are directed downward and backward from the subcostal groove of the rib above to the upper border of the rib below.

22. Intercostal Muscles • The innermost intercostal muscle forms the deepest layer. It is an incomplete muscle layer and crosses more than one intercostal space within the ribs. It is related internally to endothoracic fascia and parietal pleura and externally to the intercostal nerves and vessels . • Between the chondral portions of the ribs and the sternum; only one layer of intercostal muscles, the parasternal intercostals, is present. Dorsally, from the angles of the ribs to the vertebrae, the only fibers come from the external intercostals muscles. These latter, however, are duplicated in each interspace by a thin, spindle-shaped muscle that runs from the tip of the transverse process of the vertebra cranially to the angle of the rib caudally; this muscle is the “levator costae”. • All intercostal muscles are innervated by the intercostals nerves .

23. LEVATORES COSTARUM • Origin: Vertebrae (C7, T1-11 transverse processes) • Insert: Ribs (below origin) • Action: Raise ribs in Inspiration • Innervation: Dorsal primary rami of thoracic spinal nerves

25. Intercostal Muscles • External intercostals are inspiratory in action, and the internal intercostals are expiratory in action. • As the fibers of the external intercostal muscle slope downward and forward from the rib above to the rib below, their lower insertion is more distant from the center of rotation of the ribs (the costovertebral articulations) than their upper insertion. Consequently, when this muscle contracts, the torque acting on the lower rib is greater than that acting on the upper rib, so its net effect would be to raise the ribs and to inflate the lung . The elevation of the ribs in this way increases both the lateral and anteroposterior diameter of the ribcage resulting in a ‘bucket handle’ action .

26. Diagram illustrating the actions of the intercostal muscles. The hatched area in each panel represents the spine, and the two bars oriented obliquely represent two adjacent ribs, these are linked to each other by the sternum (right). • The external (A) and internal (B) intercostal muscles are depicted as single bundles, and the torques acting on the ribs during contraction of these muscles are represented by arrows .

27. Intercostal Muscles • As the fibers of the internal intercostals muscle slope downward and backward from the rib above to the rib below, their lower insertion is less distant from the center of rotation of the ribs than the upper one. As a result, when this muscle contracts, the torque acting on the lower rib is smaller than that acting on the upper rib, so its net effect would be to lower the ribs and to deflate the lung .

28. Bucket handle movement of ribs

29. The scalene muscles • Three pairs of muscles in the lateral neck, namely the anterior scalene, middle scalene, and posterior scalene. • They originate from the transverse processes from the cervical vertebrae of C2 to C7 and insert onto the first and second ribs. Thus they are called the lateral vertebral muscles.[3] • They are innervated by the fourth, fifth, and sixth cervical spinal nerves (C4-C6). • The action of the anterior and middle scalene muscles is to elevate the first rib and laterally flex (bend) the neck to the same side;the action of the posterior scalene is to elevate the second rib and tilt the neck to the same side. • They also act as accessory muscles of inspiration, along with the sternocleidomastoids.

30. The scalene muscles • The action of these muscles is to raise the first two ribs. The orientation of their axis in the neck causes upward motion of these ribs (“pump handle” motion) . Moreover, the scalenes are consistently active during quiet breathing in normal individuals and contribute to chest wall expansion. • They may be very important in the case of spinal cord injury. When the injury is below C4-C8, the scalenes’ function is entirely or partially preserved, and they contribute importantly to upper rib cage motion in these patients.

32. Pump handle movement of ribs and sternum

33. Sternocleidomastoid • The muscle is attached inferiorly by two heads. The medial or sternal head, arises from the upper part of the anterior surface of the manubrium sterni. The lateral or clavicular head, ascends almost vertically from the superior surface of the medial third of the clavicle. • It inserts superiorly by a strong tendon into the lateral surface of the mastoid process from its apex to its superior border, and by a thin aponeurosis into the lateral half of the superior nuchal line. • It is supplied by the spinal part of the accessory nerve. Branches from the ventral rami of the second, third, and sometimes fourth, cervical spinal nerves also enter the muscle .

35. Sternocleidomastoid • In humans, these muscles are electrically silent during quiet breathing, but they may be recruited with increased ventilatory load. • These muscles are particularly important in high quadriplegics. They also may be recruited in patients with poliomyelitis and diaphragmatic dysfunction. • These muscles are thought to be important in moving the upper rib cage in patients with COPD, even though a clinical experimental study failed to demonstrate consistent activity in these muscles in these patients

36. The Shoulder Girdle and Neck Muscles • Several shoulder girdle and neck muscles may contribute to inspiration under particular circumstances. • Most of these muscles run from the rib cage to an extrathoracic extension. When the rib cage is fixed in the lean-forward position— a position commonly employed by patients with COPD—these muscles contribute to expansion of the rib cage during inspiration. • Muscles that may contribute to inspiration include the trapezius, latissimus dorsi, pectoralis major and minor, erector spinae, teres major, serratus anterior, platysma, mylohyoid, and sternohyoid. • Since these muscles commonly contribute to inspiration in patients with severe airflow obstruction, using these muscles for other activities, such as hair combing, may considerably increase dyspnea in these patients .

37. Transversus thoracis • Transversus thoracis (also called the triangularis sterni) spreads over the internal surface of the anterior thoracic wall • It arises from the lower one-third of the posterior surface of the sternum, the xiphoid process and the costal cartilages of the lower three or four true ribs near their sternal ends. • Its fibres diverge and ascend laterally as slips that pass into the lower borders and inner surfaces of the costal cartilages of the second, third, fourth, fifth and sixth ribs. • The lowest fibres are horizontal, and are contiguous with the highest fibres of transversus abdominis; the intermediate fibres are oblique; the highest are almost vertical. • It is supplied by the adjacent intercostal nerves.

39. Transversus thoracis • The transversus thoracis is the most important expiratory muscle of the rib cage, and its action is to lower the ribs relative to the sternum and thus to cause expiration. • It is electrically silent in humans breathing quietly, but it is recruited during speech, laughing, or expiration below FRC .

40. The muscles of the anterior abdominal wall • The rectus abdominisarises from the 5th, 6th and 7th costal cartilages and is inserted into the crest of the pubis. • The external oblique arises from the outer surfaces of the lower eight ribs and fans out into the xiphoid, linea alba, the pubic crest, pubic tubercle and the anterior half of the iliac crest. • The internal oblique arises from the lumbar fascia, the anterior two-thirds of the iliac crest and the lateral two-thirds of the inguinal ligament. It is inserted into the lowest six costal cartilages, linea alba and the pubic crest. • The transversus abdominisarises from the lowest six costal cartilages (interdigitating with the diaphragm), the lumbar fascia, the anterior two thirds of the iliac crest and the lateral one-third of the inguinal ligament; it is inserted into the linea alba and the pubic crest.

42. The muscles of the anterior abdominal wall • With the exception of gas within the bowel lumen, the abdomen is an incompressible volume held between the diaphragm and the abdominal muscles. • Contraction of either will cause a corresponding passive displacement of the other. Thus abdominal muscles are generally expiratory. Contraction of these muscles results in an increase in abdominal pressure displacing the diaphragm in a cephalad direction. • Their insertion into the costal margin results in a caudal movement of the ribcage, so assisting expiration by opposing the ribcage muscles .

43. The Shoulder Girdle and Neck Muscles • Several shoulder girdle and neck muscles may contribute to inspiration under particular circumstances. Most of these muscles run from the rib cage to an extrathoracic extension. • When the rib cage is fixed in the lean-forward position— a position commonly employed by patients with COPD—these muscles contribute to expansion of the rib cage during inspiration. • Muscles that may contribute to inspiration include the trapezius, latissimus dorsi, pectoralis major and minor, erector spinae, teres major, serratus anterior, platysma, mylohyoid, and sternohyoid. • Since these muscles commonly contribute to inspiration in patients with severe airflow obstruction, using these muscles for other activities, such as hair combing, may considerably increase dyspnea in these patients .

45. Extrinsic Shoulder Muscles

46. Extrinsic Shoulder Muscles

47. Muscles of the Pelvic Floor: Pelvic Diaphragm