List the muscles involved in respiration and briefly describe their function.

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College Answer

Most candidates correctly identified the diaphragm, intercostals, abdominals and accessory
muscles as important. However, the majority of candidates omitted to mention the muscles of the
larynx, pharynx and airway and thus failed to achieve a good score. Good answers included a
detailed description of their function including differentiating between external and internal
intercostals.
Syllabus: B1b,1, 2a
References: Nunn’s respiratory physiology P76 – 80 and West, Respiratory Physiology: the
Essentials, pgs 93 - 95

Discussion

As one only has a few minutes to come up with a coherent answer to this SAQ under exam conditions, this is probably the wrong scenario for a longform prose answer discussing the relations innervation and blood supply of every single muscle. It would probably be best to group them anatomically and present their function in a table. 

Muscles of Respiration
Muscle group Inspiratory function Expiratory function

Pharyngeal muscles

  • Genioglossus
  • Palatal muscles
  • Hyoid muscles
  • Dilate the upper airway as a reflex response to negative pressure
  • Relax passively

Laryngeal muscles

  • Vocal cords abduct to decrease resistance to airflow
  • Vocal cords adduct to increase airway resistance (and prevent lower airway collapse)

Chest wall muscles

  • Intercostals
  • Levator costae
  • Transversus thoracis
  • Scalenes
  • "Bucket handle" movement: elevation of the ribs (mainly by the external intercostals)
  • "Pump handle" movement: elevation of the sternum (by the sternomastoid muscle)
  • Internal intercostals have a predominantly expiratory role (i.e. by contracting they depress the ribs).

Diaphragm

  • The main inspiratory muscle (by contracting, flattens and increases intrathoracic volume
  • Relaxes passively

Abdominal muscles

  • Rectus abdominis
  • Transvers abdominis
  • External and internal obliques
  • Pelvic floor muscles 
  • Apply counterpressure to the flattening diaphragm to facilitate lateral and anteroposterior expansion of the chest
  • Maintain intra-abdominal pressure, and act during expiration to push the diaphragm back up into the chest
  • Active role whenever the respiratory effort is increased

Accessory muscles

  • Sternocleidomastoid
  • Trapezius
  • Pectoralis group
  • Extensors of the vertebral column
  • Serratus anterior
  • Latissimus dorsi
  • Recruited to assist respiratory effort when the energy requirements of ventilation are increased
 

References

References

Gauthier, ALAIN P., et al. "Three-dimensional reconstruction of the in vivo human diaphragm shape at different lung volumes." Journal of applied physiology 76.2 (1994): 495-506.

De Troyer, André, Peter A. Kirkwood, and Theodore A. Wilson. "Respiratory action of the intercostal muscles." Physiological Reviews 85.2 (2005): 717-756.

Poole, DAVID C., et al. "Diaphragm structure and function in health and disease.Medicine and science in sports and exercise 29.6 (1997): 738-754.

Hart, Nicholas, et al. "Effect of severe isolated unilateral and bilateral diaphragm weakness on exercise performance." American journal of respiratory and critical care medicine165.9 (2002): 1265-1270.

Luce, John M., and Bruce H. Culver. "Respiratory muscle function in health and disease." Chest 81.1 (1982): 82-90.

Campbell, E. J. M. "The role of the scalene and sternomastoid muscles in breathing in normal subjects. An electromyographic study." Journal of anatomy 89.Pt 3 (1955): 378.

Raper, A. J., et al. "Scalene and sternomastoid muscle function." Journal of Applied Physiology 21.2 (1966): 497-502.

Danon, Joseph, et al. "Function of the isolated paced diaphragm and the cervical accessory muscles in C1 quadriplegics." American Review of Respiratory Disease 119.6 (1979): 909-919.

Drummond, Gordon. "Like breathing out and breathing in…." The Journal of physiology 588.Pt 18 (2010): 3345.

Montes, António Mesquita, et al. "Abdominal muscle activity during breathing with and without inspiratory and expiratory loads in healthy subjects." Journal of Electromyography and Kinesiology 30 (2016): 143-150.

Goldman, J. M., et al. "An electromyographic study of the abdominal muscles during postural and respiratory manoeuvres." Journal of Neurology, Neurosurgery & Psychiatry 50.7 (1987): 866-869.