2010B12 Describe the function of the muscles involved in ventilation.



·         Intro

·         Inspiration

·         Expiration

·         Non-ventilatory function

·         (Airway smooth muscle)



Respiratory musculature

·   All muscle relevant to ventilation is skeletal and striated except airway smooth muscle

·   Primary ventilatory muscles mostly Type 1 = oxidative, fatigue-resistant

o Diaphragm 55% in adult, 20% neonate

o Intercostals 65% adult, 45% neonate

Control of breathing

·   Activity co-ordinated in medullary resp centre

o Dorsal inspiratory group

o Ventral expiratory group

o Pre-Botzinger pacemaker cells

·   Normally involuntary. Can be controlled voluntarily by cortex.




·   Overall: ↓intrapleural pressure -> ↑transpulmonary pressure

o ↑ AP diameter of chest (↑APD)

o ↑ transverse diameter of chest (↑TD)

o ↑ chest wall elevation (↑CWE)

o Maintain airway patency (↑AP)

Diaphragm’s contribution

·   Upright adult: 50%

·   Supine adult: 66%

·   Neonate: ↑ (cartilaginous ribs = ineffective intercostals

·   Pregnant: ↑ (upward displacement = increased excursion)

Primary muscles

·   Diaphragm:

o Dome shaped muscle between thorax and abdomen

o Attachments: L and R crura to vertebral bodies, lower 6 ribs, xiphisternum

o Central tendon fused to fibrous pericardium

o Innervated by phrenic nerve C3-4-5

o Descends 1-2cm in quiet inspiration

o Descends 10cm in max inspiration

·   External intercostals:

o ↑APD, ↑TD by “bucket handle” activity

o From rib above space to rib below

o Fibres in front pocket direction

·   Scalenes:

o ↑CWE, counteract inward movement of upper ribs by diaphragm

o From cervical transverse processes to upper ribs

·   Pharyngeal and laryngeal dilator muscles:

o Maintain airway patency

o Contraction timed with inspiration

o CNIX (larynx) and X (larynx and pharynx)

·   Dilators alae nasae:

o Maintain airway patency

o Contraction timed with inspiration

o Important in neonates (obligate nose breathes, high total resistance)

Accessory muscles

·   Sternocleidomastoid:

o ↑CWE

·   Levator scapulae:

o ↑CWE

o Also: scapular elevation

·   Trapezius:

o ↑CWE, scapular elevation

o Also: neck lateral flexion

·   Pectoralis major (when upper limb fixed):

o ↑CWE, ↑APD

o Also: shoulder flexion and adduction

·   Costal slips of latissimus dorsi (when upper limb fixed):

o ↑CWE

o Also: shoulder extension and adduction

·   Serratus posterior superior

o ↑CWE




·   Overall: ↑ intra-pleural pressure hence ↓ transpulmonary pressure

·   Opposite to inspiration


·   Relaxation of inspiratory muscles

·   Elastic recoil due to surface tension and elastic lung tissue

·   Energy stored during inspiration


·   Internal intercostals

o ↓ APD, ↓ TD by reverse Bucket handle activity

o From rib below to rib above

o Back pocket direction

·   Abdominals

o ↓ APD, ↓ TD, ↓CWE

o Rectus, transversus, internal and external obliques

o Pull down chest wall

o ↑intra-abdo pressure, push up diaphragm

·   Latissimus dorsi

o ↓ APD, ↓ TD, ↓ CWE

·   Serratus posterior inferior

o ↓CWE

Quadratus lumborum

o ↓CWE


Non-ventilatory functions:


·   Inspiration, glottic closure, expiratory muscle contraction, glottic opening


·   Expiration with partial glottic closure

Increased intra-abdominal pressure

·   Expiratory muscle contraction with closed glottis

o Splint vertebral column during exertion

o Abdominal or pelvic evacuation (e.g. vomiting, micturition, defecation, parturition)



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