Respiratory Muscles

Introduction

respiratory musc = skeletal musc so have F vs L relationship

optimal length of inspiratory musc = FRC
optimal length of expiratory musc = TLC

MIP (max inspir press) = 90-100cmH₂O

MEP (max expir press) = 200cmH₂O; for a cough, need to inhale to near TLC to generate enough stretch so you are near MEP

Fiber Composition of the Respiratory Muscles

Type I: highly oxidative, slow twitch fibers (55% of fibers in diaphragm; 65% in intercostals)

Type IIa: highly oxidative, fast twitch fibers

Type IIb: glycolytic, fast twitch fibers

Mechanical Actions of the Respiratory Muscles: The Diaphragm

costal diaphragm

origin: lower rib cage

insertion: Central tendon

action: form diaphragmatic dome; moves rib cage

increases abdominal pressure Þ zap Þ forces lower ribs outward

pulls ribs upward and rotates them out

decreases pleural pressure Þ pulls upper rib cage inward

crural diaphragm

origin lumbocostal spine
insertion. Central tendon
action passes around aorta and esophogus; doesn’t move rib cage; relaxes during swallowing

dome of diaphragm

contracts Þ shorts and descends into abdominal cavity Þ decrease pressure in the thorax

zone of apposition (zap)

when diaphragm contracts, pulls up on zap region Þ lateral expansion

C3, 4, 5 Þ keep diaphragm alive; origin of phrenic nerve

Mechanical Actions of the Respiratory Muscles: The Intercostal Muscles

upper rib Þ pump handle motion Þ anterior portion has up and down movement

lower rib Þ bucket handle motion Þ lateral part moves outward / inward

external intercostal muscles: \\\\\\\ ////////

origin: superior border of rib below
insertion: Inferior border of rib above
innervation: external intercostal nerve
action: pulls ribs up; aids in inspiration

internal intercostal muscles: //////// \\\\\\\\\

origin: inferior portion of rib above
insertion: Superior portion of rib below
innervation: internal intercostal nerve
action: pulls ribs down; aids in expiration

parasternal intercostal muscles

origin: ribs adjacent to sternum
insertion: Sternum
innervation: internal intercostal nerve
action: inspiratory; lifts rib below

Mechanical Actions of the Respiratory Muscles: Other Respiratory Muscles of the Rib Cage

not usually used except in times of extreme stress

sternocleidomastoid muscles

origin: mastoid process
insertion: Sternum and clavical
action: aids inspiration

scalene muscles

origin: cervical spine
insertion: Upper ribs
action: lifting action of upper 2-3 ribs

Mechanical Actions of the Respiratory Muscles: The Abdominal Muscles

transversus abdominus

external oblique

internal oblique

rectus abdominus

these muscles attach to the lower ribs and, during contraction, act to pull these ribs downwards, producing a deflationary action

contraction of these muscles results in an increase in abdominal pressure Þ expands lower rib cage via zona of apposition

when coordinated, they have an expiratory action

Posture effects

standing: gravity pulls down on diaphragm; intercostal muscles function best

supine: diaphragm is maximally stretched so can exert maximal force

Loss of Diaphragmatic Function

loss of this muscle reduces the magnitude of the forces applied to the lower rib cage during inspiration

loss of the fixator function of this muscle, ie much of the force generated by the inspiratory intercostal muscles during inspiration is dissipated across the flaccid diaphragm

patients with bilateral diaphragmatic paralysis are usually unable to breathe while lying down and often must sleep sitting in a chair

Emphysema

hyper-inflation

flattened diaphragm; not dome-shaped

contraction of diaphragm

causes rib cage to move in