2013C09 Describe the cough reflex.

Definition

·   Forceful expiration to expel mucous or foreign material from airways. Keeps lower resp tract sterile.

·   Can be voluntary or involuntary

Sequence

·   Fast inspiration

·   Close glottis

·   Contract chest and abdo wall

·   ↑Intrathoracic pressure (ITP) ~100mmHg

·   Release glottis

·   Expiration fast (80km/h) and turbulent (to expunge wall-adherent material)

Purpose

·   Keep airways sterile

Significance to anaesthesia

·   Suppressed under anaesthesia. Risk of aspiration

Sensors

·   Mucosal irritant receptors on C fibres

·   Mechanical or chemical stimuli. E.g. water, food, dust

·   From pharynx to resp bronchioles, esp carina

Pathway

·   General visceral afferents: via dorsal horn and spinomedullary tract

·   CNIX: pharynx -> nucleus tractus solitarius (NTS)

·   CNX: -> the NTS

o Pharynx: own branch x 2

o Larynx above vocal cords: internal branch of superior laryngeal nerve x 2

o Larynx below vocal cords, trachea, bronchi: recurrent laryngeal nerve x 2

Afferents to

·   Nucleus tractus solitarius

Integration

·   Medullary respiratory centre

Efferents from

·   Nucleus ambiguus (pharynx, larynx)

·   Dorsal motor nucleus of vagus

Muscle

Innervation

Inspiration

Diaphragm

C3-5

External intercostals

Intercostal nerves

Pharyngeal dilators

CNIX

CNX pharyngeal branches

Glottic closure

Lateral cricoarytenoid

Interarytenoid

CNX recurrent laryngeal

Increased intrathoracic pressure

Internal intercostals

Intercostal nerves

Abdominals (rectus, transversus, internal and external obliques)

Lower intercostal nerves

Subcostal nerves

Iliohypogastric

Ilioinguinal

Latissimus dorsi

Long thoracic nerve

Serratus posterior inferior

Intercostal nerves

Quadratus lumborum

Subcostal nerve

Glottic release

Posterior cricoarytenoid

(and relax glottic closure muscles)

CNX recurrent laryngeal

Forceful expiration

(as for increased intrathoracic pressure)