Trachea

The trachea is a tubular structure that begins opposite the sixth cervical vertebra at the level of the thyroid cartilage. It is flattened posteriorly and supported along its 10- to 15-cm length by 16 to 20 horseshoe-shaped cartilaginous rings until bifurcating into right and left main bronchi at the level of the fifth thoracic vertebra. The cross-sectional area of the trachea is considerably larger than that of the glottis and may be more than 150 mm² and as large as 300 mm² .

Several types of receptors in the trachea are sensitive to mechanical and chemical stimuli. Slowly adapting stretch

TABLE 42-1 -- Laryngeal innervation

Nerve	Sensory	Motor
Superior laryngeal (internal division)	Epiglottis, base of tongue	None
	Supraglottic mucosa
	Thyroepiglottic joint
	Cricothyroid joint
Superior laryngeal (external division)	Anterior subglottic mucosa	Cricothyroid (adductor, tensor)
Recurrent laryngeal	Subglottic mucosa	Thyroarytenoid
	Muscle spindles	Lateral cricoarytenoid
		Interarytenoid (adductors)
		Posterior cricoarytenoid (abductor)

receptors are located in the trachealis muscle of the posterior tracheal wall. These are involved in regulating the rate and depth of breathing, but they also produce dilation of upper airways and the bronchi by decreasing vagal efferent activity. Other rapidly adapting irritant receptors lie all around the tracheal circumference. They are usually considered to be cough receptors, although their other reflex actions consist of bronchoconstriction.