Significance of Sensory Receptors, Afferent and Efferent Pathways in Respiration

Diagram of contraction of smooth muscle fiber 

Significance of the Sensory Receptors and Afferent Pathways

Slowly adapting stretch receptors control respiration via the respiratory centre. The other receptors that are important are

1. Unmyelinated sensory C fibres
2. Rapidly adapting irritant receptors that are associated with myelinated vagal fibres.

Physical and chemical stimuli acting on irritant receptors on myelinated fibres in the upper airways and C-fibre receptors in the lower airways cause

• Coughing
• Bronchoconstriction
• Mucus secretion

The stimuli may include cold air and irritants such as ammonia, sulphur dioxide, cigarette smoke, experimental tool capsaicin and endogenous inflammatory mediators.

Significance of Efferent Pathways

Autonomic innervation: The autonomic innervation of human airways is reviewed by vander Velden and Hulsmann.

Parasympathetic innervation: Bronchial smooth muscles are predominantly innervated by the parasympathetic system and these ganglia are embedded in the walls of the bronchi and bronchioles and the postganglionic fibres innervate airway smooth muscle, vascular smooth muscle and glands.
There are three types of muscarinic (M) receptors namely M1, M2 and M3 receptors.

M1 receptors: These receptors are present locally in the ganglia and on postsynaptic cells. M1 receptors facilitate nicotinic neurotransmission.

M2 receptors: These receptors are inhibitory autoreceptors and mediate negative feedback on acetylcholine release by postganglionic cholinergic nerves.
M3 receptors: These are the most pharmacologically important receptors and are found on the bronchial smooth muscles and glands and mediate bronchoconstriction and mucus secretion.
The stimulation of the vagus causes bronchoconstriction mainly in the larger airways.

Sympathetic innervation: The sympathetic nerves innervate tracheobronchial bloodvessels and glands.

Note: The sympathetic nerves do not innervate human airway smooth muscles.
B-Adrenoceptors are abundantly expressed on human airway smooth muscles, mast cells, epithelium, glands and alveoli. B-adrenoceptors in the airways are of the B2 variety (B-beta).
B-agonists relax the bronchial smooth muscle, inhibit mediator release from the mast cells and increase the mucociliary clearance.

Non-noradrenergic non-cholinergic nerves:

Inhibitory NANC nerves, releasing vasoactive intestinal peptide and nitric oxide are the important neural bronchodilator pathways in the human airways.

Excitatory NANC nerves are also present in the human airways and release neuropeptides, which initiate the neurogenic inflammation and influence the accumulation, proliferation and activation of the inflammatory cells which inturn modulate the neuronal function.

In animal models, these nerves cause an inflammatory response that consists of mucus secretion, bronchoconstriction, increase in vascular permeability, cough and vasodilatation.

The main excitatory neuropeptides present in the lung are tachykinins: substance P and neurokinin A.