Cold and flu are common and crappy. This paper is worth reading especially when suffering from one.
Summary:
Most symptoms are due to the inflammatory response, rather than the pathogen itself. Cytokines released from damaged tissue and activated white cells diffuse locally, and circulate systemically, causing the myriad of symptoms we all know.
Symptoms can be split by timing;
early - come on and dissipate over 2-3 days: sore throat, sneeze, chills, malaise, headache
late - develop over days and last over a week; nasal congestion + discharge, malaise, cough
Summary:
Most symptoms are due to the inflammatory response, rather than the pathogen itself. Cytokines released from damaged tissue and activated white cells diffuse locally, and circulate systemically, causing the myriad of symptoms we all know.
Symptoms can be split by timing;
early - come on and dissipate over 2-3 days: sore throat, sneeze, chills, malaise, headache
late - develop over days and last over a week; nasal congestion + discharge, malaise, cough
persisting long after infection: cough
Each symptom has its own mechanism;
Sore throat / Pharyngitis
Cytokines - especially bradykinin and prostaglandins - released locally, acting on trigeminal nerves.
Sneezing
A reflex, probably initiated by histamine acting on trigeminal nerves to trigger afferent signal to sneezing centre in brainstem. The efferent reflex is to motor and parasympathic (PS) parts of the facial nerve, and respiratory muscles.
Simultaneous lacrimation and nasal secretions occur via PS facial nerve activity, while eye closing is by somato-motor facial nerve activity. Note relation of eyes to nose, which is reversed in photic sneezing.
Runny nose / rhinorrhoea
Components: neuro-reflex glandular secretion (blocked by antimuscarinics) and capillary proteinaceous ooze due to inflammation induced endothelial junction loosening.
The colour of discharge and sputum does not relate directly to cause (bacterial vs viral) but more the intensity of inflammation, representing recruitment of white cells into the airway.
Neutrophils and monocytes contain azureophilic granules containing myeloperoxidase, giving green-yellow tinge at low concentration or murky green at higher ones.
nasal congestion
Due to venous plexus dilation at the narrow nasal valve region, caused by inflammatory mediators. Usually cycles between each nostril, under the influence of sympathetic vasconstrictor nerves.
Sinus pain
Pain is not dependent on patent ostia, but can be due to increased pressure in sinus, and distension of draining blood vessels.
Watery eyes / epiphora
Obstruction of naso-lacrimal duct as it opens in the nose, due to venous plexus engorgement.
Cough
Stimulated by vagal nerve activity, always at level of larynx or below. Hyper-reactivity of the reflex, which is normally to protect the airway, occurs due to inflammation.
Influenza damages airway epithelium > most cold viri, leading to more cough, rather than a simple head cold.
The progression from sneezing to cough can be explained by progressive inflammation spreading down the respiratory tract, stimulating trigeminal nerves, then vagus nerves.
Probably from effects of circulating cytokines on central (i.e. hypothalamic or brainstem) nervous system.
Chills
Chills occur regardless of skin temperature, due to central nervous activity, connected to shivering, changing appraisal of somatic sensation. Chills are not from peripheral vasoconstriction causes skin temperature decrease, perceived as coldness.
Fever
Occurs more in novel exposures, i.e. pandemic viri, or in infants. Interleukins 1 and 6 are most important, circulating to the hypothalamus or to vagus nerve neurons, effecting a change in temperature set point.
Malaise and psychological effects
As well as the cumulative effects of all the above, there are separate effects on neurological activity from circulating cytokines.
Features include: psychomotor slowing, ahedonia, sleep disturbance, hyperalgesia.
Interleukins 1, 2, 6 and tumour necrosis factor are thought responsible.
Anorexia
seems to be a protective response, saving energy, starving pathogens of zinc and iron, and stimulating monocytes sand macrophages. It occurs via effects on the hypothalamic feeding centre.
Muscle aches and pains / myalgia
Probably mediated through prostaglandin E2, causing myocyte catabolism and nerve stimulation. This releases amino acids for the liver to create immune molecules like complement. the pain and breakdown can be blocked with cyclooxygenase inhibitors.
Understanding the symptoms of the common cold and influenza.
Eccles R.
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