The Pathophysiology of Pain
Fig. 2
Tissue damage
Tissue damage at a cellular level results in a release of chemicals which stimulate pain receptors. These pain receptors are able to produce an electrical impulse which is transmitted via a peripheral nerve fibre to the spinal cord and then to the brain where the impulse is perceived as pain.
Tissue damage may be:
- Mechanical
- Thermal
- Chemical
- Electrical
- Metabolic (e.g. hypoxaemia, hypoglycaemia)
Chemicals are released as a result and stimulate pain receptors. Prostaglandins, which may be produced as a consequence of the injury, sensitize the pain receptors to these chemicals.
Tissue damage drives pain hypersensitivity peripherally through the release of chemicals due to direct cell damage or release from nearby platelets or mast cells. These include the following: ATP, bradykinin, prostaglandin, serotonin, histamine and hydrogen ions.
Chemicals such as substance P and Calcitonin Gene Related Peptide (CGRP) increase the release of these chemicals via localised vasodilation and mast cell degranulation.
Pain signals to the brain
Pain receptors, sometimes called nocioreceptors, are distributed throughout the body – skin, viscera, joints, meninges, muscle etc.
Think of them as free nerve endings.
They produce an electrical impulse and connect to peripheral nerve fibres.
Fig. 3 Pain receptors, sometimes called nocioreceptors
Peripheral nerve fibres transmit the electrical impulse to the dorsal horns of the spinal cord.
Fig. 4 Nerves of the thorax
Ascending tracts within the spinal cord transmit impulses to the brain.
Fig. 5 Spinal cord with spinothalamic tract (ascending)
The axons of the tract cross over (decussate) to the other side of the spinal cord 1-2 nerve roots above their point of entry and travel up to the brainstem via the spinothalamic tract.
Visceral pain pathways
Visceral pain pathways involve the gut, heart and lungs etc. They are more crude and result in poorer localisation of pain (referred pain).
The axons of the tract cross over (decussate) to the other side of the spinal cord 1-2 nerve roots above their point of entry and travel up to the brainstem via the spinothalamic tract.
Examples include patients with ischaemic heart pain presenting with jaw, shoulder or arm pain and patients with free peritoneal fluid (such as in ruptured ectopic pregnancy) presenting with shoulder tip pain.
Fig. 6 Visceral pain pathways