Pain receptors are activated by extremes of pressure and temperature as well as a variety of chemicals released from injured tissue. Histamine, K+, ATP, acids and bradykinin are among the most potent pain-producing chemicals.

• Sharp pain is carried by the smallest of the myelinated sensory fibres – the delta A fibres.

• Burning pain is carried more slowly by small nonmyelinated C fibres. Both types of fibres release the neurotransmitters glutamate and substance P. Somatic pain: can either be superficial which is the stimulation of receptors in the skin or deep which is stimulation of receptors in muscles, joints, tendons and bone.

• Visceral pain: results from noxious stimulation of receptors in the organs of the thorax (lungs, heart) and the abdominal cavity. Like deep somatic pain it is usually a vague sensation of dull aching, gnawing or burning. Important stimuli for visceral pain are extreme stretching of tissue, ischaemia, irritating chemicals and muscle spasm. Visceral pain can also be referred pain.

• Referred pain: pain stimuli arising in one part of the body are perceived as coming from another part. The fact that visceral pain afferents travel along the same pathway as somatic pain fibres helps explain the phenomenon of referred pain

Phantom limb pain: pain perceived in tissue that is no longer present. Is an example of hyperalgesia. Caused most often when limb amputations are conducted under general anaesthesia only and the spinal cord still experienced the pain of amputation. Epidural anaesthetics block neurotransmission in the spinal cord and largely reduce phantom limb pain.