• Nociceptor pain
(Latin: nocere, harm; capere, take)
This type of pain results from a direct irritation of the pain receptors, the nociceptors, which are the sensory nerve endings of low-velocity afferent nerve pathways. Most nociceptors react to the following harmful stimuli:
- Mechanical stimuli (impact, pressure)
- Chemical stimuli (acids)
- Thermal stimuli (cold, heat)
Nociceptors that react to various different stimuli are called polymodal, while those that react to only one type of stimulus are referred to as unimodal. The nociceptors in the skin are responsible for surface pain and the pain receptors of the inner organs, bones, muscles and joints, for example, are responsible for deeper-lying pain.
• Neuropathic/neurogenic pain
This type is usually described as a burning or shooting pain. Neuropathic pain is caused by damage to the central nervous system or the peripheral nerves. The nerve structures can be damaged either mechanically (for example by a herniated intervertebral disc with compression on the spinal nerve roots) or toxically (for example by poisonous substances).
• Deafferentation pain
This is a special case of neuropathic pain involving a partial or complete severance of the afferent connection to the central nervous system. A typical example of this type of pain is the phantom pain experienced following amputation of an appendage, such as a feeling of pain in the toes following a thigh amputation.
• Somatovisceral pain
This type of pain sensation occurs in bones, joints, skeletal muscles and the inner organs of the thorax and abdomen. Visceral pains in the thoracic or abdominal organs are often felt in parts of the body other than the point where the pain actually originates. For example, pain caused by a cardiac infarction or stenosis of the coronary arteries (angina pectoris) can feel like pain radiating through the left shoulder into the left arm. Areas of the skin in which a visceral pain is experienced, distanced from the actual point of origin, are known as ”Head’s areas.” This mechanism can be explained by the fact that the organic structures where the pain originates (e.g. inner organs) are innervated by afferent nerve pathways that originate in the same spinal cord segment as the nerve supply of the body regions in which the specific pain is experienced.
• Central pain
Damage to the supraspinal and spinal nociceptive systems may cause intense pain, since the damage may raise the stimulation level of these nerve structures. The pain as it is experienced can consist of various combinations of the above-described types of pain.