BASIC SCIENCE The brain The thalamus is the key area for processing somatosensory information. Axons travelling in the lateral and medial spinothalamic tracts terminate in their respective medial and lateral to the primary and nuclei and from here neurons project secondary somatosensory cortices, the anterior cingulate cortex and the prefrontal cortex. These areas play various roles in the perception of pain and also interact with other areas of the brain, for example the cerebellum and basal ganglia (which are areas more traditionally known to be associated with motor function rather than pain). the insula, Descending tracts These pathways (see Figure 1) have a role in the modulation of pain. Noradrenaline and 5-HT are the key neurotransmitters involved in descending inhibition. Two important areas of the brainstem are involved in reducing pain; the periaquaductal grey (PAG) and the nucleus raphe magnus (NRM).

PAG This region surrounds the cerebral aqueduct in the midbrain and is important in the control of pain. Electrical stimulation of the PAG produces profound analgesia and injection of morphine here has a far greater analgesic effect than injections anywhere else in the central nervous system (CNS). The PAG receives inputs from the thalamus, hypothalamus and cortex and also collaterals from the spinothalamic tract. PAG (anti-nociceptor) neurons excite cells in the NRM that in turn project down to the spinal cord to block pain transmission by dorsal horn cells.

NRM A second descending system of serotonin-containing neurons exists. The cell bodies of these neurons are located in the raphe nuclei of like the noradrenalin-containing neurons, the axons synapse on cells in lamina II. They also synapse on cells in lamina III. Stimulation of the raphe nuclei produces a powerful analgesia and it is thought that the serotonin released by this stimulation activates inhibitory interneurons even more powerfully than noradrenaline and thus blocks pain transmission. the medulla and, Brainstem neurons may control nociceptive transmission by: (cid:2) direct action on dorsal horn cells (cid:2) inhibition of excitatory dorsal horn neurons (cid:2) excitation of inhibitory neurons.

Visceral pain This is the pain arising from internal organs. In comparison to somatic pain, visceral pain is poorly localized, because the density of nociceptors on viscera is lower and afferent bres are less well represented in cortical mapping. Just as with somatic pain, the bres that transmit pain from visceral nociceptors are Ad and C bres and travel with autonomic afferents. Visceral pain pathways are shared with somatic pathways in the same ascending tracts of the spinal cord. The result is that pain from an internal organ can be interpreted as arising from converging somatic afferents and therefore visceral pain may be referred to the corresponding somatic tissue. Visceral pain can also be referred to a site far away form the source of stimulation. An example of this is when deep pain from the bladder is referred in 509 (cid:2) 2009 Elsevier Ltd. All rights reserved. BASIC SCIENCE