ABSTRACT
Pain sensation arises from receptors in peripheral tissues and is conducted centrally by small diameter axons of the A delta and C subtypes. In the spinal cord, the substantia gelatinosa is the main relay site for onward transmission. The modulation of incoming nociceptive afferent fibre action potentials by larger A class axons is the basis of the gate theory of pain transmission. The lateral spinal thalamic tracts in the spinal cord contain the second order neurones and conduct pain sensation to the brain stem via the medial lemniscus. In the pons and mid brain, the peri aquaductal grey matter and nucleus of raphe magnus are the sites of maximum opiate receptor concentration, and the locus cerueleus is a secondary centre. The ventrolateral thalamic nuclei contain relay sites for onward transmission of nociceptive input to the cerebral cortex and conscious appreciation of pain sensation.
