Transmitters involved in central processing of nociceptive information.

Distribution studies suggest that substance P is a transmitter released by nociceptive primary afferent fibres. Acceptance of this role, however needs more electro-physiological comparisons of the effects of substance P and the transmitter released by primary afferents on the membrane properties of dorsal horn neurones. The transmission of impulses from nociceptive afferents to supraspinal areas is subject to inhibition of both segmental and supraspinal origin. Segmental inhibition probably involves glycine and gamma-aminobutyric acid as transmitters. Of supraspinal inhibitions investigated in the cat none have been shown to involve amino acids, enkephalins or 5-hydroxytryptamine but there is evidence to suggest a role for noradrenaline.