A neuropeptide comprising 11 amino-acid residues that is found widely in tissues, especially in the nervous system and gut. It mediates the effects of the parasympathetic nervous system by stimulating contractions of the gut, causing dilation of blood vessels (vasodilation), and increasing the flow of saliva from salivary glands. Substance P is also released as a neurotransmitter within the spinal cord by sensory neurons responding to pain and other noxious stimuli (see nociception).