In Surgery of the Anus, Rectum & Colon (Third Edition), 2008 Anal sensation Pain and itch are served by the same receptors and neural pathways, which explains the effectiveness of pain (scratching) in relieving itch. It is questionable whether tertiary neurons relay the itch sensation to the cortex. Slow-conducting neurons transmit the itch sensation to the lateral spinothalamic tracts through synapses that connect with secondary fibers and send the sensation to the thalamus. Damage to cells in proximity to these nerve endings causes a release of diffusible mediators that may stimulate the receptors.
#NERVE ENDINGS SKIN#
The receptor apparatus for both itch and pain is located at the dermoepidermal junction of the skin and consists of a plexus of free nerve endings. 10 These sensations may be elicited by local irritation from excoriation, alkaline secretions, and various chemical irritants. The perianal skin is richly supplied with sensory nerve endings that mediate a variety of sensations. Beck, in Shackelford's Surgery of the Alimentary Tract (Seventh Edition), 2013 Pathophysiology The relaxant effects of CGRP appear to be closely linked to K +-channel activation and to increased production of cAMP within vascular smooth muscle.ĭavid E. Additionally, CGRP may inhibit neurogenic vasoconstriction via a prejunctional effect on adrenergic neurons. Depending on the vascular bed, CGRP causes relaxation either through direct action on the vascular smooth muscle or through an endothelium-dependent mechanism. 28 It is present in perivascular nerves of the brain and several peripheral organs including the heart. Calcitonin Gene-Related PeptideĬalcitonin gene-related peptide (CGRP) is localized in sensory neurons, often together with substance P, and has strong vasodilator properties. In some blood vessels, relaxations induced by VIP may be endothelium dependent. It also is a potent vasodilator and contributes to the atropine-resistant vasodilation observed during parasympathetic-nerve stimulation. Vasoactive intestinal polypeptide (VIP) is present in postganglionic cholinergic neurons innervating many tissues. The relaxation elicited by substance P is endothelium dependent (see later), which raises the possibility that endothelial cells may contribute to the vasodilation that accompanies axon reflexes. It is a potent vasodilator, although in high doses the peptide causes constriction of some blood vessels. Substance P is widely distributed in sensory nerve endings localized to the adventitia of large and small blood vessels.
16 However, functional significance of these fibers remains to be determined. Neurons releasing substance P, vasoactive intestinal peptide (VIP), and calcitonin gene-related peptide (CGRP) are detected in some cerebral and human epicardial coronary arteries and veins. Miller, in Vascular Medicine, 2006 Peptigergic Nerves Itch may be exclusively central rather than cutaneous in some conditions, such as cholestasis. Histamine is considered to be the most important mediator of itch, but many other mediators are capable of producing this sensation. Itch is a sensation related to pain and is greatest close to transitions of mucous membranes. 13 A number of chemicals injected into the skin may also elicit pain. Pain may also be induced by pressure greater than 50 g/mm 2 and by disruption of skin. At temperatures below 18☌ and above 45☌, pain is produced. Thermal sensitivity is highest on the face. 1 Changes in temperature of 0.03☌ can be detected, especially if the skin temperature changes faster than 0.007☌/sec. Sensations of cold persist continuously when skin temperature is below 30☌, and sensations of warmth persist continuously when it is above 37☌. The skin has a high sensitivity to rapid mechanical stimulation, with positional movements of less than 1 μm detectable. Inflow of cutaneous sensory information is strongly controlled and modulated by the cerebral cortex. Terminals from a single axon may serve an area as broad as 1 cm 2 and overlap with nerve endings from other axons. Unmyelinated branches from either plexus terminate in nerve endings that may be simple or specialized. 1, 12, 13 Nerve branches enter the dermis from the subcutaneous fat and form both a superficial and a deep nerve plexus. The skin is supplied by both myelinated and unmyelinated branches of spinal nerves.
Sensory nerve endings in the skin can elicit all of the principal sensations: touch, pain, itch, warmth, and cold. Morelli MD, in Color Textbook of Pediatric Dermatology (Fourth Edition), 2007 Cutaneous Nerves
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