C-fibres carries

(D) Pain # Primary afferents are classified by their diameter, degree of myelination, and conduction velocity.> Largest-diameter afferent fibers, A-beta (Ab), respond maximally to light touch and/or moving stimuli; they are present primarily in nerves that innervate the skin. In normal individuals, the activity of these fibers does not produce pain.> There are two other classes of primary afferents: the small-diameter myelinated A-delta (Ad) and the unmyelinated (C fiber) axons.Schallanger & Glassers ClassificationaProprioception, somatic motorbTouch, pressure, motorgMotor to muscle spindledPain, cold, TouchBPreganglionic autonomicCDorsal root: Pain; Temperature, vibration Sympathetic: Post... sympathetic> These fibers are present in nerves to the skin and to deep somatic and visceral structures. Some tissues, such as the cornea, are innervated only by A6 and C fiber afferents.> Most Ad and C fiber afferents respond maximally only to intense (painful) stimuli and produce the subjective experience of pain when they are electrically stimulated; this defines them as primary afferent nociceptors (pain receptors). The ability to detect painful stimuli is completely abolished when conduction in Ad and C fiber axons is blocked.> A delta fibers, or Ad fibers, are a type of sensory fiber.> They are associated with cold and pressure, and as nociceptors stimulation of them is interpreted as fast/first pain information.> They are thinly myelinated, so conduct signals more rapidly than unmyelinated C fibers, but more slowly than other, more thickly myelinated "A" class fibers.> Fibers terminate at Rexed lamina I.> Because of their higher conduction velocity, Ad fibers are responsible for the sensation of a sharp first pain.> They respond to a weaker intensity of stimulus. C fibers respond to stimuli which have stronger intensities and are the ones to account for the slow, dull, longer-lasting, second pain.> C fibers are considered polymodal because they can respond to thermal, mechanical, and chemical stimuli.> C fibers respond to all kinds of physiological changes in the body.> For example, they can respond to hypoxia, hypoglycemia, hypo-osmolarity, the presence of muscle metabolic products, and even light or sensitive touch.> C fiber receptors include: C fiber nociceptors: responsible for the second, burning pain> C fiber warming specific receptors: responsible for warmth> Ultra-slow histamine-selective C fibers: responsible for itch> Tactile C fibers: sensual touch> C mechano- and metabo- receptors in muscles or joints: responsible for muscle exercise, burn and cramp> This variation of input signals calls for a variety of cells of the cortex in lamina 1 to have different modality-selectiveness and morphologies.> These varying neurons are responsible for the different feelings we perceive in our body and can be classified by their responses to ranges of stimuli.> The brain uses the integration of these signals to maintain homeostasis in the body whether it is temperature related or pain related.