Mechanosensitivity of peripheral nociceptors in the normal joint and muscle

In single-fiber recordings, primary afferent neurons have been classified according to their mechanosensitivity. These recordings showed types of primary afferent neurons that encode noxious stimuli applied to joint and muscle and are thus suitable for signaling noxious

Thalamocortical systems

Thalamocortical systems

Figure 2.1 Sequence of neuronal events induced by inflammation in deep tissue.

mechanical events which cause pain sensations in awake individuals.

In the joint nerve, more than 50 percent of the Ad-fibers and most C-fibers with a detectable receptive field are able to encode noxious mechanical stimuli applied to the joint. These fibers are either weakly activated by innocuous stimuli and strongly activated by noxious stimuli, or they are exclusively activated by noxious stimuli.21,22 Innocuous stimuli are light to moderate pressure and movements within the normal working range of the joint. Noxious stimuli are high-intensity pressure (that causes pain when applied to humans) and movements against the resistance of the tissue beyond the limit of the normal working range. Fibers activated by these noxious mechanical stimuli are thought to be the nociceptors which cause pain upon twisting the normal joint against the resistance of the tissue. Some A8-fibers and a significant proportion of C-fibers do not respond to any mechanical stimulus applied to the normal joint. These fibers are "initially mechanoinsensitive'' or "silent nociceptors'' that are only activated during inflammation (see below under Changes of mechanosensitivity during inflammation (peripheral sensitization)).23,24,25 In contrast, most Ap-fibers and about half of the Ad-fibers are low threshold units that are strongly activated by innocuous pressure, such as light pressure and movements in the working range.26, 27 Their responses to innocuous stimuli might be used to control movements and to prevent unphysiological movements. Although these units may show their highest discharge rate upon noxious stimuli, they do not discriminate between innocuous and noxious stimuli. In fact, the most adequate innocuous mechanical stimulus can evoke a stronger response than a noxious mechanical stimulus, e.g. a noxious movement into another direction.26, 27

In the muscle nerve, numerous sensory Ad- and C-fibers are only activated by noxious mechanical stimuli. These muscle nociceptors do not respond to everyday stimuli, such as weak local pressure, contractions, and muscle stretch within the physiological range. They require potentially noxious stimuli to be readily activated, and the best stimulus is noxious squeezing of the muscle belly or tendon at intensities that elicit pain in humans. Nociceptors may also respond to unphysiological stretch and maximal contraction. The threshold of a nociceptor may lay below frankly tissue-damaging intensities (small response to moderate pressure). Similar units have been found in the cat, dog, rat, and humans.28,2930,31,32,33 Electrical stimulation of such nerve fibers in human muscle nerves evokes cramp-like sensations. Electrical stimulation frequencies of 5-6 Hz are required to elicit pain sensations.34 Why such high frequencies are needed is unknown. It may be speculated that a small number of muscle afferents drive spinal cord neurons only during temporal facilitation, either because they form fewer synapses on neurons than cutaneous afferents or because descending inhibition of nociceptive spinal cord neurons is stronger for deep input than for cutaneous input (see below under Descending influences on spinal neurons with deep input).

Only a proportion of the sensory units with free nerve endings in muscle nerves are nociceptors. Other slowly conducting units are more sensitive and respond strongly to physiological stimuli, such as stretch and contraction. These low threshold units are considered to be ergo-receptors. Presumably they are important for respiratory and circulatory adjustments during physical exercise.7 In addition, as in the joint nerve, some units are mechanoinsensitive and may thus be silent nociceptors, because they respond to intra-arterial injection of bradykinin into the muscle.

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