title: Mechano- and thermosensitivity of injured muscle afferents.
abstract: Injury of limb nerves leading to neuropathic pain mostly affects deep somatic nerves including muscle nerves. Here, we investigated the functional properties of injured afferent fibers innervating the lateral gastrocnemius-soleus muscle 4-13 h [time period (TP) I] and 4-7 days (TP II) after nerve crush in anesthetized rats using neurophysiological recordings from either the sciatic nerve (165 A-, 137 C-fibers) or the dorsal root L(5) (43 A-, 28 C-fibers). Ongoing activity and responses to mechanical or thermal stimulation of the injury site of the nerve were studied quantitatively. Of the electrically identified A- and C-fibers, 5 and 38% exhibited ectopic activity, respectively, in TP I and 51 and 61%, respectively, in TP II. Thus all afferent fibers in an injured muscle nerve developed ectopic activity since ∼ 50% of the fibers in a muscle nerve are somatomotor or sympathetic postganglionic. Ongoing activity was present in 50% of the afferent A-fibers (TP II) and in 53-56% of the afferent C-fibers (TP I and II). In TP II, mechanical, cold, and heat sensitivity were present in 91, 63, and 52% of the afferent A-fibers and in 50, 40, and 66% of the afferent C-fibers. The cold and heat activation thresholds were 5-27 and 35-48°C, respectively, covering the noxious and innocuous range. Most afferent fibers showed combinations of these sensitivities. Mechano- and cold sensitivity had a significantly higher representation in A- than in C-fibers, but heat sensitivity had a significantly higher representation in C- than in A-fibers. These functional differences between A- and C-fibers applied to large- as well as small-diameter A-fibers. Comparing the functional properties of injured muscle A- and C-afferents with those of injured cutaneous A- and C-afferents shows that both populations of injured afferent neurons behave differently in several aspects.
authors.affiliation: Physiologisches Institut, Christian-Albrechts-Universität zu Kiel, Olshausenstrasse 40, 24098 Kiel, Germany.
authors.forename: Irina
authors.initials: I
authors.lastname: Kirillova
authors.forename: Vanessa H
authors.initials: VH
authors.lastname: Rausch
authors.forename: Jan
authors.initials: J
authors.lastname: Tode
authors.forename: Ralf
authors.initials: R
authors.lastname: Baron
authors.forename: Wilfrid
authors.initials: W
authors.lastname: Jänig
mesh_terms: D000818:Animals; D003080:Cold Temperature / Q000009:adverse effects; D006358:Hot Temperature / Q000009:adverse effects; D008297:Male; D008465:Mechanoreceptors / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology; D009409:Nerve Crush / Q000379:methods; D009413:Nerve Fibers, Myelinated / Q000502:physiology*; D036421:Nerve Fibers, Unmyelinated / Q000502:physiology*; D009437:Neuralgia / Q000503:physiopathology*; D009475:Neurons, Afferent / Q000502:physiology*; D010812:Physical Stimulation / Q000379:methods; D051381:Rats; D017208:Rats, Wistar; D013697:Thermosensing / Q000502:physiology*