Table 1.

Contractile and membrane properties are equally affected in experimental muscles (nerve block + distal stimulation) and control muscles undergoing natural impulse activity

Muscle treatmentTwitch durationb (msec)RMP (mV)Resistance of action potentials to TTXe(V/sec)Extrajunctional AChRse (femtomoles of125I-αBtx/mg)F-CSA (μm2)% polyneuronal innervation
Nerve block + distal stimulation97.5  ± 7.2576.5  ± 1.173.7  ± 1.0015.9  ± 1.62f2376  ± 929 SDg54.6  ± 5.91
(5) [32–36](3, 60) [32–36](6, 83) [15](5) [15](3, 1763) [32–34](16) [10–34]
78.4  ± 1.58c
(3, 26)
Control (natural impulse activity)96.2  ± 8.3873.3  ± 0.813.2  ± 1.4216.8  ± 3.06f2592  ± 1001 SDg18.3  ± 2.65
(11) [32–36](11, 213) [32–36](4, 52) [15](5) [15](3, 1792) [32–34](34) [10–36]
TTX-block alone161  ± 11.5761.9  ± 2.4d76.7  ± 4.0558.1  ± 3.48598  ± 380 SD53.6  ±  10.01
(5) [32–36](5, 63) [32–36](2, 27)(3) [15](3, 7635) [32–34](12) [10–34]
Stimulation alone2221  ± 998 SDg19.9  ± 4.01
(3, 2357) [32–33](6) [10–15]
Denervationa85.5  ± 4.8869.8  ± 11.67
(7, 25) [15](10) [15]
Normal98.5  ± 6.9575.3  ± 1.050  ± 02.7  ± 0.532194  ± 656 SD
(4)(2, 20)(3, 30)(5)(1, 1333)
  • Fiber cross-sectional area (F-CSA) and percentage of polyneuronal innervation are here repeated for reference. First and second figures in parentheses are number of muscles and fibers, respectively. Figures in square brackets are number of days after section of the original soleus axons.

  • aDenervated regions of experimental and control muscles.

  • bWith reference to the values of experimental and control muscles, note that at times longer than those explored here, twitches faster than those of normal soleus have been evoked through the foreign nerve (Lømo and Waerhaug, 1985), a change induced by reinnervation of a slow muscle by mostly fast motoneurons: the development of this change takes time, also because of the progressive character of muscle reinnervation. With reference to the value of TTX-blocked muscles, prolonged values are usually observed in these as well as in denervated muscles, including soleus at times corresponding to those used here (Buffelli et al., 1997).

  • cPolyneuronally innervated fibers only.

  • dTTX-blocked muscles are known to have the low RMPs typical of denervated muscles (Cangiano et al., 1977).

  • eTTX-resistance and extrajunctional AChRs develop in parallel in denervated and disused myofibers, both in a graded manner (Redfern and Thesleff, 1971; Lømo and Westgaard, 1975; Pasino et al., 1996). In denervated soleus muscles they start ∼2 d after nerve section, rise to a maximum, and gradually return to normal after reinnervation by the fibular nerve (Lømo and Slater, 1980a). TTX resistance is given by the maximum rate of rise of spikes under TTX 10−6m. For chronic stimulation in these experiments, we used the main patterns of this study (three patterns, two rats each: 86,400, 34,560, and 11,520 pulses per day; 80, 15, and 80 Hz in the train, respectively).

  • fPresence in the muscle fragments measured in the gamma counter of some fiber layers underneath the superficial one, which are more recently reinnervated, partly explains why the values, though low, have not yet reached those of normal muscles.

  • gOnly large, putatively innervated fibers underneath the foreign nerve implant (see Fig. 5a).