Extrafusal and intrafusal muscle effects in experimental botulinum toxin-A injection

Muscle Nerve. 1996 Apr;19(4):488-96. doi: 10.1002/(SICI)1097-4598(199604)19:4<488::AID-MUS9>3.0.CO;2-8.


The effects of botulinum toxin-A was compared on both extrafusal and intrafusal muscle fibers in the biceps femoris of Wistar rats. Four days after injection no action potentials were elicited with stimulation single-fiber electromyography on the injected side. Fourteen days after injection, jitter became measurable and these values were increased on the injected side. Extrafusal muscle fibers began to atrophy on the 4th day and this continued to the 14th day postinjection. Atrophy was also evident and progressive in intrafusal muscle fibers. Increased terminal innervation ratios, end-plate spread of cholinesterase, and increased density of very small myelinated fibers in large intramuscular nerves were observed 14 days postinjection. Both extrafusal and intrafusal fibers are cholinergically innervated, and both were progressively affected by botulinum toxin, perhaps varying in degree only. In addition to partial denervation, Botulinum toxin effects in dystonia may also be related to modified spindle afferent discharge.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenosine Triphosphatases / metabolism
  • Animals
  • Botulinum Toxins / pharmacology*
  • Electromyography
  • Electrophysiology
  • Injections, Subcutaneous
  • Muscle Fibers, Skeletal / drug effects
  • Muscle Fibers, Skeletal / metabolism
  • Muscle Fibers, Skeletal / ultrastructure
  • Muscle Spindles / drug effects
  • Muscle Spindles / metabolism
  • Muscles / drug effects*
  • Muscles / innervation
  • Muscles / physiology
  • Nervous System / drug effects
  • Nervous System / pathology
  • Rats
  • Rats, Wistar


  • Botulinum Toxins
  • Adenosine Triphosphatases