Responses of muscle spindles in feline dorsal neck muscles to electrical stimulation of the cervical sympathetic nerve

Exp Brain Res (2005) 165: 328342

DOI 10.1007/s00221-005-2309-7RESEARCH ARTICLEF. Hellstrom S. Roatta J. Thunberg M. PassatoreM. DjupsjobackaResponses of muscle spindles in feline dorsal neck muscles

to electrical stimulation of the cervical sympathetic nerveReceived: 12 January 2005 / Accepted: 8 February 2005 / Published online: 10 May 2005

Springer-Verlag 2005Abstract Previous studies performed in jaw muscles of

rabbits and rats have demonstrated that sympathetic

outow may aect the activity of muscle spindle aerents (MSAs). The resulting impairment of MSA information has been suggested to be involved in the genesis

and spread of chronic muscle pain. The present study

was designed to investigate sympathetic inuences on

muscle spindles in feline trapezius and splenius muscles

(TrSp), as these muscles are commonly aected by

chronic pain in humans. Experiments were carried out in

cats anesthetized with alpha-chloralose. The eect of

electrical stimulation (10 Hz for 90 s or 3 Hz for 5 min)

of the peripheral stump of the cervical sympathetic nerve

(CSN) was investigated on the discharge of TrSp MSAs

(units classied as Ia-like and II-like) and on their responses to sinusoidal stretching of these muscles. In

some of the experiments, the local microcirculation of

the muscles was monitored by laser Doppler owmetry.

In total, 46 MSAs were recorded. Stimulation of the

CSN at 10 Hz powerfully depressed the mean discharge

rate of the majority of the tested MSAs (73%) and also

aected the sensitivity of MSAs to sinusoidal changes of

muscle length, which were evaluated in terms of amplitude and phase of the sinusoidal tting of unitary

activity. The amplitude was signicantly reduced in Ialike units and variably aected in II-like units, while in

general the phase was aected little and not changed

signicantly in either group. The discharge of a smaller

percentage of tested units was also modulated by 3-Hz

CSN stimulation. Blockade of the neuromuscular junctions by pancuronium did not induce any changes in

MSA responses to CSN stimulation, showing that these

responses were not secondary to changes in extrafusal or

fusimotor activity. Further data showed that the sympathetically induced modulation of MSA discharge was

not secondary to the concomitant reduction of muscle

blood ow induced by the stimulation. Hence, changes

in sympathetic outow can modulate the aerent signals

from muscle spindles through an action...