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The Journal of Neurophysiology Vol. 81 No. 4 April 1999, pp. 1666-1684
Copyright ©1999 by the American Physiological Society
Laboratorio de Neurociencia, Facultad de Biología, Universidad de Sevilla, 41012 Seville, Spain
Trigo, José A.,
Agnès Gruart, and
José M. Delgado-García.
Discharge profiles of abducens, accessory abducens, and
orbicularis oculi motoneurons during reflex and conditioned blinks in
alert cats. The discharge profiles of identified abducens, accessory abducens, and orbicularis oculi motoneurons have been recorded extra- and intracellularly in alert behaving cats during spontaneous, reflexively evoked, and classically conditioned eyelid responses. The movement of the upper lid and the electromyographic activity of the orbicularis oculi muscle also were recorded. Animals were conditioned by short, weak air puffs or 350-ms tones as
conditioned stimuli (CS) and long, strong air puffs as unconditioned
stimulus (US) using both trace and delayed conditioning paradigms.
Motoneurons were identified by antidromic activation from their
respective cranial nerves. Orbicularis oculi and accessory abducens
motoneurons fired an early, double burst of action potentials (at 4-6
and 10-16 ms) in response to air puffs or to the electrical
stimulation of the supraorbital nerve. Orbicularis oculi, but not
accessory abducens, motoneurons fired in response to flash and tone
presentations. Only 10-15% of recorded abducens motoneurons fired a
late, weak burst after air puff, supraorbital nerve, and flash
stimulations. Spontaneous fasciculations of the orbicularis oculi
muscle and the activity of single orbicularis oculi motoneurons that
generated them also were recorded. The activation of orbicularis oculi
motoneurons during the acquisition of classically conditioned eyelid
responses happened in a gradual, sequential manner. Initially, some
putative excitatory synaptic potentials were observed in the time
window corresponding to the CS-US interval; by the second to the fourth conditioning session, some isolated action potentials appeared that
increased in number until some small movements were noticed in eyelid
position traces. No accessory abducens motoneuron fired and no abducens
motoneuron modified their discharge rate for conditioned eyelid
responses. The firing of orbicularis oculi motoneurons was related
linearly to lid velocity during reflex blinks but to lid position
during conditioned responses, a fact indicating the different neural
origin and coding of both types of motor commands. The power spectra of
both reflex and conditioned lid responses showed a dominant peak at
20 Hz. The wavy appearance of both reflex and conditioned eyelid
responses was clearly the result of the high phasic activity of
orbicularis oculi motor units. Orbicularis oculi motoneuron membrane
potentials oscillated at
20 Hz after supraorbital nerve stimulation
and during other reflex and conditioned eyelid movements. The
oscillation seemed to be the result of both intrinsic (spike
afterhyperpolarization lasting
50 ms, and late depolarizations) and
extrinsic properties of the motoneuronal pool and of the circuits
involved in eye blinks.
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