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 orbi cularis 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 electro myographic activity of the orbicularis oculi muscle also Here recorded. Ani mals were conditioned by short, weak air puffs or 350-ms tones as condition ed stimuli (CS) and long, strong air puffs as unconditioned stimulus (US) u sing both trace and delayed conditioning paradigms. Motoneurons wen: identi fied by antidromic activation from their respective cranial nerves. Orbicul aris 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 t he electrical stimulation of the supraorbital nerve. Orbicularis oculi, but not accessory abducens, motoneurons fired in response to hash and tone pre sentations. Only 10-15% of recorded abducens motoneurons fired a late, weak burst after air puff, supraorbital nerve, and flash stimulations. Spontane ous fasciculations of the orbicularis oculi muscle and the activity of sing le orbicularis oculi motoneurons that generated them also wee recorded. The activation of orbicularis oculi motoneurons during the acquisition of clas sically conditioned eyelid responses happened in a gradual, sequential mann er. Initially, some putative excitatory synaptic potentials were observed i n 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 posi tion traces. No accessory abducens motoneuron fired and no abducens motoneu ron modified their discharge rate for conditioned eyelid responses. The fir ing of orbicularis oculi motoneurons was related Linearly to lid velocity d uring reflex blinks but to lid position during conditioned responses, a fac t indicating the different neural origin and coding of both types of motor commands. The power spectra of both reflex and conditioned lid responses sh owed a dominant peak at approximate to 20 Hz. The wavy appearance of both r eflex and conditioned eyelid responses was clearly the result of the high p hasic activity of orbicularis oculi motor units. Orbicularis oculi motoneur on membrane potentials oscillated at approximate to 20 Hz after supraorbita l nerve stimulation and during other reflex and conditioned eyelid movement s. The oscillation seemed to be the result of both intrinsic (spike afterhy perpolarization lasting approximate to 50 ms, and late depolarizations) and extrinsic properties of the motoneuronal pool and of the circuits involved in eye blinks.