Long-term increases in excitability of facial motoneurons and other neurons in and near the facial nuclei after presentations of stimuli leading to acquisition of a Pavlovian conditioned facial movement

Alteration of central motor excitability in a patient with hemimasticatory spasm after treatment with botulinum toxin injections

Hemimasticatory spasm (HMS) is a condition characterized by paroxysmal involuntary contraction of masticatory muscles. We performed an electrophysiological investigation of a single patient with HMS to identify any pathophysiological changes associated with the condition. We identified a delayed M wave and jaw jerk on the affected side and an absent masseteric silent period during spasm. Botulinum toxin injections successfully treated the clinical symptoms and resulted in a significant reduction in the excitability of the blink reflex recovery cycle. These data suggest that HMS may be due to ectopic activity in the motor portion of the trigeminal nerve that is capable of inducing changes in the excitability of central reflex pathways. These changes can be altered by successful treatment with botulinum toxin.

Inactivation of the brachium conjunctivum prevents extinction of classically conditioned eyeblinks

It is well established that the intermediate cerebellum is involved in the acquisition of classically conditioned eyeblink responses (CRs). Recent studies that inactivated the interposed nuclei (IN) demonstrated that blocking the intermediate cerebellum also interrupts CR extinction. Is this extinction deficit related to interrupting the information flow to efferent targets of the IN? To address this question, we inactivated axons of IN neurons in the brachium conjunctivum (BC). This treatment blocked the output of the intermediate cerebellum without directly affecting neurons in the deep cerebellar nuclei. Rabbits were trained in a delay classical conditioning paradigm, using a tone as the conditioned stimulus (CS) and a corneal air puff as the unconditioned stimulus (US). Then, the BC was microinjected with a sodium channel blocker, tetrodotoxin, during 4 extinction sessions in which rabbits were presented only with the CS. Tests performed after the 4-day injection period revealed that CRs did not extinguish in BC inactivation sessions but extinguished at a normal rate in the absence of the drug. CRs were then re-acquired. These data show that the normal flow of information along axons of cerebellar nuclear cells is required for CR extinction.

Gradient of adaptability in four different motor systems performing the same learned motor task in cats

The ability of four different brainstem motoneuron pools to perform a newly acquired motor task was studied in alert cats. A classical conditioning of eyelid responses was carried out in (i). unoperated animals, and in animals with (ii). transection, 180 degrees rotation, and re-suture of the zygomatic facial nerve branch, (iii). a crossed anastomosis of the buccal to the zygomatic facial nerve branch and (iv). a hypoglossal-facial nerve anastomosis. Animals were conditioned with a delay paradigm using a tone (350 ms, 600 Hz, 90 dB) as conditioned stimulus, followed 250 ms later by an air puff (100 ms, 3 kg/cm2) as unconditioned stimulus. Animals with zygomatic nerve rotation performed conditioned responses (CRs) at control rate, with significantly larger amplitude, area and velocity, but a de-synchronized oscillatory pattern. Animals with buccal-zygomatic anastomosis acquired CRs at control rate, but these CRs had significantly smaller amplitude than those of controls and a de-synchronized pattern. Animals with a hypoglossal-facial anastomosis were unable to perform CRs. The trigeminal hyper-reflexia triggered by the axotomy was probably the origin of the large CRs after zygomatic nerve rotation. Trigeminal hyper-reflexia could also contribute to generation of the small CRs recorded after buccal-zygomatic anastomosis. Although trigeminal hyper-reflexia was also present following hypoglossal-facial anastomosis, hypoglossal motoneurons did not reach their firing threshold to perform CRs. In accordance with the embryonic origin of involved motoneurons, animals with buccal-zygomatic and hypoglossal-facial anastomoses moved the ipsilateral eyelid synchronously to mouth-related activities. It is suggested that there is a gradient of adaptability in motoneuron pools forced to perform new motor tasks through foreign muscles, which depends on their embryological origins and functional properties.

The role of interpositus nucleus in eyelid conditioned responses

One of the most widely used experimental models for the study of learning processes in mammals has been the classical conditioning of nictitating membrane/eyelid responses, using both trace and delay paradigms. Mainly on the basis of permanent or transitory lesions of putatively-involved structures, and using other stimulation and recording techniques, it has been proposed that cerebellar cortex and/or nuclei could be the place/s where this elemental form of associative learning is acquired and stored. We have used here an output-to-input approach to review recent evidence regarding the involvement of the cerebellar interpositus nucleus in the acquisition of these conditioned responses (CRs). Eyelid CRs appear to be different in profile, duration, and peak velocity from reflexively-evoked blinks. In addition, CRs are generated in a quantum manner across conditioning sessions, suggesting a gradual neural process for their proper acquisition. Accessory abducens and orbicularis oculi motoneurons have different membrane properties and contribute differently to the generation of CRs, with significant species differences. In particular, facial motoneurons seem to encode eyelid velocity during reflexively-evoked blinks and eyelid position during CRs, two facts suggestive of a differential somatic versus dendritic arrival of specific motor commands for each type of movement. Identified interpositus neurons recorded in alert cats during classical conditioning of eyelid responses show firing properties suggestive of an enhancing role for CR performance. However, as their firing started after CR onset, and because they do not seem to encode eyelid position during the CR, the interpositus nucleus cannot be conclusively considered as the place where this acquired motor response is generated. More information is needed regarding neural signal transformations taking place in each involved neural center, and it its proposed that more attention should be paid to functional states (as opposed to neural sites) able to generate motor learning in mammals. The contribution of feedforward mechanisms normally involved in the processing activities of related centers and circuits, and the possible functional interactions within neural systems subserving the associative strength between the conditioned and unconditioned stimuli, are also considered.

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 approximately 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 approximately 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 approximately 50 ms, and late depolarizations) and extrinsic properties of the motoneuronal pool and of the circuits involved in eye blinks.

Hebbian-like functional plasticity in the auditory cortex of the behaving monkey

In this study, the necessary conditions, including those related to behavior, for lasting modifications to occur in correlated activity ('functional plasticity') were examined in the behaving monkey. Previously, in-vitro studies of neuronal plasticity yielded important information about possible mechanisms of synaptic plasticity, but could not be used to test their functionality in the intact, behaving brain. In-vivo studies usually focused on analysis of the responsiveness of single cells, but did not examine interactions between pairs of neurons. In this study, we combined the two approaches. This was achieved by recording extracellularly and simultaneously the spike activity of several single cells in the auditory cortex of the behaving monkey. The efficacy of neuronal interactions was estimated by measuring the correlation between firing times of pairs of single neurons. Using acoustic stimuli, a version of cellular conditioning was applied when the monkey performed an auditory discrimination task and when it did not. We found that: (i) functional plasticity is a function of the change in correlation, and not of the correlation or covariance per se, and (ii) functional plasticity depends critically on behavior. During behavior, an increase in the correlation caused a short-lasting strengthening of the neuronal coupling efficacy, and a decrease caused a short-lasting weakening. These findings indicate that neuronal plasticity in the auditory cortex obeys a version of Hebb's associative rule under strong behavioral control, as predicted by Thorndike's "Law of Effect".

Evidence of the origin of specific spontaneous head turns during intertrial intervals

Direction and the frequency of spontaneous head movements during the ITIs following forward and backward paired trials were compared to an acquisition of a conditioned orienting (alpha) response directed to the side of the tone source. The head movements were analyzed from video recordings using classification of head turns to preferred and to nonpreferred directions. The results showed a significant increase in the alpha responses during the forward paired conditioning to the preferred direction and rapid extinction during the subsequent backward conditioning sessions. Spontaneous head movements during the ITIs increased to the same preferred direction as the conditioned alpha responses. The results of this experiment suggest that the response initially elicited by the CS can later appear as "spontaneous," instrumental behavior, the form and the nature of which is determined by the characteristics of the conditioned alpha response developing as a result of classical conditioning.

Orbicularis oculi responses to stimulation of nerve afferents from upper and lower limbs in normal humans

A brief mechanical or electrical stimulus to peripheral nerve afferents from the upper and lower limbs elicited a small and inconsistent EMG response of the orbicularis oculi muscles. This response was facilitated when the stimuli were delivered at fixed leading time intervals, of 45-300 ms, with respect to a supraorbital nerve electrical stimulus. Also, the peripheral nerve stimulus modified the conventional blink reflex responses, inducing facilitation of R1 and inhibition of R2. These results suggest a complex processing of sensory inputs from the face and the limbs at the brainstem, where they are probably integrated in a network of interneurons influencing the excitability of facial motoneurons.

Changes in membrane currents during Pavlovian conditioning of single cortical neurons

Single electrode voltage clamp recordings were made during Pavlovian conditioning of single units of the motor cortex of cats. Units that developed a conditioned spike discharge in response to a click conditioned stimulus (CS) after pairing the click with glabella tap and local ionophoretic application of glutamate showed increases in input resistance and reductions of an early outward current induced by depolarizing commands and by return to holding potentials after hyperpolarizing commands. Changes in later currents were also found in some cells. Units that failed to develop a conditioned response did not show these changes. The decreases in membrane currents could contribute to an increased spike discharge in response to the CS as could the increased input resistance observed after conditioning. Conductance changes of this type may serve as engrams by which some forms of memory and learning are expressed across both vertebrate and invertebrate species.

Sustained excitatory synaptic input to motor cortex neurons in awake animals revealed by intracellular recording of membrane potentials

1. Most of the intracellular electrophysiological data on cortical neurons has been obtained in anesthetized or reduced preparations, and differs from observations in awake, intact animals. To determine whether these differences are due to experimental techniques or physiological factors, we recorded membrane potentials intracellularly from motor cortex neurons in chronically prepared cats and monkeys under Nembutal-anesthetized, Halothane-anesthetized, and unanesthetized conditions, or during transitions between anesthetized and awake conditions. 2. Resting membrane potentials were found to depend on the anesthetic state of the animal. Membrane potentials of neurons recorded in awake animals were more depolarized than those recorded in the anesthetized state. In the awake state membrane potentials were all less than -65 mV. 3. The input resistance of neurons recorded in awake animals were significantly smaller than those measured in the anesthetized state. Action potentials recorded in awake animals typically showed an undershoot (i.e. negative values at peak), implying that voltage-dependent conductances may be altered. Undershoot of the action potential was more prominent in pyramidal tract neurons (PTNs) than non-PTNs. 4. These data suggested that in awake animals motor cortex neurons, especially PTNs, receive sustained excitatory synaptic input or neuro-modulatory activities.