Rubral inputs to the facial motoneurons in cat

Comparative anatomy of the facial motor nucleus in mammals, with an analysis of neuron numbers in primates

The facial motor nucleus (VII) contains motoneurons that innervate the facial muscles of expression. In this review, the comparative anatomy of this brainstem nucleus is examined. Several aspects of the anatomical organization of the VII appear to be common across mammals, such as the distribution of neuron types, general topography of muscle representation, and afferent connections from the midbrain and brainstem. Phylogenetic specializations are apparent in the proportion of neurons allocated to the representation of subsets of muscles and the degree of differentiation among subnuclei. These interspecific differences may be related to the elaboration of certain facial muscles in the context of socioecological adaptations such as whisking behavior, sound localization, vocalization, and facial expression. Furthermore, current evidence indicates that direct descending corticomotoneuron projections in the VII are present only in catarrhine primates, suggesting that this connectivity is an important substrate for the evolution of enhanced mobility and flexibility in facial expression. Data are also presented from a stereologic analysis of VII neuron numbers in 18 primate species and a scandentian. Using phylogenetic comparative statistics, it is shown that there is not a correlation between group size and VII neuron number (adjusted for medulla volume) among primates. Great apes and humans, however, display moderately more VII neurons that expected for their medulla size.

Neuronal premotor networks involved in eyelid responses: retrograde transneuronal tracing with rabies virus from the orbicularis oculi muscle in the rat

Retrograde transneuronal tracing with rabies virus from the right orbicularis oculi muscle was used to identify neural networks underlying spontaneous, reflex, and learned blinks. The kinetics of viral transfer was studied at sequential 12 hr intervals between 3 and 5 d after inoculation. Rabies virus immunolabeling was combined with the immunohistochemical detection of choline acetyltransferase expression in brainstem motoneurons or Fluoro-Ruby injections in the rubrospinal tract. Virus uptake involved exclusively orbicularis oculi motoneurons in the dorsolateral division of the facial nucleus. At 3-3.5 d, transneuronal transfer involved premotor interneurons of trigeminal, auditory, and vestibular reflex pathways (in medullary and pontine reticular formation, trigeminal nuclei, periolivary and ventral cochlear nuclei, and medial vestibular nuclei), motor pathways (dorsolateral quadrant of contralateral red nucleus and pararubral area), deep cerebellar nuclei (lateral portion of interpositus nucleus and dorsolateral hump ipsilaterally), limbic relays (parabrachial and Kölliker-Fuse nuclei), and oculomotor structures involved in eye-eyelid coordination (oculomotor nucleus, supraoculomotor area, and interstitial nucleus of Cajal). At 4 d, higher order neurons were revealed in trigeminal, auditory, vestibular, and deep cerebellar nuclei (medial, interpositus, and lateral), oculomotor and visual-related structures (Darkschewitsch, nucleus of the posterior commissure, deep layers of superior colliculus, and pretectal area), lateral hypothalamus, and cerebral cortex (particularly in parietal areas). At 4.5 and 5 d the labeling of higher order neurons occurred in hypothalamus, cerebral cortex, and blink-related areas of cerebellar cortex. These results provide a comprehensive picture of the premotor networks mediating reflex, voluntary, and limbic-related eyelid responses and highlight potential sites of motor learning in eyelid classical conditioning.

Electrical properties of facial motoneurons in brainstem slices from guinea pig

Electrical properties of guinea pig facial motoneurons (FMNs) were studied in a brainstem slice preparation. FMNs were identified histologically and by antidromic activation. They displayed time-varying responses and inward rectification during both subthreshold depolarization and hyperpolarization. The depolarizing rectification was caused by a persistent Na+ current (INaP); the Cs+-sensitive hyperpolarizing response had a different mechanism. Hyperpolarizing prepulses caused a 4-aminopyridine-sensitive delay of spike initiation. An evoked spike was followed by a fast- and a medium-duration hyperpolarization (the fAHP and mAHP, respectively). Blockade of Ca2+ influx abolished the mAHP without affecting spike duration, whereas spikes were prolonged and the fAHP was abolished by TEA or 4-AP. Adequate depolarization evoked tonic repetitive firing characterized by a steep F-I slope and fast adaptation. Abolition of the mAHP was associated with reduced fast adaptation and increased F-I slope, whereas the mAHP was enhanced and firing rate was slowed after TEA application. Three outward ionic currents were identified during voltage clamp: a rapidly inactivating current, a slowly inactivating current and a slow persistent Ca2+-mediated current (IK(Ca]. We conclude that spike repolarization and the fAHP are governed mainly by fast voltage-dependent currents, whereas progressive activation of IK(Ca) causes fast adaptation and, together with INaP, regulates firing rate.

Mechanisms regulating the activity of facial nucleus motoneurons--III. Synaptic influences from the cerebral cortex and subcortical structures

Peculiarities of synaptic processes in facial motoneurons evoked by stimulation of various regions of the cerebral cortex and subcortical structures were studied in acute experiments on cats by intracellular recording technique. Stimulation of the motor cortex as well as gyrus proreus and pyramidal tract was shown to evoke polysynaptic excitatory and inhibitory postsynaptic potentials in facial motoneurons. Stimulation of the lateral hypothalamus produced exclusively excitatory polysynaptic effects. It was also found that stimulation of the head of nucleus caudatus and globus pallidus evokes a polysynaptic activation in facial motoneurons, while stimulation of nucleus amygdala centralis leads to mono- and polysynaptic excitation of these neurons. Convergence of the above effects on the same motoneurons is shown to exist. Possible pathways and mechanisms of descending influences on the activity of facial motoneurons is discussed.

Mechanisms regulating the activity of facial nucleus motoneurons--IV. Influences from the brainstem structures

Peculiarities of synaptic processes of facial motoneurons evoked by stimulation of brainstem structures were studied in acute experiments on anaesthetized and immobilized cats by intracellular recording technique. It was shown that stimulation of nucleus reticularis parvocellularis of the medulla oblongata as well as interstitial nucleus of Cajal, nucleus Darkschewitsch, periaqueductal gray and pretectal area evokes in facial motoneurons monosynaptic excitatory postsynaptic potentials accompanied by single action potentials. Somatic localization of synapses under study is supposed. Convergence of inputs is revealed. Functional role of the midbrain structures as intermediate relays transmitting descending signals to the facial nucleus is discussed.

Red nucleus inputs to retractor bulbi motoneurones in the cat

Rubral projections to retractor bulbi motoneurones were investigated by intracellular recording in the accessory abducens nucleus in the cat. Their post-synaptic effects were compared with those of trigeminal afferent pathways. Post-synaptic potentials evoked by stimulation of the contralateral red nucleus were predominantly excitatory, with latencies in the monosynaptic range. Post-synaptic potentials evoked by stimulation of the ipsilateral red nucleus, at threshold, were predominantly inhibitory with latencies indicating a di- or trisynaptic pathway. At above threshold intensity, an earlier excitatory post-synaptic potential (e.p.s.p.) preceded the inhibitory post-synaptic potential (i.p.s.p.) in about half the recorded neurones. Convergence of rubral and trigeminal inputs was also observed in other neurones, not identified as retractor bulbi motoneurones, but situated within the accessory abducens nucleus.

Peculiarities of cerebellar excitation of facial nucleus motoneurons

Excitatory postsynaptic potentials (EPSPs) of facial nucleus motoneurons evoked by stimulation of the cerebellar nuclei, were studied in anesthetized cats. It is shown that the stimulation of nucleus interpositus and dentatus evoked mono-, oligo- and polysynaptic EPSPs in facial motoneurons. From the experiments with lesions in the red nucleus it is suggested that this structure is implicated in the mechanisms and pathways through which the cerebellar control over facial nucleus motoneurons is realized.

Distribution and peculiarities of axon collateral branching of rubro-spinal neurones in brainstem structures

Peculiarities of axon branching and distribution of rubro-spinal neurones in various brainstem structures were studied in acute cats using the technique of intracellular recording of antidromic action potentials as well as collision testing. Axon collaterals of rubro-spinal neurones into the main sensory trigeminal nucleus, facial nerve nucleus, descending (inferior) vestibular nucleus, lateral reticular nucleus, external cuneate nucleus, gracile and main cuneate nuclei were identified. Correlation between the antidromic impulse conduction time along the stem axon before and after collateral branching and the time of impulse conduction in the collaterals themselves was analysed. The number of axon collaterals of individual rubro-spinal neurones to particular brainstem structures was studied. A tendency was observed for the synchronous arrival of rubro-spinal impulses to various brainstem centres, due to an increase in conductance velocity the further away these centres were from the red nucleus.

The facial nucleus of the rat: representation of facial muscles revealed by retrograde transport of horseradish peroxidase

The representation of facial muscle groups in the facial nucleus of rat was examined by retrograde transport of HRP. Motoneurons supplying muscle groups are arranged in longitudinal columns. Those supplying nasolabial muscles are located in the lateral and ventral intermediate segments, posterior auricular muscles in a medial column, platysma in an intermediate column; the lower lip and ocular muscles are in the ventral and dorsal segments respectively of the intermediate column. The posterior belly of the digastric muscle is supplied by motoneurons extending from the dorsal aspect of the facial nucleus to the caudal pole of the trigeminal motor nucleus.

Mechanisms regulating the activity of facial nucleus motoneurones--1. Antidromic activation

The antidromic activation of facial nucleus montoneurones has been studied in acute experiments on cats by means of extra- and intracellular recording techniques. Time and amplitude characteristics of separate components of the antidromic action potential and the after--potentials accompanying it have been analyzed. A correlation is found between the duration of the falling phase of the soma-dendritic component of the action potential, the duration of its after-hyperpolarization and impulse conduction time in the axon. The dendritic origin of after-depolarizing processes is shown. It is concluded that, since there is no recurrent collateral pathway in facial motoneurones, the modulating effect of spike after-potentials on the proper excitability of facial motoneurones acquires great functional significance.

Serotonergic facilitation of facial motoneuron excitation

The effect of serotonin (5-HT) on motoneurons located in the facial nucleus of the rat was investigated in the present study. Microiontophoretic application of 10--200 nA pulses of 5-HT lasting from 1 to 10 min failed to excite facial motoneurons. However, small amounts of 5-HT facilitated the subthreshold and threshold excitatory effects of iontophoretically applied glutamate on these cells. Typically, the current of glutamate required to produce an activation of facial motoneurons was reduced by at least 50% in the presence of 5-HT. In addition, 5-HT markedly shifted to the left the cumulative dose-response curve of glutamate-induced excitation of motoneurons. The 5-HT releasing agent p-chloroamphetamine (PCA) facilitated the excitatory effects of glutamate on montoneurons in control animals, but not in those pretreated with the 5-HT also facilitated the subthreshold and threshold excitation of motoneurons produced by stimulation of the motor cortex and the red nucleus. The facilitating effect of 5-HT was blocked by methysergide. Norepinephrine also facilitated facial motoneuron excitation but this effect was not blocked by methysergide. It is concluded that 5-HT in the facial nucleus functions in a manner that is not analagous to direct excitation, but rather acts as a gain setter to enhance the effects of excitatory afferent inputs.