Antidromic and synaptic activation of the facial nucleus of cat

Afferent projections in the spinal accessory nerve to the facial motoneurons of the cat

Stimulation of the accessory nerve evoked polysynaptic excitatory postsynaptic potentials (EPSPs) in the facial nucleus (FN) neurons of anesthetized cats. From the experiments with severance of C1-C3 dorsal roots, it is suggested that accessory afferents enter the brainstem through the accessory nerve. It was also found that stimulation of the solitary tract nucleus produced exclusively monosynaptic EPSPs in the FN neurons and the afferent volleys are most likely to be relayed at the solitary tract nucleus.

Hemifacial spasm: ephaptic transmission or hyperexcitability of the facial motor nucleus?

Hemifacial spasm is a rare disorder that is caused by a blood vessel cross-compressing the facial nerve near the nerve's entry into the brain stem. Electrical stimulation of one branch of the facial nerve on the affected side in patients elicits a response from muscles that are innervated by another branch. Intraoperative recording of this abnormal muscle response, together with measurements of conduction times in parts of the facial nerve, in earlier studies have shown evidence that the cross-transmission of antidromic activity that causes this response occurs central to the site of vascular compression of the facial nerve. In this study we show that in rare circumstances during microvascular decompression operations, a small EMG potential precedes the regular abnormal muscle response. This early response has a latency that is equal to the sum of the conduction times of the portions of the facial nerve that would have been involved if the cross-transmission had occurred at the site of vascular compression of the facial nerve, and it may thus be a result of ephaptic transmission in the nerve at that location. Thus, cross-transmission of antidromic activity can indeed occur in the facial nerve at the site of compression, although we have observed this early potential in only 3 of more than 50 patients who were operated upon for hemifacial spasm; further, in those three patients it occurred only for a short time and only during surgical manipulation of the facial nerve. In view of the fact that this early response occurs only after the nerve has been manipulated, and because it is of such small magnitude, it seems unlikely that such cross-transmission (ephaptic transmission) at the site of the vascular compression plays a direct role in generating the spasm and the antidromic activity that spreads from one branch of the facial nerve to another.

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.

Facial nerve evoked potentials

Evoked potentials representing both intracranial and extracranial activity have been recorded from the canine scalp after retrograde stimulation of peripheral facial nerve branches. A negative electrode at the vertex and a positive electrode at the ipsilateral mastoid (grounded at the contralateral mastoid) show a monophasic negative stylomastoid evoked potential that appears to be generated from the facial nerve trunk at the stylomastoid foramen. When the positive electrode is moved from the mastoid to the posterior scalp (overlying the foramen magnum), a similar potential with a latency 1.5 msec greater is obtained. This brain stem evoked potential is generated from fibers in the vicinity of the root entry zone and the initial trajectory into the brain stem. A potential of similar latency but greater magnitude is recorded with the positive electrode in the cerebrospinal fluid of the posterior fossa.

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.

Mechanisms regulating the activity of facial nucleus motoneurones--2. Synaptic activation from the caudal trigeminal nucleus

Field and postsynaptic potentials of facial motoneurones evoked by stimulation of the caudal trigeminal nucleus were studied in cats by means of extra- and intracellular recording. Mono- and polysynaptic input onto facial motoneurones from the caudal trigeminal nucleus were shown. Four types of responses were distinguished: excitatory postsynaptic potentials generating a single action potential; a gradual shift of depolarization inducing multiple discharges; a rhythmic discharge of action potentials appearing at a low level of depolarization; excitatory postsynaptic potentials or a sequence of excitatory and inhibitory postsynaptic potentials. Multiple discharge was shown to appear as a result of effective summation of high frequency excitatory influences from efferent neurones of the caudal trigeminal nucleus projecting into the facial nucleus. Factors facilitating the development of gradual depolarization are: dendritic localization of synaptic terminals, dendritic origin of after-depolarizing processes and the high input resistance of the facial motoneurone membrane. It is thought that specific features of facial motoneurones and properties of afferent inputs are supposed to provide high sensitivity of neuronal organization of the facial nucleus to afferent signals as well as wide diversity in controlling its activity.

Distribution of sensory ganglion cells innervating facial muscles in the cat. An anatomical study with the horseradish peroxidase technique

The question of a possible sensory component in branches of the facial nerve innervating facial mimetic muscles in the cat was examined by the technique of retrograde axonal transport of horseradish peroxidase (HRP). HRP was applied to the proximal cut end of facial nerve branches innervating different facial muscle groups. Following survival periods of 71-75 h the animals were fixed by perfusion. Certain craniospinal sensory ganglia and the brain stem were processed histochemically for demonstration of HRP. HRP-labelled cell bodies, structurally resembling sensory neurons, were consistently observed ipsilaterally in the geniculate and proximal vagal ganglia and under certain conditions in the trigeminal ganglion. Measurements of HRP-labelled neurons in the geniculate and proximal vagal ganglia showed a wide size range but a unimodal size distribution with peaks in the small size range. These findings support the view that facial nerve branches innervating the mimetic muscles contain different types of sensory fibers.

The seventh cranial nerve of the rat. Visualization of efferent and afferent pathways by cobalt precipitation

The cobalt sulphide precipitation technique, in conjunction with Timm's intensification procedure, was used to delineate the afferent and efferent intramedullary pathways of the seventh cranial nerve complex in the rat. The branchial motor nucleus with the accompanying first part, genu, and second part of the root are described. The motor branches to the superficial facial musculature do not contain fibres of geniculate ganglion origin or fibres which terminate in the spinal trigeminal nucleus. The motor branches to the deep facial muscles arise from the dorsal part of the branchial motor nucleus and traverse to the midline medial to the genu, then project under the genu into the lateral reticular formation before exiting with the facial nerve. The salivatory and lacrimal nuclei and their intramedullary pathways are described. Sensory fibres from the cutaneous auricular branch enter the spinal trigeminal tract and most of the chorda tympani gustatory fibres enter the fasciculus solitarius. A smaller number of gustatory fibres extend medially to the region of the salivatory nucleus. Fibres of greater superficial petrosal origin also enter the fasciculus solitarius as well as the medial reticular formation. These findings are discussed in relation to previous anatomical, physiological and clinical reports.