Anatomical connections of the ventral, but not the dorsal part of the striatum with the parvicellular reticular formation: Implications for the anatomical substrate of oral movements

Cortical involvement in the StartReact effect

The rapid release of prepared movements by a loud acoustic stimulus capable of eliciting a startle response has been termed the StartReact effect (Valls-Solé et al., 1999), and premotor reaction times (PMTs) of <70 ms are often observed. Two explanations have been given for these short latency responses. The subcortical storage and triggering hypothesis suggests movements that can be prepared in advance of a "go" signal are stored and triggered from subcortical areas by a startling acoustic stimulus (SAS) without cortical involvement. Alternatively, it has been hypothesized that the SAS can trigger movements from cortical areas through a faster pathway ascending from subcortical structures. Two experiments were designed to examine the possible role of the primary motor cortex in the StartReact effect. In Experiment 1, we used suprathreshold transcranial magnetic stimulation (TMS) during the reaction time (RT) interval to induce a cortical silent period in the contralateral primary motor cortex (M1). Thirteen participants performed 20° wrist extension movements as fast as possible in response to either a control stimulus (82 dB) or SAS (124 dB). PMTs for startle trials were faster than for control trials, while TMS significantly delayed movement onset compared to No TMS or Sham TMS conditions. In Experiment 2, we examined the StartReact effect in a highly cortically represented action involving speech of a consonant-vowel (CV) syllable. Similar to previous work examining limb movements, a robust StartReact effect was found. Collectively, these experiments provide evidence for cortical (M1) involvement in the StartReact effect.

Behavioral pharmacology of orofacial movement disorders

Dysfunction in orofacial movement is evident in patients with schizophrenia, Parkinson's disease and Huntington's disease. In animal studies on orofacial dyskinesia, these neurological disorders have been considered as a starting point to examine the pathophysiology and mechanisms underlying the symptoms. There is circumstantial evidence that orofacial dyskinesia in humans might be the consequence of hyperfunctioning mesolimbic-pallidal circuitry, in which the mesolimbic region occupies a central role, in contrast to typical Parkinson-like symptoms which involve hypofunction in the nigrostriato-nigral circuity. Studies in animals suffer from technical difficulties concerning the assessment of orofacial behaviors. There are some experimental designs that provide detailed information on the amplitude and the frequency of the jaw movements. By using such methods, the involvement of neurotransmitter systems and functional neural connections within the basal ganglia has been studied in rat rhythmical jaw movements. Regarding neurotransmitter systems, dopaminergic, cholinergic, γ-aminobutyric acid (GABA)ergic and glutamaterigic systems have been shown to be involved in rat rhythmical jaw movements. The involved neural connections have also been investigated, focusing on the differential role between the dorsal and ventral part of the striatum, the shell and core of the nucleus accumbens and the output pathways from the striatum and the nucleus accumbens. Taking available clinical and experimental evidence, the orofacial dyskinesias are thought to arise when hierarchically lower order output stations of the mesolimbic region start to dysfunction as a consequence of the arrival of distorted information sent by the mesolimbic region. This review seeks to provide an overview of prior and recent findings across several orofacial movement disorders and interpret new insights in the context of the limitations of behavioral pharmacology and prior knowledge of the regulation of behavior by dopamine receptors and other related neuronal systems.

Input-output organization of jaw movement-related areas in monkey frontal cortex

The brain mechanisms underlying mastication are not fully understood. To address this issue, we analyzed the distribution patterns of cortico-striatal and cortico-brainstem axon terminals and the origin of thalamocortical and intracortical fibers by injecting anterograde/retrograde tracers into physiologically and morphologically defined jaw movement-related cortical areas. Four areas were identified in the macaque monkey: the primary and supplementary orofacial motor areas (MIoro and SMAoro) and the principal and deep parts of the cortical masticatory area (CMaAp and CMaAd), where intracortical microstimulation produced single twitch-like or rhythmic jaw movements, respectively. Tracer injections into these areas labeled terminals in the ipsilateral putamen in a topographic fashion (MIoro vs. SMAoro and CMaAp vs. CMaAd), in the lateral reticular formation and trigeminal sensory nuclei contralaterally (MIoro and CMaAp) or bilaterally (SMAoro) in a complex manner of segregation vs. overlap, and in the medial parabranchial and Kölliker-Fuse nuclei contralaterally (CMaAd). The MIoro and CMaAp received thalamic projections from the ventrolateral and ventroposterolateral nuclei, the SMAoro from the ventroanterior and ventrolateral nuclei, and the CMaAd from the ventroposteromedial nucleus. The MIoro, SMAoro, CMaAp, and CMaAd received intracortical projections from the ventral premotor cortex and primary somatosensory cortex, the ventral premotor cortex and rostral cingulate motor area, the ventral premotor cortex and area 7b, and various sensory areas. In addition, the MIoro and CMaAp received projections from the three other jaw movement-related areas. Our results suggest that the four jaw movement-related cortical areas may play important roles in the formation of distinctive masticatory patterns.

Role of GABA(A) receptors in the retrorubral field and ventral pallidum in rat jaw movements elicited by dopaminergic stimulation of the nucleus accumbens shell

The role of gamma-aminobutyric acid (GABA)(A) receptors in the retrorubral field in the production of rat repetitive jaw movements was examined, as this nucleus receives a GABAergic, inhibitory input from the nucleus accumbens and is connected with the parvicellular reticular formation, a region that is directly connected with the orofacial motor nuclei. The GABA(A) receptor antagonist bicuculline (150 ng/0.2 microl per side) significantly produced repetitive jaw movements when injected bilaterally into the retrorubral field, but not the ventral pallidum. The effects of bicuculline were GABA(A) receptor specific, because the effects were abolished by muscimol, a GABA(A) receptor agonist, given into the same site. The bicuculline-induced jaw movements differed qualitatively from those elicited by injection of a mixture of (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF 82958; 5 microg) and quinpirole (10 microg), agonist at dopamine D1 and D2 receptors respectively, into the nucleus accumbens shell. Nevertheless, bilateral injections of muscimol (10 ng, 25 ng and 50 ng/0.2 microl per side) into the retrorubral field significantly inhibited jaw movements evoked by the dopamine D1/D2 receptor stimulation in the nucleus accumbens shell. Bilateral injections of bicuculline (50 ng and 150 ng/0.2 microl per side) also reduced the dopamine D1/D2 receptor-mediated jaw movements. Essentially similar effects were obtained when muscimol and bicuculline were given into the ventral pallidum, a region that is also known to receive GABAergic inhibitory inputs from the nucleus accumbens. In conclusion, GABA(A) receptor blockade in the retrorubral field elicits characteristic repetitive jaw movements, and the GABA(A) receptors in that region as well as in the ventral pallidum modulate the accumbens-specific, dopamine D1/D2 receptor-mediated jaw movements.

Projections from bed nuclei of the stria terminalis, posterior division: implications for cerebral hemisphere regulation of defensive and reproductive behaviors

The posterior division of the bed nuclei of the stria terminalis has three major nuclei: principal, interfascicular, and transverse, which receive topographically ordered inputs from the medial amygdalar nucleus. The overall pattern of axonal projections from each nucleus was determined in male rats with the Phaseolus vulgaris-leucoagglutinin method. Together, these nuclei project topographically back to the medial amygdalar nucleus, to the adjacent lateral septal nucleus, to the nucleus accumbens and substantia innominata, to hypothalamic parts of the behavior control column, and to the hypothalamic periventricular region, which controls patterned neuroendocrine and autonomic responses. The principal nucleus preferentially innervates septal and hypothalamic regions that control reproductive behavior and visceromotor responses, confirming a similar analysis by Gu et al. (J Comp Neurol [2003] 460:542-562). In contrast, the interfascicular and transverse nuclei differentially innervate septal and hypothalamic regions that control defensive as well as reproductive behaviors. In addition, the transverse nucleus projects significantly to midbrain parts of the behavior control column concerned with foraging/exploratory behavior. All three posterior division nuclei also project to thalamocortical feedback loops (by means of the nucleus reuniens and paraventricular nucleus). These structural data may be interpreted to suggest that the bed nuclei posterior division forms part (pallidal) of a corticostriatopallidal system involved in controlling two major classes of social (defensive and reproductive) behavior.

Projections from the rhomboid nucleus of the bed nuclei of the stria terminalis: implications for cerebral hemisphere regulation of ingestive behaviors

The basic organization of an exceptionally complex pattern of axonal projections from one distinct cell group of the bed nuclei of the stria terminalis, the rhomboid nucleus (BSTrh), was analyzed with the PHAL anterograde tract-tracing method in rats. Brain areas that receive a strong to moderate input from the BSTrh fall into nine general categories: central autonomic control network (central amygdalar nucleus, descending hypothalamic paraventricular nucleus, parasubthalamic nucleus and dorsal lateral hypothalamic area, ventrolateral periaqueductal gray, lateral parabrachial nucleus and caudal nucleus of the solitary tract, dorsal motor nucleus of the vagus nerve, and salivatory nuclei), gustatory system (rostral nucleus of the solitary tract and medial parabrachial nucleus), neuroendocrine system (periventricular and paraventricular hypothalamic nuclei, hypothalamic visceromotor pattern generator network), orofaciopharyngeal motor control (rostral tip of the dorsal nucleus ambiguus, parvicellular reticular nucleus, retrorubral area, and lateral mesencephalic reticular nucleus), respiratory control (lateral nucleus of the solitary tract), locomotor or exploratory behavior control and reward prediction (nucleus accumbens, substantia innominata, and ventral tegmental area), ingestive behavior control (descending paraventricular nucleus and dorsal lateral hypothalamic area), thalamocortical feedback loops (medial-midline-intralaminar thalamus), and behavioral state control (dorsal raphé and locus coeruleus). Its pattern of axonal projections and its position in the basal telencephalon suggest that the BSTrh is part of a striatopallidal differentiation involved in modulating the expression of ingestive behaviors, although it may have other functions as well.

Efferent subcortical projections of the laryngeal motorcortex in the rhesus monkey

In order to better understand the descending voluntary vocal control pathway, the efferent subcortical projections of the laryngeal motorcortex were studied in the rhesus monkey (Macaca mulatta). For this purpose, the left motorcortex was exposed in three animals under narcosis. By electrical brain stimulation, sites were identified yielding vocal fold adduction. Effective sites were injected with the anterograde tracer biotin dextran amine. Subcortical projections could be traced within the forebrain to the putamen, caudate nucleus, claustrum, zona incerta, field H of Forel and a number of thalamic nuclei, with the heaviest projections to the nuclei ventralis lateralis, ventralis posteromedialis, including its parvocellular part, medialis dorsalis, centralis medialis, centrum medianum and reuniens. In the midbrain, labeling was found in the deep mesencephalic nucleus. In the lower brainstem, fibers terminated in the pontine and medullary reticular formation, locus coeruleus, nucleus subcoeruleus, medial parabrachial nucleus, nucleus of the spinal trigeminal tract, solitary tract nucleus and facial nucleus. No projections were found to the nucl. ambiguus. The fact that monkeys, in contrast to humans, lack a direct connection of the motorcortex with the laryngeal motoneurons suggests that this connection has evolved in the last few million years and might represent one of the factors that made speech evolution possible.

The superior colliculus contains a discrete region involved in the control of jaw movements: role of GABAA receptors

The role of GABA(A) receptors in the superior colliculus in the production of rat repetitive jaw movements was examined, as this nucleus receives tonic GABAergic inhibitory inputs from the dorsolateral part of the substantia nigra pars reticulata and the entopeduncular nucleus. Both regions are also connected with the ventrolateral striatum where stimulation of either dopamine or acetylcholine receptors has been found to elicit distinct types of jaw movements in rats. The GABA(A) receptor antagonist bicuculline (50 and 150 ng/0.2 microl per side) dose-dependently produced repetitive jaw movements only when injected bilaterally into a circumscribed region (A 3.0) of the lateral deeper layers of the superior colliculus; this region is known to receive input predominantly from the dorsolateral part of the substantia nigra pars reticulata. The effects of bicuculline were GABA(A) receptor specific because the effects were abolished by muscimol, a GABA(A) receptor agonist, given into the same site. The bicuculline-induced jaw movements differed qualitatively from those elicited by injection of a mixture of (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF 82958; 5 microg) and quinpirole (10 microg), agonist at dopamine D1 and D2 receptors, respectively, or carbachol (2.5 microg), an acetylcholine receptor agonist, into the ventrolateral striatum. Nevertheless, injection of muscimol into the lateral deeper layers of the superior colliculus (A 3.0) inhibited jaw movements evoked by the dopamine D1/D2 receptor stimulation. Conversely, the jaw movements evoked by acetylcholine receptor stimulation were enhanced by injection of muscimol into the superior colliculus. In conclusion, GABA(A) receptor blockade in a circumscribed region (A 3.0) of the lateral deeper layers of the superior colliculus elicits characteristic repetitive jaw movements, and the GABA(A) receptors in that region modulate the dopamine D1/D2 receptor-mediated and acetylcholine receptor-mediated jaw movements in an opposite manner.

Dopaminergic and cholinergic stimulation of the ventrolateral striatum elicit rat jaw movements that are funnelled via distinct efferents

It has been reported that two distinct types of jaw movements can be elicited by bilateral injections of drugs into the ventrolateral striatum: (1) dopamine receptor-mediated jaw movements that are elicited by a mixture of (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF 82958; 5 microg) and quinpirole (10 microg), and (2) acetylcholine receptor-mediated jaw movements that are elicited by carbachol (2.5 microg). In the present study, electromyographic analysis was used to characterise these movements: the dopamine receptor-mediated jaw movements were marked by a dominant digastric activity during jaw opening and a dominant masseter activity during jaw closing (digastric/masseter type), whereas the acetylcholine receptor-mediated jaw movements were marked by a dominant digastric activity during jaw opening without any significant change in masseter activity during jaw closing (digastric type). The main goal was to (in)validate the hypothesis that these two types of jaw movements are funnelled via distinct gamma-aminobutyric acid (GABA)ergic output channels. Bilateral injections of both muscimol (25 and 50 ng/0.2 microl per side) and bicuculline (50 and 150 ng/0.2 microl per side) into the ventral pallidum, entopeduncular nucleus or dorsolateral part of the substantia nigra pars reticulata essentially inhibited dopamine receptor-mediated jaw movements to various degrees. In contrast, acetylcholine receptor-mediated jaw movements were inhibited by muscimol given into the entopeduncular nucleus and dorsolateral part of the substantia nigra pars reticulata, whereas these movements were enhanced by bicuculline. The acetylcholine receptor-mediated jaw movements were not affected by muscimol injections into the ventral pallidum, but were inhibited by bicuculline injections. Studies on such injections into the ventral pallidum, entopeduncular nucleus or dorsolateral part of the substantia nigra pars reticulata of naive rats revealed that jaw movements of the digastric/masseter type were elicited either by muscimol injections into the dorsolateral part of the substantia nigra pars reticulata or by combined injections of muscimol and bicuculline into the entopeduncular nucleus, and that jaw movements of the digastric type were elicited only by combined injections of muscimol and bicuculline into the entopeduncular nucleus. Together, the data allow the conclusion that dopamine receptor-mediated and acetylcholine receptor-mediated jaw movements are two distinct types of jaw movements that are funnelled via separate GABAergic output channels. It is suggested that the three different profiles of responses to GABAergic drugs in animals showing either dopamine receptor-mediated or acetylcholine receptor-mediated jaw movements reflect the involvement of three distinct types of output neurons of the striatum, namely: type I neurons with collateralised axons to the ventral pallidum, entopeduncular nucleus and dorsolateral part of the substantia nigra pars reticulata, mediating the dopamine receptor-mediated jaw movements; type II neurons with collateralised axons to the globus pallidus that, in turn, project to the entopeduncular nucleus and the dorsolateral part of the substantia nigra pars reticulata, mediating directly the acetylcholine receptor-mediated jaw movements; and type III neurons with a single axon to the ventral pallidum, mediating indirectly the acetylcholine receptor-mediated movements. It is evident that future studies are required to provide direct evidence in favour of the latter hypothesis.

Neural pathways underlying vocal control

Vocalization is a complex behaviour pattern, consisting of essentially three components: laryngeal activity, respiratory movements and supralaryngeal (articulatory) activity. The motoneurones controlling this behaviour are located in various nuclei in the pons (trigeminal motor nucleus), medulla (facial nucleus, nucl. ambiguus, hypoglossal nucleus) and ventral horn of the spinal cord (cervical, thoracic and lumbar region). Coordination of the different motoneurone pools is carried out by an extensive network comprising the ventrolateral parabrachial area, lateral pontine reticular formation, anterolateral and caudal medullary reticular formation, and the nucl. retroambiguus. This network has a direct access to the phonatory motoneurone pools and receives proprioceptive input from laryngeal, pulmonary and oral mechanoreceptors via the solitary tract nucleus and principal as well as spinal trigeminal nuclei. The motor-coordinating network needs a facilitatory input from the periaqueductal grey of the midbrain and laterally bordering tegmentum in order to be able to produce vocalizations. Voluntary control of vocalization, in contrast to completely innate vocal reactions, such as pain shrieking, needs the intactness of the forebrain. Voluntary control over the initiation and suppression of vocal utterances is carried out by the mediofrontal cortex (including anterior cingulate gyrus and supplementary as well as pre-supplementary motor area). Voluntary control over the acoustic structure of vocalizations is carried out by the motor cortex via pyramidal/corticobulbar as well as extrapyramidal pathways. The most important extrapyramidal pathway seems to be the connection motor cortex-putamen-substantia nigra-parvocellular reticular formation-phonatory motoneurones. The motor cortex depends upon a number of inputs for fulfilling its task. It needs a cerebellar input via the ventrolateral thalamus for allowing a smooth transition between consecutive vocal elements. It needs a proprioceptive input from the phonatory organs via nucl. ventralis posterior medialis thalami, somatosensory cortex and inferior parietal cortex. It needs an input from the ventral premotor and prefrontal cortex, including Broca's area, for motor planning of longer purposeful utterances. And it needs an input from the supplementary and pre-supplementary motor area which give rise to the motor commands executed by the motor cortex.