Tectal and tegmental excitation in dorsal neck motoneurones of the cat

Dissecting the Tectal Output Channels for Orienting and Defense Responses

Electrical stimulation and lesion experiments in 1980’s suggested that the crossed descending pathway from the deeper layers of superior colliculus (SCd) controls orienting responses, while the uncrossed pathway mediates defense-like behavior. To overcome the limitation of these classical studies and explicitly dissect the structure and function of these two pathways, we performed selective optogenetic activation of each pathway in male mice with channelrhodopsin 2 (ChR2) expression by Cre driver using double viral vector techniques. Brief photostimulation of the crossed pathway evoked short latency contraversive orienting-like head turns, while extended stimulation induced body turn responses. In contrast, stimulation of the uncrossed pathway induced short-latency upward head movements followed by longer-latency defense-like behaviors including retreat and flight. The novel discovery was that while the evoked orienting responses were stereotyped, the defense-like responses varied considerably depending on the environment, suggesting that uncrossed output can be influenced by top-down modification of the SC or its target areas. This further suggests that the connection of the SCd-defense system with non-motor, affective and cognitive structures. Tracing the whole axonal trajectories of these two pathways revealed existence of both ascending and descending branches targeting different areas in the thalamus, midbrain, pons, medulla, and/or spinal cord, including projections which could not be detected in the classical studies; the crossed pathway has some ipsilaterally descending collaterals and the uncrossed pathway has some contralaterally descending collaterals. Some of the connections might explain the context-dependent modulation of the defense-like responses. Thus, the classical views on the tectal output systems are updated.

Convergent synaptic inputs from the caudal fastigial nucleus and the superior colliculus onto pontine and pontomedullary reticulospinal neurons

The caudal fastigial nucleus (FN) is known to be related to the control of eye movements and projects mainly to the contralateral reticular nuclei where excitatory and inhibitory burst neurons for saccades exist [the caudal portion of the nucleus reticularis pontis caudalis (NRPc), and the rostral portion of the nucleus reticularis gigantocellularis (NRG) respectively]. However, the exact reticular neurons targeted by caudal fastigioreticular cells remain unknown. We tried to determine the target reticular neurons of the caudal FN and superior colliculus (SC) by recording intracellular potentials from neurons in the NRPc and NRG of anesthetized cats. Neurons in the rostral NRG received bilateral, monosynaptic excitation from the caudal FNs, with contralateral predominance. They also received strong monosynaptic excitation from the rostral and caudal contralateral SC, and disynaptic excitation from the rostral ipsilateral SC. These reticular neurons with caudal fastigial monosynaptic excitation were not activated antidromically from the contralateral abducens nucleus, but most of them were reticulospinal neurons (RSNs) that were activated antidromically from the cervical cord. RSNs in the caudal NRPc received very weak monosynaptic excitation from only the contralateral caudal FN, and received either monosynaptic excitation only from the contralateral caudal SC, or monosynaptic and disynaptic excitation from the contralateral caudal and ipsilateral rostral SC, respectively. These results suggest that the caudal FN helps to control also head movements via RSNs targeted by the SC, and these RSNs with SC topographic input play different functional roles in head movements.

The pedunculopontine tegmental nucleus: implications for a role in modulating spinal cord motoneuron excitability

There is evidence that deep brain stimulation (DBS) of the pedunculopontine tegmental nucleus (PPTg) improves parkinsonian motor signs. The mechanisms that mediate these effects and the modifications that occur in the PPTg in Parkinson's disease (PD) are not fully known and are the object of current debate. The aim of this paper was to critically review available data with respect to (1) the presence of PPTg neurons linked to reticulospinal projections, (2) the involvement of these neurons in modulating spinal reflexes, and (3) the participation of fibers close to or within the PPTg region in such modulation. The PPTg neurons are distributed in a large pontotegmental region, stimulation of which can evoke activity in hindlimb, shoulder and neck muscles, and potentiate motor responses evoked by stimulation of dorsal roots. This influence seems to be carried out by fast-conducting descending fibers, which likely run in the medial reticulospinal pathway. It is yet unclear which neurotransmitters are involved and on which elements of the gray matter of the spinal cord PPTg fibers synapse. The modulation of spinal cord activity which can be achieved by stimulating the PPTg region seems to be mediated not only by PPTg neurons, but also by tecto-reticular fibers which run in the pontotegmental area, and which likely are activated during PPTg-DBS. The importance of these fibers is discussed taking into account the degeneration of PPTg neurons in PD and the benefits in gait and postural control that PPTg-DBS exerts in PD. The potential usefulness of PPTg-DBS in other neurodegenerative disorders characterized by neuronal loss in the brainstem is also considered.

Long descending motor tract axons and their control of neck and axial muscles

It has been tacitly assumed that a long descending motor tract axon consists of a private line connecting the cell of origin to a single muscle, as a motoneuron innervates a single muscle. However, this notion of a long descending motor tract referred to as a private line is no longer tenable, since recent studies have showed that axons of all major long descending motor tracts send their axon collaterals to multiple spinal segments, suggesting that they may exert simultaneous influences on different groups of spinal interneurons and motoneurons of multiple muscles. The long descending motor systems are divided into two groups, the medial and the lateral systems including interneurons and motoneurons. In this chapter, we focus mainly on the medial system (vestibulospinal, reticulospinal and tectospinal systems) in relation to movement control of the neck, describe the intraspinal morphologies of single long descending motor tract axons that are stained with intracellular injection of horseradish peroxidase, and provide evidence that single long motor-tract neurons are implicated in the neural implementation of functional synergies for head movements.

Overlapping gaze shifts reveal timing of an eye–head gate

The ability to dissociate eye movements from head movements is essential to animals with foveas and fovea-like retinal specializations, as these species shift the eyes constantly, and moving the head with each gaze shift would be impractical and energetically wasteful. The processes by which the dissociation is effected remain unclear. We hypothesized that the dissociation is accomplished by means of a neural gate, which prevents a common gaze-shift command from reaching the neck circuitry when eye-only saccades are desired. We further hypothesized that such a gate would require a finite period to reset following opening to allow a combined eye-head saccade, and thus the probability of generating a head movement during a saccade would be augmented when a new visual target (the 'test' target) appeared during, or soon after, a combined eye-head saccade made to an earlier, 'conditioning' target. We tested human subjects using three different combinations of targets-a horizontal conditioning target followed by a horizontal test target (H/H condition), horizontal conditioning followed by vertical test (H/V), and vertical conditioning followed by horizontal test (V/H). We varied the delay between the onset of the conditioning head movement and the presentation of the test target, and determined the probability of generating a head movement to the test target as a function of target delay. As predicted, head movement probability was elevated significantly at the shortest target delays and declined thereafter. The half-life of the increase in probability averaged 740, 490, and 320 ms for the H/H, H/V, and V/H conditions, respectively. For the H/H condition, the augmentation appeared to outlast the duration of the conditioning head movement. Because the augmentation could outlast the conditioning head movement and did not depend on the head movements to the conditioning and test targets lying in the same directions, we could largely exclude the possibility that the augmentation arises from mechanical effects. These results support the existence of the hypothetical eye-head gate, and suggest ways that its constituent neurons might be identified using neurophysiological methods.

The neural control of orienting: role of multiple-branching reticulospinal neurons

This chapter emphasizes the functional significance of the multiple-branching patterns of descending axons implicated in the control of movement. The example provided concerns orienting head movements, which are controlled by pathways from the superior colliculus (SC). Such control is mediated via cervical reticulospinal neurons (C-RSNs), which take origin in the nucleus reticularis pontis caudalis and nucleus reticularis gigantocellularis, and give off multiple collaterals along the full length of their axonal trajectory. Their projection is not only to lamina IX neck motor nuclei in upper cervical segments, but also to laminae VII-VIII in lower cervical segments. Thus, SC commands for head orienting are transmitted to both neck motoneurons and lower cervical spinal circuitry, which latter network controls appropriate postural adjustments by the coordinated control of motoneurons supplying the four limbs.

Premotoneuronal and direct corticomotoneuronal control in the cat and macaque monkey

The literature on premotoneuronal and direct corticomotoneuronal (CM) control in the cat and macaque monkey is reviewed. The available experimental findings are not in accordance with a recently proposed hypothesis that direct CM connections have "replaced" the premotoneuronal pathways. Instead, we propose that premotoneuronal CM control plays an important role in motor control also in primates and that the direct CM connection has been added during phylogeny.

Neck muscle responses to stimulation of monkey superior colliculus. I. Topography and manipulation of stimulation parameters

The role of the primate superior colliculus (SC) in orienting head movements was studied by recording electromyographic (EMG) activity from multiple neck muscles following electrical stimulation of the SC. Combining SC stimulation with neck EMG recordings provides an objective and sensitive measure of the SC drive onto neck muscle motoneurons, particularly in relation to evoked gaze shifts. In this paper, we address how neck EMG responses to SC stimulation in head-restrained monkeys depend on the rostrocaudal, mediolateral, and dorsoventral location of the stimulating electrode within the SC and vary with manipulations of the eye position prior to stimulation onset and changes in stimulation current and duration. Stimulation predominantly evoked EMG responses on the muscles obliquus capitis inferior, rectus capitis posterior major, and splenius capitis. These responses became larger in magnitude and shorter in onset latency for progressively more caudal stimulation locations, consistent with turning the head. However, evoked responses persisted even for more rostral stimulation locations usually not associated with head movements. Manipulating initial eye position revealed that the magnitude of evoked responses became stronger as the eyes attained positions contralateral to the side of stimulation, consistent with a summation between a generic command evoked by SC stimulation and the influence of eye position on tonic neck EMG. Manipulating stimulation current and duration revealed that the relationship between gaze shifts and evoked EMG responses is not obligatory: short-duration (<20 ms) or low-current stimulation evoked neck EMG responses in the absence of gaze shifts. However, long-duration stimulation (>150 ms) occasionally revealed a transient neck EMG response aligned on the onset of sequential gaze shifts. We conclude that the SC drive to neck muscle motoneurons is far more widespread than traditionally supposed and is relayed through intervening elements which may or may not be activated in association with gaze shifts.

Brainstem control of head movements during orienting; organization of the premotor circuits

When an object appears in the visual field, animals orient their head, eyes, and body toward it in a well-coordinated manner (orienting movement). The head movement is a major portion of the orienting movement. Interest in the neural control of head movements in the monkey and human have increased in the 1990's, however, fundamental knowledge about the neural circuits controlling the orienting head movement continues to be based on a large number of experimental studies performed in the cat. Thus, it is crucial now to summarize information that has been clarified in the cat for further advancement in understanding the neural control of head movements in different animal species. The superior colliculus (SC) has been identified as the primary brainstem center controlling the orienting. Its output signal is transmitted to neck motoneurons via two major separate pathways: one through the reticulospinal neurons (RSNs) in the pons and medulla and the other through neurons in Forel's field H (FFH) in the mesodiencephalic junction. The tecto-reticulo-spinal pathway controls orienting chiefly in the horizontal direction, while the tecto-FFH-spinal pathway controls orienting in the vertical direction. In each pathway, a subgroup of neurons functions as premotor neurons for both extraocular and neck motoneurons, while others are specified for each, which allows both coordinated and separate control of eye and head movements. Head movements almost always produce shifts in the center of gravity that might cause postural disturbances. The postural equilibrium may be maintained by transmitting the orienting command to the limb segments via descending axons of the reticulospinal and long propriospinal neurons. The SC and brainstem relay neurons receive descending inputs from higher order structures such as the cerebral cortex, cerebellum, and basal ganglia. These inputs may serve context-dependent control of orienting by modulating the activities of the primary brainstem pathways.

Effects of lesion of pontomedullary reticular formation on visually triggered vertical and oblique head orienting movements in alert cats

The role of the nucleus reticularis pontis caudalis (NRPC) and the nucleus reticularis gigantocellularis (NRG) in control of vertical and oblique head orienting movements was investigated in alert cats by lesion of these nuclei with kainic acid. Cats were trained to orient the head vertically or obliquely to various targets. Following unilateral lesion of these nuclei, vertical orienting could be performed correctly with a slight decrease in velocity, while oblique orienting tended to exhibit zigzag course because of severe impairment of horizontal orienting. The horizontal and vertical components became coordinated in the course of experiments due to a significant decrease in vertical component velocity, resulting in smooth oblique trajectories. Results suggest that horizontal and vertical components of head orienting are controlled separately, but impairment of horizontal component causes adaptive change of vertical component velocity in oblique orienting.

Axonal trajectories of the nucleus reticularis gigantocellularis neurons in the C2-C3 segments in cats

Axonal trajectories in the C2-C3 segments of the nucleus reticularis gigantocellularis neurons projecting to the lower cervical cord (C-RSNs) and excited monosynaptically from cortico- and tectofugal fibers were studied by mapping thresholds of antidromic excitation and intra-axonal staining in cats. The C-RSNs descended in various sites of the spinal funiculi, and the projection area of individual cells varied with the funicular location of the stem axon. C-RSNs descending in the ventrolateral funiculus (inRSNs) projected mainly to lamina VIII-IX, those descending in the lateral funiculus (IRSNs) mainly to laminae VI-VIII, and those descending in the contralateral funiculus (coRSNs) chiefly to laminae VIII-IX on that side. It is suggested that inRSNs and coRSNs mediate disynaptic effects from cortico- and tectofugal fibers to dorsal neck motoneurons bilaterally.

Axonal branching and termination of cervical reticulospinal neurons in the cat brachial segments

Axonal branching patterns in the brachial segments of cervical reticulospinal neurons (C-RSNs) were examined in cats using intraaxonal injection of horseradish peroxidase (HRP). Axons of these neurons were electrophysiologically identified by their projection to the lower cervical but not to the lumbar segments and monosynaptic activation after tectal and pyramidal stimulation. Six axons were stained up to terminals. Their stem axons descended in the ventral funiculus near the boundary of the spinal gray. The majority of collateral axons crossed lamina VIII and distributed terminals in the whole area of lamina VIII, the middle part of lamina VII, the lateral 2/3 of lamina VI, the ventral part of lamina V, and sparsely in the limb motor nuclei. Their possible functional role in head orienting movements is discussed.

Post-spike facilitation of neck EMG by cat tectoreticulospinal neurones during orienting movements

1. The activity of fourteen tectoreticulospinal neurones (TRSNs) was recorded intraaxonally in the caudal pons of alert cats during orienting movements towards visual stimuli. TRSN spikes were used to compute the spike-triggered average (STA) of rectified EMG of dorsal neck muscles. 2. Eight TRSNs for which 400-2532 spikes were available were analysed with the STA technique. When the STA was computed from all spikes, significant post-spike facilitation (PSF) was obtained for six of eighteen cell-muscle pairs investigated (5 TRSNs). The mean relative amplitude of PSFs was 7.4% (S.D. 3.7). The onset latencies ranged from 1.1 to 5.0 ms and mean duration was 11.4 +/- 3.1 ms (mean +/- S.D.). 3. Interspike interval distributions were unimodal, with modes between 2.7 and 12.7 ms. Spike trains of TRSNs that produced significant PSFs contained 5-13% of the interspike intervals < or = 5 ms and 22-37% of the intervals < or = 10 ms. To evaluate the contribution of short intervals to PSF, STAs were computed separately for spikes preceded by 'short' (< or = 5 or < or = 10 ms) and 'long' (> 5 or > 10 ms) intervals. 4. When computed from spikes preceded by 'long' intervals, PSF amplitudes were small (mean +/- S.D., 5.3 +/- 2.7%) and onset latencies measured by cusum ranged between 2.4 and 5.4 ms. This is longer than the estimated minimal latency of monosynaptic facilitatory effect on neck EMG (1.9-2.1 ms). 5. Relative amplitudes of PSF obtained with spikes preceded by 'short' intervals were much larger (mean +/- S.D., 14.8 +/- 7.4%), but cusums indicated negative latencies for four of six PSFs. The unrealistically short onset latencies could be accounted for by the summation of facilitation from the trigger spike with that of the preceding spikes. In four of five TRSNs a large increase of PSF amplitude (from 3.2 to 7.2 times the amplitude obtained from 'long' intervals) suggests the presence of frequency-dependent potentiation of synaptic transmission. 6. This study unequivocally demonstrates that some TRSNs produce significant post-spike facilitation of neck motoneurones. This facilitation could be mediated by monosynaptic tectomotoneuronal connections although a contribution by disynaptic connections cannot be definitively ruled out. The high instantaneous firing rates of TRSNs produce a potentiation of the otherwise weak facilitatory action of TRSNs that presumably contributes to a rapid recruitment of motoneurones during initiation of head orienting movements.

Distribution of motoneurons supplying dorsal and ventral suboccipital muscles in the feline neck

A combination of retrograde tracers was used to compare the relative distributions of motoneurons supplying the ventral and lateral suboccipital muscles, rectus capitis anterior (RCA), and rectus capitis lateralis (RCL), with those supplying dorsal muscles, including rectus capitis posterior muscles (RCP), complexus (CM), and the medial head of obliquus capitis superior (OCS). Three of the tracers, horseradish peroxidase, fluororuby, and fluorescein-conjugated dextran, were applied to cut nerve ends. Fast blue was applied by intramuscular injection, and fluorogold was delivered both by injection and by cut nerve exposure. Motoneurons supplying RCA and RCL were clustered on the medial wall of the ventral horn in a restricted region defined previously as the commissural nucleus. Labelled cells supplying RCL were confined to the C1 segment, but those supplying RCA were distributed from C1 to rostral C4. Motoneurons supplying RCA tended to lie more dorsomedially than those supplying RCL, but there was substantial overlap between the two populations. Motoneurons supplying dorsal muscles had a separate, more ventral distribution. RCP motoneurons were located primarily in the ventromedial nucleus, but a small proportion of cells was found in the white matter of the ventral funiculus or the gray matter surrounding the central canal. Motoneurons supplying CM and OCS were located dorsomedially to the RCP cell population. These results suggest that neck motoneurons are arranged according to a "musculotopic" pattern in which dorsal muscles have the most ventral locations, and progressively more lateral and then ventral muscles are layered dorsomedially along the medial wall of the ventral horn.

Tracing premotor brain stem networks of orienting movements

Methods allowing a direct matching of movement-related firing patterns and connectivity of individual neurons have been used in the analysis of premotor networks controlling orienting movements. Advances have been made in the description of coding properties of orienting-related tectal output neurons, as well as in specifying their distributed connections in the brain stem and possible modes of coupling to saccadic pattern generators in the reticular formation. New data on the properties of signals and connectivity patterns have also been obtained for the tecto-recipient reticulo-spinal neurons. At least a small portion of the network performing the spatio-temporal transformations of orienting-related tectal efferent signals can now be described both in functional and in morphological terms.

Descending pathways mediating disynaptic excitation of dorsal neck motoneurones in the cat: facilitatory interactions

Facilitatory interactions between disynaptic EPSPs evoked from the contralateral tectum, ipsilateral tegmentum and contra- and/or ipsilateral pyramid have been investigated in dorsal neck motoneurones of the cat. Monosynaptic convergence on common intercalated neurones was found from ipsi- and contralateral pyramidal, contralateral tectal and ipsilateral tegmental fibres. In addition, disynaptic facilitation was observed from ipsilateral pyramidal fibres on disynaptic contralateral pyramidal EPSPs. Transection of cortico-fugal fibres in the pyramid showed that the location of the interactions occurred in the lower brain stem, suggesting that reticulospinal neurones are mediating the effects.

Descending pathways mediating disynaptic excitation of dorsal neck motoneurones in the cat: brain stem relay

The location of intercalated neurones mediating disynaptic excitation from tectum, tegmentum and pyramids to dorsal neck motoneurones has been investigated by: (a) recording field potentials in the lower brain stem evoked from the above systems, (b) systematic stimulation in the brain stem during intracellular recording from motoneurones innervating the splenius, biventer cervicis and complexus muscles, and (c) comparing the effects of lesions of the brain stem with kainic acid on the disynaptic EPSPs elicited from the above three systems. Electrical stimulation of the contralateral superior colliculus evoked monosynaptic field potentials which were largest in the caudal pontine reticular formation rostral to the abducens nucleus and in the rostral part of the medullary reticular formation caudal to the abducens nucleus. Likewise, stimulation of the ipsilateral tegmentum (the cuneiform and subcuneiform nucleus) evoked field potentials which were large in the caudal medulla and small in the pons. In contrast, stimulation of the contralateral tegmentum was ineffective in evoking field potentials. Stimulation of the pyramid 2-3 mm rostral to the obex elicited monosynaptic field potentials in the reticular formation of the lower brain stem that were only about 25% of those from the superior colliculus. In contrast to the field potentials from the superior colliculus, the pyramidal ones were large in the medulla and small in the pons. Lesions of the reticular formation in the lower brain stem by unilateral kainic acid injection caused disappearance of disynaptic EPSPs in motoneurones from the above three systems. These results strongly suggest that the intercalated neurones mediating pyramidal, tectal and tegmental EPSPs are reticulospinal neurones in the lower brain stem. Systematic stimulation in various locations of the lower brain stem showed that monosynaptic EPSPs were evoked from the regions of the reticular formation which received projection from the above three descending systems. The effective regions for evoking the EPSPs in splenius (SPL) were located somewhat more dorsally than for biventer cervicis and complexus (BCC) motoneurones. The descending axons of presumed reticulospinal neurones were stimulated with electrodes placed in medial, middle and lateral positions at the spinomedullary junction. Monosynaptic EPSPs in SPL and BCC motoneurones were evoked from the medial and middle electrodes but not from the lateral electrode.