K. Fukushima

Reticulospinal connections with limb and axial motoneurons.

SummaryResponses of motoneurons supplying muscles of the forelimbs, hindlimbs, back, and neck to stimulation of the medial pontomedullary reticular formation were studied with intracellular recording in cere-bellectomized cats under chloralose anesthesia.Stimulation of the midline or of a reticular region consisting of nucleus reticularis (n.r.) pontis caudalis and the dorsorostral part of n.r. gigantocellularis produced monosynaptic excitation of ipsilateral motoneurons supplying axial muscles and flexor and extensor muscles in both proximal and distal parts of the limbs. This widespread excitation appears to have been produced by rapidly conducting medial reticulospinal fibers.Stimulation of a second region consisting of n.r. ventralis and the ventrocaudal part of n. r. gigantocellularis produced monosynaptic excitation of ipsilateral neck and back motoneurons but only longer latency, apparently multisynaptic excitation of limb motoneurons. Collision tests indicated that this monosynaptic excitation did not involve fibers descending along the midline. It therefore appears to have been produced by lateral reticulospinal fibers.Reticular stimulation also produced short latency, monosynaptic inhibition of neck motoneurons, long latency, apparently polysynaptic inhibition of limb motoneurons and intermediate latency inhibition of back motoneurons. The latencies and properties of inhibitory responses of back motoneurons indicated that they were produced either disynaptically by fast fibers or monosynaptically by slower fibers.The data indicate that the medial pontomedullary reticular formation can be divided into a number of different zones each with a distinct pattern of connections with somatic motoneurons. These include the dorsorostrally located medial reticulospinal projection area, from which direct excitation of a wide variety of motoneurons can be evoked, the ventrocaudally located lateral reticulospinal projection area from which direct excitation of neck and back and direct inhibition of neck motoneurons can be evoked and the dorsal strip of n.r. gigantocellularis which has direct excitatory and inhibitory actions only on neck motoneurons.

Reticulospinal excitation and inhibition of neck motoneurons

SummaryResponses of neck motoneurons to electrical stimulation of the pontomedullary reticular formation were recorded intracellularly in cerebellectomized cats anesthetized with chloralose. Stimulation of nucleus reticularis (n.r.) ventralis and the dorsal part of n.r. gigantocellularis evoked short latency, monosynaptic inhibitory postsynaptic potentials (IPSPs) in the majority of motoneurons supplying the ipsilateral splenius, biventer cervicis and complexus muscles and in 25% of motoneurons projecting in the ipsilateral spinal accessory nerve. Monosynaptic IPSPs were also evoked by stimulating the medial longitudinal fasciculus (MLF) but lesion and collision experiments indicated that these IPSPs were independent of those evoked by reticular stimulation. Monosynaptic IPSPs were also occasionally observed following stimulation of the contralateral reticular formation, especially of the dorsal part of n.r. gigantocellularis.Monosynaptic excitatory postsynaptic potentials (EPSPs) were evoked in all classes of neck motoneurons studied by stimulation of n.r. pontis caudalis, gigantocellularis and ventralis. Each reticular nucleus appeared to contribute to this excitation. The excitation was bilateral but large monosynaptic EPSPs were most often seen in motoneurons ipsilateral to the stimulus site. Data indicated that pontine EPSPs were mediated by ventromedial reticulospinal fibers while medullary EPSPs were mediated by ventrolateral reticulospinal fibers. Neck motoneurons thus receive at least three distinct direct reticulospinal inputs, two excitatory and one inhibitory.

Vestibulospinal, reticulospinal and interstitiospinal pathways in the cat.

Publisher Summary This chapter reviews the properties and motor actions of three descending systems: the vestibulospinal tracts, the reticulospinal tracts, and the interstitiospinal tract. The vestibulospinal tracts are the most direct pathways between the labyrinth and spinal motoneurons. The medial vestibulospinal tract (MVST) is the predominant direct pathway to axial motoneurons, the lateral vestibulospinal tract (LVST) the only direct pathway to limb motoneurons; not much is known about the recently discovered caudal vestibulospinal tract. The role of these direct pathways in functionally meaningful vestibulospinal reflexes remains to be determined. The reticulospinal tracts consist of three groups of descending fibers: one descending in the ventromedial funiculus (RST m ), one in the ipsilateral ventrolateral funiculus (RST i ), and one in the contralateral ventrolateral funiculus (RST c ). Excitatory RST m neurons scattered throughout nucleus reticularis (n.r.) pontis candalis and the dorsal part of n.r. gigantocellularis establish direct synaptic connections with motoneurons supplying a wide variety of muscles throughout the body. The reticulospinal systems receive major direct inputs from many different regions including vestibular nuclei, suggesting that they participate in vestibulospinal reflexes. The interstitiospinal tract, which has not been studied extensively, includes neurons that establish direct excitatory connections with neck motoneurons, but do not establish direct connections with limb and back motoneurons.

Direct excitation of neck motoneurons by interstitiospinal fibers

Summary1.Responses of neck motoneurons to stimulation of the interstitial nucleus of Cajal (INC) were recorded intracellularly in cats under chloralose anesthesia. When stimuli were applied within or close to the INC, short latency, monosynaptic excitatory postsynaptic potentials (EPSPs) were evoked in many neck motoneurons. Such EPSPs were not evoked by stimulating mesencephalic regions outside the INC.2.Stimulation of the ipsilateral INC produced monosynaptic EPSPs consistently in biventer cervicis-complexus (BCC) motoneurons, while such EPSPs were observed in about two thirds of the splenius (SP) motoneurons and half of the trapezius (TR) motoneurons tested. Stimulation of the contralateral INC produced weak monosynaptic EPSPs in about half the BCC motoneurons and in a few SP and TR motoneurons. All types of motoneurons also received longer latency, apparently polysynaptic, PSPs from both INCs. In BCC and TR motoneurons these were mainly EPSPs, in SP, mixed excitatory and inhibitory PSPs.3.Monosynaptic EPSPs evoked by INC stimulation were not eliminated by acute and chronic parasagittal and transverse lesions placed to interrupt the bifurcating axons of all vestibulospinal and many reticulospinal neurons. No significant collision was observed between EPSPs evoked by INC and vestibular or reticular stimulation. The EPSPs evoked by stimulation of the INC therefore appear to have been produced by activation of interstitiospinal neurons rather than by an axon reflex mechanism.4.The properties of a number of interstitiospinal neurons were observed while recording extracellularly from the mesencephalon to map the location of the INC. One third of the interstitiospinal neurons activated antidromically from the C4 segment could also be activated antidromically from L1. These lumbar-projecting neurons had conduction velocities ranging from 15–123 m/s. Several interstitiospinal neurons sending axons to the ventral horn of the neck segments were identified and two of these were found to be branching neurons that projected both to the neck and to lower levels of the spinal cord.

Direct fastigiospinal fibers in the cat

Projections o f the fastigial nucleus to the vestibular nuclear complex 6,8, the pontomedul lary reticular format ion 6,9 and the ventral thalamic complex1,3, 6 are well known. In 1956, Thomas et al. 6 reported that fastigial fibers also project to the upper cervical spinal cord, a finding which has since been overlooked. Following injection o f horseradish peroxidase (HRP) into the upper cervical spinal cord we have found numerous labeled neurons within the fastigial nucleus, indicating the existence o f a significant direct fastigiospinal pathway. To obtain further information on this pathway, we utilized retrograde axonal t ransport o f H R P to investigate the following points: (1) Is the projection f rom the fastigial nucleus to the spinal cord bilateral or unilateral? (2) Where in the fastigial nucleus are fastigiospinal cells located? (3) To what level of the spinal cord do they primarily project? Our results, described in this paper, are confirmed and extended by the physiological study which foUows 1°. Experiments were performed on 8 cats anesthetized with Nembuta l (40 mg/kg, i.p.). In 4 cats 1-6 injections of 0.2-1.0 #1 of a 50 ~ solution of H R P (Sigma, Type VI) were made into the grey matter on one side of the C8 segment using a microsyringe fitted with a glass micropipette having a tip diameter of 20-100 #m. In 2 cats similar injections were made into the grey matter at the L5 and L7 segments. In 2 other cats 0.2 /~1 injections were made into the grey and white matter on one side o f the C~ and C5 segments with a horizontal and vertical spacing of 0.3 mm to facilitate uptake of H R P by damaged axons 4. Such injections gave rise to labeling o f cells th roughout the red

Compartmentalization of the cervicocollic reflex in cat splenius muscle

SummaryWe have examined the cervicocollic reflex (CCR), evoked by horizontal rotation of the head of decerebrate cats, in the dorsal neck extensor muscle splenius. This muscle is divided into compartments which are innervated by three or four spinal segments; an analogous Compartmentalization may be observed in the CCR.When the CCR is evoked by rotation of the head about a vertical axis centered over C1–C2, the modulation of EMG activity is higher in the rostral than in the caudal compartments; in some cases, the rostral compartments can be modulated selectively. The rostrocaudal gradient of modulation is absent if the axis of rotation is shifted caudally to C4–C5.In muscles which had been completely detached from their origin and insertion, the pattern of activation of the CCR was similar to that observed in intact muscle, although the gain of the reflex fell by two thirds. This suggests that significant inputs to this reflex arise both from within splenius itself and from receptors outside this muscle.The typical CCR disappears if the C1–C4 dorsal roots ipsilateral to splenius are cut; furthermore, the reflex appears normal in animals with spinal transections above C1. A significant component of the CCR in splenius appears to be a segmental stretch reflex, originating partly in splenius and partly from receptors outside the muscle.

Properties and connections of cat fastigiospinal neurons

Summary1.Neurons in the cat fastigial nucleus that project to the upper cervical spinal segments (fastigiospinal neurons) were fired by antidromic stimulation of the contralateral spinal cord. Dye ejection from the recording electrode was used to show that most neurons were in the rostral half of the fastigial nucleus.2.Fastigiospinal neurons can be excited and/or inhibited by stimulation of forelimb and hindlimb nerves and by stimulation of the vestibular nerve. These inputs converge on many neurons.3.Antidromic microstimulation was used to trace fastigiospinal axons to the vicinity of motor nuclei in C2-C3.4.The rostral fastigial nucleus was stimulated in preparations with the medial longitudinal fasciculus transected by a wide lesion that impinged on the medial reticular formation in the caudal medulla, to eliminate some potential axon reflexes. Short-latency EPSPs were recorded in some trapezius and biventer-cervicis motoneurons. In many cases there was little or no occlusion between these EPSPs and others evoked by stimulation of the vestibular nerve ipsilateral to the motoneurons.5.Movement of the stimulating electrode and placement of this electrode lateral to the fastigial nucleus show that the zone from which low threshold EPSPs can be evoked is localized.6.Latency measurements and lack of temporal facilitation with double shocks suggest that the EPSPs are monosynaptic. The evidence suggests that they are caused by fastigiospinal fibers terminating on motoneurons.

Properties of a new vestibulospinal projection, the caudal vestibulospinal tract

SummaryNeurons in the caudal portions of the medial and descending vestibular nuclei and in vestibular cell group f that project to the cervical or lumbar spinal cord were located by antidromic spinal stimulation. These caudal Vestibulospinal tract (CVST) neurons have a median conduction velocity of 12 m/sec, which is well below the conduction velocities of typical lateral or medial Vestibulospinal tract (LVST, MVST) axons. The descending fiber trajectories of CVST neurons, determined by comparing thresholds for activation of each neuron from six points in the spinal white matter, were remarkably diverse. Unlike LVST and MVST axons, which are located in the ipsilateral ventral funiculi, CVST axons can be found in both the ventral and dorsolateral funiculi on both sides of the spinal cord. The CVST system is thus both anatomically and physiologically different from the LVST and MVST.

Interstitiospinal action on forelimb, hindlimb, and back motoneurons.

SummaryResponses of motoneurons supplying muscles of the forelimbs, hindlimbs, and back to stimulation of the interstitial nucleus of Cajal (INC) were recorded intracellularly in cats under chloralose anesthesia. Stimulation of the ipsilateral and contralateral INC evoked predominantly excitatory postsynaptic potentials in these motoneurons. Response latencies and properties of responses to multiple shock stimuli indicated that the responses were evoked by a di- or polysynaptic pathway.Stimulation of the anterior MLF (P2), which should have activated the entire interstitiospinal tract, but few reticulospinal or vestibulospinal fibers, evoked only polysynaptic responses. These results indicate that the INC does not establish direct synaptic connections with limb and back motoneurons.