Victor J. Wilson

Mammalian vestibular physiology

1 Introduction.- 2 Peripheral Morphology.- 3 Biophysics of the Peripheral End Organs.- 4 Mechanoneural Transduction and the Primary Afferent Response.- 5 Labyrinthine Input to the Vestibular Nuclei and Reticular Formation.- 6 The Vestibular System and the Cerebellum.- 7 The Vestibulospinal System.- 8 The Vestibuloocular System.- References.

Precise Localization of Renshaw Cells with a New Marking Technique

FOR some years Renshaw cells in the spinal cord have been extensively investigated. While their physiological and pharmacological properties are relatively well known, knowledge concerning their location in the spinal cord is only approximate and is mostly derived from electrophysiological observations1 and indirect anatomical evidence2. From all this work it has been concluded that Renshaw cells are located in the ventral horn medial to motoneurones. For accurate localization of these and other cells within the central nervous system, however, it is necessary to record the electrical activity with microelectrodes which have satisfactory electrical properties, and then to indicate the location of the electrode tip by making, while causing a minimum of damage to the area, a small and easily identifiable mark. A number of techniques have been used for locating the position of microelectrode tips in nervous tissue3. However, most of these techniques require special histological procedures, may be unreliable, or can only be used at the end of an experiment. While the method of Galifret and Szabo4 needs no special histological procedure and can be used several times in an experiment, it makes rather large marks (several hundred μ) and has not proved completely reliable. We have therefore developed a new technique for routinely making precise, easily located marks and have used this technique in localizing Renshaw cells.

Cervical branching of lumbar vestibulospinal axons

1. We have investigated the possibility that individual lateral vestibulospinal tract (LVST) axons branch so as to innervate more than one spinal cord level.

Localization of proprioceptive reflexes in the splenius muscle of the cat.

Activity of the cat splenius muscle was modulated by sinusoidal rotation of the head around the C1-C2 joint in decerebrate cats with labyrinth intact or with all semicircular canals plugged, or, in one intact and alert cat, by rotation of the body with the head fixed in space. EMG modulation, recorded from the areas of splenius innervated by the C1-C4 nerves, was due to the cervicocollic reflex. Modulation was not uniform, but decreased with progressively more caudal recording locations; with stimuli of small amplitude it was often possible to obtain modulation of the rostral part of the muscle only. The results demonstrate localization of proprioceptive reflexes, including the stretch reflex, within the splenius muscle.

Marking Single Neurons by Staining with Intracellular Recording Microelectrodes

Glass microelectrodes filled with a saturated solution of methyl blue or fast green in 1.0M potassium acetate can be used to mark penetrated ne. urons from which intracellular recordings have been made. Many cells can be marked in one experiment and can easily be located in subsequent histologic section.

Dynamic properties of vestibular reflexes in the decerebrate cat

SummaryVestibulocollic (VCR) and vestibulo-ocular (VOR) reflexes were studied during angular rotation in the horizontal plane in precollicular decerebrate cats. Angular position was modulated by sinusoids or sums of sinusoids with frequencies ranging from 0.05 to 5 Hz.Reflex motor output was measured by recording electromyographic (EMG) activity of the lateral rectus and dorsal neck muscles and discharge of abducens motoneurons. Measured with respect to input angular acceleration VCR motor output displayed a second order lag at low frequencies, bringing mean EMG phase (−136 °) and gain slope (−35 dB/ decade) close to those of an angular position signal at 0.2 Hz. At higher frequencies the lag was counteracted by a second order lead bringing mean phase (−52 °) and gain slope (−5.6 dB/decade) back close to those of an angular acceleration signal at 3 Hz. By contrast, mean phase (−113 ° to −105 °) and gain slope (−21 to −28 dB/decade) of the VOR motor output remained close to those of an angular velocity signal across the entire frequency range.The data suggest that neural pathways producing the VCR receive selective input from “irregular type” horizontal semicircular canal afferents which provide one lag and one lead in the overall transfer function while the other lag and lead are produced by central pathways.Transaction of the medial longitudinal fasciculus (MLF), which eliminates all of the most direct (three neuron) arcs of the horizontal VCR, did not cause any detectable change in the horizontal VCR at either low or high frequencies. Reductions in overall gain occurred in some cases but these could be attributed to damage to axons outside the MLF. Less direct pathways, probably including vestibulo-reticulospinal pathways, are thus able to produce both the low-frequency, phase-lagging and high-frequency, phase-leading components of the horizontal VCR.

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.

Properties of vestibular neurones projecting to neck segments of the cat spinal cord.

1. Vestibular neurones projecting to the upper cervical grey matter (vestibulocollic neurones) were identified by localized microstimulation in the C3 segment of the cat spinal cord.

Synaptic actions of individual vestibular neurones on cat neck motoneurones.

1. Unitary synaptic potentials evoked by the activity of single vestibulocollic neurones were recorded by means of spike‐triggered signal averaging in neck extensor motoneurones of decerebrate cats. Properties of the vestibulocollic neurones which produced the potentials were examined.

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