Spatial distribution of the vestibulospinal neurons in the frog vestibular nuclei.

In experiments on the preparation of a frog perfused brain (Rana ridibunda), intracellular potentials were recorded from neurons of the vestibular nuclei following stimulation of the vestibular nerve and the spinal cord. The vestibulospinal neurons were identified on the basis of excitatory postsynaptic potentials evoked by the stimulation of the ipsilateral vestibular nerve and antidromic activation from the stimulation of the cervical and lumbar enlargements of the spinal cord. The cells that could be activated antidromically only by cervical cord stimulation have been designated as C cells, and the cells that could also be activated antidromically as a result of lumbar stimulation have been termed L cells. The average conduction velocity determined for C neurons was 10.67 m/s and for L neurons 15.84 m/s. The ratio of C and L neurons over the vestibular nuclear complex was very similar to each other: 52% C neurons and 48% L neurons. The majority of both types of neurons were localized in the lateral vestibular nucleus (58.6%), to a lesser extent in the descending vestibular nucleus (30.7%) and very little in the medial vestibular nucleus (10.6%). In the lateral vestibular nucleus, C neurons prevailed in the caudal part of the nucleus and L neurons prevailed in the rostral part. By contrast, in the descending and medial vestibular nuclei there was a gradual increase of C and L cells quantitatively from the rostral to the caudal part. Fast and slow cells were detected among the vestibulospinal neurons. The fast neurons of L cells did not prevail greatly over the slow ones, whereas the slow neurons of C cells prevailed comparatively largely over the fast neurons. Thus, it became possible to reconstruct the spatial distribution of the identified vestibulospinal neurons. The results of spatial distribution of C and L vestibulospinal neurons in the frogs failed to conform to definite somatotopy, which is characteristic of mammalian vestibular nuclei. The results of this study have confirmed an earlier assumption that C and L neurons in the frog's vestibular nuclei as a source of vestibulospinal fibers, are scattered separately or more frequently in groups, so that they establish a 'patch-like' somatotopy and do not form a distinctly designed field as in mammals.

Functional characteristics of the input-output correlation in the vestibular nuclear complex of the frog.

Experiments on perfused frog brains were used to record focal and intracellular potentials of neurons in the vestibular nuclear complex produced in response to stimulation of the anterior branch of the ipsilateral vestibular nerve and the spinal cord. Stimulation of the vestibular nerve evoked mono- and polysynaptic EPSP with orthodromic action potentials. These were accompanied by recordings of antidromic activation (with a mean latent period of 0.75 sec) of neurons which send their axons into the labyrinth. Antidromic action potentials from vestibular neurons arose with latent periods of the order of 1.43 msec in response to stimulation of the cervical thickening and 2.19 msec in response to stimulation of the lumbar thickening of the spinal cord. Bursts from the spinal cord often evoked EPSP with orthodromic action potentials in vestibular neurons. The characteristics of the functional correlation between the vestibular input and the vestibulospinal system are discussed.

Electrophysiological properties of the somatotopic organization of the vestibulospinal system in the frog.

In experiments on the preparation of a frog perfused brain (Rana ridibunda), field and intracellular potentials were recorded from neurons of the vestibular nuclear complex following stimulation of the ipsilateral vestibular nerve and different levels of the spinal cord. Stimulation of the vestibular nerve evoked mono- and polysynaptic excitatory postsynaptic potentials and orthodromic action potentials. In parallel, an antidromic activation of vestibular neurons sending their axons to the labyrinth was recorded. Vestibulospinal neurons sending their axons to the cervical (C neurons) and lumbar (L neurons) enlargements of the spinal cord were identified by their antidromic activation. A rather high conduction velocity along vestibulospinal fibres (mean 15.47 m/s) was observed. A somatotopic arrangement of the vestibulospinal system was established in spite of extremely large overlapping zones for the fore- and hindlimb representations in the vestibular nuclear complex. The hindlimbs were represented more poorly than the forelimbs. Antidromic potentials of C and L neurons were recorded in the medial, descending and with the highest density in the lateral vestibular nuclei (Deiters' nucleus). C neurons were evenly distributed in the other vestibular nuclei studied, while L neurons were located predominantly in the caudal parts of the vestibular nuclear complex. The multiplicity of the origin of the vestibulospinal axons was established. Peculiarities of the functional correlation between the vestibular input and vestibulospinal system are discussed.