type-II Neurons Research Articles

Applications of neuronal labeling techniques to the study of the peripheral auditory system.

Horseradish peroxidase (HRP) techniques have recently been applied to the study of the peripheral auditory system. These techniques make it possible to trace single, auditory-nerve fibers from their peripheral terminals on sensory cells to their central terminations within the cochlear nucleus. For the large, myelinated axons of type-I neurons contacting inner hair cells, HRP can be injected intracellularly through the same electrode used to record neural activity, allowing correlations to be made between structure and function on a single-cell basis. From these data, a cochlear frequency map has been constructed, and morphological correlates of differences in spontaneous discharge have been identified. For the small, unmyelinated axons of type-II neurons contacting outer hair cells, single-unit recordings have not yet been made; however, these neurons have been labeled by gross extracellular injections of HRP. Extracellular injections have also been used to study the peripheral terminations of single efferent neurons projecting to the outer hair cell region. The afferent fibers to the outer hair cell (OHC) region contact primarily OHCs from the third row, while efferent fibers primarily contact cells from the first row. Single efferent fibers innervate OHCs over a significantly larger cochlear length than single afferent fibers of either type. Possible functional interpretations of these morphological differences are discussed.

Some morphological features of neurons in the rat spinal ganglion.

By means of serial sections in different planes, the perikarya in the rat spinal ganglion were found to be of an irregular spherical shape, with the largest maximal diameter and perimeter in tangential sections. Type-I and type-II neurons have a mean size of about 18 micrometers and 13 micrometers, respectively. Since some type-I perikarya are in the range of type-II cells, it is not possible to distinguish them from type-II cells in unstained thick sections. Some type-I cell perikarya exhibit areas with a reduced number of organelles. These cells are considered as being in a generating state, a fact finally leading to an age-dependent loss of neurons.

Associative and non-associative changes in unit activity of the rat hippocampus

Two main types of neurons of the dorsal hippocampus were recorded in chronic rats during the classical conditioning of an arousal phenomenon (neocortical EEG desynchrony). The relative importance of the associative and non-associative factors was assessed by a differentiation procedure. In naive rats, during the “acquisition” session most type-I neurons rapidly acquired an “inhibitory” response closely parallel to the EEG response. Neither of these responses showed differentiation. Some type-II neurons acquired an “excitatory” response which was independent of the EEG response and which had a significant tendency to differentiation. These data were confirmed in rats submitted to several “retention” sessions. All the type-I neurons were undifferentiated while 11 out of 41 type-II neurons were differentiated. Type-I neurons were characterized by a bursting mode of discharge. Their activity was higher during slow wave sleep (SWS) than during wakefulness (W) or paradoxical sleep (PS). Most of the differentiated type-II neurons were more active during W and/or PS than during SWS.

Organization of the Commissural Connections : Physiology

Synaptic nature of the commissural inhibition and the neuronal organization of the related pathway mediating the inhibitory action on kinetic and tonic type-I neurons of the horizontal canal have been studied with the unanesthetized and decerebrate cat. These studies were carried out by correlating the intracellular records of the postsynaptic potentials in the vestibular nucleus neurons with the extracellular records of functionally identified type-I neurons. The commissural inhibition is caused by a production of the inhibitory postsynaptic potential (IPSP) in the recipient vestibular neuron. The commissural pathway acting on the kinetic type-I neurons is composed of two neuron chains from the vestibular nerve; impulses along the primary afferent fibers excite monosynaptically the inhibitory commissural neurons, impulses from which in turn cross the midline, eventually producing the IPSP monosynaptically in the contralateral kinetic neurons. The commissural pathway for tonic type-I neurons has one or two more synapses along its path from the primary vestibular nerve, and the inhibitory neuron (probably type-II neuron) is located on the same side as the type-I neuron that receives the commissural inhibition. Extra commissural crossed effects on Deiters neurons responding to lateral tilt of the head are briefly described here; they are consistently excitatory.