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Notes: Abstract 1. The activity of presumably inhibitory reticulospinal neurons with cell bodies located in the medial aspects of the medullary reticular formation and axons projecting to lumbosacral cord has been recorded in decerebrate cats and their response characteristics to sinusoidal stimulation of labyrinth receptors (134 neurons) and neck receptors (110 neurons) have been related to cell size inferred from the conduction velocity of the corresponding axons. 2. No significant correlation was found between resting discharge and conduction velocity of the axons. 3. Among the recorded reticulospinal neurons, 64/134 (i.e. 47.8%) units responded to roll tilt, while 66/110 (i.e. 60.0%) units responded to neck rotation (0.026 Hz, ±10°). A positive correlation was found between gain (imp./s/deg) of the labyrinth and neck responses and conduction velocity of the axons. Thus, due to absence of correlation between resting discharge and conduction velocity of the axons, larger neurons exhibited a greater percentage modulation (sensitivity) to the labyrinth and the neck input than smaller neurons. These findings are attributed to an overall increase in density or efficacy of the synaptic contacts made by the vestibular and neck afferent pathways on reticulospinal neurons of increasing size. 4. Units receiving neck-macular vestibular convergence showed on the average an higher gain of the neck (G N) response with respect to the labyrinth (G L) response (G N/G:L: 1.95±1.49, S.D.;n=43); however, due to a parallel increase in gain of the reticulospinal neurons to both neck and labyrinth inputs, the relative effectiveness of the two inputs did not vary in different units as a function of cell size. 5. The reticulospinal neurons were mainly excited by the direction of animal orientation and/or neck displacement. In particular, most of these positional sensitive units were excited by side-up animal tilt (37/58, i.e. 63.8%) and by sidedown neck rotation (47/60, i.e. 78.3%). These predominant response patterns were particularly found between large size neurons, whereas small size neurons tended to show also other response patterns. 6. The evidence indicates that in addition to intrinsic neuronal properties related to cell size, the quantitative and qualitative organization of synaptic inputs represents the critical factor controlling the responsiveness of reticulospinal neurons to vestibular and neck stimulation.

Notes: Abstract In decerebrate cats, rotation about the longitudinal axis of the whole animal at 0.15 Hz,±10° produced an increased electromyogram (EMG) activity of the triceps brachii during side-down tilt and a decreased activity during side-up tilt. This vestibulospinal reflex (VSR) was tested before, during and after a sustained (3-h) period of roll tilt of the head at the parameters indicated above, associated with a synchronous roll tilt of the body at 0.15 Hz, but at the peak amplitude of either 12.5° or 7.5°. This additional stimulus led to 2.5° of neck rotation, which was respectively out of phase (condition A) or in-phase (condition B) with head rotation. In a few instances the peak amplitude of neck rotation was increased to 5°. In the first experimental condition A, the gain of the VSR (tested every 10–15 min) progressively increased, starting from the first hour of out of phase neck-vestibular stimulation to reach, on average, 241% of the control value at the end of the third hour of stimulation. On the other hand, in the second experimental condition B, the mean gain of the VSR first decreased to 82% during the first hour of in-phase neck-vestibular stimulation, but then increased to 165% of the corresponding control during the last hour of recording. In other experiments an adaptive increase in gain of the pure VSR occurred during a sustained (3-h) period of selective roll tilt of the whole animal, but it was less consistent and, on average, smaller in amplitude than that obtained during out of phase neck-vestibular stimulation. The adaptive changes in gain of the VSR described above were not associated with changes in the phase angle of the responses, and were also observed during the post-adaptation period. Further experiments indicated that the gain of the N-VSR, i. e. of the EMG responses to combined neck-vestibular stimulation, displayed a prominent adaptive increase during the sustained out of phase stimulation, but not during the inphase stimulation.

Notes: Abstract 1. The activity of lateral vestibular nucleus (LVN) neurons, antidromically identified by stimulation of the spinal cord at T12 and L1, thus projecting to the lumbosacral segments of the spinal cord (lVS neurons), was recorded in precollicular decerebrate cats during rotation about the longitudinal axis either of the whole animal (labyrinth input) or of the body only while the head was kept stationary (neck input). 2. Among the lVS neurons tested for vestibular stimulation, 76 of 129 units (i.e. 58.9%) responded to roll tilt of the animal at the standard parameters of 0.026 Hz, ±10°. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.47±0.44, SD, impulses·s−1·deg−1 and 3.24±3.15, SD, %/deg, respectively. As to the response patterns, 51 of 76 units (i.e. 67.1%) were excited during side-down and depressed during side-up tilt, whereas 15 (i.e. 19.7%) showed the opposite behavior. In both instances the peak of the responses occurred with an average phase lead of about +21.0±27.2., SD, deg with respect to the extreme side-down or side-up position of the animal. Moreover, the former group of units showed almost a twofold larger gain with respect to the latter group (t-test,p〈0.05). 3. Among the lVS neurons tested for neck stimulation, 75 of 109 units (68.8%) responded to neck rotation at the standard parameters. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.49±0.40, SD, impulses·s−1·deg−1 and 3.30±3.42, SD, %/deg, respectively, thus being similar to the values obtained for the labyrinth responses. However, 59 of 75 units (i.e. 78.6%) were excited during side-up neck rotation and depressed during side-down neck rotation, while 8 of 75 units (i.e. 10.7%) showed the opposite pattern. In both instances the peak of the responses occurred with an average phase lead of +52.0±18.3, SD, deg for the extreme side-up or side-down neck displacements. Further, the former group of units showed a larger gain than the latter group. 4. Histological controls indicated that 102 of 129 (i.e. 79.0%) lVS neurons tested for labyrinth stimulation and 86 of 109 (i.e. 78.9%) lVS neurons tested for neck stimulation were located in the dorsocaudal part of LVN, the remaining lVS neurons being located in the rostroventral part of LVN. 5. The observation that the predominant response pattern of the lVS neurons to roll tilt was just opposite to that of lVS neurons to neck rotation indicates that the motoneurons innervating ipsilateral hindlimb extensors were excited by an increased discharge of vestibulospinal neurons during side-down tilt but they were disfacilitated by the reduced discharge of vestibulospinal neurons during side-down neck rotation; the opposite would occur during side-up animal tilt or neck rotation. These findings were compared with those of previously recorded LVN neurons, whose descending axons were not identified as projecting to upper or lower segments of the spinal cord. It was then possible to evaluate the role that the LVN exerts not only in the control of the limb but also of the neck extensor musculature.

Notes: Abstract Extracellular recordings were obtained in precollicular decerebrate cats from 90 neurons located in the noradrenergic area of the dorsal pontine tegmentum, namely in the dorsal (LCd,n=24) and the ventral part (LCα,n=40) of the locus coeruleus (LC) as well as in the locus subcoeruleus (SC,n=26). Among these units of the LC complex, 13 were coerulospinal (CS) neurons antidromically identified following stimulation of the spinal cord at T12-L1. Some of these neurons showed the main physiological characteristics of the norepinephrine (NE)-containing LC neurons, i.e., a slow and regular resting discharge and a typical biphasic response to fore- and hindpaw compression consisting of a short burst of excitation followed by a period of quiescence, due, in part at least, to recurrent and/or lateral inhibition. Unit firing rate was analyzed under separate stimulation of macular vestibular, neck, or combined receptors by using sinusoidal rotation about the longitudinal axis at 0.15 Hz, ±10° peak amplitude. Among the 90 LC-complex neurons, 60 (66.7%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 67 (74.4%) responded to neck rotation. Convergence of macular and neck inputs was found in 52/90 (57.8%) LC-complex neurons; in these units, the gain and the sensitivity of the first harmonic of the response corresponded on the average to 0.34±0.45, SD, impulsed·s−1·deg−1 and 3.55±2.82, SD, %/deg for the neck responses and to 0.23±0.29, SD, impulses·s−1·deg−1 and 3.13±3.04, SD, %/deg for the macular responses. In addition to these convergent units, 8/90 (8.9%) and 15/90 (16.7%) LC-complex units responded to selective stimulation either of macular or of neck receptors only. These units displayed a significantly lower response gain and sensitivity to animal tilt and neck rotation with respect to those obtained from convergent units. Most of the convergent LC-complex units were maximally excited by the direction of stimulus orientation, the first harmonic of responses showing an average phase lead of about +31.0° with respect to neck position and +17.6° with respect to animal position. Two populations of convergent neurons were observed. The first group of units (43/52, i.e., 82.7%) showed reciprocal (“out of phase”) responses to the two inputs; moreover, most of these units were excited during side-down neck rotation, but inhibited during side-down animal tilt. The second group of units (9/52, i.e., 17.3%) showed parallel (“in phase”) responses to the two inputs and they were excited by side-down or side-up neck rotation and animal tilt. The response characteristics of LC-complex neurons to combined neck and macular inputs, elicited during head rotation, corresponded to those predicted by a vectorial summation of the individual neck and macular responses. In particular, “out of phase” units displayed small amplitudes and large phase shifts of their responses with respect to those obtained during individual neck or macular stimulation. In contrast, “in phase” units displayed large responses during head rotation. Some nonlinearities of the responses to combined stimulation of neck and macular receptors, however, were observed. The possibility that the CS neurons contributed, with the vestibulospinal (VS) neurons, to the postural adjustments of the limb musculature during labyrinth and neck reflexes was discussed.