Injections of a beta-adrenergic antagonist in pontine reticular structures modify the gain of vestibulospinal reflexes in decerebrate cats

1. The norepinephrine (NE)-containing locus coeruleus (LC) neurons control posture as well as the gain of the vestibulospinal reflexes either through direct coeruleospinal (CS) projections or by inhibiting the dorsal pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system. The question whether these inhibitory influences on the pRF are mediated through beta-adrenoceptors was investigated by injecting in precollicular decerebrate cats small doses of the non-selective beta-adrenergic antagonist propranolol in different pontine tegmental structures. 2. Injection of propranolol (usually 0.25 microliters at the concentration of 4.5 micrograms/microliters of saline) in dorsal pontine structures, which decreased the tonic contraction of limb extensors ipsilateral to the side of the injection, greatly increased the amplitude of the multiunit EMG responses of the ipsilateral triceps brachii to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. Correspondingly, the response gain of the forelimb extensor to labyrinth stimulation increased. Moreover, a slight decrease in phase lead of the responses was observed. These responses were always characterized by an increased EMG activity during ipsilateral tilt and a decreased activity during contralateral tilt, as shown in the control records (alpha-responses). The same injection also produced in some instances an increase of the extensor tonus of the contralateral limbs, associated with an increased EMG activity of the contralateral triceps brachii; on the other hand, the amplitude of modulation and thus the response gain of this muscle to the same parameters of labyrinth stimulation decreased, while the response pattern reversed (beta-responses), thus being opposite to that displayed by the triceps brachii ipsilateral to the side of the injection. 3. The changes in posture and reflexes described above appeared 10-20 min after unilateral injection of propranolol in the pRF, reached in highest values in about 60-100 min and persisted for more that three hours before returning to the control level. These effects were not due to irritative phenomena following injection of the fluid, since neither changes in posture nor in the response gain of the triceps brachii to labyrinth stimulation were observed after injection of an equal volume of saline in the pRF of that side. Moreover, the magnitude of the effects increased to some extent in relation to the dose of the beta-adrenergic blocker. 4. Histological controls indicated that the structure responsible for these postural and reflex changes was located in the dorsal pontine tegmental region immediately ventral to the LC and included the peri-LC alpha and the surrounding dorsal pRF.(ABSTRACT TRUNCATED AT 400 WORDS)     

Pontine reticular origin of cholinergic excitatory afferents to the locus coeruleus controlling the gain of vestibulospinal and cervicospinal reflexes in decerebrate cats

1. Previous experiments had shown that the medullary inhibitory reticulospinal (mRS) neurons act 180 degrees out-of-phase with respect to the excitatory vestibulospinal (VS) neurons during the vestibular and the neck reflexes involving the limb extensor motoneurons. This finding suggested that the higher the firing rate of the medullary inhibitory RS neurons in the animal at rest, the greater the disinhibition which affects the limb extensor motoneurons during side-down roll tilt of the animal or side-up neck rotation, thus leading to an increased gain of response of limb extensors to sinusoidal stimulation of labyrinth and neck receptors. The gain of these postural reflexes would then represent a sensitive test to evaluate the background discharge of the inhibitory reticulospinal system of the medulla. 2. The discharge of the inhibitory mRS neurons is under the tonic excitatory control of cholinergic pontine reticular formation (pRF) neurons which are also self-excitatory, while these cholinergic pontine neurons are in turn inhibited by the norepinephrine (NE)-containing locus coeruleus (LC) neurons, which are also self-inhibitory due to mechanisms of recurrent and/or lateral inhibition. The present experiments were performed to find out whether cholinergic and cholinoceptive pontine reticular neurons, which are under the inhibitory control of the LC neurons, also send axons to the LC on which they may exert an excitatory influence. This excitatory effect would then counteract the self-inhibitory influence mediated by the NE, which acts on the alpha 2-adrenoceptors distributed on the somatodendritic membrane of the LC neurons. 3. In precollicular decerebrate cats, local injection into the dorsal aspect of the pontine tegmentum of 0.25 microliter of a solution of the muscarinic blocker atropine sulphate at the concentration of 6 micrograms/microliter of sterile saline did neither modify the postural activity in the ipsilateral limbs nor the response gain of the ipsilateral forelimb extensor triceps brachii to sinusoidal stimulation of labyrinth receptors (roll tilt of the animal at 0.15 Hz, +/- 10 degrees). These negative results were attributed to the fact that in these preparations the activity of the cholinergic and cholinoceptive pRF neurons and the related inhibitory mRS neurons is very low, due to the tonic discharge of the NE-containing LC neurons, which exert a prominent inhibitory influence on the underlying reticular structures.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inhibition of vestibulospinal reflexes during the episodes of postural atonia induced by unilateral lesion of the locus coeruleus in the decerebrate cat

1. The spontaneous EMG activity of the forelimb extensor triceps brachii of both sides as well as their responses to roll tilt of the animal at 0.15 Hz, +/- 10 degrees leading to sinusoidal stimulation of labyrinth receptors were tested in precollicular decerebrate cats, before and after unilateral electrolytic lesion of the locus coeruleus (LC). 2. Lesion of the LC of one side decreased the tonic contraction of the ipsilateral limb extensors, but greatly increased the amplitude of modulation and the response gain of the corresponding triceps brachii to animal tilt; however, no change in the phase angle of the responses was observed. A slight increase in the response gain affected also the contralateral triceps brachii. 3. The postural asymmetry described above was followed from time to time by short-lasting episodes of postural atonia, which affected not only the ipsilateral but also the contralateral limb extensors. These episodes were also associated with a suppression of the EMG responses of the triceps brachii of both sides to sinusoidal stimulation of labyrinth receptors. 4. The episodes of postural atonia which appeared after unilateral lesion of the LC were not associated with rapid eye movements; however, the slow horizontal eye movements, which may occur in normal decerebrate animals, increased in amplitude throughout these episodes. Both the postural atonia as well as the related suppression of the vestibulospinal reflexes, which lasted for 5-10 min, disappeared either spontaneously or following acoustic or somatosensory stimulations. 5. Histological controls indicated that unilateral lesions limited to the caudal part of the LC produced only a permanent decrease in postural activity of the ipsilateral limbs, associated with an increase in gain of the vestibulospinal reflex. However, in order to elicit episodes of bilateral postural atonia associated with the suppression of the vestibulospinal reflexes it was necessary to extend the lesion to more rostral aspects of the LC. 6. Since the effects described above were similar to those elicited in decerebrate cats by local injection of cholinergic agonists into the dorsal part of the pontine reticular formation, we postulated that the postural atonia as well as the related suppression of the vestibulospinal reflexes was due to transient release from LC inhibition of these dorsal pontine reticular structures, which might in turn excite the medullary reticulospinal neurons, thus leading to inhibition of the extensor motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of microinjection of cholinergic agonists into the pontine reticular formation on the gain of vestibulospinal reflexes in decerebrate cats

1. The question of which pontine neuronal groups and related receptors can mediate the cholinergic induction of the increased gain of vestibulospinal reflexes elicited by sinusoidal stimulation of labyrinth receptors was investigated by injecting in precollicular decerebrate cats either carbachol, which is a mixed muscarinic-nicotinic agonist, or bethanechol, which is a pure muscarinic agonist, via a cannula stereotaxically oriented in different pontine tegmental structures. 2. Injection of 0.1-0.2 microliter of carbachol solution (0.01-0.2 microgram/microliter of sterile saline) into the dorsal aspect of the pontine reticular formation (pRF), which slightly decreased the tonic contraction of limb extensors ipsilateral to the side of the injection, greatly increased the amplitude of the multiunit EMG response of the ipsilateral triceps brachii to roll tilt of the animal at 0.15 Hz, +/- 10 degrees, leading to selective stimulation of labyrinth receptors. Correspondingly, the response gain of the forelimb extensor to labyrinth stimulation increased. Moreover, a slight decrease in phase lead of the responses was observed. These findings were not attributable to decreased postural activity, since they were still observed when postural EMG activity was reflexly maintained by an increased static stretch of the muscle. No changes in the dynamic characteristics of the responses were observed in the contralateral triceps brachii. 3. The changes in posture as well as in response gain produced by the carbachol injection appeared suddenly, but partially declined to reach a plateau level which persisted for several hours before returning to the control level. Moreover, the magnitude of the effects increased in relation to the dose of the cholinergic agonist. 4. Histological controls indicated that the structure responsible for these postural and reflex changes was located in the dorsal aspect of the pontine tegmentum immediately ventral to the principal locus coeruleus (LC); this area corresponds to the peri-LC region and the surrounding pRF including the dorsal aspect of the central tegmental field. The effects were still obtained after chronic kainic acid lesioning of the gigantocellular area of the medulla. 5. An increase in gain of the vestibulospinal reflex which was as potent, dose-dependent, and site-specific as that previously observed with carbachol, appeared after injection of the pure muscarinic agonist bethanechol.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tonic inhibitory influences of locus coeruleus on the response gain of limb extensors to sinusoidal labyrinth and neck stimulations

Previous experiments had shown that in decerebrate cats activation of limb extensor motoneurons during side-down roll tilt of the animal or side-up neck rotation depends on both an increased discharge of excitatory vestibulospinal (VS) neurons and a reduced discharge of inhibitory reticulospinal (RS) neurons of the medulla, thus leading to disinhibition of limb extensor motoneurons. The present experiments were performed to find out whether the locus coeruleus (LC) complex keeps under its tonic inhibitory control the medullary inhibitory RS neurons and, if so, whether this structure intervenes in the gain regulation of the vestibular and neck reflexes acting on the limb extensor musculature. In precollicular decerebrate cats with good postural rigidity of the four limbs, the amplitude of modulation and thus the response gain of the first harmonic component of multiunit EMG responses of limb extensors to sinusoidal stimulation of labyrinth and neck receptors (at 0.15 Hz, +/- 10 degrees) were quite small in forelimb muscles (triceps brachii) and almost negligible or absent in hindlimb muscles (triceps surae). Electrolytic lesion limited to the LC complex decreased the tonic contraction of limb extensors, but greatly increased in the forelimbs (and brought to the light in the hindlimbs) the response modulation of extensor muscles to the same parameters of labyrinth or neck stimulation. Correspondingly, the response gain increased, but no change in the phase angle of the responses was observed. Both changes in posture, as well as in response gain of the limb extensors to labyrinth and neck stimulation, fully developed some time after the LC lesion. This increase in response gain of the vestibular and neck reflexes acting on the limb extensor muscles did not depend on the decrease in postural activity following the LC lesion, since it was still obtained when an increased static stretch of the extensor muscle following passive flexion of the limb compensated for the reduced EMG activity. Moreover, the slope of the regression line relating the gain of the multiunit EMG response of the triceps brachii to animal tilt with the base frequency greatly increased following lesioning of the LC, thus indicating that for the same background discharge of the muscle the amplitude of modulation, and thus the response gain, increased significantly. The effects described above involved mainly, but not exclusively, the limbs ipsilateral to the side of the lesion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tonic facilitatory influences of dorsal pontine reticular structures on the response gain of limb extensors to sinusoidal labyrinth and neck stimulations

Experiments were performed to find out whether changes in resting discharge of the inhibitory reticulospinal (RS) neurons of the medulla, produced either by selective destruction or by cholinergic activation of a pontine tegmental reticular system, may modify the response gain of limb extensor muscles to given parameters of roll tilt of the animal or neck rotation. In precollicular decerebrate cats, an electrolytic lesion of the dorsal aspect of the pontine tegmentum, which slightly increased the tonic contraction of limb extensors, greatly decreased the amplitude of the multiunit EMG response of forelimb extensor muscles, i.e. of the medial head of the triceps brachii, to roll tilt of the animal and neck rotation (at 0.15 Hz, +/- 10 degrees), leading to selective stimulation of labyrinth or neck receptors. Correspondingly, the response gain of the forelimb extensors to labyrinth and neck stimulation decreased, but no change in the phase angle of the responses was observed. These findings did not depend on the increased postural activity, since they were still observed in the absence of any change in spontaneous EMG activity of the triceps brachii following the lesion. The changes in posture as well as in response gain of the forelimb extensors to labyrinth and neck stimulation produced by the pontine lesion appeared suddenly, and persisted for several hours throughout the survival period. Moreover, these changes involved mainly, but not exclusively, the limbs ipsilateral to the side of the lesion. Histological controls indicated that the structure responsible for the postural and reflex changes described above corresponded to the dorsal aspect of the pontine tegmentum located immediately ventral to the locus coeruleus (LC); this area corresponded to the peri-LC region as well as the surrounding pontine reticular formation (RF), including the dorsal aspect of the central tegmental field. This region closely corresponds to the area from which a tegmentoreticular tract, ending on the medullary inhibitory area, originates. It was previously shown that unilateral or bilateral lesion of the LC, which decreased the extensor tonus in the ipsilateral limbs, greatly enhanced the response gain of the triceps brachii to sinusoidal stimulation of labyrinth and neck receptors. These findings were attributed to suppression of an inhibitory influence that the LC exerts on the dorsal pontine reticular structures described above.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of microinjection of a cholinergic agonist into the locus coeruleus on the gain of vestibulospinal reflexes in decerebrate cats

1. Experiments were performed in precollicular decerebrate cats to determine whether activation of locus coeruleus (LC) neurons elicited by local injection of the cholinergic agonist carbachol modifies the dynamic characteristics of responses of forelimb extensors to selective stimulation of labyrinth receptors resulting from roll tilt of the animal. 2. Injection of 0.1-0.4 microliter (usually 0.25 microliter) of carbachol at a concentration of 0.02-0.1 micrograms/microliter of sterile saline into the LC of one side, which slightly increased the tonic contraction of limb extensors ipsilateral to the side of the injection, greatly decreased the amplitude of the multiunit EMG response of the ipsilateral triceps brachii to animal tilt at 0.15 Hz, +/- 10 degrees. Correspondingly, the response gain of this forelimb extensor decreased. Moreover, a significant increase in phase lag of the responses was observed. These findings did not result from the increased postural activity, since they were still observed when the limb position was adjusted so that the spontaneous EMG activity remained constant throughout the experiments. 3. The changes in posture as well as in response characteristics of the forelimb extensor to labyrinth stimulation produced by carbachol injection appeared a few min after the injection and soon reached a plateau level which persisted for several hours before returning to the control levels. 4. The effects described above involved mainly, if not exclusively, the limbs ipsilateral to the side of the injection. However, the effects of local injection into the LC of one side could be reproduced on the contralateral side following injection into the LC of that side. 5. The increase in phase lag of the multiunit EMG responses of the triceps brachii to labyrinth stimulation appeared at a threshold lower than that required to decrease the response gain of this extensor muscle. These findings suggest that different neuronal populations within the LC complex, one projecting directly to the spinal cord, the other projecting indirectly through the pontine reticular formation, are involved in the control of phase angle and gain of the vestibulospinal reflexes, respectively. However, as soon as the threshold was reached the effects described above were dose-dependent. 6. Histological controls indicated that the structure responsible for the postural and reflex changes described above corresponded to the LC. In fact, postural and reflex changes opposite in sign to those described above were obtained when the same amount of carbachol was injected into the dorsal aspect of the pontine reticular formation (pRF) located immediately ventral to the LC.(ABSTRACT TRUNCATED AT 400 WORDS)     

Injections of beta-adrenergic substances in the locus coeruleus affect the gain of vestibulospinal reflexes in decerebrate cats

1. The tonic discharge of the noradrenergic locus coeruleus (LC) neurons is dampened by norepinephrine (NE) which acts not only on alpha2-adrenoceptors located on the somatodendritic membrane, through mechanisms of recurrent inhibition, but also on beta-receptors. Experiments were performed to find out whether inactivation of LC neurons by local injection of the beta-adrenergic agonist isoproterenol into the LC complex of one side produced changes in posture as well as in the gain of vestibulospinal reflexes acting on forelimb extensors. 2. In precollicular decerebrate cats the amplitude of modulation and thus the gain of the multiunit EMG responses of the forelimb extensor triceps brachii to animal tilt at 0.15 Hz, +/- 10 degrees, leading to sinusoidal stimulation of labyrinth receptors, were quite small. Microinjection of 0.25 microliter of a solution of the beta-adrenergic agonist isoproterenol at the concentration of 4.5-9.0 microgram/microliter of sterile saline into the LC complex of one side decreased the extensor rigidity in the ipsilateral limbs and to a lesser response gain of the ipsilateral triceps brachii to the same parameters of labyrinth stimulation greatly increased (t-test, P less than 0.001); moreover, a slight but significant increase in phase lead of the responses was observed. These findings appeared within 5-10 min after the injection of isoproterenol, fully developed within 20-30 min and persisted for about 2-3 hours after the injection. 3. The increased gain of the vestibulospinal reflexes acting on the triceps brachii did not depend on the decreased postural activity following injection of the beta-adrenergic agonist, since it was still observed if the reduced EMG activity of the extensor muscle following the injection was compensated for by an increased static stretch of the muscle. The positive correlation (t-test, P less than 0.001) between gain of the multiunit EMG response of the triceps brachii to animal tilt and base frequency observed in the control experiment disappeared and was substituted by a slight negative correlation (t-test, P less than 0.05) after injection of isoproterenol into the LC complex, probably due to a more prominent recruitment of motor units for low level of background discharge of the muscle. 4. In addition to the effects which involved the triceps brachii ipsilateral to the side of the injection, a smaller but significant increase in response gain affected the contralateral extensor muscle. This increase in gain was also associated with a slight increase in phase lead of the responses.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inhibition of vestibulospinal reflexes following cholinergic activation of the dorsal pontine reticular formation

1. The multiunit EMG activity of the forelimb extensor muscle triceps brachii was recorded in precollicular decerebrate cats, either at rest or during roll tilt of the animal at 0.15Hz, +/- 10 degrees leading to sinusoidal stimulation of labyrinth receptors. Both the spontaneous EMG activity as well as the labyrinthine-induced EMG responses were tested before and after pontine microinjection of a cholinergic agonist. 2. Local injection of the cholinergic agonist carbachol into the dorsal aspect of the pontine tegmentum (usually 0.25 microliter, 0.01-0.2 microgram/microliter) produced a state of postural atonia, and abolished both the spontaneous EMG activity as well as the EMG responses of the triceps brachii to sinusoidal stimulation of labyrinth receptors. This suppression was generally ipsilateral to the side of the injection and persisted throughout the episode of postural atonia, but sometimes it involved also the contralateral limbs. In these instances it could be accompanied by a spontaneous nystagmus, interspersed at regular intervals with bursts of rapid eye movements. 3. Similar effects were also obtained following injection of carbachol in the gigantocellular tegmental field (FTG) (0.25 microliter, 0.5-1.0 microgram/microliter). However, this structure was not critically responsible for the phenomena reported above, which persisted unaltered after kainic acid lesion of the FTG performed ipsilaterally to the side of the pontine injection. 4. Local infusion of the muscarinic blocker atropine sulphate reversed the effects of carbachol injection into the dorsal aspect of the pontine tegmentum, thus indicating that muscarinic receptors were involved. 5. It is postulated that the postural atonia as well as the tonic depression of vestibulospinal reflexes, which occur in the decerebrate cat after local injection of a cholinergic agonist depends, at least in part, on the activation of cholinoceptive neurons located in dorsal pontine reticular structures. These may in turn excite medullary reticulospinal neurons, which are finally responsible for the inhibition of extensor motoneurons.     

Microinjections of beta-noradrenergic substances in the cerebellar vermis of decerebrate cats modify the gain of the vestibulospinal reflexes.

1. The noradrenergic (NA) afferent system, which originates mainly from the locus coeruleus and projects to the cerebellar cortex, may act on the corresponding neurons by utilizing not only alpha- but also beta-adrenoceptors. Since the vermal cortex of the cerebellar anterior lobe receives a labyrinth input and projects to the lateral vestibular nucleus (LVN), experiments were performed in precollicular decerebrate cats to find out whether the noradrenergic system intervenes in the control of posture as well as of the dynamic characteristics of vestibulospinal (VS) reflexes elicited by recording the multiunit EMG responses of the forelimb extensor triceps brachii of both sides to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. In particular, we used the method of local microinjection into the vermal cortex of the cerebellar anterior lobe of the non-selective beta-adrenergic agonist ((+/-) -isoproterenol hydrochloride) or antagonist (dl-propranolol hydrochloride) to act on both beta 1- and beta 2-adrenoceptors. 2. Unilateral injection into the vermal cortex of the culmen of isoproterenol (0.25-0.50 microliters at the concentration of 8-16 micrograms/microliter of saline stained with pontamine 5%) decreased the extensor tonus in the ipsilateral forelimb, while the postural activity either remained unmodified or slightly increased in the contralateral fore-limb. The same injection significantly increased the gain (imp./sec/deg) of the first harmonic component of the EMG responses of the ipsilateral and to a lesser extent also of the contralateral triceps brachii to animal tilt. This effect was also associated with slight changes in the phase angle of the responses, which remained positional throughout the experiments. The effects described above occurred within 5-10 min after the injection and reached the highest values after 20-30 min; they were then followed for about 2 hours after the injection, before disappearing. 3. In contrast to these findings, injection in other experiments of 0.25-0.50 microliter of a solution of propranolol at the concentration of 16 micrograms/microliter of saline increased the extensor tonus in the ipsilateral limbs, while the decerebrate rigidity either remained unmodified or slightly decreased in the contralateral limbs. In addition, the amplitude of modulation and thus the response gain of the ipsilateral triceps brachii to the same parameters of animal tilt decreased. This effect was associated with slight changes in the phase angle of the responses. There was also a slight but insignificant decrease in gain of the responses recorded contralaterally to the side of the propranolol injection.(ABSTRACT TRUNCATED AT 400 WORDS)     

Muscarinic receptors in the cerebellar vermis modulate the gain of the vestibulospinal reflexes in decerebrate cats.

1. The Purkinje (P)-cells of the cerebellar vermis, which exert a prominent influence on posture as well as on the gain of vestibulospinal (VS) reflexes, are under the control not only of the classic mossy fibers and climbing fibers which liberate excitatory amino acids as neurotransmitter, but also of cholinergic afferents. The role of these afferents was investigated in precollicular decerebrate cats by using the method of local microinjection of cholinergic agents into appropriate areas of the cerebellar cortex. 2. Unilateral injection into the vermal cortex of the culmen of the non-selective cholinergic agonist carbachol (0.25 microliters at 0.5 micrograms/microliters saline) produced a postural asymmetry, characterized by a slight decrease of the extensor tonus in the ipsilateral forelimb and an increased tonus in the contralateral forelimb. Moreover, the gain of the EMG responses of the ipsilateral and the contralateral triceps brachii to animal tilt increased significantly, while no significant changes in the phase angle of the responses were observed. These effects started 5-10 min after the injection and persisted for at least 2 hours before disappearing. Similar but smaller effects were obtained after injection of eserine, an inhibitor of acetylcholinesterase. Thus, the effects could be produced by increasing the naturally present amount of acetylcholine (ACh). 3. The changes in posture and gain of the VS reflexes described above utilized in part at least muscarinic receptors, since effects similar to those induced by carbachol injection were also obtained after unilateral microinjection into the vermal cortex of the culmen of the muscarinic agonist bethanechol (0.25 microliters at 0.1 micrograms/microliters). On the other hand opposite effects, characterized by an increased postural activity in the ipsilateral forelimb associated with a decreased activity in the contralateral forelimb, as well as by a reduced gain of the EMG responses of the triceps brachii of both sides to animal tilt were observed in other experiments after local microinjection of the muscarinic antagonist scopolamine (0.25 microliter at 4-8 micrograms/microliters saline). Evidence for muscarinic supersensitivity was obtained following repetitive injections of scopolamine into the cerebellar vermis. 4. The area which upon injection of the cholinergic agents modified the postural activity as well as the gain of the VS reflexes was located within the third and/or the fourth folium rostral to the fissura prima (culmen), at the laterality of 1.4-1.8 mm with respect to the midline.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of passive flexion of the forepaw on the response gain of limb extensors to sinusoidal stimulation of labyrinth receptors

The main aim of the present study was to find out whether the dynamic characteristics of responses of limb extensor muscles to labyrinth stimulation were modified by the proprioceptive input elicited by appropriate displacements of the corresponding limb extremity. In cats decerebrated at precollicular or intercollicular level, the multiunit EMG activity of the medial head of the triceps brachii was recorded during roll tilt of the animal at the frequency of 0.15 Hz, +/- 10 degrees leading to selective stimulation of labyrinth receptors. This stimulation was then tested several times at regular intervals of 2 to 6 min for several hours while maintaining the ipsilateral forelimb in the horizontal extended position, i.e. with the plantar surface of the foot lying on the tilting table, or during passive flexion of the forepaw in plantar or dorsal direction. In all the experiments in which the forelimb was in the control position, the multiunit EMG responses of the triceps brachii were characterized by an increased activity during side-down tilt of the animal and a decreased activity during side up tilt. These responses were related to animal position and not to the velocity of animal displacement, thus being attributed to stimulation of macular, utricular receptors. Static displacement of limb extremities following plantar flexion of the forepaw greatly decreased the amplitude of the EMG modulation and thus the gain of the first harmonic component of the multiunit EMG responses of the ipsilateral triceps brachii to animal tilt. This reduced gain was due not only to a reduced number of motor units recruited during labyrinth stimulation, but also to a reduced modulation of firing rate of the active motor units, as shown by recording the activity of individual motor units. On the other hand, displacement of the same extremity in the opposite direction, i.e. following dorsiflexion of the forepaw, enhanced the amplitude of the EMG modulation and thus the gain of the multiunit EMG responses of the ipsilateral triceps brachii to animal tilt. This finding was mainly due to an increased recruitment of motor units during side-down tilt, although an increased modulation of the firing rate of individual motor units could not be excluded. In both instances, no changes in the phase angle to the responses were observed. The changes in response gain described above depended on the amount of passive displacement of the forepaw and persisted unmodified throughout the new maintained position.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cholinergic mechanism controlling the response gain of forelimb extensor muscles to sinusoidal stimulation of macular labyrinth and neck receptors

The multiunit EMG activity of the triceps brachii was recorded in precollicular decerebrate cats during roll tilt of the animal or neck rotation at the frequencies of 0.026-0.15 Hz and at the peak amplitude of 10 degrees, leading to selective stimulation of labyrinth or neck receptors. The first harmonic component of the EMG responses to labyrinth stimulation was characterized by an increased activity during side-down tilt of the animal and a decreased activity during side-up tilt; however, just the opposite changes were elicited for the same directions of neck rotation. The peak of the responses was closely related to the extreme animal or neck displacement, thus being attributed to stimulation of position-sensitive macular labyrinth and receptors. Moreover, the modulation as well as the gain of the EMG responses were small in amplitude. Intravenous injections of an anticholinesterase at a dose which in some instances slightly decreased the extensor tonus as well as the background activity of the triceps brachii (eserine sulphate, 0.05-0.075 mg/kg), greatly enhanced the response gain of this extensor muscle to animal tilt or neck rotation at the parameters reported above. This finding was also observed in the absence of any decrease in spontaneous EMG activity of the extensor muscle after injection of the anticholinesterase. In no instance did the phase angle of the response change following these injections. The increased gain of the EMG response of the forelimb extensor muscle to sinusoidal stimulation of labyrinth and neck receptors was first observed 5-10 min after the injection and reached the highest value in about one hour. This effect, was not only time-dependent, but also state-dependent. In fact, the increase in response gain described above either did not occur or was negligible during the sudden recovery of the extensor rigidity which occurred either spontaneously or after somatosensory stimulations. The effects elicited by eserine sulphate were reversed within seconds by a 0.1-0.5 mg/kg dose of atropine sulphate, an anticholinergic drug. It is postulated that for the same labyrinthine or neck signal giving rise to excitatory vestibulospinal volleys acting on extensor motoneurons, the amplitude of the EMG modulation of limb extensor muscles depends on the activity of a cholinergic system.(ABSTRACT TRUNCATED AT 400 WORDS)     

Microinjections of alpha 1- and alpha 2-noradrenergic substances in the cerebellar vermis of decerebrate cats affect the gain of the vestibulospinal reflexes.

1. In addition to mossy and climbing fibers, the Purkinje (P)-cells of the cerebellar cortex receive noradrenergic (NA) afferents which originate mainly from the locus coeruleus. Since these fibers impinge also on the vermal cortex of the cerebellar anterior lobe, which receives a labyrinth input and projects to the lateral vestibular nucleus, experiments were performed in precollicular decerebrate cats to find out whether unilateral injection of alpha-adrenergic substances into the vermal cortex of the cerebellar anterior lobe exerted some influence on posture as well as on the dynamic characteristics of vestibulospinal (VS) reflexes evaluated by recording the multiunit EMG responses of the forelimb extensor triceps brachii of both sides to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. 2. Unilateral injection into the vermal cortex of the culmen of the alpha 1-adrenergic agonist metoxamine or the alpha 2-adrenergic agonist clonidine (0.25 microliters at the concentration of 4 micrograms/microliters of saline) produced a postural asymmetry, characterized mainly by a slight decrease of the extensor tonus in the ipsilateral forelimb and an increased tonus in the contralateral forelimb. The same substances significantly increased the gain (imp./sec/deg) of the first harmonic component of the EMG responses of the ipsilateral and the contralateral triceps brachii to animal tilt. The crossed effects were more prominent for the alpha 2- than for the alpha 1-agonist. However, no significant changes in the phase angle of the responses were observed in both instances. The effects described above occurred within 5-10 min after the injection, reached the peak values after 15-20 min, and disappeared within 2 hours. 3. The postural and reflex changes described above were not due to irritative events following the injection, since they were not observed in control experiments after injection of 0.25 microliter of saline into the same corticocerebellar area prior to the administration of the alpha 1- or the alpha 2-adrenergic agonist. Moreover, the resulting effect were dose-dependent. 4. Both the ipsilateral as well as the contralateral effects induced by the alpha 1- or the alpha 2-adrenergic agonist metoxamine or clonidine were impaired by previous injection into the same corticocerebellar area of the corresponding alpha 1- or alpha 2-adrenergic antagonist prazosin or yohimbine, respectively (0.25 microliter at the concentration of 8-16 micrograms/microliters in both cases).(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of pressure stimulation of the body surface on posture and vestibulospinal reflexes

The effects of pressure stimulation of the body surface on postural activities as well as on the response gain of limb extensors to natural stimulation of labyrinth receptors were investigated in intact, as well as in decerebrate cats. In intact, unanesthetized cats, slight pressure applied symmetrically to the body surface at the chest level decreased the tonic activity of the axial (neck) and limb extensor musculature, as well as the proprioceptive reflexes induced by passive flexion of the limbs. The positive supporting reaction caused by pressure applied to the pad of the foot was also depressed. If the cats were suspended in the air by their nape, slight pressure applied to the upper part of the body greatly reduced the tonic contraction of the forelimb extensors to linear acceleration after downward movement of the animal, a response which can be attributed to stimulation of macular receptors located in the sacculus. Moreover, the prominent myotatic reflexes which occurred in all four limbs as soon as the animal touched the floor were greatly depressed, as shown by the fact that the forelimbs displayed only a slight tonic contraction of the extensor musculature during landing, while the hindlimbs collapsed under the weight of the body. In precollicular decerebrate cats there was a good postural activity in all four limbs. Moreover, the multiunit EMG activity of the medial head of the triceps brachii responded to roll tilt of the animal (at 0.15 Hz, +/- 10 degrees) leading to selective stimulation of labyrinth receptors. These responses, characterized by an increased EMG activity during side-down tilt and a decreased activity during side-up tilt, were related to animal position and not to velocity of animal displacement, and are thus attributable to stimulation of macular, utricular receptors. Slight pressure applied to the chest greatly decreased not only the postural activity of the limbs, but also the amplitude of EMG modulation and then the gain in the first harmonic component of the multiunit EMG responses of the triceps brachii to animal tilt. This reduced gain was due, in particular, to a reduced number of motor units being recruited during labyrinth stimulation, although a reduced modulation of firing rate of the active motor units should not be ruled out. However, no changes in the phase angle of the responses were observed.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cholinoceptive pontine reticular structures modify the postural adjustments during the limb movements induced by cortical stimulation

1. Activation of the pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system decreases the postural activity. This effect can be achieved either by local injection into the dorsal pontine tegmentum of cholinergic agonists which excite cholinoceptive pRF neurons, or by injection of noradrenergic agents which block the inhibitory influence exerted by the locus coeruleus (LC) neurons on the pRF. The main aim on the present study was to analyze the effects of tonic activation of these pRF neurons on the postural adjustments accompanying limb movements induced by motor cortex stimulation. In particular, electrodes were implanted chronically in the motor cortex of cats and stainless steel guide tubes of small size, later used for drug injection, were set bilaterally into sites just above the responsive regions. 2. Limb flexion elicited by stimulation of the motor cortex was accompanied by a diagonal pattern of postural adjustment, characterized by a decreased force exerted by the limb diagonally opposite to the moving one and an increased force exerted by the other two. 3. Microinjection into the pRF of both sides of 0.25 microliter of the muscarinic agonist bethanechol at the concentration of 8 or 16 micrograms/microliters in buffered artificial cerebrospinal fluid produced a short-lasting episode of postural atonia followed by a period of reduced postural activity, during which the cats were still able to stand on the measurement platform. Under this condition no changes in threshold, latency and amplitude of the flexion response were observed in the performing limb; however, the postural responses were considerably affected. In particular, when the performing limb was a forelimb, the other anterior limb showed a dissociation of the postural response in two distinct components. The first anticipatory component, which had a short latency (12-15 msec) and was considered to be centrally triggered, decreased in amplitude after injection of bethanechol and sometimes disappeared; on the other hand the second component, which had a long latency (50-60 msec) and was thus considered to be of reflex origin, increased in amplitude, due to the instability resulting from the depression of the early postural response. Similar results also affected to a lesser extent the hindlimbs. Moreover, body oscillations were observed and monitored from the force platforms following the late component of the postural responses.(ABSTRACT TRUNCATED AT 400 WORDS)     

GABA receptors in Deiters nucleus modulate posturokinetic responses to cortical stimulation in the cat.

The early component of the postural responses which accompany the limb flexion during unilateral stimulation of the motor cortex in the cat is not of reflex origin, but results from a central command. These postural adjustments are characterized by a decreased force under the limb diagonally opposite to the moving one and an increased force under the other two. Since the lateral vestibular nucleus (LVN) exerts an excitatory influence on ipsilateral limb extensor motoneurons, experiments were performed in cats to establish whether the cortical-induced postural changes were mediated through the LVN. This structure is tonically inhibited by GABAergic synapses originating from Purkinje cells of the cerebellar vermis, so that local microinjection into the LVN of GABA agonists or antagonists should either decrease or increase the spontaneous discharge of their neurons. Unilateral microinjection of 0.25 microliters of the GABA-A agonist muscimol or the GABA-B agonist baclofen (at 2-4 micrograms/microliters saline) into the LVN produced a short-lasting episode of ipsilateral postural hypotonia and contralateral hypertonia, during which the cats were unable to stand on the measurement platform. When, shortly after, some recovery of the postural activity appeared, no changes in threshold, latency or amplitude of the cortical-induced flexion movement were observed; however, the early component of the postural responses decreased in the other three limbs. Moreover, the slope of the response curve of the moving limb remained unmodified, while that of the early component of the postural responses, which involved the remaining limbs, decreased following stimulation of the motor cortex at different stimulus intensities. These effects started a few min after the injection and lasted for about 2-3 h. The effects described above were dose-dependent. Moreover, histological controls indicated that the structure responsible for these postural changes corresponded to the middle part of the LVN. The specificity of the results was shown by the fact that unilateral microinjection of 0.25 microliters of the GABA-A antagonist bicuculline or the GABA-B antagonist phaclofen (at 5-8 micrograms/microliter saline) into the LVN produced a postural asymmetry opposite in sign to that elicited in the same experiments by the corresponding agonists. These injections did not modify the amplitude of the cortical-induced limb flexion, but rather enhanced the amplitude of the early component of the postural responses in the other three limbs.(ABSTRACT TRUNCATED AT 400 WORDS)     

Early postnatal maturation in vestibulospinal pathways involved in neck and forelimb motor control

To assess the organization and functional development of vestibulospinal inputs to cervical motoneurons (MNs), we have used electrophysiology (ventral root and electromyographic [EMG] recording), calcium imaging, trans‐synaptic rabies virus (RV) and conventional retrograde tracing and immunohistochemistry in the neonatal mouse. By stimulating the VIIIth nerve electrically while recording synaptically mediated calcium responses in MNs, we characterized the inputs from the three vestibulospinal tracts, the separate ipsilateral and contralateral medial vestibulospinal tracts (iMVST/cMVST) and the lateral vestibulospinal tract (LVST), to MNs in the medial and lateral motor columns (MMC and LMC) of cervical segments. We found that ipsilateral inputs from the iMVST and LVST were differentially distributed to the MMC and LMC in the different segments, and that all contralateral inputs to MMC and LMC MNs in each segment derive from the cMVST. Using trans‐synaptic RV retrograde tracing as well as pharmacological manipulation of VIIIth nerve‐elicited synaptic responses, we found that a substantial proportion of inputs to both neck and forelimb extensor MNs was mediated monosynaptically, but that polysynaptic inputs were also significant. By recording EMG responses evoked by natural stimulation of the vestibular apparatus, we found that vestibular‐mediated motor output to the neck and forelimb musculature became more robust during the first 10 postnatal days, concurrently with a decrease in the latency of MN discharge evoked by VIIIth nerve electrical stimulation. Together, these results provide insight into the complexity of vestibulospinal connectivity in the cervical spinal cord and a cogent demonstration of the functional maturation that vestibulospinal connections undergo postnatally. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1061–1077, 2016     

Modulation of the Cutaneous Silent Period in the Upper-Limb with Whole-Body Instability

The silent period induced by cutaneous electrical stimulation of the digits has been shown to be task-dependent, at least in the grasping muscles of the hand. However, it is unknown if the cutaneous silent period is adaptable throughout muscles of the entire upper limb, in particular when the task requirements are substantially altered. The purpose of the present study was to examine the characteristics of the cutaneous silent period in several upper limb muscles when introducing increased whole-body instability. The cutaneous silent period was evoked in 10 healthy individuals with electrical stimulation of digit II of the right hand when the subjects were seated, standing, or standing on a wobble board while maintaining a background elbow extension contraction with the triceps brachii of ~5% of maximal voluntary contraction (MVC) strength. The first excitatory response (E1), first inhibitory response (CSP), and second excitatory response (E2) were quantified as the percent change from baseline and by their individual durations. The results showed that the level of CSP suppression was lessened (47.7 ± 7.7% to 33.8 ± 13.2% of baseline, p = 0.019) and the duration of the CSP inhibition decreased (p = 0.021) in the triceps brachii when comparing the seated and wobble board tasks. For the wobble board task the amount of cutaneous afferent inhibition of EMG activity in the triceps brachii decreased; which is proposed to be due to differential weighting of cutaneous feedback relative to the corticospinal drive, most likely due to presynaptic inhibition, to meet the demands of the unstable task.     

Modulation of slow and fast elbow extensor EMG tonic activity by stretch reflexes in man

Reflex EMG responses to sudden passive flexion of the elbow were recorded from anconeus and triceps brachii in 5 human volunteers. While the subjects were required not to resist the flexion movement, they were required to maintain an extension torque of 3.5 or 7.0 Nm prior to its onset. Under these isotonic conditions, the latency and amplitude of the reflex activities from anconeus and triceps brachii did not differ significantly, in contrast to the findings of Le Bozec (1986) in actively relaxed subjects. The myotatic/postmyotatic EMG amplitude ratio did not provide a further quantitative way to distinguish between these muscles. The absence of a difference between the reflex activities of a slow (anconeus) and a fast (triceps brachii) muscle is interpreted as resulting from a strong drive of spindle activity on the whole extensor motoneuron pool, which outweights the differences in recruitment due to the differing relative amounts of type I and type II fibres in the two muscles. Differences like those described between finger and calf muscles by other authors are thought to be due to the relative degree of corticalization of these muscles. All short and long latency responses of the muscles increased in magnitude and decreased in latency with increasing background EMG activity as well as with increasing initial length. The position and tonic activity dependency of these responses is explained in terms of alpha-gamma coactivation.