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)     

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 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)     

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)     

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)     

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)     

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)     

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)     

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)     

Effects of microinjection of vasopressin in dorsal pontine reticular structures on the gain of vestibulospinal reflexes in decerebrate cats.

1. The possibility that vasopressin (VP) acts on the dorsal pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system to control posture as well as the vestibulospinal reflexes has been investigated by injecting small doses of VP in precollicular decerebrate cats. 2. Unilateral microinjection of VP (0.25 microliters at the concentration of 10(-11) micrograms/microliters saline) in the pRF decreased the extensor rigidity in the ipsilateral limbs, while that of the contralateral limbs either decreased or increased. The same injection also produced a moderate or a prominent increase in gain of the multiunit EMG responses of the ipsilateral triceps brachii to roll tilt of the animal (t-test, P less than 0.001 for either group of responses). In the first instance the response gain of the contralateral triceps brachii to animal tilt slightly increased, while the pattern of response remained always of the alpha-type, as shown for the ipsilateral responses (increased EMG activity during ipsilateral tilt and decreased activity during contralateral tilt). In the second instance, however, the response gain showed only slight changes, while the pattern of responses reversed from the alpha- to the beta-type. These findings occurred 5-20 min after the injection, fully developed within 30-60 min and disappeared in about 2-3 hours. 3. The structures responsible for the postural and reflex changes described above were located in the dorsal pontine tegmental region immediately ventral to the LC, and included the peri-LC alpha and the surrounding dorsal pRF. The induced effects depended upon the injected neuropeptide, since previous injection of an equal volume of saline stained by the pontamine sky blue dye into the same dorsal pontine area was ineffective. 4. We postulated that VP exerts an excitatory influence on ipsilateral dorsal pRF neurons. The increased discharge of these neurons and the related medullary inhibitory RS neurons would lead to a decreased postural activity in the ipsilateral limbs. However, since these inhibitory RS neurons fire out of phase with respect to the excitatory vestibulospinal neurons, it appears that the higher the firing rate of the RS neurons in the animal at rest, the greater the disinhibition that affects the limb extensor motoneurons during ipsilateral tilt. These motoneurons would then respond more efficiently to the same excitatory volleys elicited by given parameters of stimulation, thus leading to an increased gain of the EMG responses.(ABSTRACT TRUNCATED AT 400 WORDS)     

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)     

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)     

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.     

Facilitation of triceps brachii muscle contraction by tendon vibration after chronic cervical spinal cord injury.

One way to improve the weak triceps brachii voluntary forces of people with chronic cervical spinal cord injury may be to excite the paralyzed or submaximally activated fraction of muscle. Here we examined whether elbow extensor force was enhanced by vibration (80 Hz) of the triceps or biceps brachii tendons at rest and during maximum isometric voluntary contractions (MVCs) of the elbow extensors performed by spinal cord-injured subjects. The mean +/- SE elbow extensor MVC force was 22 +/- 17.5 N (range: 0-23% control force, n = 11 muscles). Supramaximal radial nerve stimuli delivered during elbow extensor MVCs evoked force in six muscles that could be stimulated selectively, suggesting potential for force improvement. Biceps vibration at rest always evoked a tonic vibration reflex in biceps, but extension force did not improve with biceps vibration during triceps MVCs. Triceps vibration induced a tonic vibration reflex at rest in one-half of the triceps muscles tested. Elbow extensor MVC force (when >1% of control force) was enhanced by vibration of the triceps tendon in one-half of the muscles. Thus triceps, but not biceps, brachii tendon vibration increases the contraction strength of some partially paralyzed triceps brachii muscles.     

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)     

Changes of postural tonus produced by unilateral deafferentation of the musculature of the neck in pigeons

In intact or decerebrate pigeons, unilateral functional deafferentation of the dorsal neck muscles of one side produced a postural asymmetry, characterized by an increase in flexor tonus of the ipsilateral wing and leg and an increase in extensor tonus of the contralateral limbs. This postural asymmetry was just opposite in sign to that described recently in cats, where unilateral section of the cervical dorsal roots C1-C3 produced ipsilateral hypertonia and contralateral hypotonia of the limb extensors. The striking increase in flexor tonus of the wing following deafferentation of the ipsilateral neck extensors contrasts with the decrease in flexor tonus of the wing which occurs after deafferentation of the ipsilateral leg extensors. It appears, therefore, that the proprioceptive input from the neck and that from the leg extensors exert an antagonistic influence on the flexor tonus of the ipsilateral wing.     

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.     

EMG and MMG of agonist and antagonist muscles as a function of age and joint angle.

The aim of this study was to determine the effect of elbow joint position on electromyographic (EMG) and mechanomyographic (MMG) activities of agonist and antagonist muscles in young and old women. Surface EMG and MMG were recorded from the triceps and biceps brachii, and brachioradialis muscles during isometric elbow extensions in young and old women. The measurements were carried out at an optimal joint angle (A(o)), as well as at smaller (A(s) = A(o) - 30 degrees ) and larger (A(l) = A(o) + 30 degrees ) angles. The normalized to force EMG amplitude (RMS-EMG/F) was smaller in old women compared to young in all muscles. The RMS-EMG/F of the triceps brachii muscle was not affected by muscle length while that of the biceps brachii and brachioradialis muscles increased at shortest muscle length in both groups. The normalized to force MMG amplitude (RMS-MMG/F) was smaller in old than in young in the triceps brachii muscle only. There was an increase in RMS-MMG/F with triceps brachii and biceps brachii muscle shortening in both groups, and in the brachioradialis muscle -- in young only. Compared to young, older women exhibited a bigger force fluctuation during maximum voluntary contraction, but these did not contribute significantly to the RMS-MMG. Skinfold thickness accounted for the RMS-EMG/F and RMS-MMG/F differences seen between old and young women in the biceps brachii muscle only. It is concluded that, the EMG and MMG response to muscles length change in agonist and antagonist muscles is generally similar in old and young women but the optimal angle shifts toward a bigger value in older women.     

Relationship between muscle coordination and forehand drive velocity in tennis

This study aimed at investigating the relationship between trunk and upper limb muscle coordination and stroke velocity during tennis forehand drive. The electromyographic (EMG) activity of ten trunk and dominant upper limb muscles was recorded in 21 male tennis players while performing five series of ten crosscourt forehand drives. The forehand drive velocity ranged from 60% to 100% of individual maximal velocity. The onset, offset and activation level were calculated for each muscle and each player. The analysis of muscle activation order showed no modification in the recruitment pattern regardless of the velocity. However, the increased velocity resulted in earlier activation of the erector spinae, latissimus dorsi and triceps brachii muscles, as well as later deactivation of the erector spinae, biceps brachii and flexor carpi radialis muscles. Finally, a higher level of activation was observed with the velocity increase in the external oblique, latissimus dorsi, middle deltoid, biceps brachii and triceps brachii. These results might bring new knowledge for strength and tennis coaches to improve resistance training protocols in a performance and prophylactic perspective.     

Effects of Local and Remote Muscle Pain on Stretch ReflexActivities in the Elbow Joint Flexors and Extensors of Unanesthetized Cats

In experiments on unanesthetized cats, we compared the effects of experimentally induced pain in the m. biceps brachii or in the neck muscles on EMG activity of the flexors and extensors of the elbow joint (mm. biceps et triceps brachii, respectively) evoked by a passive extension-flexion of the above joint. Muscle pain was induced by injections of 0.5 ml of a hypertonic (7%) NaCl solution into the above-mentioned muscles. In the case of pain in the biceps, i.e., in the muscle directly involved in realization of the reflex, we observed an increase in the amplitude and significant shortening of the latency of EMG responses of this muscle. The amplitude of a short-latency (supposedly monosynaptic) component of the biceps reflex (М1 response) increased by 65%, while an increment of the latter (supposedly polysynaptic) М2 component was 117%. When pain was induced in anatomically remote neck muscles, the stretch reflex in the biceps was considerably suppressed. The maximum amplitudes of the М1 and М2 components decreased by 25 and 30%, respectively, but the latencies of these components decreased significantly, similarly to what was observed in the case of induction of experimental pain in the biceps. Under both conditions of experimental pain, changes in the parameters of EMG responses of the forearm extensor (m. triceps brachii) demonstrated similarity with those of the biceps responses. The maximum effect of pain induction was observed within the first 5 min after injections of the hypertonic solution; full recovery of the stretch reflex parameters was observed on the 20th to 30th min. We conclude that the effects of pain induction on the reflex under study are not generalized. They depend on the site of such induction with respect to the muscle where the stretch reflex is elicited. Unidirectional effects of both types of pain on the antagonist muscles allow us to suppose that modulation of the reflex reactions upon pain induction is mediated by influences from the supraspinal CNS structures. Induction of pain in the biceps increased the amplitude of EMG manifestations of the stretch reflex, while such induction in the neck muscles decreased such responses; nonetheless, in both cases the latency of the reflexes decreased. This fact allows us to believe that the sensitivity of muscle spindles increased under both conditions of the pain influence.