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PubMed2002

Sex differences in genioglossus muscle response to changes in pharyngeal resistance.

The genioglossus (GG) muscle's response to partial oropharyngeal occlusion was studied in age-matched, healthy awake men (n = 16) and women (n = 15). A miniature balloon was placed in the retroglossal pharynx, and the GG's electromyographic (EMG) reflexive response was evaluated in different body postures. We assumed that inflating the pharyngeal balloon and changing the body posture from upright to supine would increase pharyngeal airway resistance. Our hypothesis was that the change in airway resistance would elicit a different response in the GG muscle depending on sex. Our results showed that GG activity during natural breathing was higher in women than in men. GG EMG activity did not change upon inflation of the balloon in women. In contrast, when the balloon was inflated in the men, the GG's basal activity increased (P <.01) in both the upright and the supine positions. Women appeared to show a higher GG baseline EMG activity during spontaneous breathing at rest, while men were more responsive to the partial occlusion of the pharyngeal airway.

Adult↗
PubMed1999

EMG responses in lower leg muscles elicited by head-taps.

Forehead-taps elicited short-latency motor responses in the lower leg muscle which, together with vestibulocollic reflexes, might contribute to multisensory control of posture. We tested this paradigm in human subjects standing upright in order to determine if the responses differed depending on whether the taps were to the forehead or temporal bone. Forehead-taps elicited short-latency inhibitory EMG responses in both gastrocnemius muscles with a mean latency of 48 to 99 ms. The right or left temporal bone-taps elicited short-latency inhibitory EMG responses in the ipsilateral gastrocnemius muscle of 47 to 77 ms, short-latency excitatory EMG responses in the contralateral gastrocnemius muscle of 46 to 78 ms, and short-latency excitatory EMG responses in the contralateral tibialis anterior muscle of 48 to 78 ms. The temporal bone-taps elicited short-latency inhibitory EMG responses in the ipsilateral gastrocnemius muscle and excitatory EMG responses in the contralateral gastrocnemius muscle, which were thought to be generated by vestibular afferents and convergent with vestibular input for the multisensory control of posture.

Adult↗
PubMed1987

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)

Animals↗
PubMed2000

Posture-related changes of soleus H-reflex excitability.

We investigated whether the modulatory effects of segmental and descending inputs on the soleus H reflex are modified by postural conditions. Fourteen healthy volunteers received a transcranial magnetic stimulus (TMS) or percutaneous electrical stimulation of the posterior tibial nerve (PTN), preceding by 0 to 400 ms the elicitation of the soleus H reflex in supine, sitting, and standing positions. In all positions, TMS induced an early period of facilitation at interstimulus intervals (ISIs) ranging between 5 and 35 ms. In supine and sitting positions, there was a second period of facilitation at ISIs between 60 and 90 ms, which was absent or significantly reduced in the standing position. PTN induced a strong inhibition of the H reflex in all positions up to 125 ms. In supine and sitting positions, inhibition continued up to 400 ms, whereas it was significantly reduced or completely absent beyond 125 ms in the standing position. These results demonstrate posture-related differences in the modulatory effects of descending and segmental inputs on the excitability of the H-reflex circuit.

Adult↗
PubMed1991

Task-dependent changes in the responses to low-threshold cutaneous afferent volleys in the human lower limb.

1. In seven human subjects who were standing without support the sural nerves were stimulated electrically using trains of non-painful stimuli (five pulses at 300 Hz), designed to activate afferents from cutaneous mechanoreceptors. The reflex effects of the stimulus train on different muscles of the ipsilateral and contralateral legs were sought in post-stimulus averages of rectified EMG. Changes in the pattern of reflex influence were investigated when the subjects maintained different postures. 2. Clear reflex responses were seen in ipsilateral tibialis anterior, soleus, biceps femoris and vastus lateralis, but only when the muscles were actively contracting. In each muscle, inhibition was the dominant reflex response within the first 100 ms. In four of the seven subjects, reflex changes were detectable in the contralateral tibialis anterior and soleus, the peak-to-peak modulation within the first 200 ms being 25-50% of that for the homologous ipsilateral muscle. 3. When subjects attempted to stand on a tilted platform, an unstable platform or on one leg with the other flexed, different combinations of muscles were active, involving both flexors and extensors or predominantly flexors or predominantly extensors. In each posture the reflex effects were demonstrable only in the active muscles. 4. With ipsilateral tibialis anterior, there were task-dependent changes in the short-latency components of the EMG response, approximately 60 ms and 80 ms after the stimulus. When seated performing voluntary contractions these components were difficult to define, and when standing on a platform tilted toe-up they were small. When the ipsilateral leg was flexed or when standing on an unstable base, these early components were more prominent in each subject. With contralateral tibialis anterior, the dominant reflex pattern was inhibition when seated and contracting voluntarily, and facilitation during bipedal stance tilted toe-up. These changes in reflex pattern could not be explained by different levels of background contraction. 5. It is concluded that cutaneous mechanoreceptors of the foot have widespread reflex actions on muscles throughout both limbs, particularly the ipsilateral limb, and that the reflex pattern in different muscles and within a single muscle may change dependent on the task that the subject is undertaking. These task-dependent changes indicate plasticity in the expression of cutaneous reflex activity, affecting both short-latency spinal as well as long-latency pathways.

Adult↗
PubMed1996

Gain of the triceps surae stretch reflex in decerebrate and spinal cats during postural and locomotor activities.

1. The triceps surae (TS) stretch reflex was measured in decerebrate cats during crossed extensor stimulation and after spinalization during rhythmic locomotor activity induced by clonidine and manual perineal stimulation. The TS force in response to sinusoidal stretch was measured at a given contraction level before and after deafferentation, and the 'reflex force' was computed by subtracting these two responses. Reflex 'gain' was computed as the ratio of the reflex and deafferented force responses (a unitless estimate of the open loop feedback gain). 2. Prior to locomotion the spontaneous muscle activity was low (less than 15% of maximum), but the reflex gain was relatively high (close to 1.0 with a 5 Hz stretch). When locomotion commenced the reflex gain was markedly lowered when measured at the same contraction level as before locomotion (25% of the gain prior to locomotion). At higher contraction levels the reflex gain was not significantly increased. The reflex force and EMG responses to stretch increased with the contraction level, but their effect on the total reflex gain was cancelled by an associated increase in the intrinsic muscle stiffness. 3. In the decerebrate cat, during weak tonic contractions (spontaneous), the reflex gain was high and comparable with the gain in the resting spinal cat. However, with increased tonic contractions produced by crossed extensor stimulation the reflex gain dropped. At higher contraction levels the gain was not significantly different from the gain during spinal locomotion. 4. When the frequency of stretch was increased from 3 to 20 Hz, EMG responses to stretch increased, but the reflex force decreased, since a more fused contraction developed with the more frequent reflex activations. Overall, the reflex gain decreased with frequency in both spinal and decerebrate cats. The phase lag of the reflex force, relative to the intrinsic muscle force, increased with increasing frequency, due to reflex delays, with a 180 deg lag occurring between 12 and 18 Hz (tremor frequencies). The mean gain was significantly lower and the phase lag was significantly greater during locomotion than during tonic crossed extensor contractions, suggesting different reflex mechanisms. 5. In conclusion, during locomotion in spinal cats afferent feedback from low frequency ankle movements, similar to those occurring during the normal step cycle, reflexly produces a small but significant fraction of the extensor force (about a quarter of the stretch-related force modulation). This fraction is remarkably constant at the different contraction levels of the step cycle. Afferent feedback during higher frequency movement is less effective, minimizing the chance of instability and tremor. In contrast during tonic contractions afferent feedback produces half of the total muscle force during perturbations, clearly contributing to the maintenance of posture.

Afferent Pathways↗
PubMed1975

[Vascular responses to tilting in paraplegic subject compared to normal (author's transl)].

1 Vascular tone is higher in paraplegics than in normals, both in capacitance and resistance vessels. This is possibly correlated with the increase in circulating catecholamines which has recently been reported. 2 Tilting at 30 degrees from horizontal induces a hydrostatic increase in transmural pressure in the affected vascular bed. This pressure change causes: an initial decrease in resistance, followed by a progressive increase which can be explained by the Bayliss reflex. The time sequence and amplitude of the responses are comparable for normal and paraplegic subjects; an increase in vascular tone of the capacitance vessels (increase in venous pressure, decrease in local blood volume). This response was constantly observed in paraplegic subjects and was absent or weak in normal subjects. 3 In conclusion, reflex changes in vascular tone due to upright posture persist after traumatic section of the spinal chord. Orthostatic hypotension and blood pooling in the lower limbs in paraplegic subjects is probably due primarly to a deficit of the pumping action of the leg muscles.

Adult↗
PubMed1989

Clinical application of vestibulospinal reflex tests in peripheral vestibular disorders.

The influence of peripheral vestibular disturbances upon postural behaviour can be measured by posturography (PG). The results show that both otolithic and canal dysfunction can have some influence, whereas central compensation tries to suppress it. In this way, central compensation can be assessed by PG, which adds complementary information sui generis to the data of the 'classical' evaluation of a dizzy patient.

Humans↗
PubMed1993

The effect of intra-uterine breech position on postnatal motor functions of the lower limbs.

The effect of intra-uterine movement restriction on the development of motor functions was studied longitudinally by comparing infants born after uncomplicated breech position (n = 13) with control infants (vertex position, n = 5-10). Before birth, fetal leg posture was studied at regular intervals by means of real time ultrasound observations, and classified as complete (n = 1), inconsistent (n = 6), or incomplete (n = 6) breech position. Limited extension of the hips, preference posture and joint position in percentage of time (each until 12 weeks), withdrawal reflex and magnet response (until 26 weeks) and posture while sitting, standing and walking without support (up to 12-18 months) were assessed longitudinally. The results showed statistically significant, positive relationships between intra-uterine breech position and neonatal limited extension of the hip-joint, between limited extension of the hip-joint and the percentage of time that the hips are in flexion during the first 12 weeks, between this flexion of the hips (in percentage of time) and an abnormally 'flexed' walking pattern at 12-18 months, and finally, between a positive magnet response at 6 months and an abnormal walking pattern at 12-18 months. These findings suggest that intra-uterine movement restriction of the legs can cause long term alterations in the development of motor functions (leg posture, reflexes and posture while walking), possibly mediated by alterations in proprioceptive feedback mechanisms.

Adult↗
PubMed2001

Reflex vascular defects in the orthostatic tachycardia syndrome of adolescents.

Dependent pooling occurs in postural orthostatic tachycardia syndrome (POTS) related to defective vasoconstriction. Increased venous pressure (Pv) >20 mmHg occurs in some patients (high Pv) but not others (normal Pv). We compared 22 patients, aged 12-18 yr, with 13 normal controls. Continuous blood pressure and strain-gauge plethysmography were used to measure supine forearm and calf blood flow, resistance, venous compliance, and microvascular filtration, and blood flow and swelling during 70 degrees head-up tilt. Supine, high Pv had normal resistance in arms (26 +/- 2 mmHg x ml(-1) x 100 ml x min) and legs (34 +/- 3 mmHg x ml(-1) x 100 ml x min) but low leg blood flow (1.5 +/- 0.4 ml x 100 ml(-1) x min(-1)). Supine leg Pv (30 +/- 2 vs. 13 +/- 1 mmHg in control) exceeded the threshold for edema (isovolumetric pressure = 19 +/- 3 mmHg). Supine, normal Pv had high blood flow in arms (4.1 +/- 0.2 vs. 3.5 +/- 0.2 ml x 100 ml(-1) x min(-1) in control) and legs (3.8 +/- 0.4 vs. 2.7 +/- 0.3 ml x 100 ml(-1) x min(-1) in control) with low resistance. With tilt, calf blood flow increased steadily in POTS with high Pv and transiently increased in normal Pv. Calf volume increased in all POTS patients. Arm blood flow increased in normal Pv only with forearm maintained at heart level. These data suggest that there are (at least) two subgroups of POTS characterized by high Pv and low flow or normal Pv and high flow. These may correspond to abnormalities in local or baroreceptor-mediated vasoconstriction, respectively.

Adolescent↗