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At least 109 records · Page 6Linked to original sources
PubMed1973

Modulation of postural reflexes by voluntary movement. II. Modulation at an inactive joint.

We have observed that there is a brief, 400 msec period of widespread facilitation of both the Hoffmann and the tendon-jerk reflexes associated with the initiation of a variety of local, voluntary, isometric efforts. It is suggested that the site of this facilitation lies within the spinal cord. We have also observed that this period is followed, for the Hoffmann reflex, by a period of spinal inhibition. By contrast, the tendon-jerk is facilitated, at a reduced level, for at least another 500 msec by mechanisms probably working at the muscle spindle. These results are discussed in comparison with experiments involving activation of the reflex agonist and antagonist muscles. It is proposed that voluntary control of the locomotor system is organized through separate mechanisms for muscle recruitment and for regulation of the sensitivity of the myotatic reflex loop.

Ankle Joint↗
PubMed2004

Classically conditioned postural reflex in cerebellar patients.

The aim of the current study was to compare postural responses to repetitive platform-evoked perturbations in cerebellar patients with those of healthy subjects using a classical conditioning paradigm. The perturbations consisted of tilting of the platform (unconditioned stimulus: US) at random time intervals, preceded by an auditory signal that represented the conditioning stimulus (CS). Physiological reactions were recorded biomechanically by measuring the vertical ground forces, yielding the center of vertical pressure (CVP), and electrophysiologically by EMG measurements of the main muscle groups of both legs. The recording session consisted of a control section with US-alone trials, a testing section with paired stimuli and a brief final section with US-alone trials. Healthy control subjects were divided into those establishing conditioned responses (CR) in all muscles tested (strategy I) and those with CR in the gastrocnemius muscles only (strategy II), suggesting an associative motor-related process is involved. Patients with a diffuse, non-localized disease were almost unable to establish CR. This was also true for a patient with a focal surgical lesion with no CR on the affected side but who, simultaneously, showed an essentially normal CR incidence on the intact side. During US-alone trials healthy controls exhibited a remarkable decay of the UR amplitude due to a non-associative motor-related process such as habituation. The decay was most prominent in the paired trials section. In contrast, patients showed no significant differences in the UR amplitude throughout the entire recording session. Analysis of the CVP supported the electrophysiological findings, showing CR in the controls only. The differences between the responses of control subjects and those of the cerebellar patients imply strongly that the cerebellum is involved critically in controlling associative and non-associative motor-related processes.

Adult↗
PubMed1992

Visceral and postural reflexes evoked by genital stimulation in urethane-anesthetized female rats.

The present study describes several muscular reflexes produced by genital stimulation, the nerves that subserve them, and the visceral and postural effects induced by these reflexes. Electrical stimulation of the iliococcygeus (ic) and pubococcygeus (pc) (striated) muscles produced movement of the vaginal orifice and wall, membranous urethra, tail and pelvis. Electrical stimulation of the psoas major (pm) or iliacus (i) (striated) muscles produced movements of the lumbar vertebrae and extension of the ipsilateral hindlimb. Sensory mechanostimulation elicited responses of these muscles as follows: stimulation of the perineal skin, clitoral sheath or distal vagina produced reflex contraction of the ic and pc muscles. Stimulation of the cervix produced reflex contraction of the pm and i muscles and also blocked the above reflex contraction of the ic and pc muscles. Both the cervical stimulation-induced blockage of the ic and pc reflex response, and the cervical stimulation-induced activation of pm and i muscles was prevented by bilateral transection of the viscerocutaneous branch of the pelvic nerve. Based on the above observations, it is proposed that stimulation of the vaginal surface of the cervix resulting from penile intromission and/or seminal plug deposition during mating behavior in the rat may reflexively active pm and i, thereby contributing to the hindleg postural rigidity and lordotic dorsiflexion that are characteristic of the normal mating posture in female rats.

Abdominal Muscles↗
PubMed2006

Training induced adaptations in characteristics of postural reflexes in elderly men.

The aging neuromuscular system is affected by structural and functional changes which lead to a general slowing down of neuromuscular performance and an increased risk of falling. The impact of heavy resistance (HR) training in the elderly on maximum voluntary contraction (MVC) and rate of force development (RFD) has been investigated in the past. However, the influence of sensorimotor (SENSO) training and HR training on the ability to compensate for gait perturbations has not yet been investigated in the elderly. Therefore, the purpose of the study was to examine the impact of HR and SENSO training in elderly men on unexpected treadmill perturbations. Functional reflex activity was recorded by means of surface EMG in 40 male subjects (>60 years) before and after 13 weeks of HR and SENSO training and in another 20 male subjects (>60 years), which served as a CONTROL-group. SENSO training resulted in a decrease in onset latency, an enhanced reflex activity in the prime mover as well as a decrease in maximal angular velocity of the ankle joint complex during the perturbation impulses. No significant changes were observed in the HR- and in the CONTROL-group. The results clearly indicate that SENSO training has an impact on spinal motor control mechanisms in the elderly. Training induced improvements in perception and procession of afferent information could be a possible reason for the increase in reflex contraction. Due to these adaptive processes, SENSO training could be a well-suited method for fall preventive programs in elderly people.

Adaptation, Physiological↗
PubMedDate not supplied

Postural reflexes in cranial muscles in man.

Using electromyographic recording techniques, tonic neck reflexes and tonic labyrinth reflexes have been analysed in cranial muscles of healthy man. The frontalis and orbicularis oculi (pars orbitalis) muscles are the site of positional reflexes of cervical origin, which are involved in the control of eyelid position during head movements. At a mandibular level, postural influences are set up in the masseter and digastric muscles upon ventral and dorsal flexion of the head. Their reciprocal character contrasts with the lack of evidence concerning reciprocal proprioceptive jaw reflexes. The genioglossal muscle is the site of an intense postural activity when the head is flexed dorsally. The latter influences appear related to the preservation of free air passage to the lungs.

Adult↗
PubMed1980

Postural reflex gelastic seizures.

A 26-year-old man had gelastic seizures ("laughing attacks") preceded by an aura of sexual pleasure. They were reflexly induced by volitional hyperextension of his neck and trunk. His EEGs showed a right temporal spike focus. We discuss the characteristics of this unique case.

Adult↗
PubMed1983

Variability of postural "reflexes" in humans.

The functional role of spinal and supraspinal EMG-responses for the maintenance of upright human posture was investigated in ten healthy subjects standing on a force measuring platform, which could be rotated in pitch around an axis aligned with the subject's ankle joint. Voluntary changes of body posture prior to the platform movement by leaning forward or backward led to a change in the amplitude and temporal organization of EMG-responses as compared to platform movements starting from a neutral position. Tilting the platform toe-up while leaning backward led to an increase of the latency of the short- and medium-latency responses in the triceps surae muscle and to a decrease of the latency of the stabilizing response in the anterior tibial muscle. Functionally, a cocontraction of both antagonistic muscles could be observed which partly compensated for the destabilizing action of the "reflex" response in the stretched triceps surae muscle. In analogy, leaning forward and tilting the platform toe-down led to a cocontraction of the two antagonistic muscles. The observed changes of latencies of short-, medium-, and long-latency response show the functional variability of segmental and suprasegmental "reflex" mechanisms. EMG-activities, which are functionally destabilizing posture, can be suppressed or compensated by reflexive cocontractions of antagonists.

Adult↗
PubMed1973

Modulation of postural reflexes by voluntary movement. I. Modulation of the active limb.

Experiments were performed involving isometric activation/relaxation of the agonist-antagonist muscle groups about the ankle joint. It is shown that phasic changes of foot torque in the dorsal direction are associated with a transient inhibition of the soleus H-reflex which is independent of anterior tibial contraction. During phasic changes in foot torque in the plantar direction, facilitation, as opposed to disinhibition of the soleus H-reflex, is graded by soleus contraction. These results support the hypothesis that the sensitivity of the monosynaptic reflex arc is one of the controlled variables of the motor system. The stretch reflex of a shortening muscle is generally facilitated, while that of a lengthening muscle is inhibited. These hypotheses are in keeping with our present knowledge of the linkage between skeletomotor and fusimotor neurones for the organization of voluntary movements.

Ankle Joint↗
PubMed1989

Human postural reflexes and gravity--an under water simulation.

This study represents the first attempt to investigate the influence of gravity on postural adjustments. Subjects were displaced while standing under water on a movable platform, while the buoyancy of the body was adjusted by using a variety of lead vests. Under water, an approximately linear relationship was found between body weight and impulse directed electromyographic response amplitudes in the leg and thigh muscles. Loading of the subjects out of water resulted in a saturation of the response amplitude. The biomechanical signals recorded during the displacements indicated that neither vestibulospinal nor muscle proprioceptive reflex mechanisms can account for the effect observed under water. It is suggested that the EMG responses are mediated by reflexes which are activated by pressure receptors within the body in order to hold the centre of gravity over the feet.

Adult↗
PubMed1975

Postural reflexes in chronic Chagas's heart disease. Heart rate and arterial pressure responses.

Studies of heart rate and blood pressure responses to 70 degrees head-up tilt and 30 degrees head-down tilt were conducted in normal subjects and in chronic cardiac chagasic patients without past or present cardiac decompensation. During steady state of tilt, heart rate alterations were less marked in the chagasic than in the control group. Vasodepressor syncope supervened in three cardiac chagasic patients, two of whom showed the distinctive feature of hypotension without reflex bradycardia. In these patients, vagal control of cardiac rate could not be demonstrated either following a full blocking dose of atropine or during sudden elevation of systemic arterial blood pressure. These results are interpreted in the context of the pathologic and functional derangements of autonomic control which occur in Chagas' heart disease.

Adult↗
PubMed1991

Influence of an acoustic preparatory signal on postural reflexes of the distal leg muscles in humans.

In standing subjects the sudden displacement of a platform toe-up evokes a stabilizing postural response of the anterior tibial muscle (long-latency reflex). A preparatory signal (acoustic warning stimulus) preceding the postural perturbation by 1 s results in a significant decrease in latency of this long-latency response. There is no parallel modulation of the afferent cortical input in terms of evoked cerebral potentials following platform tilts toe-up nor of efferent cortico-spinal functions as evaluated by means of transcranial magnetic stimulation. Conceivable underlying mechanisms include faster intraspinal and/or intracortical processing of afferent input or switching to a shorter spinal reflex pathway.

Acoustic Stimulation↗
PubMed2001

Postural reflexes evoked by tapping forehead and chest.

We investigated whether a tap with a reflex hammer to the forehead can elicit responses in the leg muscles and whether vestibular stimulation is the crucial prerequisite for eliciting these responses. We also measured the postural changes caused by the tap and by the compensatory, presumably reflex-like reactions of the subject. Tap-evoked activity of leg muscles was easily elicited during upright stance in normal subjects and was also seen in two subjects without vestibular function. The pattern of muscle activation clearly showed a counteraction to the tap-evoked perturbation of stance. Taps applied to the chest elicited similar reflexes. Since these two conditions imply a different activation of the vestibular apparatus, the vestibular input alone cannot account for the observed leg muscle reflexes. We suggest that multisensory reflex pathways that integrate vestibular and proprioceptive inputs account for these reflexes.

Adult↗