Showing papers on "Reflex published in 1985"

Dynamic modification of the vestibulo-ocular reflex by the nodulus and uvula.

Abstract: The time constant of the decay of slow-phase eye velocity of postrotatory nystagmus or optokinetic after-nystagmus is reduced during exposure to a stationary visual surround (visual suppression). It is also reduced after tilting the head (tilt suppression). A "dump" mechanism in the vestibulo-ocular reflex has been proposed to rapidly discharge activity from the central vestibular system during both types of suppression. Monkeys lost this mechanism after lesions of the nodulus and uvula. They also lost the ability to habituate the time constant of nystagmus on repeated exposure to optokinetic and vestibular stimuli. Periodic alternating nystagmus, which is believed to represent an instability in the vestibulo-ocular reflex, was observed in two of three monkeys. These data indicate that the nodulus and uvula play an important role in suppressing, habituating, and stabilizing the vestibulo-ocular reflex.

Pathophysiology of blepharospasm and oromandibular dystonia

Abstract: The pathophysiology of reflexes mediated by the fifth and seventh cranial nerves has been studied in 16 patients with blepharospasm and oromandibular dystonia compared with normal age-matched subjects. The EMG activity of the dystonic spasms in the periocular and jaw muscles was similar to that described in other muscles in patients with generalized torsion dystonia. The latency of the R1 and R2 components of the blink reflex and of the corneal reflex was normal. However, the amplitude and the duration of the R1 and R2 and the duration of the corneal reflex were increased. In some patients the R1 component was also present on the side contralateral to the stimulus, while in normal subjects it was present only on the ipsilateral side. The excitability cycle of recovery of the R2 component of the blink reflex after a prior conditioning shock was enhanced in the patients. There were no EEG potentials preceding blepharospasms in the patients, although a Bereitschaftspotential was seen beginning some 500 ms prior to voluntary blinks in the same individuals. Exteroceptive suppression in the contracting masseter and orbicularis oculi muscles was absent in 40 to 50 per cent of the patients. The jaw jerk was present in all the patients with normal latency. These results indicate that the neuronal arcs of the facial reflexes in blepharospasm and oromandibular dystonia are normal. However, there is an abnormal excitatory drive, perhaps from the basal ganglia, to the facial motoneurons and the interneurons which mediate the facial reflexes in the brainstem.

Motor control in humans with large-fiber sensory neuropathy.

Abstract: Upper limb motor control was evaluated in a series of patients with a large-fiber sensory neuropathy associated with impaired position, vibration and cutaneous sensation and absence of deep tendon reflexes. Muscular strength was normal or only minimally affected. In studies of wrist movement it was found that both postural maintenance and accuracy of wrist displacement were heavily dependent on visual guidance. Without vision the limb would drift in almost random directions, and during intended movements both the trajectory and movement end-point were abnormal. The defects in posture and voluntary movement control were reflected in the inability of patients to maintain consistent levels or emit consistent patterns of muscle activity. It is concluded that whereas central motor commands are sufficient to initiate movements proprioceptive afferent inputs are important for accurate postural maintenance and the fine control of movement.

Dynamics of Postural Control in the Child with Down Syndrome

Abstract: We examined the development of neural control processes underlying stance balance in both developmentally normal children and children with Down syndrome to test the hypothesis that motor deficiencies in children with Down syndrome are associated with deficits within the automatic postural control system. We compared children with Down syndrome and developmentally normal children in two age groups (1-3 and 4-6 years) by using displacements of a platform and measuring electromyograms from leg muscles. The automatic muscle response pattern in both normal children and children with Down syndrome were directionally specific, although the pattern were more variable than in adults. Responses in children with Down syndrome showed no adaptive attenuation to changing task conditions. Onset latencies of responses in children with Down syndrome were significantly slower than in normal children. Presence of the monosynaptic reflex during platform perturbations at normal latencies suggests that balance problems in children with Down syndrome do not result from hypotonia, which researchers have defined as decreased segmental motoneuron pool excitability and pathology of stretch reflex mechanisms, but rather result from defects within higher level postural mechanisms.

The systemic administration of local anaesthetics produces a selective depression of C-afferent fibre evoked activity in the spinal cord

Abstract: An electrophysiological analysis of the antinociceptive effects of systemic lidocaine and its longer acting primary amine congener, tocainide, has been performed in the decerebrate-spinal unanaesthetised rat. Neither of these local anaesthetic drugs when administered systemically in doses of up to 10 mg/kg (lidocaine) or 100 mg/kg (tocainide), produced any evidence of a block in the conduction of action potentials in A beta, A delta or C primary afferents. The local anaesthetics also failed to reduce mustard oil induced neurogenic extravasation, a test of cutaneous C-fibre terminal function. Lidocaine produced a transient (1-2 min) depression in monosynaptic reflexes at doses of greater than or equal to 1 mg/kg while tocainide had no effect on this reflex at any dose up to a 100 mg/kg. Both drugs, however, significantly suppressed the C-fibre evoked polysynaptic reflex generated by stimulating the sural nerve. The tocainide effect was longer lasting with less action on the short latency A beta-evoked reflex than lidocaine. The reflex activity in hamstring flexor alpha-motoneurones evoked by pinching the toes of the ipsilateral hind paw was reduced by both drugs but not abolished. Thermal and noxious chemical evoked reflexes were, however, completely suppressed by the local anaesthetic drugs, again with a longer action from tocainide. These results demonstrate that the systemic administration of drugs which increase the inactivation of sodium channels can produce a selective central block of certain types of afferent evoked activity in the spinal cord. There are resemblances between the selective C-fibre suppressing actions of systemically administered local anaesthetic and the pharmacological actions of narcotic opiates which may represent a similar mechanism for the analgesic action of these quite different classes of drugs.

The contribution of neurogenic inflammation in experimental arthritis.

Abstract: The release of the peptide neurotransmitter substance P from the peripheral terminals of nociceptive afferent neurons and the release of catecholamines from postganglionic sympathetic efferent neurons produce physiologic changes associated with acute inflammation The contribution of these neurogenic mechanisms to inflammatory diseases has not been determined Activation of central neural circuits elicits similar physiologic changes, and lesions of the peripheral and central nervous system are associated with alteration in activity of inflammatory diseases We have evaluated the contribution of neurogenic inflammation to the severity of joint injury in experimentally induced arthritis in the rat The finding of a greater density of substance P-containing nociceptive afferents in a joint that develops more severe arthritis (ankle) suggests a role of substance P in joint injury Direct evidence that the proinflammatory factor released from these nociceptors is substance P is provided by the finding that the injection of substance P into a joint which normally develops less severe arthritis (knee) increases the severity of arthritis in that joint A contribution of catecholamines to the severity of joint injury was suggested by the finding that both guanethidine-induced sympathectomy and reserpine-induced depletion of catecholamines attenuated the severity of joint injury Finally, a contribution of central neural circuits to inflammatory processes was studied in a model in which activation of nociceptive afferents elicited swelling and tenderness at a remote site This reflex neurogenic inflammation was inhibited by intracerebroventricular injections of morphine, which also attenuated the severity of arthritis These studies provide evidence that elements of the peripheral afferent and sympathetic efferent neurons and of descending supraspinal, opioid-mediated, circuits in the central nervous system modulate the severity of joint injury in experimental arthritis in the rat

Reflex neurogenic inflammation. I. Contribution of the peripheral nervous system to spatially remote inflammatory responses that follow injury

Abstract: Recent studies of the mechanism of neurogenic inflammation have focused on the contribution of neuropeptides released from peripheral terminals of primary afferent sensory neurons. In this study we addressed the contribution of humoral and neural factors to the hyperalgesia and swelling that are produced contralateral to an injured hindpaw, a phenomenon which we refer to as reflex neurogenic inflammation. The contralateral inflammatory response develops gradually, over a period of hours, and shows no tachyphylaxis with repeated application of the same stimulus. Denervation of either limb significantly attenuated the contralateral responses. Selective lesions of small-diameter, presumed nociceptive afferent fibers with capsaicin, or of sympathetic postganglionic efferents by immunosympathectomy, also reduced swelling and hyperalgesia of the uninjured paw. Interruption of venous circulation to the injured limb by vein ligation did not alter the response in the contralateral paw. Taken together, these data suggest that reflex neurogenic inflammation is neurally mediated, via connections across the spinal cord.

Long-latency spinal reflexes in humans

Abstract: Stretching human muscles with a mechanical device gave rise to multiple peaks in the rectified and averaged electromyogram. In the first dorsal interosseous the latency of the first peak (M1) was 32.4 +/- 2.4 ms (SD) and the latency of the second peak (M2) was 55.1 +/- 11.3 ms, in both cases measured from the time of the stimulus to the take-off point of the peak. Often a third peak (M3) was seen, having a considerably longer latency. The origin of peak M1 was considered to be in the stretch reflex arc because of its latency and its invariable association with muscle movement. Peak M2 was due to stimulation of afferent terminals in the skin and/or subcutaneous tissues by the mechanical device producing the muscle stretch. The conduction velocity of the pathway involved in the generation of the M1 component is the same as that for M2. This implies that central processing in the spinal cord delays the M2 response. The M2 mechanism does not involve a transcortical (long-loop) pathway because in foot muscles the M1-M2 delay remains the same as is found for hand muscles, although M1 latency is prolonged (to 39.4 +/- 6.2 ms for extensor digitorum longus). This indicates that there is not time for M2 impulses to traverse a pathway any longer than that passing to and from the spinal cord.

Voluntary and reflexive recruitment of flexor carpi radialis motor units in humans.

Abstract: The order of recruitment of flexor carpi radialis (FCR) motor units was studied during voluntary and reflexive activation of the motoneuron pool for two adult subjects. During slow "voluntary" activation, the recruitment threshold for tonic motoneuron firing was determined, and then the twitch profile of the motor unit was computed by the spike-triggered averaging technique. A positive correlation (r = 0.79 and 0.68 for the two subjects, respectively) between recruitment threshold and twitch amplitude implies that recruitment of FCR motoneurons during slow ramp isometric contractions proceeds in order of increasing size. The accompanying paper describes the behavior of single motor units during the short- and long-latency periods of the stretch reflex. When the effects of sufficient voluntary facilitation (preload) combined with a sufficiently large torque step were just adequate to cause a motor unit to fire during the stretch reflex, its response was virtually always confined to the long-latency period. In addition, the first unit to begin responding to muscle stretch always had the lowest voluntary recruitment threshold relative to other units "visible" at that recording site. By making this unit tonic, the reflex response to the same load increased substantially during the short-latency reflex period, while a second unit increased its reflex response probability during the long-latency period. Thus the voluntary recruitment order of two or more motor units is preserved during the stretch reflex, and is in fact maintained within first the long-latency and then short-latency components of this reflex.

Thixotropic behaviour of human finger flexor muscles with accompanying changes in spindle and reflex responses to stretch.

Abstract: Prompted by previous reports on muscle thixotropy, we have investigated changes in inherent and reflex stiffness of the finger flexor muscles of human subjects at rest, following transient conditioning manoeuvres involving contractions and/or length changes of the finger flexors. The stiffness measurements were combined with electromyographic recordings from forearm and hand muscles and with microneurographic recordings of afferent stretch responses in finger flexor nerve fascicles. Finger flexor stiffness was evaluated by measuring (a) the flexion angle of the metacarpo-phalangeal joints at which the system during rest balanced the force of gravity and (b) the speed and amplitude of angular finger extensions induced by recurrent extension torque pulses of constant strength delivered by a torque motor. In the latter case, extension drifts in the resting position of the fingers were prevented by a weak flexion bias torque holding the fingers in a pre-determined, semiflexed position against a stop-bar. Stiffness changes following passive large amplitude finger flexions and extensions were studied in subjects with nerve blocks or nerve lesions preventing neurally mediated contractions in the forearm and hand muscles. Inherent stiffness was enhanced following transient finger flexions and reduced following transient finger extensions. The after-effects gradually declined during observation periods of several minutes. Similar results were obtained in subjects with intact innervation who succeeded during the pre- and post-conditioning periods in keeping the arm and hand muscles relaxed (i.e. showed no electromyographic activity). In these subjects it was also found that the after-effects were similar for active and passive finger movements and that isometric voluntary finger flexor contractions loosened the system in a way similar to finger extensions. In some subjects electromyographic reflex discharges appeared in the finger flexors in response to the extension test pulses. When elicited by small ramp stretch stimuli of constant amplitude, the stretch reflex responses were found to vary in strength in parallel with the changes in inherent stiffness following the various conditioning manoeuvres. The strength of the multi-unit afferent stretch discharges in the muscle nerve, used as index of muscle spindle stretch sensitivity, varied in parallel with the changes in inherent stiffness. Post-manoeuvre changes in muscle spindle stretch sensitivity were seen also when the spindles were de-efferented by a nerve block proximal to the recording site. The results can be explained in terms of thixotropic behaviour of extra- and intrafusal muscle fibres.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiovascular control during exercise: Central and reflex neural mechanisms☆

Abstract: Both reflex neural and central command mechanisms have been postulated to explain the cardiovascular responses that occur during exercise. The 2 mechanisms appear to affect the same neural circuits and to be capable of working either in conjunction with one another or independently.

Cervicocollic reflex: its dynamic properties and interaction with vestibular reflexes.

Abstract: Electromyographic activity of dorsal neck muscles elicited by sinusoidal rotations of the body and head was studied in decerebrate cats over a wide range of rotational frequencies and amplitudes. Rotation of the body with the head held fixed in space elicited a cervicocollic reflex (CCR) in the biventer cervicis, complexus, obliquus capitis inferior, rectus capitis major, and splenius muscles. As stimulus amplitude increased, CCR amplitude increased first rapidly and then more slowly, displaying two linear incremental sensitivity ranges. In contrast, the vestibulocollic reflex (VCR) elicited by whole body rotation had a minimum stimulus threshold below which no response was observed, whereas the vestibuloocular reflex (VOR) saturated at intermediate stimulus intensities. When stimulus frequency was varied, the CCR exhibited second-order dynamic behavior. At frequencies below 0.5 Hz, muscle EMG activation was in phase with peak platform angular deviation in the direction that stretched the muscle, and the gain measured as the percent modulation of EMG activity per degree of rotation remained constant. As frequency increased to 3-4 Hz, response phase advanced by 120 deg or more and gain increased with a slope approaching 40 dB/decade. The data were well-fitted by second-order transfer functions containing two zeros. Both the dynamic behavior of the CCR and its high sensitivity to small stimuli resemble the properties of muscle spindle primary afferents, suggesting that the latter may provide the major input responsible for the CCR. Dynamic properties and gains of the CCR and VCR were quite similar at frequencies between 0.2 and 3-4 Hz. Transfer functions of both reflexes contained two zeros whose time constants were correlated in a population of 11 cats, suggesting that reflex dynamics may be matched to the mechanical properties of each animal's head-neck system. Interaction of the CCR and VCR was studied under two conditions. When the head was driven by a servomotor while the body remained stationary, EMG activation by the two reflexes added linearly to produce a large response. When the body was rotated with the head allowed to counterrotate about the C1-C2 joint, the two reflexes combined linearly in an antagonistic fashion: the CCR acted to oppose head rotations produced by the VCR, thus preventing the ratio of head counterrotation to body rotation from exceeding 0.5. The data indicate that the CCR and VCR behave approximately linearly, both individually and in combination. Acting together, the two reflexes assist each other in preventing oscillation of the head on a stationary body.(ABSTRACT TRUNCATED AT 400 WORDS)

Three forms of the scratch reflex in the spinal turtle: central generation of motor patterns

Abstract: A turtle with a complete transection of the spinal cord, termed a spinal turtle, exhibits three types or “forms” of the scratch reflex: the rostral scratch, pocket scratch, and caudal scratch (21)....

Three forms of the scratch reflex in the spinal turtle: movement analyses.

Abstract: The scratch reflex is a motor task exhibited by the hindlimb of a turtle after complete transection of the spinal cord just posterior to the forelimb enlargement. The scratch is elicited by applyin...

The distal hindlimb musculature of the cat. Cutaneous reflexes during locomotion.

Abstract: In order to better understand the organization of the locomotor control system, we examined the temporal patterns of distal hindlimb muscle responses to brief electrical stimulation of cutaneous nerves during walking on a treadmill Electromyographic recordings were made from twelve muscles; stimuli were applied individually to three nerves at random times throughout the step cycle A new graphical technique was developed to assist detailed examination of the time course and gating of complex reflex patterns The short latency reflexes were of two primary types: inhibition of extensors and excitation of flexors; these responses were only evident during locomotor phases in which the respective motoneuron pools were active Longer-latency response components were gated in a similar but not identical manner, suggesting some independence from the basic locomotory influence on the motoneuronal pool The phase-dependent gating of reflexes appeared to be consistent with a functional role for reflex responses during locomotion The reflex responses of muscles with complex anatomical actions were often correspondingly complex

Increasing gracilis muscle interstitial potassium concentrations stimulate group III and IV afferents.

Abstract: Static muscular contraction reflexly increases arterial blood pressure and heart rate. One possible mechanism evoking this reflex is that potassium accumulates in the interstitial space of a working muscle to stimulate group III and IV afferents whose activation in turn evokes a pressor response. The responses of group III and IV muscle afferents to increases in interstitial potassium concentrations within the range evoked by static contraction are unknown. Thus we injected potassium chloride into the gracilis artery of anesthetized dogs while we measured both gracilis muscle interstitial potassium concentrations with potassium-selective electrodes and the impulse activity of afferents in the gracilis nerve. We found that increasing interstitial potassium concentrations to levels similar to those seen during static contraction stimulated 14 of 16 group III and 29 of 31 group IV afferents. The responses of the afferents to potassium were concentration dependent. The typical response to potassium consisted of a burst of impulses, an effect that returned to control firing rates within 26 s, even though interstitial potassium concentrations remained elevated for several minutes. Although our results suggest that potassium may play a role in initiating the reflex cardiovascular responses to static muscular contraction, the accumulation of this ion does not appear to be solely responsible for maintaining the pressor response for the duration of the contraction.

The sagittal vestibulocollic reflex and its interaction with neck proprioceptive afferents in the decerebrate cat.

Abstract: The sagittal vestibulocollic reflex (v.c.r.) evoked by nose-up, nose-down movements of the head, has been studied in the neck extensor muscle biventer cervicis in the decerebrate cat. Nose-down movements of the head increased, and nose-up movements decreased, electromyographic (e.m.g.) activity in the biventer cervicis muscles of the left and right sides. At low frequencies of sinusoidal head movement (0.1-0.5 Hz), the gain of the sagittal v.c.r. was approximately constant, and e.m.g. modulation showed a phase lead of about 40 deg with respect to head position. At higher frequencies (2-5 Hz), v.c.r. gain increased at a rate close to 40 dB/decade, and phase lead increased to approach 150 deg. The relation between head movement and v.c.r. activity may be described by a transfer function containing two lead terms, with time constants of 0.07 and 0.23 s, and two lag terms, with time constants of 5.3 and 9.9 s. When movements of the head were accompanied by stretching of the biventer cervicis muscles, the gain of the sagittal v.c.r. was increased threefold, at all frequencies between 0.1 and 5 Hz, with no substantial change in phase. Sinusoidal stretching of the biventer cervicis muscles, with the head stationary, evoked a stretch reflex (cervicocollic reflex, c.c.r.) the behaviour of which was similar to that of a second-order system described by a transfer function containing two lead terms with time constants of 0.07 and 0.16 s. This difference in dynamics between the v.c.r. and the c.c.r. indicates that the lag terms in the v.c.r. transfer function reflect the frequency-response properties of the vestibular pathway to the biventer cervicis muscles, as they do not appear when the same muscles participate in the c.c.r. The vectorial differences between the frequency-response of the sagittal v.c.r. with and without concomitant stretching of the biventer cervicis muscles is quantitatively similar to the frequency-response of the c.c.r. evoked by sinusoidal stretching. The inputs from the vestibular and stretch receptors thus appear to sum linearly to produce the increase in v.c.r. gain, at least over the frequency range 0.1-1 Hz. Since most head movements, and all voluntary head movements, involve a rotation of the head in relation to the neck, the potentiation of the gain of the v.c.r. by afferents from stretch receptors (presumably muscle spindles) in the neck muscles is an important factor in the normal reflex stabilization of head position.

Activity-dependent presynaptic facilitation: an associative mechanism in Aplysia.

Abstract: In studying the classical conditioning of the siphon withdrawal reflex in Aplysia, we have identified a neuronal mechanism that plays an important role in this conditioning: activity-dependent presynaptic facilitation. This review describes our analysis of the cellular basis of this associative mechanism. During the conditioning of the withdrawal reflex, the unconditioned stimulus, a tail shock, produces presynaptic facilitation of synaptic transmission from the siphon sensory neurons in the conditioned stimulus pathway. The facilitation is enhanced if a sensory neuron has fired action potentials just prior to receiving facilitatory input, as occurs during training when the conditioned stimulus precedes the unconditioned stimulus. This activity-dependent enhancement of presynaptic facilitation provides a mechanism for the temporal specificity in conditioning of the reflex. Activity-dependent facilitation appears to involve the same cyclic AMP (cAMP)-dependent cascade that underlies presynaptic facilitation in these neurons in the absence of paired spike activity. Our evidence suggests that it is the transient elevation of intracellular Ca2+ that is responsible for the enhancement of the facilitation response by paired spike activity. Moreover, our preliminary results indicate that Ca2+/calmodulin is able to potentiate the activation of adenylate cyclase in Aplysia neurons by facilitatory transmitter. Thus, the dual activation of the calmodulin-dependent cyclase by Ca2+ and transmitter may give this enzyme an important associative role in learning. In the conclusion, the possible phylogenetic generality of this associative mechanism is discussed as well as its possible role in activity-dependent processes in neuronal development.

Partial neuromuscular blockade and cardiovascular responses to static exercise in man.

Abstract: In human subjects sustained static contractions of the quadriceps femoris in one leg were performed with the same absolute and the same relative intensity before and after partial neuromuscular blockade with either decamethonium or tubocurarine which reduced strength to about 50% of the control value. During the contractions performed with the same absolute force, the magnitude of the cardiovascular responses (heart rate and blood pressure) was greater during neuromuscular blockade than during control contractions. During the contractions involving the same relative force the magnitude of the cardiovascular responses was almost the same with and without neuromuscular blockade. These findings were independent of the drug used. The metabolic part of the exercise pressor reflex was assessed by the application of an arterial cuff 1/2 min before cessation of exercise and for the following 3 min of rest. Although heart rate and blood pressure decreased after cessation of exercise, application of the tourniquet resulted in higher post-exercise values and this effect was seen both with and without neuromuscular blockade. Muscle biopsies from the subjects' m. vastus lateralis were analysed for fast- and slow-twitch fibre composition showing 27-66% slow-twitch fibres. No correlation was found between cardiovascular responses to static exercise, with or without neuromuscular blockade, and fibre type predominance. The results suggest that the involvement of fast- or slow-twitch muscle fibres does not play a dominant role in the cardiovascular responses to static exercise in man. Both central command and reflex neural mechanisms are of importance, and it appears that these two control mechanisms are redundant and that neural occlusion may be operative. However, when partial neuromuscular blockade induces a disproportion between an increase in central command and a constant or decreasing muscle tension and metabolism, the larger signal arising from central command determines the magnitude of the cardiovascular responses.

Long-loop reflexes in small hand muscles studied in normal subjects and in patients with Huntington's disease.

Abstract: Long-latency electromyographic (EMG) responses of the first dorsal interosseus muscle were evoked by short displacements of the index finger in healthy subjects and in patients with Huntington's disease (HD). In all 20 healthy subjects the early spinal response (mean latency 31.5 ms) was followed by a reproducible later reflex response with a mean latency of 56.5 ms. The activity pattern of single motor units of the stretched muscle was similar to that of the surface EMG. Thus all single motor units tested could be active during either the first or second response phase, but never in both in a given trial. Of the 50 patients with HD, the late EMG response was missing completely in all but 7, but the early spinal component was almost identical to that of the control group. Double stretches at an interval of 25 ms evoked two similar EMG responses in these patients, proving that the motoneuron pool is still excitable during the time at which the second response would have appeared in healthy subjects. The reflex responses of the thenar muscles evoked by electrical stimulation of the median nerve were examined during a voluntary opposing contraction of the thumb in both groups of subjects. In normal subjects, two EMG responses could be distinguished with latencies similar to those of the mechanically elicited responses. Patients with HD again lacked the second response, although the first spinal response was always present. The results are discussed with respect to different proposals concerning the origin of long-latency responses in human muscles. At least for distal hand muscles, the results suggest that the long-latency responses are long-loop reflexes.

Interaction of cardiopulmonary and carotid baroreflex control of vascular resistance in humans.

Abstract: Previous studies in experimental animals indicate an important inhibitory interaction between cardiopulmonary and arterial baroreflexes. In the dog, for example, cardiopulmonary vagal afferents modulate carotid baroreflex control of vascular resistance. On the other hand, previous studies in human subjects have not produced convincing evidence of a specific interaction between these baroreceptor reflexes. The purpose of this study was to determine whether unloading of cardiopulmonary baroreceptors in humans with nonhypotensive lower body negative pressure selectively augments the reflex vasoconstrictor responses to simulated carotid hypotension produced by neck pressure. In nine healthy subjects, we measured forearm vascular responses with plethysmography during lower body negative pressure alone (cardiopulmonary baroreflex), during neck pressure alone (carotid baroreflex), and during concomitant lower body negative pressure and neck pressure (baroreflex interaction). Lower body negative pressure produced a greater than twofold augmentation of the forearm vasoconstrictor response to neck pressure. This increase in resistance was significantly greater (P less than 0.05) than the algebraic sum of the increase in resistance from lower body negative pressure alone plus that from neck pressure alone. In contrast, lower body negative pressure did not potentiate the forearm vasoconstrictor responses either to intra-arterial norepinephrine or to the cold pressor test. Thus, the potentiation of the vasoconstrictor response to neck pressure by lower body negative pressure cannot be explained by augmented reactivity to the neurotransmitter or to a nonspecific augmentation of responses to all reflex vasoconstrictor stimuli. In conclusion, nonhypotensive lower body negative pressure selectively augments carotid baroreflex control of forearm vascular resistance. These experiments demonstrate a specific inhibitory cardiopulmonary-carotid baroreflex interaction in humans.

Spatial and temporal response properties of the vestibulocollic reflex in decerebrate cats.

Abstract: Vestibulocollic reflex responses of several neck muscles in decerebrate cats were studied during angular rotations of the whole body in a large number of vertical and horizontal rotation planes, at frequencies from 0.07 to 1.6 Hz. Vestibulocollic responses were compared to eye muscle and forelimb muscle vestibular responses. Electromyographic activity was recorded by fine wires inserted in biventer cervicis, complexus, longus capitis, obliquus capitis inferior, occipitoscapularis, rectus capitis major, splenius, lateral rectus, and triceps brachii. At frequencies of approximately 0.5 Hz and above, neck muscle electromyographic response gains were sinusoidal functions of stimulus orientation within a set of vertical or horizontal planes, and a muscle's response phase remained constant across rotation planes, or reversed by 180 degrees. Response patterns at high frequencies were consistent with vestibulocollic reflex activation by semicircular canals through brain circuitry that modifies canal dynamics. At frequencies of approximately 0.5 Hz and above, the stimulus orientation in which a given neck muscle's response was maximal remained nearly constant across frequencies. Thus, we used responses to rotations at high frequencies to calculate axes of maximal response of each muscle in three-dimensional space. Lateral rectus, obliquus, and to a lesser extent, splenius and longus capitus were activated predominantly by horizontal rotations. Biventer was activated predominantly by pitch, triceps predominantly by roll, and complexus, occipitoscapularis, and rectus major significantly excited by rotations in all three coordinate planes. In some cases, at frequencies less than 0.5 Hz, neck muscle response phase varied depending on the vertical plane in which the cat was rotated, and the optimal response plane was poorly defined and varied with frequency. These responses indicated that, at some frequencies, neck muscle activity can result from summation of inputs with differing spatial orientation and dynamics (spatial-temporal convergence). Differences between responses to vertical and horizontal rotations suggested that low-frequency spatial-temporal convergence behavior of the vestibulocollic reflex during vertical rotations was due to convergent semicircular canal and otolith receptor inputs.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of the monosynaptic stretch reflex in the rat: an in vitro study.

Abstract: The properties and development of the stretch reflex pathway were investigated in new-born and fetal rats using the isolated spinal cord-hind limb preparation. Muscle afferent discharge was elicited by small stretch of the triceps surae muscle in the new-born rat and in the fetus. It appeared as early as embryonic day 18.5. Ramp-and-hold stretch elicited only phasic discharges in most afferent fibres. A phasic reflex response was evoked in the triceps surae muscle by brief or ramp-and-hold stretch of the muscle in the new-born rat. The threshold stretch required for evoking the reflex response was close to that for eliciting the afferent discharge. A reflex response in the triceps surae muscle was also evoked by electrical stimulation of the triceps surae muscle nerve or the sciatic nerve in the new-born rat. Excitatory post-synaptic potentials (e.p.s.p.s) in the triceps surae motoneurones were evoked by stimulation of the muscle nerve in the new-born rat. The amplitude of the e.p.s.p.s was large enough to generate spike potentials. Homonymous e.p.s.p.s were significantly larger than heteronymous e.p.s.p.s. The amplitudes of the e.p.s.p.s were very susceptible to the rate of stimulus repetition. At a stimulus frequency of 10 Hz they were depressed to less than 10% of the control value. Presynaptic impulses evoked by stimulation of afferents in the muscle nerve appear in the motor nucleus less than 1.0 ms before the onset of synaptically evoked field potentials. The interval between the arrival of impulses evoked by dorsal root stimulation and the onset of e.p.s.p.s in motoneurones was 0.56 +/- 0.16 ms, indicating monosynaptic transmission from the primary afferents to the motoneurones. In the fetus, a reflex response in the triceps surae muscle was observed following a small stretch of the muscle (or electrical stimulation of the sciatic nerve) in all preparations at embryonic day 20.5 and in about half of those examined at embryonic day 19.5. Neither stimulation evoked a reflex response at embryonic day 18.5. Latencies of the reflex responses evoked by muscle stretch or by nerve stimulation were similar to those in the new-born rat. It is concluded that the monosynaptically evoked stretch reflex response in the triceps surae muscle first appear at embryonic day 19.5. Natural and electrical stimulation of the plantar skin evoked a reflex response with long latencies in flexor muscles. Such a cutaneous reflex was first present at embryonic day 17.5, two days earlier than the onset of the stretch reflex.