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PubMed1983

Stretch reflexes of triceps surae in patients with upper motor neuron syndromes.

Electromyographic responses of triceps surae to dorsiflexion stretch were studied in 47 patients with a variety of lesions producing an upper motor neuron syndrome. The short latency spinal reflexes, both when the patient was at rest and when he was exerting a voluntary plantarflexion, were frequently enhanced in magnitude and the rate of increase with acceleration was also enhanced. Long-latency reflexes were uncommon at rest. With background force long-latency reflexes were present unless the short latency reflex was very large. Long latency reflexes often were normal, but in some patients they were either excessively larger or even of abnormal shape with prolonged continuous activity. The clinical assessment of the ankle jerk correlated with the magnitude of the short latency reflex. The clinical assessment of tone correlated with the magnitude of the short latency reflex, the magnitude of the long latency reflex and the duration of the long latency reflex. There appear to be multiple physiological mechanisms underlying the clinical phenomenon of spasticity.

Cerebral Cortex↗
PubMed1993

The limitations of the tendon jerk as a marker of pathological stretch reflex activity in human spasticity.

The motor disorders associated with human spasticity arise, partly from a pathological increase in the excitability of muscle stretch reflexes. In clinical practice, reflex excitability is commonly assessed by grading the reflex response to a blow delivered to the tendon of a muscle. This is a much simpler response than the complex patterns of activity which may be elicited following muscle stretch caused by active or passive movement. Changes in the biceps brachii tendon jerk response have been followed over the first year after stroke in a group of hemiparetic patients and compared with changes in short and medium latency reflex responses elicited by imposed elbow flexion of initially relaxed spastic muscle and with the development of the late reflex responses which contribute to spastic hypertonia. A progressive increase in tendon jerk responses occurred over the first year following stroke, whereas reflex responses to imposed displacement, in particular the late reflex responses contributing to muscle hypertonia, reached their peak excitability one to three months after stroke, with a subsequent reduction in activity. The tendon jerk reflex therefore provides an incomplete picture of the pathological changes in the reflex responses in spasticity.

Adult↗
PubMed1979

The development of central nervous system control of the gill withdrawal reflex evoked by siphon stimulation in Aplysia.

In older Aplysia, the central nervous system (CNS) (abdominal ganglion) exerts suppressive and facilitatory control over the peripheral nervous system (PNS) which initially mediates the gill withdrawal reflex and its subsequent habituation evoked by tactile stimulation of the siphon. In young animals, both the suppressive and facilitatory CNS control were found to be absent. In older animals, removal of branchial nerve (Br) input to the gill resulted in a significantly reduced reflex latency and, with ctenidial (Ct) and siphon (Sn) nerves intact, a significantly increased reflex amplitude and an inability of the reflex to habituate with repeated siphon stimulation. In young animals, removal of Br had no effect on reflex latency and with Ct and Sn intact, the reflex amplitude latency was not increased and the reflex habituated. Older animals can easily discriminate between different intensity stimuli applied to the siphon as evidenced by differences in reflex amplitude, rates of habituation, and evoked neural activity. On the other hand, young animals cannot discriminate well between different stimulus intensities. The lack of CNS control in young animals was found to be due to incompletely developed neural processes within the abdominal ganglion and not the PNS. The lack of CNS control in young Aplysia results in gill reflex behaviours being less adaptive in light of changing stimulus conditions, but may be of positive survival value in that the young will not habituate as easily. The fact that CNS control is present in older animals strengthens the idea that in any analysis of the underlying neural mechanisms of habituation the entire integrated CNS-PNS must be taken into account.

Animals↗
PubMed1981

Transcortical reflexes and servo control of movement.

Sherrington proposed that the major role of proprioceptors is in processing afferent inputs generated by the active movements of the animal itself, and noted that the reflex effects of proprioceptive inputs are "mild." Current experimental results are consistent with the view that the major role of both segmental and transcortical proprioceptive reflexes is in small active movements and active postural stability, with muscle afferent inputs reducing "...errors of muscle length produced by fluctuating levels of motor discharge..." as stated by Goodwin and coworkers in 1978. Exteroceptive reflexes generate intense muscular responses and are of critical importance in prompt reprogramming essential for effective responses to environmental stimuli. Within the motor cortex (MI) there is a caudal region (MI/c) which receives exteroceptive cutaneous inputs and a rostral region (MI/r) which receives proprioceptive inputs. Transcortical reflexes mediated via pyramidal tract neurons (PTNs) of MI/r have properties which are analogous to segmental proprioceptive reflexes: changes of muscle length elicit PTN discharges which oppose the length change and so act to maintain stability. Furthermore, MI/r PTNs which are recruited earliest for small active movements are most sensitive to proprioceptive inputs. Data are not yet available concerning transcortical reflexes via MI/c during voluntary movement, but it is speculated that the cutaneous reflexes via MI/c might be functionally analogous to segmental cutaneous reflexes. Short-latency reflex responses also occur in postcentral (PoC) PTNs, and in this report we present results concerning the properties of PoC PTNs during active and passive movement. Caudal (area 2-5) PoC PTNs were similar to MI PTNs in that they often discharged prior to electromyogram (EMG) activity with active movement, and had different discharge frequencies with different steady state loads, but were unlike most MI PTNs in having the same changes of discharge with active and passive movement. Our finding of PoC discharge prior to movement onset, confirming that of Soso and Fetz in 1980, is discussed in connection with the concept of corollary discharge.

Afferent Pathways↗
PubMed1979

Interaction between the baroreceptor and Bezold-Jarisch reflexes.

The interaction between the carotid baroreflex and Bezold-Jarisch (BJ) reflex (intravenously administered veratridine) was studied in anesthetized rabbits after aortic nerve section. The carotid sinuses were vascularly isolated to regulate the intrasinus pressure (ISP). The extent of BJ reflex bradycardia and hypotension was progressively diminished as the ISP was elevated stepwise. When the carotid baroreflex was not operative by holding the ISP constant at control, the BJ reflex changes in heart rate (HR) and systemic arterial pressure (SAP) were not significantly different from those induced at the normal condition. Thus the calculated baroreflex static loop gain was greatly decreased during the BJ reflex. However, sinus denervation, analogous to keeping ISP below 50 mmHg, significantly enhanced the BJ reflex effects. A steady-state infusion of veratridine remarkably reduced the slope of the baroreflex function ISP-SAP and ISP-HR curves. The results indicate that the BJ reflex effects are affected by the prevailing arterial baroreceptor input, varying inversely with the ISP level. An attenuation in the baroreflex sinsitivity, in terms of the loop gain or slope of the transfer function curve, was observed during the BJ reflex. The presence of tonic cardiovascular inhibitions exerted by the arterial baroreceptors tends to reduce the BJ reflex bradycardia and hypotension, but the baroreceptors do not function adequately in buffering the cardiovascular inhibition produced by the cardiogenic reflex.

Animals↗
PubMed2006

Roles of glutamatergic and serotonergic mechanisms in reflex control of the external urethral sphincter in urethane-anesthetized female rats.

This study was conducted to examine reflex mechanisms that mediate urinary bladder and external urethral sphincter (EUS) coordination in urethane-anesthetized female Sprague-Dawley rats. We investigated the properties of EUS reflexes elicited by electrical stimulation of pelvic nerve afferent axons (pelvic-EUS reflex). The changes in the reflexes induced by bladder distension and administration of agonists or antagonists for glutamatergic or serotonergic receptors were examined. The reflexes consisted of an early response (ER, 18- to 22-ms latency) and a late, long-duration (>100-ms latency) response (LR), which consisted of bursts of activity at 20- to 160-ms interburst intervals. In a few experiments, a reflex with an intermediate (40- to 70-ms) latency was also identified. With the bladder empty, the ER, but not the LR, was detected in the majority of experiments. The LR was markedly enhanced when the bladder was distended. The ER remained, but the LR was abolished, after spinal cord transection at T8-T9. The ER and LR were significantly decreased 75 and 35%, respectively, by the N-methyl-D-aspartate receptor antagonist MK-801 (0.3 mg/kg iv), but only decreased 18 and 14%, respectively, by the alpha-amino-5-methylisoxazole-4-propionate receptor antagonist LY-215490 (3 mg/kg iv). The serotonin (5-HT1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (1 mg/kg iv) enhanced spontaneous EUS activity and the pelvic-EUS reflex. WAY-100635 (0.1-1 mg/kg iv), a 5-HT1A antagonist, reversed the effect of 8-hydroxy-2-(di-n-propylamino)-tetralin and suppressed EUS activity and the pelvic-EUS reflex. These results indicate that glutamatergic and serotonergic mechanisms are important in the reflex pathways underlying bladder- sphincter coordination in rats.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed1990

Micturition reflexes in decerebrate and spinalized neonatal rats.

Micturition in neonatal rats is mediated by a spinal reflex pathway activated by the mother licking the perineum (the perineal-to-bladder reflex, P-Bld). Micturition in adult rats is mediated by a spinobulbospinal reflex pathway activated by bladder distension (the bladder-to-bladder reflex, Bld-Bld). This study examines the postnatal development of the Bld-Bld reflex in decerebrate or spinalized unanesthetized and urethan-anesthetized rat pups 2-26 days of age. Urethan anesthesia depressed both the Bld-Bld and P-Bld micturition reflexes. Bld-Bld micturition reflexes (peak intravesical pressures of 13 +/- 6 cmH2O and durations of 35 +/- 11 s) were noted in 54% of 2-day-old decerebrate pups and in all of the 6- and 9-day-old decerebrate pups but in none of the 2- or 9-day-old spinalized pups. We conclude that a weak supraspinal Bld-Bld reflex is present during the early postnatal period; however, the P-Bld reflex is the primary mediator of micturition in the neonatal rat.

Anesthesia↗
PubMed1979

Abolition of a viscerosomatic reflex during oxygen deprivation.

Effect of hypoxia was studied on the J reflex, a term used for the reflex inhibition of muscular exercise by activation of type J pulmonary endings. Hypoxia was induced by ventilating the animal with gas mixtures varying from 18 to 9% oxygen in nitrogen. The blood gas tensions of the arterial blood (PaO2) were measured before, during, and after hypoxia. It was found that the J reflex is sensitive to a reduction in oxygen. A fall in PaO2 between 67 and 47 Torr abolished this reflex. Studies were also undertaken to exclude the possibility that the effect of hypoxia was predominantly exerted on monosynaptic reflex itself. A similar fall in PaO2 also depressed the monosynaptic reflex. Minimum time required for the significant depression was 32 s. On the contrary the J reflex was abolished within this period suggesting that the abolition of J reflex is independent of the changes in monosynaptic reflex during hypoxia.

Action Potentials↗
PubMed2004

Contribution of muscle afferents to prolonged flexion withdrawal reflexes in human spinal cord injury.

The contribution of force-sensitive muscular afferents to prolonged flexion withdrawal reflexes, or flexor spasms, after human spinal cord injury (SCI) was investigated. In three separate experimental conditions, flexion reflexes were triggered in subjects with SCI using trains of electrocutaneous stimuli delivered at the foot and lower leg and compared with reflexes elicited via intramuscular (i.m.) electrical stimuli. In the first experiment, flexion reflexes were elicited using i.m. stimuli to the tibialis anterior (TA) in the majority of subjects tested. The ratio of peak isometric ankle to hip torques during i.m.-triggered reflexes were proportionally similar to those evoked by electrocutaneous foot or shank stimulation, although the latency to onset and peak flexion torques were significantly longer with i.m. stimulation. In the second experiments, the amplitude and frequency of i.m. TA stimulation were varied to alter the stimulus-induced muscle torque. Peak ankle and hip torques generated during the flexion reflex responses were correlated to a greater extent with stimulus-induced muscle torques as compared with the modulated stimulus parameters. In the third experimental series, i.m. stimuli delivered to the gastrocnemius (GS) elicited flexion reflexes in approximately half of the subjects tested. The combined data indicate a potentially prominent role of the stimulus-induced muscle contraction to the magnitude and latency of flexor reflex behaviors after i.m. TA stimulation. Results after i.m. GS stimulation indicate multi-joint flexion reflexes can also be elicited, although to a lesser extent than i.m. TA stimulation.

Adult↗
PubMed2006

Context-dependent modulation of interlimb cutaneous reflexes in arm muscles as a function of stability threat during walking.

Cutaneous reflexes evoked in the muscles of the arms with electrical stimulation of nerves of the foot ("interlimb reflexes") are observed during walking. These reflexes have been suggested to coordinate the actions of the legs and arms when walking is disturbed. Recently, we showed that cutaneous reflexes evoked in the leg muscles after stimulation at the foot are modulated according to the level of postural threat during walking. We hypothesized that the amplitude of interlimb cutaneous reflexes would similarly be modulated when subjects walk in unstable environments. Subjects walked on a treadmill under four walking conditions: 1) normal; 2) normal with unpredictable anterior-posterior (AP) perturbations; 3) arms crossed; and 4) arms crossed with unpredictable AP perturbations. Interlimb reflexes evoked from electrical stimulation of the right superficial peroneal or sural nerves were recorded bilaterally, at four points of the step cycle. These reflexes were compared between conditions in which the arms were moving in a similar manner: 1) normal versus AP walking and 2) arms crossed versus arms crossed with AP perturbations. Differences in reflex amplitudes between arms-crossed conditions were observed in most upper limb muscles when subjects were perturbed while walking compared with undisturbed walking. This effect was less apparent when the arms were swinging freely. The results indicate that the strength of interlimb connections is influenced by the level of postural threat (i.e., the context of the behavior), thereby suggesting that these reflexes serve a functional link between the legs and arms during locomotion.

Adult↗
PubMed2004

Neuromuscular reflexes contribute to knee stiffness during valgus loading.

We have previously shown that abduction angular perturbations applied to the knee consistently elicit reflex responses in knee joint musculature. Although a stabilizing role for such reflexes is widely proposed, there are as of yet no studies quantifying the contribution of these reflex responses to joint stiffness. In this study, we estimate the mechanical contributions of muscle contractions elicited by mechanical excitation of periarticular tissue receptors to medial-lateral knee joint stiffness. We hypothesize that these reflex muscle contractions will significantly increase knee joint stiffness in the adduction/abduction direction and enhance the overall stability of the knee. To assess medial-lateral joint stiffness, we applied an abducting positional deflection to the fully extended knee using a servomotor and recorded the torque response using a six degree-of-freedom load-cell. EMG activity was also recorded in both relaxed and preactivated quadriceps and hamstrings muscles with surface electrodes. A simple, linear, second-order, delayed model was used to describe the knee joint dynamics in the medial/lateral direction. Our data indicate that excitation of reflexes from periarticular tissue afferents results in a significant increase of the joint's adduction-abduction stiffness. Similar to muscle stretch reflex action, which is modulated with background activation, these reflexes also show dependence on muscle activation. The potential significance of this reflex stiffness during functional tasks was also discussed. We conclude that reflex activation of knee muscles is sufficient to enhance joint stabilization in the adduction/abduction direction, where knee medial-lateral loading arises frequently during many activities.

Adult↗
PubMed2003

Windup of flexion reflexes in chronic human spinal cord injury: a marker for neuronal plateau potentials?

The physiological basis of flexion spasms in individuals after spinal cord injury (SCI) may involve alterations in the properties of spinal neurons in the flexion reflex pathways. We hypothesize that these changes would be manifested as progressive increases in reflex response with repetitive stimulus application (i.e., "windup") of the flexion reflexes. We investigated the windup of flexion reflex responses in 12 individuals with complete chronic SCI. Flexion reflexes were triggered using trains of electrical stimulation of plantar skin at variable intensities and inter-stimulus intervals. For threshold and suprathreshold stimulation, windup of both peak ankle and hip flexion torques and of integrated tibialis anterior electromyographic activity was observed consistently in all patients at inter-stimulus intervals < or =3 s. For subthreshold stimuli, facilitation of reflexes occurred only at intervals < or =1 s. Similarly, the latency of flexion reflexes decreased significantly at intervals < or =1 s. Patients that were receiving anti-spasticity medications (e.g., baclofen) had surprisingly larger windup of reflex responses than those who did not take such medications, although this difference may be related to differences of spasm frequency between the groups of subjects. The results indicate that the increase in spinal neuronal excitability following a train of electrical stimuli lasts for < or =3 s, similar to previous studies of nociceptive processing. Such long-lasting increases in flexion reflex responses suggest that cellular mechanisms such as plateau potentials in spinal motoneurons, interneurons, or both, may partially mediate spinal cord hyperexcitability in the absence of descending modulatory input.

Adult↗
PubMed2006

Sensorimotor cortex ablation prevents H-reflex up-conditioning and causes a paradoxical response to down-conditioning in rats.

Operant conditioning of the H-reflex, a simple model for skill acquisition, requires the corticospinal tract (CST) and does not require other major descending pathways. To further explore its mechanisms, we assessed the effects of ablating contralateral sensorimotor cortex (cSMC). In 22 Sprague-Dawley rats, the hindlimb area of left cSMC was ablated. EMG electrodes were implanted in the right soleus muscle and a stimulating cuff was placed around the right posterior tibial nerve. When EMG remained in a specified range, nerve stimulation just above the M response threshold elicited the H-reflex. In control mode, no reward occurred. In conditioning mode, reward occurred if H-reflex size was above (HRup mode) or below (HRdown mode) a criterion value. After exposure to the control mode for > or = 10 days, each rat was exposed for another 50 days to the control mode, the HRup mode, or the HRdown mode. In control and HRup rats, final H-reflex size was not significantly different from initial H-reflex size. In contrast, in HRdown rats, final H-reflex size was significantly increased to an average of 136% of initial size. Thus like recent CST transection, cSMC ablation greatly impaired up-conditioning. However, unlike recent CST transection, cSMC produced a paradoxical response to down-conditioning: the H-reflex actually increased. These results confirm the critical role of cSMC in H-reflex conditioning and suggest that this role extends beyond producing essential CST activity. Its interactions with ipsilateral SMC or other areas contribute to the complex pattern of spinal and supraspinal plasticity that underlies H-reflex conditioning.

Animals↗
PubMed1976

Regulatory actions of human stretch reflex.

1. The stretch reflex in the elbow flexor musculature was studied in 23 human subjects. The subjects were required to establish an initial force equivalent to 10% maximum at a prescribed initial length; mechanical disturbances delivered at random times increased load force to 15% or reduced it to 5%. We measured arm force, displacement, and EMG (usually biceps); acceleration was calculated from displacement, and average responses from sets of 10 like trials. 2. Modification of the stretch reflex was studied by comparing average responses obtained with different instructions, but with the same disturbance. The usual introductions were "compensate for arm deflection" and "do not intervene voluntarily". The initial response did not depend on instruction; changes in response that depended on instruction began abruptly after a latent period which ranged from 70 to 320 ms (measured from force and acceleration), depending on conditions and subject. The latency became longer (10-50 ms) and more variable when the subject did not know the direction of disturbance in advance. This and other observations indicate that modifications of the stretch reflex are not produced by servo actions. They are produced by triggered reactions, which occur at both short and long latencies and which have properties resembling the movements produced in a reaction-time task. 3. We confirmed that most subjects can suppress triggered reactions when the instruction calls for no intervention, leaving an unmodified reflex response. This response consists of a compliant deflection of the arm in the direction of the disturbance. 4. The compensatory actions associated with unmodified stretch (and unloading) reflexes were assessed from EMG responses of biceps. During a 300-ms transient phase, EMG changes were notably asymmetric when responses to symmetric disturbances were compared. Increased force stretched biceps and produced a prominent increase in EMG, whereas decreased force allowed biceps to shorten and produced either an EMG decrease of smaller magnitude or an actual increase. These asymmetric reflex actions produced quite symmetric mechanical responses (arm displacements and forces), which implies the existence of and compensation for nonlinear muscle mechanical properties. This result is discussed in relation to the hypothesis that the function of the stretch reflex is to compensate for variations in muscle properties, thus maintaining stiffness. 5. Effective control of muscle length or joint position does not result from servo action by the stretch reflex. Errors in position are corrected only when triggered reactions are superimposed on the reflex response.

Adult↗
PubMed1979

Response to sudden torques about ankle in man: myotatic reflex.

1. Sudden dorsiflexions and plantarflexions of the foot were imposed on normal human subjects under various states of voluntary activity. 2. Under conditions of constant muscle contraction, the myotatic reflex in soleus and lateral gastrocnemius muscles is linearly and highly correlated with the rate of muscle stretch. The slope of this curve characterizes part of the reflex arc "gain." 3. The gain is linearly proportional to the level of tonic voluntary activation. 4. The gain is reduced by tonic contraction of antagonists. 5. The above statements can be summarized by the following equation (formula: see text), where d theta/dt is the rate of joint rotation. Ts and Tat are measures of voluntary contraction (tension) of all the extensor and flexor muscles acting at the ankle. The term S represents the level of preexisting spinal excitability that can be altered by prior instruction to the subject. 6. A phasic voluntary contraction of the soleus muscle, which leads to muscle shortening, will alter the reflex gain. The gain initially increases with increasing rates of shortening, but at higher rates the gain is reduced. This is in contradiction to the observation for tonic activation as stated above and may be due to an inability of the coactivated fusimotor system to produce sufficiently rapid cocontraction of the spindle fibers. 7. During lengthening of a muscle caused by voluntary contraction of its antagonists, the myotatic reflex gain is reduced. 8. The above facts are interpreted to imply that a functional role for the myotatic reflex in the leg extensors is limited to conditions of postural maintenance or slow, precise movement. During rapid movement, the myotatic reflex is ineffective and load-compensating reactions are mediated by longer latency loops. 9. The duration of the myotatic reflex EMG is from 10 to 40 ms, too brief to be a simple response to a velocity-sensing receptor organ. Either the response is in large measure due to the initial burst of spindle activity that occurs at the start of a ramp stretch, or motoneuron pool dynamics act as a high-pass filter on afferent inputs. 10. In the anterior tibial muscle, the relationships between stretch velocity and reflex amplitude and tonic voluntary contraction and reflex gain are qualitatively similar to those found in the ankle extensors.

Ankle↗
PubMed1979

Quantitative analysis of relative contribution of central and peripheral neurons to gill-withdrawal reflex in Aplysia californica.

1. There is general agreement that the gill-withdrawal reflex elicited by weak tactile stimuli (less than 2 g to the siphon skin) is mediated almost entirely by the central nervous system (CNS) (13, 15, 18, 23). However, there was disagreement concerning the effects of moderate intensity (2--4 g) stimuli. Kupfermann et al. (18) found that the CNS mediates approximately 94% of the reflex elicited by moderate-intensity stimuli, whereas Peretz et al. (23) found that in this stimulus range, the amplitude of the reflex was, on average, unaltered when the CNS was removed. 2. To resolve this difference we first carried out pilot experiments in collaboration with B. Peretz, J. W. Jacklet, and K. Lukowiak using isolated mantle preparations, and then performed a systematic study using both the isolated mantle and intact animals. The main difference that seemed to account for the discrepancy in the results was the magnitude of the reflex response that was selected for study. Previous studies from this laboratory used a minimum-response criterion whereby only brisk and clearly observable responses of at least 35% maximum were examined. By contrast, Peretz et al. (23) examined all responses, even those that were extremely small. In addition, the two groups used different methods of stimulation so that stimulus intensities could not really be compared. 3. By comparing the effects of moderate-intensity stimuli in experiments with and without a response criterion in isolated mantle preparations, we found that when a minimal response-amplitude criterion is imposed the CNS mediates 90--95% of the gill-withdrawal reflex, whether it is elicited by the "tapper" stimulus used by Peretz et al. (23) or by the servo-controlled probe previously used in this laboratory. On the other hand, when no minimal response criterion is used and small responses are also examined, the response to the probe is still significantly reduced by 85% when the CNS is removed, whereas the reflex response to the tapper is more variable, sometimes increasing and other times decreasing with deganglionation. 4. We have also tested, in intact animals, the role of the CNS in mediating gill withdrawal. Water-jet stimuli were delivered to the siphon to elicit the reflex, as in previous behavioral studies. As was the case with probe stimulation in the isolated mantle preparation, in intact animals the CNS mediates 90% of the reflex evoked by moderate-intensity stimuli when a minimal response-amplitude criterion is imposed. 5. Our experiments indicate that, using response criteria and methods of stimulation, one can reliably study the reflex as predominantly mediated by the CNS.

Animals↗
PubMed1987

Changes in size of the stretch reflex of cat and man attributed to aftereffects in muscle spindles.

1. This is a report of experiments carried out on the cat and on man, which demonstrate that conditioning of a muscle by contraction and movement can lead to changes in amplitude of stretch reflexes elicited in that muscle. 2. In triceps surae of the cat, the reflex response to a brief stretch was recorded after conditioning with a whole-muscle contraction followed by a pause at a length either 5 mm longer or shorter than the length at which the reflex was elicited. Following conditioning at the long length the reflex response was less than half as large as that following conditioning at the short length. 3. The changes in reflex amplitude could be correlated with an altered stretch responsiveness of muscle spindles in the soleus muscle. When the muscle had been held long during conditioning, a subsequent brief stretch applied at an intermediate length elicited fewer impulses in primary endings of spindles than after conditioning at a short length. 4. The same kind of experiment was then carried out on adult human subjects. When a tendon tap was applied to the Achilles tendon after a voluntary contraction and relaxation of triceps surae with the muscle at a long length, (foot dorsiflexed) the reflex was frequently less than half the size it had been after a contraction at a short length (foot plantarflexed). It was concluded that the same kind of spindle aftereffects as observed for cat soleus spindles were responsible for the changes in reflex amplitude. 5. It was found both in the cat and in human subjects that the changes in reflex amplitude after conditioning became progressively less as the test length was made longer. 6. The explanation put forward to account for these observations is that stable cross-bridges form between actin and myosin filaments of passive intrafusal (and extrafusal) fibers. When the muscle is shortened several seconds after a contraction at a long length, the intrafusal fibers, stiffened by the presence of cross-bridges, fall slack. Slack does not develop after a contraction at a short muscle length, as the fiber is stretched to the test length. Since any slack must first be taken up by the test stretch, there is a smaller afferent response and consequently a smaller reflex contraction in response to a tendon tap after conditioning at a long length.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1990

Ipsilateral and contralateral effects on cutaneous reflexes in a back muscle of the female rat: modulation by steroids relevant for reproductive behavior.

1. Multiunit EMG recordings of cutaneous reflexes--evoked in the back extensor, lateral longissimus (LL), by bilateral stimulation of nerves to dorsal flank skin--were studied in ovariectomized female rats with and without estrogen pretreatment. 2. Poststimulus time (PST) histograms of data from rats with and without estrogen pretreatment show that the axial EMG response (10-30 ms) to ipsilateral (ipsi) flank skin nerve stimulation is significantly shorter in latency (1.4 ms) and 67% larger than the response to contralateral (contra) flank skin nerve stimulation recorded at the same site (n = 29 pairs). 3. When late EMG responses were also evoked at 50-120 ms in 37% of ipsi and 29% of contra cutaneous reflexes, the incidence of multiunit activity in the late peak was significantly greater in rats pretreated with silastics containing 100% estradiol (E) compared with 10% E or cholesterol controls. 4. When an ipsi cutaneous reflex in LL was conditioned by a stimulus to the contra flank skin nerve at a condition-test interval of 30 ms (C-T 30 ms), the average number of discharges in the early peak of the histogram was double that in the histogram obtained from the unconditioned ipsi reflex, independent of estrogen pretreatment. 5. In 12 out of 19 cases in which a contra cutaneous reflex was conditioned by a stimulus to the ipsi flank skin nerve (C-T 30 ms), the number of discharges in the early peak of the histogram was less than that in the early peak of the histogram derived from the unconditioned contra response, independent of estrogen pretreatment. 6. Intravenous injections of progesterone (P) or its metabolite 5 alpha-pregnane-3 alpha-ol-20-one (tetrahydraprogesterone, THP) decreased the magnitude of the early peak of the ipsi cutaneous reflex and the contra cutaneous reflex in LL, independent of estrogen pretreatment. At the same time, it did not reduce the magnitude of the early peak evoked by either ipsi or contra nerves after conditioning from the other side at C-T 30 ms. 7. As a consequence, the percentage facilitation of ipsi cutaneous reflexes by contra cutaneous conditioning was significantly increased after P or THP. This suggests that these steroids can selectively enhance behaviors involving bilateral inputs. 8. An unchanged axial motoneuron pool output to bilateral cutaneous inputs after P and THP, in spite of reduced motoneuron responses to cutaneous inputs from each side of the body, implies an increased gain in the reflex circuit.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗