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Biomedical subjects

H Hultborn

Publications and source records attributed to H Hultborn.

At least 55 records · Page 3Linked to original sources
PubMed1987

Proprioceptive input resets central locomotor rhythm in the spinal cat.

The reflex regulation of stepping is an important factor in adapting the step cycle to changes in the environment. The present experiments have examined the influence of muscle proprioceptors on centrally generated rhythmic locomotor activity in decerebrate unanesthetized cats with a spinal transection at Th12. Fictive locomotion, recorded as alternating activity in hindlimb flexor and extensor nerves, was induced by administration of nialamide (a monoamine oxidase inhibitor) and L-DOPA. Brief electrical stimulation of group I afferents from knee and ankle extensors were effective in resetting fictive locomotion in a coordinated fashion. An extensor group I volley delivered during a flexor burst would abruptly terminate the flexor activity and initiate an extensor burst. The same stimulus given during an extensor burst prolonged the extensor activity while delaying the appearance of the following flexor burst. Intracellular recordings from motoneurones revealed that these actions were mediated at premotoneuronal levels resulting from a distribution of inhibition to centres generating flexor bursts and excitation of centres generating extensor bursts. These results indicate that extensor group I afferents have access to central rhythm generators and suggest that this may be of importance in the reflex regulation of stepping. Experiments utilizing natural stimulation of muscle receptors demonstrate that the group I input to the rhythm generators arises mainly from Golgi tendon organ Ib afferents. Thus an increased load of limb extensors during the stance phase would enhance and prolong extensor activity while simultaneously delaying the transition to the swing phase of the step cycle.

Afferent Pathways↗
PubMed1987

Reciprocal Ia inhibition between ankle flexors and extensors in man.

1. Reciprocal inhibition between antagonist muscle groups at the ankle has been investigated in sixty healthy subjects. Hoffmann reflexes (H reflexes) in the soleus and tibialis anterior muscles were used to assess changes in reciprocal inhibition evoked by electrical stimulation of antagonist muscle nerves. 2. Inhibition of the soleus H reflex was evoked by a single conditioning stimulus to the common peroneal nerve, and inhibition of the tibialis anterior H reflex was elicited by one conditioning stimulus to the posterior tibial nerve. Symmetrical central connections between the antagonist flexors and extensors were assumed and under this assumption the central delay for the inhibition, in addition to the delay for monosynaptic Ia excitation, was calculated to be about 1 ms. The inhibition was evoked by weak stimuli to the nerves from antagonist muscle groups; the threshold for the inhibition was around 0.6 X threshold for a direct motor response (M-threshold). Furthermore, tendon taps to the Achilles tendon facilitated the soleus H reflex and inhibited the tibialis anterior reflex at short latencies. The short central delay, the low electrical threshold and the.actions of Achilles tendon taps strongly suggest that the early reciprocal inhibition is homologous to the disynaptic Ia inhibition previously studied in animal experiments. 3. With the test soleus H reflex kept at 15-25% of the maximum directly evoked motor response (M-response) and the strength of the conditioning peroneal nerve stimulation kept at 1.0 X M-threshold, the inhibition from the peroneal nerve ranged between 0 and 40% (mean, 14.9%) at rest. 4. Changes in the amount of reciprocal inhibition from the peroneal nerve were studied both during tonic and dynamic dorsi- and plantarflexion. During tonic dorsiflexion there was no significant change of inhibition as compared to rest, while inhibition decreased during tonic plantarflexion. However, during ramp-and-hold dorsiflexion there was a transient increase in reciprocal inhibition of the soleus H reflex. This increase in inhibition from the peroneal nerve could be seen 50 ms prior to the onset of contraction. The increase in inhibition before and at the very beginning of the contraction cannot be due to sensory feed-back during contraction, but must depend on a supraspinal control of the spinal cord. 5. At conditioning-test intervals of 4-6 ms, the inhibition of the soleus H reflex from the peroneal nerve was considerably larger during tonic dorsiflexion than at rest. Thus, tonic dorsiflexion revealed an inhibition with long latency from the peroneal nerve, which was not seen at rest.(ABSTRACT TRUNCATED AT 400 WORDS)

Achilles Tendon↗
PubMed1987

Assessing changes in presynaptic inhibition of I a fibres: a study in man and the cat.

1. A method to assess changes in presynaptic inhibition of I a afferent terminals in man is proposed. The soleus H reflex was facilitated by a heteronymous I a volley from quadriceps and the amount of reflex facilitation was used to estimate the size of the conditioning I a excitatory post-synaptic potential (e.p.s.p.). It is argued that the size of this e.p.s.p. as measured by the resulting amount of reflex facilitation reflects the amount of presynaptic inhibition on the corresponding I a fibres. A decrease in the reflex facilitation may then be ascribed to an increase in presynaptic inhibition of the I a fibres mediating the conditioning volley. 2. That the heteronymous I a facilitation from quadriceps to soleus is caused by a purely monosynaptic e.p.s.p. is a prerequisite for the validity of the method. Experimental evidence is therefore given in an Appendix that in man the earliest part (first 0.5 ms) of this heteronymous I a facilitation is mediated through a monosynaptic pathway. Evidence is also given that this earliest facilitation is not yet contaminated by any polysynaptic effects from I a or I b afferents. 3. The validity of the method was established in animal experiments in which presynaptic inhibition of I a fibres and post-synaptic events in motoneurones could be assessed by direct tests. It was found that the amount of test reflex facilitation produced by a conditioning I a volley was decreased when I a fibres were subjected to presynaptic inhibition but remained unchanged when the motoneurone pool in which the test reflex was elicited received pure post-synaptic inhibition. 4. In man, presynaptic inhibition of I a fibres was evoked by a short-lasting (three shocks at 200 Hz) vibration applied to the tibialis anterior tendon. Such a vibratory burst reduced the efficiency of the heteronymous I a volley in facilitating the soleus H reflex. By contrast, during a pure post-synaptic inhibition of soleus motoneurones the efficiency of the conditioning volley in facilitating the test reflex remained unchanged. It is therefore argued that the amount of heteronymous I a facilitation can indeed be used to assess the amount of ongoing presynaptic inhibition exerted onto heteronymous I a fibres from the quadriceps muscle to soleus motoneurones. 5. The short-lasting tibialis anterior vibration used here evoked a long-lasting (300-500 ms) depression of soleus and quadriceps H reflexes.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1987

Changes in presynaptic inhibition of Ia fibres at the onset of voluntary contraction in man.

1. Two independent methods were used, in man, to assess changes in presynaptic inhibition of I a terminals at the onset of selective voluntary contractions: (1) measurement of the amount of heteronymous monosynaptic I a facilitation (from the quadriceps muscle to soleus motoneurones) to provide an assessment of the amount of ongoing presynaptic inhibition exerted on the I a fibres responsible for the facilitation; (2) measurement of the inhibition of H reflexes 40-60 ms after a short vibration to the tibialis anterior tendon to estimate the excitability of the interneurones mediating presynaptic inhibition from tibialis anterior I a afferents to the I a afferents of the test H reflex (soleus or quadriceps). 2. At the onset of an isolated voluntary plantar flexion of the foot (gastrocnemius-soleus contraction) the heteronymous facilitation from quadriceps to soleus was increased, reflecting a decreased presynaptic inhibition of the quadriceps I a terminals on soleus motoneurones. Vibratory inhibition of the soleus H reflex was decreased, reflecting an inhibition of transmission of presynaptic inhibition to homonymous soleus I a afferent terminals. 3. At the onset of the same gastrocnemius-soleus contraction there was, on the contrary, an increased vibratory inhibition of the quadriceps H reflex indicating a facilitation of transmission of presynaptic inhibition to homonymous quadriceps I a afferent terminals. 4. At the onset of an isolated voluntary knee extension (quadriceps contraction) the opposite pattern was seen: the heteronymous facilitation from quadriceps to soleus was decreased and the vibratory inhibition of a soleus H reflex was increased, whereas the vibratory inhibition of the quadriceps H reflex was decreased. 5. These results indicate that presynaptic inhibition of I a afferent terminals on motoneurones of contracting muscles is decreased, permitting I a activity to contribute to excitation of voluntarily activated motoneurones. On the contrary, presynaptic inhibition of I a fibres to motoneurones of muscles not involved in the contraction is increased. It is argued that the former must be supraspinal in origin. 6. It is concluded that the control of presynaptic inhibition of I a fibres at the onset of movement may be organized so as to aid in achieving selectivity of muscle activation, i.e. so as to increase motor contrast.

Adult↗
PubMed1986

Changes in polysynaptic Ia excitation to quadriceps motoneurones during voluntary contraction in man.

Homonymous Ia facilitation of quadriceps (Q) motoneurones (MNs) was significantly larger during (and at the onset of) a very weak Q voluntary contraction than at rest. This increase in Ia facilitation only appeared with a conditioning-test interval within the narrow range of 5-9 ms, which fits the time course of the recently described polysynaptic Ia excitation to Q MNs. This suggests that interneurones mediating polysynaptic Ia excitatory effects to Q MNs receive a strong descending excitation during such a contraction. It is therefore argued that these interneurones might mediate part of the descending command to Q MNs during voluntary contraction.

Electric Stimulation↗
PubMed1985

Reciprocal Ia inhibition from the peroneal nerve to soleus motoneurones with special reference to the size of the test reflex.

The aim of the study was to examine the supraspinal control during voluntary movements of the foot in man, of the Ia inhibitory interneurones activated from the anterior tibial muscle and projecting to the soleus alpha-motoneurones. Previous studies have reported an increased inhibition of the soleus alpha-motoneurones by a constant conditioning stimulus to the common peroneal nerve during dorsiflexion of the foot. This was interpreted as a sign of supraspinal facilitation of the Ia inhibitory interneurones. However, these results could not be reproduced in the present study. The contradictory results can probably be explained by some important methodological differences in the use of the H-reflex technique.

Adult↗
PubMed1984

Labelling of interneurones by retrograde transsynaptic transport of horseradish peroxidase from motoneurones in rats and cats.

Transsynaptic transport of conjugates of wheat germ agglutinin with horseradish peroxidase (WGA-HRP) has been studied in rats and cats. WGA-HRP was injected into a muscle nerve from which it was first transported to motoneurones and along sensory fibres, and secondarily to interneurones. More extensive labelling of interneurones occurred in preparations with only relevant ventral roots intact than in preparations with only dorsal roots intact, which indicates that the transsynaptic transport of WGA-HRP is primarily in the retrograde direction, i.e. from motoneurones to interneurones. Such a transport appeared to be considerably enhanced by neuronal activity.

Animals↗
PubMed1984

Evidence for presynaptic inhibition of muscle spindle Ia afferents in man.

A new method for estimating presynaptic inhibition of soleus (Sol) Ia fibres in man is introduced based on the assessment of the amount of facilitation produced by a preceding homonymous Ia volley onto a Sol monosynaptic reflex. It was found in animal experiments that a reduction of such Ia facilitation reflects the amount of presynaptic inhibition exerted on these Ia fibres. Since, in man, a similar reduction of homonymous Ia facilitation was found after tibialis anterior vibration, this provides evidence for presynaptic inhibition of Sol Ia fibres, elicited here by the afferent discharge due to this flexor vibration.

Adult↗
PubMed1984

Afterhyperpolarization in neurones of the red nucleus.

Afterhyperpolarization (AHP) following single or short trains of spikes in rubrospinal neurones (RN neurones) of the cat has been studied with intracellular recording techniques. The AHP amplitude was potential dependent; it increased with depolarization and decreased with hyperpolarization and had an extrapolated reversal potential about 20 mV below resting membrane potential. The AHP was associated with an increase in the membrane conductance and it was concluded that the AHP is primarily caused by an increase in membrane conductance to potassium ions. The time course of the conductance change underlying the AHP was measured with short current pulses and calculated from the AHP voltage. The AHP following a single spike was conditioned at different interspike intervals by a preceding spike (or several spikes). In many RN neurones the AHP (conductance) following a spike added approximately linear to that generated by a preceding spike. In most cells, however, the AHP following a spike was instead depressed by a preceding spike. The summation of AHPs increased progressively, while the depression appeared to be already maximal with one preceding spike. The depression was then approximately constant for interspike intervals less than the AHP duration. It will be shown in a following paper that these properties of the AHP are reflected in the behaviour of the repetitive discharge evoked by constant current pulses in the same neurones.

Action Potentials↗
PubMed1984

Intrinsic membrane properties causing a bistable behaviour of alpha-motoneurones.

In decerebrate cats a train of impulses in Ia afferents may lead to a sustained increase in excitability of alpha-motoneurones of homonymous and heteronymous muscles. It was previously suggested that this long-lasting excitability increase reflects a maintained synaptic input to the motoneurones from excitatory interneurones. With intracellular recording from motoneurones we here demonstrate that the sustained increase of alpha-motoneurone activity is due to an all-or-none plateau depolarization. This plateau can be induced by a short train of excitatory synaptic potentials or a brief, intracellularly injected depolarizing current pulse and is terminated by a short train of inhibitory synaptic potentials or a hyperpolarizing current pulse. It is concluded that maintained motor unit firing triggered by a brief train of impulses in Ia afferent reflects an intrinsic bistable behaviour of alpha-motoneurones.

Animals↗
PubMed1983

Changes in segmental reflexes following chronic spinal cord hemisection in the cat. I. Increased monosynaptic and polysynaptic ventral root discharges.

The effect of a chronic spinal cord hemisection on segmental reflex transmission was studied in cats. Recordings of ventral root responses were made after a terminal transection below the initial lesion to eliminate descending influence through the intact spinal half. Procedures to ensure comparability between sides, that are lacking in earlier work on this experimental model, were introduced in the present work. It was demonstrated that there was an increase in reflex size on the lesioned (left) side, relative to the other side, of both mono- and polysynaptic reflexes. The reflex changes were found at all survival times studied (from 2 to 515 days). In control animals mono- and polysynaptic reflexes were found to be larger on the right side. It is discussed that side symmetry of reflex size is not to be presupposed even in a normal population. Detailed clinical examinations of lesioned animals were not performed, but a tendency for enhancement of the ipsilateral knee-jerk was found. There was no syndrome of spasticity and general motor recovery was very good.

Animals↗
PubMed1983

Changes in segmental reflexes following chronic spinal cord hemisection in the cat. II. Conditioned monosynaptic test reflexes.

In a companion paper (Hultborn & Malmsten 1983) it was described that ventral root discharges to stimulation of peripheral nerves became larger on the side of a chronic spinal hemisection (left) than on the other side. In the present paper, based on the same experiments, conditioning of monosynaptic test reflexes was used to study changes of both excitatory and inhibitory effects on specified motoneuronal pools. Conditioning stimulation was given to IA afferents (reciprocal Ia inhibition, presynaptic inhibition of Ia fibers), high threshold muscle afferents, low and high threshold cutaneous afferents and motor axons (recurrent inhibition). A comparison of the efficacy of conditioning stimuli on the two sides showed that facilitatory effects were larger on the side of hemisection in a clear majority of cases. Inhibition was almost always either more efficient on the side of hemisection or equally efficient on the two sides. In control cats, facilitatory effects tended to be larger on the right side, while the results for inhibitory conditioning generally showed no clear side-bias. The increase in facilitatory effects after lesions may contribute to symptoms of spasticity.

Afferent Pathways↗
PubMed1979

On the function of recurrent inhibition in the spinal cord.

Recurrent inhibition of alpha-motoneurons, via motor axon collaterals and Renshaw cells, obviously reduces the response (output) from a motor nucleus to a given synaptic input. It is proposed that the supraspinal covergence on Renshaw cells allows recurrent inhibition to serve as a variable gain regulator at motoneuronal level. This would allow for an optimal resolution in the force control during weak as well as strong contractions. Renshaw cells are not only inhibiting alpha-motoneurons but also gamma-motoneurons and IA inhibitory interneurons. It is argued that this distribution is meaningful since all these receptive neurons act together as a functional unit, forming an "output stage" of the motor system.

Animals↗
PubMed1979

Changes in recurrent inhibition during voluntary soleus contractions in man studied by an H-reflex technique.

1. The recurrent inhibition, brought about by a conditioning H-reflex discharge, was estimated in human subjects by the amplitude of a test H-reflex involving only the soleus motoneurones which fired in response to the conditioning volley. The modifications of the recurrent inhibition during contraction were evaluated by comparing the amplitude of the test H-reflex to a reference H-reflex. Both reflexes experienced the excitation underlying the voluntary contraction, but only the test H-reflex was subjected to the recurrent inhibition evoked by the conditioning H-reflex discharge. 2. Distinct differences were observed between the modifications of the test reflex and those of the reference H-reflex during both tonic and phasic voluntary contractions. Evidence is presented that these differences were due to changes in the amount of recurrent inhibition elicited by the conditioning discharge. 3. The changes in recurrent inhibition were studied while the subjects performed voluntary tonic contractions of various forces. The weakest contractions were accompanied by a decrease in the size of the test reflex. With greater contraction forces, there was no longer an inhibition of the test reflex, but instead a facilitation which grew continuously with increased contraction forces. The test reflex could eventually exceed the reference H-reflex amplitude at the strongest contractions. This is taken to indicate that the recurrent inhibition following the conditioning discharge was progressively decreasing, as the contraction force increased. 4. During ramp contractions, whatever the contraction velocity, the time courses of the variations of the test and reference H reflexes were almost inverse. Evidence is presented that these differential time courses were due to changes in the amount of recurrent inhibition elicited by the conditioning discharge. 5. The possibility of occlusion in the recurrent pathway ;as considered and it was concluded that the decrease in the recurrent inhibition elicited by the conditioning discharge was essentially due to an inhibitory control (spinal and/or suprasegmental) acting on Renshaw cells. This inhibition of Renshaw cells eventually counteracts the increasing excitatory inputs (resulting from the voluntary motor discharge) which they receive via motor axon collaterals during increasing tonic contractions and throughout ramp contractions. 6. The functional significance of the Renshaw cell inhibition during voluntary contraction is discussed in relation to the inhibition exerted through the recurrent pathway on both motoneurones and Ia inhibitory interneurones. It is suggested that the depression of Renshaw cell activity could play an important role during voluntary movements by favouring reciprocal Ia inhibition.

Adult↗
PubMed1979

Recurrent inhibition and afterhyperpolarization following motoneuronal discharge in the cat.

1. The relation between the size of a monosynaptic reflex (varying from the smallest values to the maximal motor response) and the output from Renshaw cells was investigated. This relation was extremely variable from one Renshaw cell to another. However, a linear relation between the reflex size and the early discharge emerged when the responses of all the Renshaw cells were averaged or when the summed activity of a pool of Renshaw cells was estimated by recording the recurrent inhibition in their target motoneurones. It was concluded that the lowest threshold motoneurones were efficient in producing recurrent inhibition. 2. In motoneurones, recorded intracellularly, the size of the depression caused by the afterhyperpolarization was compared to the maximum autogenetic recurrent inhibition. Under the particular experimental conditions used to mimic human experiments (Hultborn & Pierrot-Deseilligny, 1979a), it was found that recurrent inhibition had the same order of magnitude as the depression caused by afterhyperpolarization. 3. The additional depression caused by the summation of afterhyperpolarizations of two consecutive spikes was measured. It was shown that a summation of importance equal to the maximum autogenic recurrent inhibition required a mean interspike interval of 25 msec.

Animals↗