The Hoffmann reflex: a means of assessing spinal reflex excitability and its descending control in man.
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Animals↗
Biomedical subjects
M Schieppati
Publications and source records attributed to M Schieppati.
Reflex excitability of human soleus motoneurones during voluntary shortening or lengthening contractions.
1. We investigated the possibility that increase or decrease in the monosynaptic reflex excitability of the soleus muscle in man might play a role in matching the muscle mechanical output to the voluntary command aimed at performing isotonic contractions or relaxations, at various velocities. 2. Rectified and integrated electromyographic activity (e.m.g.) and the H reflex of soleus were measured during plantar flexions against a constant load (shortening contractions) or dorsal flexions resisting the load (lengthening contraction), performed without activation of pretibial muscles. 3. At the same ankle joint angle, integrated e.m.g. was larger during shortening contractions than during lengthening contractions. During shortening contractions, integrated e.m.g. increased as a function of the velocity of plantar flexion. During lengthening contractions, integrated e.m.g. decreased as a function of dorsal flexion and angular velocity and nearly disappeared in the last part of the most rapid lengthening contractions. 4. During shortening contractions, the H reflex increased beyond the extent expected for the level of e.m.g. activity; during lengthening contractions, reduction of the H reflex below control values at rest occurred in spite of background e.m.g. activity. 5. When the level of e.m.g. activity was kept constant, the above changes in H reflex were larger in both directions as a function of the velocity of the movement. 6. Passive rotation in the dorsal direction contributed to the inhibition observed during lengthening contractions. 7. It is suggested that these changes in the excitability of the H reflex, probably presynaptic in origin, serve the purpose of appropriately modulating the rate and extent of motoneurone recruitment during shortening and lengthening contractions. This allows the foot to follow a constant-velocity path in spite of the perturbing effects of the spindle afferent inputs and of the muscle characteristics described by the force--length and force--velocity diagrams.
Action Potentials↗
Inhibition of jaw-closing muscle activity by tactile air-jet stimulation of peri- and intra-oral sites in man.
There were two distinct periods of suppression in electromyograms of masseter and temporalis muscles in 8 subjects after light tactile stimulation (a puff of air) of the peri-oral zone; the latencies were 16 and 60 ms, respectively. These effects were similar to those induced by painful electrical stimulation. A single period of suppression with a latency of 50 ms, was evoked by tactile stimulation of the mucosa of the hard palate or of the tongue. Threshold for suppression was lower in mucosa than skin. Low-threshold and fast-adapting receptors are likely to be involved in this reflex inhibition.
Adult↗
Modulation of the Hoffmann reflex by rapid muscle contraction or release.
The inhibition of the H-reflex of the Soleus (Sol) muscle that takes place during and after voluntary release of Sol muscle has been attributed to presynaptic inhibition of autogenetic spindle afferences. In the present study, the time-relationship between onset of H-reflex depression and termination of Sol contraction was investigated to ascertain whether the reflex inhibition is linked to the command to release, or whether it is an accompanying phenomenon connected to changes in the neural outflow from the periphery. A parallel investigation was carried out on the temporal characteristics of the facilitation of the H-reflex that precedes onset of Sol contraction, in an attempt to point at the different functional organization of the two motor tasks. Voluntary releases from a bilateral isometric plantar torque, or bilateral plantar flexions, were performed in response to an acoustic stimulus, in a reaction time (RT) situation. The intervals from the starting signal to complete termination, or to beginning, of the Sol EMG were measured. The H-reflex was evoked at random during the tasks in one leg and its amplitude was referred in time to the end, or to the onset, of the EMG recorded from the contralateral Sol muscle. The RTs of the termination of Sol EMG had an average duration of about 100 ms, being some 20 ms shorter than those of the onset of EMG. In the release-task, the H-reflex amplitude was higher than that of the controls during the holding phase, and started to decrease about 20 ms before the cessation of the EMG.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Voluntary muscle release is not accompanied by H-reflex inhibition in patients with upper moto neuron lesions.
Changes in excitability of the soleus (Sol) monosynaptic reflex arc were investigated in spastic subjects, affected by amyotrophic lateral sclerosis (ALS), upon voluntary relaxation of tonic contraction of triceps surae muscle. Force and electromyograms (EMG) were recorded during triceps release, performed in response to an acoustic signal, in a reaction-time (RT) situation. Sol H-reflex was evoked at random during the task, and its amplitude was referred in time to the end of Sol EMG. At variance with the results for the same task in normal subjects, it was found that in ALS patients the RTs of the termination of EMG were longer than those of the beginning of the EMG, the decrease in force was prolonged and perturbed owing to intercurrent clonus-like EMG activity, and the H-reflex did not undergo the expected profound inhibition. It is suggested that the absence of the activation of presynaptic inhibitory mechanisms by the descending command to release brings about major disorders in voluntary muscle relaxation.
Amyotrophic Lateral Sclerosis↗
Excitability of reciprocal and recurrent inhibitory pathways after voluntary muscle relaxation in man.
We studied the potential contribution of postsynaptic mechanisms to the depression of reflex excitability which occurs immediately after a voluntary release from tonic muscle contraction. The excitability of the Soleus (Sol) motor pool was tested at rest and after voluntary muscle relaxation. In both cases the Sol H-reflex was conditioned by a single shock to the peroneal nerve, in order to activate the Ia interneurones (INs) mediating the reciprocal inhibition via a peripheral input, or by a short-lasting voluntary contraction of the Tibialis Anterior (TA) muscle, to activate the Ia INs via a central command. Changes in excitability of Renshaw cells were also tested at rest and after release, to assess the role of recurrent inhibition in the release-induced inhibition of the Sol H-reflex. It was demonstrated that: the excitability of the INs mediating the reciprocal inhibition was only slightly enhanced in comparison with resting conditions; the H-reflex of the antagonist muscle (TA) evoked after Sol release was not consistently facilitated with respect to rest; the command to contract the TA muscle reduced the H-reflex of the Sol muscle during rest but not after Sol release; recurrent inhibition did not increase its effect in the post-release period. Such features suggest that recurrent and reciprocal post-synaptic inhibitions do not play a major role in reducing the reflex excitability of a relaxing muscle; rather, the command to release prevents the reciprocal inhibitory effect which accompanies the contraction of the antagonist muscle.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Natural cutaneous stimulation induces late and long-lasting facilitation of extensor motoneurons in the cat.
An investigation was made of the effects of physiological cutaneous stimulation on the excitability of extensor motoneurons in spinal unanesthetized cats. The time course of changes in the monosynaptic reflex (MSR) amplitude of the soleus (Sol) and gastrocnemius medialis (GM) and lateralis (GL) was studied after conditioning stimulation with air jets (delivered to different regions of the skin of the ipsilateral hind limb), pinpricks, or stretching of the skin of the heel induced by passive rotation of the tibio-tarsal joint. Low-intensity electrical stimulation of the sural or saphenous nerves was also employed in order to condition the MSRs of the triceps surae muscles. Hair bending, skin indentation or stretching, as well as electrical nerve stimulation, can induce a similar biphasic excitability cycle of the extensor MSRs, characterized by an early inhibition followed by a late facilitatory period (LFP). The LFP started approximately 20 ms after the arrival of the cutaneous afferent volley, and lasted about 80 ms. Conditioned MSRs could attain values corresponding to 200% or more of controls. The receptive field of the LFP evoked by the air jet proved to be as large as the whole leg and foot skin surface. No significant differences were found in the extent of the late facilitation in the MSRs of Sol, GM and GL, conditioned by electrical stimulation. The LFP was also present, after conditioning stimulation of the same types as above, in intact (and spinal) chloralose-anesthetized cats.
Anesthesia, Intravenous↗
From activity to rest: gating of excitatory autogenetic afferences from the relaxing muscle in man.
The changes in reflex excitability of the motoneurones to the soleus (Sol) muscle occurring during and after voluntary releases of various duration from a constant plantar-torque level in isometric or isotonic conditions have been investigated in normal humans by means of the H-reflex and T-response. The amplitude of both reflexes during the release phase attains values lower than control (obtained in resting conditions) even in the presence of force and EMG activity; the maximal inhibition is reached at the end of the release; the reflexes recover gradually to control values over several seconds. The more abrupt is the release, the more inhibited is the reflex and the shorter is the time to recovery and vv. These results apply both in isometric and isotonic conditions. Activation of the antagonist muscles, sometimes occurring at the end of the fastest release, does not contribute to the H-reflex inhibition. Tonic isometric contractions and relative releases have also been evoked by the tonic vibration reflex (TVR). The H-reflex during the TVR-induced contractions were lower than control values, at variance with those obtained during the voluntary contractions, but their amplitudes during the releases had similar values and time-courses in both conditions, pointing to a common involved inhibitory mechanism. Any voluntary ballistic or ramp contraction taking place after a preceding release, in the period in which the H-reflexes were still inhibited, was not apparently influenced, despite the fact that H-reflexes evoked during the release-conditioned ramp contractions were significantly lower than when evoked during control ramps of similar characteristics. The results are discussed in terms of a premotoneuronal, possibly presynaptic, inhibitory mechanism.
Adult↗
Interhemispheric transfer of voluntary motor commands in man.
The lack of callosal fibres between homotopic areas of the hand in the sensorimotor cortex in man and its possible functional correlates were investigated in normal adult subjects by comparing simple reaction times (RTs) of voluntary movements triggered by a somaesthetic stimulus to the same or opposite side of the body. An air jet was delivered to the skin of distal (index finger) or proximal (shoulder) ipsi- or contralateral zones, and triggered voluntary extension of the index finger or flexion of the forearm. The RT was measured from the arrival of the stimulus to the skin to the onset of the surface EMG of the muscle extensor indicis proprius or biceps brachii. The RTs of the contralateral finger movements triggered by either proximal or distal skin stimuli were significantly longer than the RTs of the corresponding ipsilateral movements (mean difference 11.72 and 15.10 msec respectively). When the task was flexion of the forearm, the differences in RTs between contra- and ipsilateral movements were instead compatible with a transcallosal transfer (mean difference about 2 msec in both cases). It is concluded that transcallosal connections between hand sensorimotor areas are conceivably absent also in man. Furthermore, the delay in contralateral distal performance appears to be due to a lack of transfer of the command through the motor areas, rather than to a lack of transfer of the triggering cutaneous afferent information to the performing hemisphere.
Adult↗
Short-latency cortical potentials evoked by tactile air-jet stimulation of body and face in man.
Natural cutaneous stimulation was performed in 10 healthy volunteers by means of a brief, localized air jet directed to the glabrous skin of the face, finger or toe. Neurograms (from finger stimulation) and somatosensory evoked potentials (SEPs) were recorded and, in the case of finger and toe stimulation, compared with the SEPs obtained by low intensity electrical stimulation. Comparing the latencies at wrist and elbow of the respective neurograms, it appears that a 2 msec period accounts for skin indentation and build-up of the generator potential in the receptors activated by the air jet. A slightly lower conduction velocity was obtained on natural than on electrical stimulation, and the cortical SEPs accordingly had a longer latency. In spite of the much smaller amplitude of the air-jet evoked neurograms, the amplitudes of the SEPs from finger and toe were similar to the amplitudes of the SEPs on electrical stimulation of the same regions. Natural stimulation in the regions innervated by the 3 branches of the trigeminal nerve (tongue included) yielded consistent SEPs, comparable with those reported in the literature to electrical stimulation. These potentials were distinguishable from the electrical activity due to the blink reflex, which invariably takes place on air-jet stimulation of the first trigeminal branch.
Adult↗
Effects of deep barbiturate coma on acute spinal cord injury in the cat.
The effects of barbiturate administration on experimental balloon-induced spinal cord injury were tested in cats. Somatosensory evoked potentials from sciatic nerve stimulation were obtained before trauma and every 60 minutes after it up to the sixth hour, when the animals were killed. Eight cats received no barbiturate treatment. On histologic examination the traumatic lesion was found to be extensive (mean, 72.8% of total cross section of the cord area), sparing dorsal columns only in six cats. Somatosensory evoked potentials were absent in two cats and profoundly modified (that is, the late waves were absent) in six cats at the sixth hour. Eight cats were given a continuous infusion for 1 hour of intravenous thiopental sodium (total dose, 65-90 mg/kg) starting 30 minutes after trauma. In these eight cats, the extent of the traumatic lesion was significantly reduced (8.8% of the cord area). Among them, three animals presented with unaltered somatosensory evoked potentials (that is, with the presence of both primary components and late waves) at the sixth hour. It was concluded that thiopental sodium improves the response of the spinal cord to trauma, both at an anatomic and at a functional level.
Acute Disease↗
Influences of locus ceruleus, raphe dorsalis, and periaqueductal gray matter on somatosensory-recipient thalamic nuclei.
Spontaneous and evoked discharge of neurons in the nucleus ventralis posterolateralis (VPL) and spontaneous discharge of neurons in the posterior group and nucleus lateralis posterior (LP) were conditioned by brief trains of stimuli to the locus ceruleus (LC), raphe dorsalis (RD), and periaqueductal gray matter (PAG) in cats anesthetized with pentobarbital or ketamine. Stimulation of LC and RD was without effect on VPL neurons, but induced a long-latency, long-lasting inhibition of LP neurons. Stimulation of the PAG induced marked inhibition of the firing of neurons in all three thalamic nuclei. No differences were found between cats anesthetized with ketamine or pentobarbital.
Animals↗
Mesencephalic and bulbar reticular influences on somatosensory cortical neurons: short- and long-latency effects.
Somatosensory cortical neurons were intracellularly and extracellularly recorded in cats encéphale isolé, and after acute lesions of midthalamic nuclei or after chronic hemisection of the brain stem at the pretrigeminal level. Intracellular recordings showed postsynaptic facilitatory and inhibitory effects at very low latency by stimulating both the mesencephalic (MRF) and bulbar reticular formation (BRF). Inhibitory effects dominated by stimulating the BRF. Neither midthalamic lesions nor pretrigeminal hemisection changed the quality of latency of postsynaptic responses. Extracellular recordings revealed long-latency inhibition of discharge following MRF stimulation after midthalamic lesion. In these experimental conditions long-latency BRF effects were abolished. No differences were found in responses of pyramidal tract (PT) or non-PT neurons during BRF and MRF stimulation. The results are discussed on the basis of a possible extrathalamic differential reticular control, from caudal and rostral brain stem, of somatosensory cortical neurons.
Animals↗
Simulation of post-tetanic potentiation and fatigue in muscle using a visco-elastic model.
Post-tetanic potentiation (PTP) in single motor units was simulated using a simple visco-elastic model. Single isometric twitches and unfused tetani were obtained using a wide range of physiological input rates. Values of model parameters were chosen to simulate contraction times close to those of fast and slow muscle fibers. PTP has been attributed either to i) an augmented plateau level of active state or ii) an increase in time constant of active state decay. Our results show that a prolonged decay time of active state can account for most of the experimental data obtained in amphibian and mammalian preparations. In particular, potentiation is more marked in unfused tetani than in single twitches. Moreover the model accounts for PTP even in the case of a reduction of active state plateau due to fatigue.
Animals↗
Mesencephalic and bulbar reticular formation influences on somatosensory transmission through the thalamus.
The influence of the brain stem reticular formation (RF) on transfer of somatosensory information has been studied in intact cats and in cats with a chronic hemisection of the brain stem at the pretrigeminal level. An air-jet applied to the hairy skin receptive field evoked the discharge of thalamocortical relay cells in nucleus ventralis postero-lateralis, extracellularly recorded. Conditioning stimuli were brief trains of electrical pulses to mesencephalic (MRF) and bulbar (BRF) reticular formation. In intact cats both MRF and BRF induced in a small percentage of cells slight facilitation or inhibition of evoked discharge. In pretrigeminal cats MRF stimulation increased the probability of discharge in response to peripheral stimuli, whereas BRF stimulation induced a striking decrease in evoked firing in a great percentage of neurones. It is suggested that RF activation can decrease the incoming peripheral volley by means of its caudalmost part, while it is able to enhance thalamic response by way of the more rostral structures.
Afferent Pathways↗
Long-latency, nonreciprocal reflex responses of antagonistic hind limb muscles after cutaneous nerve stimulation in the cat.
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Animals↗
Effects of thoracic dorsal rhizotomy or vagotomy on inspiratory muscle activity at various levels of chemical drive.
The relationship between relative peak activity (moving average EMG) of the diaphragm (Adi) and of the cranial (2nd and 3rd) external intercostal or parasternal muscles (Aic) was assessed during rebreathing in animals before and after bilateral thoracic (T1-T4) dorsal rhizotomy (TDR) and/or bilateral vagotomy (VGT). The relationship had the form Aic=a Adib under all conditions. In intact rabbits and cats mean values for b were 1.48 and 1.79, respectively, a being unity by definition. Neither TDR nor VGT changed b; a decreased to about 0.15 with TDR and halved with VGT only if performed before TDR. Selective reflex facilitation of inspiratory intercostals with occlusions at FRC was observed after VGT and was abolished by TDR. Neither VGT nor TDR affected Adi time course. Hence: (1) central command to alpha-motoneurones of the major inspiratory muscles differs; (2) proprioceptive feedback markedly increases external intercostal activity, apparently by multiplying Aic due to central command to alpha-motoneurones by a factor independent of chemical drive; (3) vagally mediated augmentation of Aic depends entirely on intact proprioceptive feedback. The possible role of fusimotor drive is discussed.
Animals↗
Effects of stimulus intensity, cervical cord tractotomies and cerebellectomy on somatosensory evoked potentials from skin and muscle afferents of cat hind limb.
The somatosensory evoked potentials (SEPs) recorded from the sensory cortex were investigated by using graded stimulation of skin and muscle nerves from contralateral hind limb in the cat. Sections were made of the middle cervical cord to assess the pathways involved in mediating SEPs evoked by large and small diameter fibers. Dorsal column (DC) section caused a decrease of SEPs from skin group I afferents, and a small increase in those from group I muscle afferents. A subsequent section of dorso-lateral fasciculus (DLF) further decreased SEPs from skin and eliminated SEPs from muscle, evoked at low stimulus intensity. When the stimulus recruited group III fibres, SEPs were still present after DC and DLF section, both from skin and muscle nerves. Section of ALT in addition to DC confirmed a major role played by DLF (mainly spino-cervical tract of Morin) in transmitting impulses from muscle afferents; the role of DLF in mediating potentials evoked from skin is less remarkable than that of DC. Cerebellectomy did not change any SEP, however evoked. Previous results in the literature are discussed, taking into account the methodologies employed by various authors, and the possible interactions among pathways mediating SEPs.
Afferent Pathways↗