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H-Reflex

H reflexes provide nerve conduction measurements along the entire length of the nerve, demonstrating abnormalities in neuropathies and radiculopathies.

From: Handbook of Clinical Neurology, 2019

Related terms:

F Wave

Electromyography

Nerve Conduction Study

Radiculopathy

Lesion

Excitability

Stimulation

Spinal Cord

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H-Reflex and F-Response Studies

Morris A. Fisher, in Aminoff's Electrodiagnosis in Clinical Neurology (Sixth Edition), 2012

Disorders of the Peripheral Nervous System

H reflexes are a sensitive test for polyneuropathies and may be abnormal even in mild neuropathies. H reflexes involve conduction in proximal as well as distal fibers. These studies, therefore, can define proximal nerve injury and may be abnormal even when studies of distal function are unremarkable. Absent H reflexes are characteristic of acute inflammatory demyelinating polyneuropathy (Guillain–Barré syndrome). This loss of H reflexes occurs early and may be an isolated finding in patients studied within several days after onset of illness. H reflexes may be abnormal in asymptomatic patients with possible neuropathic dysfunction,30 and in plexopathies and radiculopathies. H reflexes in the forearm flexor muscles may be abnormal with C6 or C7 root injury,31 and calf H reflexes may be abnormal with S1 radiculopathies. H reflexes are affected by injury to either the posterior or anterior roots. Examination of these reflexes therefore can be important in the electrodiagnostic evaluation of radiculopathies by documenting nerve injury, even when needle EMG is unrevealing owing to sparing of the anterior roots. Eliciting the H reflex by direct stimulation of the S1 root may enhance their utility for detecting an S1 radiculopathy.32 Increase in the stimulus threshold has been noted in patients with neurogenic claudication when studied after reproduction of their symptoms with walking.33

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Electromyography and Evoked Potentials

Bernard M. Abrams, Howard J. Waldman, in Practical Management of Pain (Fifth Edition), 2014

Pitfalls and Comments

The H-reflex is somewhat more useful than the F wave, but the main reason for the study is to evaluate patients with suspected S1 radiculopathy whose history or findings on physical examination are suggestive but the EMG is normal.34 Usually, when an absent H-reflex is noted, which suggests a problem with S1 nerve root conduction, an absent or depressed ankle reflex has already been noted on the physical examination, so the study is, for many, redundant. Pitfalls occur when the opposite leg is not studied to show a normal H-reflex as a contrast. If the H-reflex is absent bilaterally, it may reflect more generalized disease, such as peripheral neuropathy. Older patients often do not have good H-reflexes as a normal finding. In addition, a unilaterally absent H-reflex with normal findings on needle EMG does not indicate when the injury occurred; the findings may have been the result of a previous injury.

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Specialized Electrodiagnostic Studies

Bashar Katirji M.D., F.A.C.P., in Electromyography in Clinical Practice (Second Edition), 2007

H Reflex

The H reflex, named after Hoffmann for his original description, is an electrical counterpart of the stretch reflex which is elicited by a mechanical tap. Group 1A sensory fibers constitute the afferent arc which monosynaptically or oligosynaptically activate the alpha motor neurons that in turn generate the efferent arc of the reflex through their motor axons. The H reflex amplitude may be occasionally as high as the M amplitude but it is often lower with the H/M amplitude ratio usually not exceeding 0.75.

The H reflex and F wave can be distinguished by increasing stimulus intensity (see Table 3-1). The H reflex is best elicited by a long-duration stimulus which is submaximal to produce an M response, whereas the F wave requires supramaximal stimulus Also, the F wave can be elicited from any limb muscle while the H reflex is most reproducible with stimulating the tibial nerve while recording the soleus muscle which assess the integrity of the S1 arc reflex and is equivalent to the Achilles reflex (Figure 3-4). Finally, the H reflex latency (and often amplitude) is constant when elicited by the same stimulus intensity, since it reflects activation of the same motor neuron pool.

The H reflex is most useful as an adjunct study in the diagnosis of peripheral polyneuropathy or S1 radiculopathy. The H reflex latency and amplitude is the most sensitive, yet nonspecific, among the nerve conduction studies in the early phases of Guillain-Barré syndrome. The H reflex may be absent in healthy elderly subjects and isolated abnormalities of the H reflex are nondiagnostic since they may reflect pathology anywhere along the reflex arc.

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Clinical Neurophysiology: Basis and Technical Aspects

Nivedita Jerath, Jun Kimura, in Handbook of Clinical Neurology, 2019

Clinical applications

The H reflex provides a measure of nerve conduction along the entire length of the tibial/S1 pathway, providing information along proximal nerve segments, including the plexus and roots (Burke, 2016). Clinical conditions with a depressed ankle reflex, such as polyneuropathy, sciatic neuropathy, or S1 radiculopathy, will show a diminished or absent H reflex. Diabetic polyneuropathy is known to increase the H-reflex latency. Table 15.2 summarizes normal values in healthy adults. In evaluating radiculopathy, a unilateral absent H reflex or a side-to-side latency difference greater than 2.0 ms supports a diagnosis of an S1 radiculopathy (Kimura, 2013).

Table 15.2. H reflex normal valuesa (Kimura, 2013)

Amplitude (mV)bDifference between right and left (mV)Latency to recording Site (ms)bDifference between right and left (ms)2.4 ± 1.41.2 ± 1.229.5 ± 2.40.6 ± 0.4

aMean ± standard deviation.bAmplitude of the evoked response measured from baseline to negative peak; latency measured to the onset of the evoked response.

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H-reflex

J.A. Robichaud, in Encyclopedia of Movement Disorders, 2010

Introduction

The H-reflex can be used to assess changes in motoneuron pool reflex excitability. This reflex, also known as the Hoffmann reflex, is an electrically stimulated reflex that excites the muscle spindles Ia afferents. When electrical stimulation is applied over a nerve, this causes action potentials to be transmitted to the spinal cord where monosynaptic or polysynaptic connections cause motoneurons to reach threshold, thereby, causing the extrafusal muscle to contract. The overall level of reflex excitability may be influenced by excitation of Ib afferents from Golgi tendon organs, group II afferents from muscle spindles, and larger cutaneous afferents. While the H-reflex was originally suggested to be analogous to the tendon jerk, the central timing of the H-reflex is shorter than the tendon response, allowing for the possibility that more spinal processing occurs with the tendon response. Therefore, it is possible that both responses use slightly different pathways, and hence, it may not be appropriate to directly compare these responses.

The H-reflex can be evaluated either through the latency of the response or the reflex size (peak to peak amplitude). While there are established values for the latency of this response (time from stimulus application until initial deflection from baseline), this is not the case for the amplitude. The validity and reliability of the H-reflex amplitude have been evaluated by Crayton and King and Hugon et al. Because of the variability of the amplitude of the H-reflex, one test measurement should consist of the average of at least 5–7 (and up to 20) H-reflex measurements. The amplitude of the H-reflex also exhibits a wide intersubject variability in reflex recording; however, there is small test/retest variability within each subject. Because of this variability, changes in the amplitude of the H-reflex should only be analyzed relative to each subject's baseline H-reflex value.

Several methods, which evaluate H-reflex amplitude changes, have been used to assess spinal reflex excitability. One method assesses the percentage change of the test H-reflex amplitude in comparison to a control (baseline) H-reflex amplitude. However, this method of analysis requires an understanding of how changes in stimulus intensity can influence the amplitude of the H-reflex. At low stimulation intensities, Ia afferents are activated first which causes activation of the muscle's motoneurons eliciting a small amplitude H-reflex. As stimulation intensity further increases, the amplitude of the H-reflex increases which activates alpha motoneurons, and the direct muscle response (M-wave) is now observed. As the stimulation intensity further increases, the amplitude of the H-reflex becomes progressively larger up to a critical point, after which the amplitude of the H-reflex becomes progressively smaller until the H-reflex is no longer present. Therefore, test H-reflex alterations elicited by a given conditioning stimulus depend not only on the conditioning volley, but also on the amplitude of the control H-reflex. This aspect of H-reflex recruitment makes it difficult to determine the 'true' amount of reflex facilitation or inhibition. For instance, a small control H-reflex could result in a large amount of facilitation, while a larger control H-reflex would result in a smaller amount of facilitation. Therefore, using percentage change may make it difficult to estimate the real amount of either facilitation or inhibition of the motoneuron pool.

The H/M ratio, which is the ratio of the size of the H-reflex to the size of the M-wave, is an alternate method of analyzing changes in motoneuron reflex excitability. The H/M ratio represents the percentage of the motoneuron pool that may be considered to be active at any particular time. The maximum M-wave represents 100% of the activity of the specified muscle's motoneuron pool. The H/M ratio allows a quantification of the percentage of the active motoneuron pool. This interpretation enhances the ability to compare changes in the H-reflex between subjects. Crone and colleagues established that regardless of differences in the maximum H-reflex, the change in sensitivity to facilitation followed the same pattern as long as the control reflex sizes are explored.

The H-reflex recovery curve, which is a variation of the H-reflex, can also be used to assess motoneuron pool reflex excitability. The recovery curve is elicited by stimulating a nerve with paired stimuli at intervals from 1 ms to 10 s. The magnitude of the recovery curve at a given interstimulus interval refers to the percentage ratio of the test H-reflex amplitude to the unconditioned H-reflex amplitude, with these amplitudes being plotted as a function of the intervals between the reflexes. The recovery curve in healthy subjects consists of several phases. The first phase occurs with interstimulus intervals from 3 to 10 ms with the test reflex varying in size with simulation intensity. Next, there is an inhibition phase which is then followed by a facilitatory phase which occurs at stimulus intervals between 50 and 400 ms (peaks at 200–300 ms). Finally there is another inhibition phase which can last from 5 to 10 s.

In addition to changes in motoneuron reflex excitability, changes in the H-reflex have also been used to evaluate reciprocal Ia inhibition, Ib inhibition/facilitation, and presynaptic inhibition. Reciprocal Ia inhibition is evaluated by using a test H-reflex and a conditioning response (paired stimulus paradigm). The H-reflex is elicited with a low intensity stimulus (designed to stimulate primarily the muscle spindle Ia afferents) supplying the antagonist muscle. If the antagonist stimulation is given at the appropriate time, the test H-reflex in the agonist can be inhibited. Ib inhibition/facilitation is also assessed using a paired stimulus paradigm. However, unlike Ia inhibition, Ib inhibition/facilitation can be measured in the antagonist or synergists muscles acting at different joints within a limb. Finally, several techniques have been established whereby the H-reflex can be used to assess presynaptic inhibition. One method assesses H-reflex amplitude changes after vibration has been applied to either the tested muscle or a different muscle. In this method, the amount of inhibition is assessed by analyzing the amplitude change between the amplitude of the H-reflex prior to, and then after vibration. Presynaptic inhibition can also be assessed while testing for reciprocal inhibition between extensors and flexors in the forearm. Finally, Hultborn and colleagues showed a more difficult, but more specific method of analyzing presynaptic inhibition. In this method, the amount of H-reflex facilitation in one muscle is assessed while a monosynaptic heteronymous Ia volley is being applied to another muscle.

H-reflex changes have been evaluated in individuals with neurological disorders. It is known that slowing down of the H-reflex latency can be detected long before nerve degeneration in hereditary and acquired demyelinating neuropathies are observed. However, the scientific and clinical implication related to changes in the H-reflex amplitude is not as clear. For instance, Angel and Hoffmann observed that the Hmax/Mmax ratio was higher in individuals with spasticity. In contrast, other studies have concluded that changes in the Hmax/Mmax ratio were not a good measurement tool to quantify changes in spasticity. The literature is also mixed regarding the H-reflex changes when assessing rigidity. While no difference was shown in the H-reflex recovery curve when Parkinson's disease (PD) subjects with rigidity were compared to healthy subjects, other studies showed that rigid PD subjects exhibited an abnormal H-reflex recovery curve. Along this line, changes in the H-reflex behavior correlated with the level of rigidity in PD subjects. Studies are also mixed regarding changes in presynaptic inhibition in individuals with neurological disorders. While studies have shown a decrease in presynaptic inhibition in subjects with upper motoneuron lesions, others researchers have shown no change in presynaptic inhibition in the same type of subjects. While changes in the amplitude of the H-reflex can be used to measure several different parameters related to changes in spinal reflex excitability, it is clear that more research is needed in order to determine the clinical relevance of H-reflex changes in individuals with neurological disorders.

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General principles

Timothy R. Dillingham, in Interventional Spine, 2008

H-reflexes

H-reflexes have commonly been used to determine whether a radiculopathy demonstrates S1 involvement.7 It is a monosynaptic reflex that is an S1-mediated response and can differentiate to some extent L5 from S1 radiculopathy. Many researchers have evaluated their sensitivity and specificity with respect to lumbosacral radiculopathies and generally found a range of sensitivities from 32% to 88%.7–12 However, many of these studies suffered from lack of a control group, imprecise inclusion criteria, or small sample sizes.

Marin et al.12 prospectively examined the H-reflex and the extensor digitorum brevis reflex in 53 normals, 17 patients with L5, and 18 patients with S1 radiculopathy. Patients included in the study had all of the following: (1) radiating low back pain into the leg, (2) reduced sensation or weakness or positive straight leg raise test, and (3) either EMG evidence of radiculopathy or structural causes of radiculopathy on magnetic resonance imaging (MRI) or computed tomography (CT) imaging. The maximal (2 SD) value for the H-reflex side-to-side latency difference was 1.8 ms as derived from the normal group. They analyzed the sensitivity of the H-reflex for side-to-side differences greater than 1.8 ms or a unilaterally absent H-reflex on the affected side. The H-reflex only demonstrated a 50% sensitivity for S1 radiculopathy, 6% for L5 radiculopathy, but had a 91% specificity. Amplitudes were not assessed in this study. These results suggest that the H-reflex has a low sensitivity for S1 root level involvement.

H-reflexes may be useful to identify subtle S1 radiculopathy, yet there are a number of shortcomings related to these responses. They can be normal with radiculopathies12 and, because they are mediated over such a long physiological pathway, they can be abnormal due to polyneuropathy, sciatic neuropathy, or plexopathy.7 They are most useful in the assessment for polyneuropathy.

In order to interpret a latency or amplitude value and render a judgment as to the probability that it is abnormal, precise population-based normative values encompassing a large age range of normal subjects must be available for comparison of these nerve conduction findings. Falco et al.13 demonstrated in a group of healthy elderly subjects (60–88 years old) that the tibial H-reflex was present and recorded bilaterally in 92%. Most elderly are expected to have normal H-reflex studies and, when abnormalities are found in these persons, the electrodiagnostician should critically evaluate these findings and the clinical scenario before attributing H-reflex abnormalities to the aging process.

In patients with upper limb symptoms suggestive of cervical radiculopathy, H-reflexes and F-waves are not useful in diagnosis but rather help exclude polyneuropathy as an underlying cause of symptoms. One study by Miller and colleagues14 examined the H-reflexes in the upper limb in a set of patients defined by a combination of clinical criteria (no imaging or EMG studies) as having definite or probable cervical radiculopathy. They tested the H-reflex for the FCR, the ECR, the APB, and the biceps heteronymous reflex. The later reflex is derived by stimulating the median nerve in the cubital fossa and recording over the biceps brachii muscle, averaging 40–100 trials. These reflex studies had a 72% sensitivity overall for the group with 100% for the subset of patients with definite cervical radiculopathy. In contrast, needle EMG demonstrated 90% sensitivity for the definite group. Although these findings suggest a possible role for these upper limb H-reflexes, they are highly specialized, time consuming, and difficult to consistently elicit. They may have a role in sensory radiculopathies where needle EMG will not be positive and imaging findings are equivocal. Further studies are necessary to clarify whether the findings of Miller et al.14 can be duplicated at other centers.

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Electromyography and Nerve Conduction Velocity

Bernard Abrams, in Pain Management, 2007

H-Reflex

Definition of the H-Reflex

The H-reflex or "Hoffmann reflex" is obtained by electrode stimulation of the posterior tibial nerve in the popliteal space—at a slow rate with long duration—submaximal electrical shock, and recorded with surface electrodes over the gastrocnemius-soleus complex. The impulse travels up the sensory fibers to the spinal cord, synapses with the alpha motor neuron and returns down the motor fibers to the calf muscle. H-reflex latencies are long, in the 40 to 45 msec range. They are mostly carried out in the S1 root distribution and cannot be recorded consistently from other muscles. To determine a delay or an asymmetry, the opposite leg should always be studied for comparison.26–29

The H-reflex is somewhat more useful than the F-wave, but the main reason for this study is in the work-up of a suspected S1 radiculopathy, in which the history and/or physical examination is suggestive, but the EMG is normal.30 In most cases, when an absent H-reflex is found, suggesting a problem with S1 conduction, an absent or depressed ankle reflex has already been seen in the physical examination, so the study is, for many, redundant.

Pitfalls occur when the opposite leg is not studied to show a normal H-reflex as a contrast. If the H-reflex is bilaterally absent, it may reflect a generalized disease—for example, peripheral neuropathy. Older patients do not have good H-reflexes as a rule and this may be a normal finding. In addition, a unilaterally absent H-reflex with a normal needle EMG examination does not indicate when the injury occurred. It could be the result of a previous remote injury.

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Late Responses

David C. Preston MD, Barbara E. Shapiro MD, PhD, in Electromyography and Neuromuscular Disorders (Third Edition), 2013

H Reflex

The H reflex derives its name from Paul Hoffmann, who first evoked the response in 1918. The H response is distinctly different from the F response in that it is a true reflex with a sensory afferent, a synapse, and a motor efferent segment. Likewise, several other properties differentiate the H and F responses (Table 4–1). Unlike the F response that can be elicited from all motor nerves, the distribution of the H reflex is much more limited. In newborns, H reflexes are widely present in motor nerves, but beyond the age of two, they can only be routinely elicited by stimulating the tibial nerve in the popliteal fossa and recording the gastroc–soleus muscle. Although there are techniques for obtaining an H reflex from the femoral nerve recording the quadriceps muscle and from the median nerve recording the flexor carpi radialis muscle, both of these have significant limitations.

The circuitry of the H reflex involves the Ia muscle spindles as sensory afferents and the alpha motor neurons and their axons as efferents (Figure 4–8). If a low submaximal stimulus with a long duration pulse is applied to a nerve, it is possible to relatively selectively activate the Ia fibers. Several adjustments must be made to the EMG machine to record an H reflex, similar to those made for the F response. The gain must be increased initially to 200 to 500 µV. The typical H reflex latency is approximately 30 ms, so the sweep speed must be increased to 10 ms. Most important, the stimulus duration must be increased to 1 ms in order to selectively stimulate the Ia fibers. The recording montage consists of G1 placed over the soleus and G2, the reference electrode, placed over the Achilles tendon (Figure 4–9). Although the H reflex can be recorded over any portion of the gastrocnemius and soleus muscles, the optimal location that yields the largest H reflex has been studied. If one draws a line from the popliteal fossa posteriorly to the Achilles tendon where the medial malleolus flares out and then divides that line into eight equal parts, the optimal location is at the fifth or sixth segment distally, over the soleus (Figure 4–10). This location is approximately two to three fingerbreadths distal to where the soleus meets the two bellies of the gastrocnemius. The tibial nerve is stimulated in the popliteal fossa, with the cathode placed proximally and beginning at very low stimulus intensities. One should stimulate at a rate no faster than once every 2 seconds (0.5 Hz) in order to avoid the effects of a previous stimulus on a subsequent response. As the current is slowly increased, an H reflex (which usually is triphasic) first appears at a latency of 25 to 34 ms. H reflexes are routinely recorded with the muscle at rest. If an H reflex cannot be elicited, having the patient slightly plantar flex the ankle can be used to enhance the H reflex. If that is not helpful, the Jendrassik maneuver, as described earlier for the F response, can be used to prime the anterior horn cells. As the stimulus intensity is slowly increased, the H reflex continues to increase in amplitude and decrease in latency. As the stimulus intensity is increased further, a direct motor (M) potential appears along with the H reflex. As the stimulus intensity is increased still further, the M potential grows in size and the H reflex decreases in size.

Obtaining the H reflexes on a rastered trace, which can be superimposed once all the responses are obtained, may be helpful in determining the minimal latency, which is generally also associated with the largest amplitude. It is best to place the latency marker on the H reflex at the point where it departs from the baseline, which most often is a positive (i.e., downward) deflection. At supramaximal stimulation, the H reflex disappears, and the M potential is seen followed by an F response, which has now replaced the H reflex. The explanation for these events is as follows. Initially, with very low stimulation, the H reflex appears without the M potential (Figure 4–11) because only the Ia afferents are selectively stimulated at low stimulus intensities. As the Ia afferents are stimulated, the sensory action potential travels orthodromically to the spinal cord, across the synapse, creating a motor potential that travels orthodromically down the motor nerve to the muscle, in turn creating the H reflex. The motor axons have not been directly stimulated at this point; therefore, there is no M potential. As the stimulus intensity is increased, both the Ia afferents and the motor axons are directly stimulated. At this point, the orthodromically traveling motor action potentials create the M potential, but the motor action potentials also travel antidromically toward the spinal cord (Figure 4–8). These antidromically traveling potentials collide with the orthodromically traveling H reflex potentials, resulting in a decrease in the size of the H reflex. At supramaximal stimulation, both the Ia afferents and the motor axons are stimulated at high levels, and there is greater collision proximally of the descending H reflex. The H reflex then disappears, often replaced by the F response, and the M potential increases in size.

Typically the H reflex with the shortest latency is measured and compared with a set of normal controls for height (Figures 4–12 and 4–13). Comparison with the contralateral side is more useful in assessing a unilateral lesion; any difference of more than 1.5 ms is considered significant. Of course, both H reflexes must be acquired using the same distance for the stimulating and recording electrodes, in order for a side-to-side difference to be considered significant. In addition, the maximal amplitude of the H response (often measured peak to peak) can be compared with the maximal amplitude of the M potential (measured peak to peak) to calculate an H/M ratio (Table 4–1), see below.

The H reflex can be useful in a couple of situations. First, the response is the electrical correlate of the S1 tendon ankle reflex. If the ankle reflex is present clinically, an H reflex should always be present. If the ankle reflex is absent, however, an H reflex may still be present in some cases. Any lesion that might decrease the ankle reflex might also prolong the H reflex. Thus, one may see a prolonged H reflex in polyneuropathy, proximal tibial and sciatic neuropathy, lumbosacral plexopathy, and lesions of the S1 nerve root. One should keep in mind that bilaterally absent H reflexes in the elderly are not necessarily abnormal, and correlate with the common clinical finding of absent ankle reflexes in a significant number of elderly patients. In addition, the H/M ratio is a crude assessment of anterior horn cell excitability. The H/M ratio often increases in upper motor neuron lesions. Likewise, the presence of H reflexes in other muscles in an adult should suggest a central disorder.

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Operant Conditioning of Reflexes

J.R. Wolpaw, X.Y. Chen, in Encyclopedia of Neuroscience, 2009

The Complex Spinal Cord Plasticity Accompanying Reflex Conditioning

H-reflex conditioning is associated with plasticity at multiple sites in the spinal cord. Downconditioning produces a positive shift in motor neuron firing threshold and a reduction in axonal conduction velocity. These two changes together suggest that a positive shift in sodium channel activation voltage occurs throughout the motor neuron soma and axon. As illustrated in Figure 3(a), the change in threshold can account in large part for the smaller reflex. Although activity-dependent synaptic plasticity is often assumed to be the basis of learning, the shift in motor neuron threshold produced by down-conditioning seems to be an example of neuronally based (as opposed to synaptically based) learning. The threshold change could also account for the decrease in motor neuron axonal conduction velocity that is associated with H-reflex down-conditioning (Figure 3(b)). In addition to this neuronal plasticity, H-reflex conditioning modifies Ia afferent motor neuron synapses and several other populations of synaptic terminals on the motor neuron. The effects on GABAergic terminals are particularly striking (Figure 3(c)). Conditioning probably changes spinal cord interneurons as well, either those in di- or trisynaptic pathways that can contribute to the H-reflex and/or those that convey descending influence to the motor neuron. At present, it is most likely that up-conditioning is due to change in oligosynaptic primary afferent input to the motor neuron. Thus, down-conditioning and up-conditioning are probably not mirror images of each other but rather have different mechanisms. In addition to this complex plasticity in the spinal cord ipsilateral to the conditioned reflex, H-reflex conditioning also causes plasticity in the contralateral spinal cord. The precise location and nature of this contralateral plasticity are as yet unknown.

Figure 3. Examples of the multisite spinal cord plasticity underlying H-reflex conditioning. (a) Motor neurons on the conditioned side of H-reflex (HR) down-conditioned animals have more positive firing thresholds and slightly smaller Ia excitatory postsynaptic potentials (EPSPs). Together, these two findings can explain why the H-reflex becomes smaller. (b) Distributions of soleus motor neuron axonal conduction velocities in unconditioned (solid) and down-conditioned (dashed) rats. Down-conditioning has a comparable effect in monkeys. The more positive motor neuron firing threshold can explain this drop in conduction velocity. (c) Soleus motor neurons (dashed lines) from an unconditioned rat (top) and a down-conditioned rat (bottom), with arrows pointing to γ-aminobutyricacidergic (GABAergic) terminals on the somatic membrane. GABAergic terminals are detected by glutamic acid decarboxylase (GAD67) immunoreactivity. After down-conditioning, soleus motor neurons have more GABAergic terminals, and these terminals are more densely labeled and occupy more of the somatic membrane. (a) Modified from Carp JS, Chen XY, Sheikh H, and Wolpaw JR (2001) Operant conditioning of rat H-reflex affects motoneuron axonal conduction velocity. Experimental Brain Research 136: 269–273. (b) Modified from Carp JS, Chen XY, Sheikh H, and Wolpaw JR (2001) Operant conditioning of rat H-reflex affects motoneuron axonal conduction velocity. Experimental Brain Research 136: 269–273.

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