Neuromuscular and biomechanical coupling in human cycling

This study exploited the alterations in pedal speed and joints kinematics elicited by changing crank length (CL) to test how altered task mechanics during cycling will modulate the muscle activation characteristics in human rectus femoris (RF), biceps femoris long head (BF), soleus (SOL) and tibialis anterior (TA). Kinetic (torque), kinematic (joint angle) and muscle activity (EMG) data were recorded simultaneously from both legs of 10 healthy adults (aged 20–38 years) during steady-state cycling at ~60 rpm and 90–100 W with three symmetrical CLs (155 mm, 175 mm and 195 mm). The CL elongation (CL) resulted in similar increases in the knee joint angles and angular velocities during extension and flexion, whilst the ankle joint kinematics was significantly influenced only during extension. CL resulted in significantly reduced amplitude and prolonged duration of BF EMG, increased mean SOL and TA EMG amplitudes, and shortened SOL activity time. RF activation parameters and TA activity duration were not significantly affected by CL. Thus total SOL and RF EMG activities were similar with different CLs, presumably enabling steady power output during extension. Higher pedal speeds demand an increased total TA EMG activity and decreased total BF activity to propel the leg through flexion into extension with a greater degree of control over joint stability. We concluded that the proprioceptive information about the changes in the cycling kinematics is used by central neural structures to adapt the activation parameters of the individual muscles to the kinetic demands of the ongoing movement, depending on their biomechanical function.     

Biomechanical and reflex responses to joint perturbations during electrical stimulation of muscle: instrumentation and measurement techniques

A test device is developed to measure ankle joint compliance and muscle activity when the ankle is subjected to perturbations in angular position (or torque) from bias positions achieved volitionally or via electrical stimulation. The ankle measurement system uses a pivoting footplate and is operable with the subject sitting or supine. A companion platform for the knee is developed that uses a rotary arm and attached leg brace and is operable with the subject’s leg in the horizontal or vertical plane. The knee fixture’s pivoting arm can slide to account for the cam-like movement of the knee during rotation. The devices use similar hardware and share common instrumentation and control. Precise torque or position perturbations are delivered by a computer-controlled torque motor to the ankle or knee. Angular displacement, torque, acceleration, knee fixture moment arm and electromyographic data are collected on analogue tape and simultaneously digitised and stored. A special stimulator/recording amplifier permits the recording of electromyographic signals from the stimulated muscle. Experimental data indicate that the ankle and knee devices, operated horizontally, are purely inertial systems. Sample ankle and knee joint responses to perturbations are presented.     

The transcortical nature of the late reflex responses in human small hand muscle to digital nerve stimulation

Summary The hypothesis that long-latency reflex activity in human small hand muscles in response to stimulation of digital nerves involves a transcortical pathway was tested by combining digital nerve stimulation and magnetic stimulation over the motor cortex in 12 studies on nine normal subjects. Postsynaptic events in human single first dorsal interosseous (FDI) motoneurones were derived from changes in the firing probability of voluntarily activated single motor units. Electromagnetic stimulation over the contralateral motor cortex resulted in a short-latency, brief facilitation of FDI motor units considered to be due to the activation of fast corticospinal neurones making monosynaptic projections to motoneurones. Stimulation of the digital nerves of the index finger produced a period of reduced firing probability (I1), a period of increased firing probability (E2) and a further period of reduced firing probability (I2) in FDI motor units. When the two stimuli were given separately and then together, timed so that the magnetic stimulus occurred at the predicted transit time of the E2 through the cortex, the facilitation of FDI motoneurones by the combined stimulation was often less than the algebraic sum of the facilitations from each stimulus alone. Thus, in contrast to the results of similar studies on the late response to muscle stretch, there is no confirmation that the E2 from digital nerve stimulation is due to a transcortical reflex.     

Bilateral responses of upper limb muscles to transcranial magnetic stimulation in human subjects

Anatomical and behavioural work on primates has shown bilateral innervation of axial and proximal limb muscles, and contralateral control of distal limb muscles. The following study examined if a clear boundary exists between the distal and proximal upper limb muscles that are controlled contralaterally or bilaterally. The right motor cortical area representing the upper limb was stimulated, while surface EMG was recorded bilaterally from various upper limb muscles during rest and phasic voluntary contractions. Peak-to-peak amplitude of motor evoked potential (MEP) was measured for each muscle on both sides. The ratio R = (ipsilateral MEP: contralateral MEP) was calculated for seven pairs of muscles. For each of the seven pairs, R was less than 1.0, implying that for each muscle and subject, the contralateral control is stronger. The boundary where R changed from almost zero to a clearly measurable magnitude depended on the subject. Ipsilateral MEPs from trapezius and pectoralis could be recorded with a small background contraction from almost all subjects; on the other hand, in deltoid and biceps brachii, ipsilateral MEPs were observed only with bimanual phasic contractions. The forearm and hand muscles, in general, did not show any ipsilateral MEPs. Major differences between subjects lay in the presence or the absence of ipsilateral MEPs in biceps brachii and deltoid, without defining a sharp boundary between proximal and distal muscles.     

Changes in muscle responses to stimulation of the motor cortex induced by peripheral nerve stimulation in human subjects

The aim of this study was to determine whether prolonged, repetitive mixed nerve stimulation (duty cycle 1 s, 500 ms on-500 ms off, 10 Hz) of the ulnar nerve leads to a change in excitability of primary motor cortex in normal human subjects. Motor-evoked potentials (MEPs) generated in three intrinsic hand muscles [abductor digiti minimi (ADM), first dorsal interosseous (FDI) and abductor pollicis brevis (APB)] by focal transcranial magnetic stimulation were recorded during complete relaxation before and after a period of prolonged repetitive ulnar nerve stimulation at the wrist. Transcranial magnetic stimuli were applied at seven scalp sites separated by 1 cm: the optimal scalp site for eliciting MEPs in the target muscle (FDI), three sites medial to the optimal site and three sites lateral to the optimal stimulation site. The area of the MEPs evoked in the ulnar-(FDI, ADM) but not the median-innervated (APB) muscles was increased after prolonged ulnar nerve stimulation. Centre of gravity measures demonstrated that there was no significant difference in the distribution of cortical excitability after the peripheral stimulation. F-wave responses in the intrinsic hand muscles were not altered after prolonged ulnar nerve stimulation, suggesting that the changes in MEP areas were not the result of stimulus-induced increases in the excitability of spinal motoneurones. Control experiments employing transcranial electric stimulation provided no evidence for a spinal origin for the excitability changes. These results demonstrate that in normal human subjects the excitability of the cortical projection to hand muscles can be altered in a manner determined by the peripheral stimulus applied.     

Inhibition of the responses of cat dorsal horn neurons to noxious skin heating by stimulation in medial or lateral medullary reticular formation

Summary Responses of single lumbar dorsal horn units to noxious radiant heating (50° C, 10 s) of glabrous footpad skin were recorded in cats anesthetized with sodium pentobarbital and 70% nitrous oxide. The heat-evoked responses of 37/40 units were reduced during electrical stimulation (100 ms trains, 100 Hz, 3/s, 25–600 A) in the medullary nucleus raphe magnus (NRM) and/or in laterally adjacent regions of the medullary reticular formation (MRF). Inhibition was elicited by stimulation in widespread areas of the medulla, but with greatest efficacy at ventrolateral sites. The magnitude of inhibition increased with graded increases in medullary stimulation intensity. Mean current intensities at threshold for inhibition or to produce 50% inhibition were higher for NRM than for MRF sites. Units' responses to graded noxious heat stimuli increased linearly from threshold (42–43° C) to 52° C. During NRM (5 units) or ipsilateral MRF stimulation (7 units), responses were inhibited such that the mean temperature-response functions were shifted toward higher temperatures with increased thresholds (1.5° and 1° C, respectively) and reduced slopes (to 60% of control). Contralateral MRF stimulation had a similar effect in 4 units. Inhibitory effects of NRM and MRF stimulation were reduced (by >25%) or abolished in 4/6 and 5/12 units, respectively, following systemic administration of the serotonin antagonist methysergide. Inhibitory effects from NRM, ipsi- and contralateral MRF were reduced or abolished in 2/9, 4/8 and 6/9 cases, respectively, following systemic administration of the noradrenergic antagonist phentolamine. These results confirm and extend previous studies of medullospinal inhibition and the role of monoamines, and are discussed in terms of analgesic mechanisms.     

Tetrodotoxin block of A-fibre conduction and its effect on reflex responses evoked by electrical stimulation of the sural nerve in the decerebrated rabbit

In the present study, we have investigated the viability of using tetrodotoxin (TTX) to induce selective blockade of myelinated fibre conduction in rabbit sural nerve, and explored some aspects of reflexes evoked by non-myelinated sural nerve afferents before and after application of TTX. In rabbits decerebrated under halothane-nitrous oxide anaesthesia, application of 30 nM TTX to the desheathed sural nerve completely blocked Abeta and Adelta waves of the compound action potential evoked by electrical stimulation of the nerve at 95 times threshold. The amplitude of C-fibre volleys evoked by these stimuli was reduced to a mean of 60 % of pre-treatment values. Reflexes evoked in medial gastrocnemius motoneurones by sural nerve stimulation showed corresponding changes after TTX treatment, with activation latency increasing from 5-7 ms in the control state to > 100 ms after TTX application. Temporal summation (wind up) in long latency reflexes (> 100 ms) was significantly enhanced after application of TTX. These data show that low concentrations of TTX can selectively block conduction in rabbit sural nerve A-fibres, providing a method for studying the central actions of non-myelinated C-fibres in isolation.     

Effects of neonatal and late unilateral enucleation on optokinetic responses and optic nerve projections in the rabbit

Summary Rabbits were unilaterally enucleated at the age of 0 or 21 days or at adult age. After survival times of 6–21 months optokinetic nystagmus (OKN) was measured and retinofugal connections were traced with anterograde transport of horseradish peroxidase or ³H leucine, injected into the eye. Non-enucleated animals served as controls. The asymmetry of monocular OKN in normal rabbits, characterized by a strong preference for pursuit of motion in the nasal (anterior) direction, was only slightly alleviated after enucleation. Responses to stimulation in the nasal direction were unchanged; responses to stimulation in the temporal direction showed modest improvements especially after enucleation at adult age and to a smaller degree after enucleation at 0 or 21 days. Redistribution of retinofugal fibers from the eye remaining after enucleation was very limited. Contralateral connections, including those to the lateral geniculate nucleus, showed a normal distribution. Of the ipsilateral connections, those to the lateral geniculate nucleus were normal in extent and density, while those to the superior colliculus were enhanced, in agreement with previous workers (Chow et al. 1973, 1981). Changes in ipsilateral pretectal projections were extremely small; particularly no connections to the nucleus of the optic tract were developed in any of the normal or enucleated animals. Of the accessory optic nuclei, the medial terminal nucleus received a very small ipsilateral projection in normal rabbits, which was markedly enhanced after enucleation especially at 0 and 21 days, but even at adult age. It is concluded that functional and anatomical plasticity of OKN circuits in the rabbit is very limited from the time of birth.     

Long-lasting modulation of human motor cortex following prolonged transcutaneous electrical nerve stimulation (TENS) of forearm muscles: evidence of reciprocal inhibition and facilitation

Several lines of evidence indicate that motor cortex excitability can be modulated by manipulation of afferent inputs, like peripheral electrical stimulation. Most studies in humans mainly dealt with the effects of prolonged low-frequency peripheral nerve stimulation on motor cortical excitability, despite its being known from animal studies that high-frequency stimulation can also result in changes of the cortical excitability. To investigate the possible effects of high-frequency peripheral stimulation on motor cortical excitability we recorded motor-evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) of the left motor cortex from the right flexor carpi radialis (FCR), extensor carpi radialis (ECR), and first dorsal interosseous (FDI) in normal subjects, before and after transcutaneous electrical nerve stimulation (TENS) of 30 min duration applied over the FCR. The amplitude of MEPs from the FRC was significantly reduced from 10 to 35 min after TENS while the amplitude of MEPs from ECR was increased. No effects were observed in the FDI muscle. Indices of peripheral nerve (M-wave) and spinal cord excitability (H waves) did not change throughout the experiment. Electrical stimulation of the lateral antebrachial cutaneous nerve has no significant effect on motor cortex excitability. These findings suggest that TENS of forearm muscles can induce transient reciprocal inhibitory and facilitatory changes in corticomotoneuronal excitability of forearm flexor and extensor muscles lasting several minutes. These changes probably may occur at cortical site and seem to be mainly dependent on stimulation of muscle afferents. These findings might eventually lead to practical applications in rehabilitation, especially in those syndromes in which the excitatory and inhibitory balance between agonist and antagonist is severely impaired, such as spasticity and dystonia.M. Tinazzi and S. Zarattini contributed equally to the work     

Vestibular nerve and nuclei unit responses and eye movement responses to repetitive galvanic stimulation of the labyrinth in the rat

Summary Two-second cathodal current pulses were applied at one-minute intervals at a point external to the round window in the ear of each albino rat subject. Responses were recorded in the vestibular nerve ganglion, the vestibular nuclei (single units), or in the eye movements (search coil recording method) of anaesthetized, decerebrated, or alert rats. The unit responses to the galvanic stimuli were characterized and compared with responses to galvanic and rotational stimuli reported in the literature. The main focus of the study, however, was effects of stimulus repetition. In both the vestibular nerve and vestibular nuclei recordings, the responses of many units were substantially larger or smaller at the end of a 13-pulse stimulus train than at the beginning. In the vestibular nuclei, but not in the nerve, there was a slight bias towards a decrease in response magnitude, with 10/88 units showing decreases great enough to be considered as reflecting an habituation process. In contrast, the eye movement responses showed more consistent response decrements, especially in the alert condition, but also in the other conditions (none of the unit recordings were done in alert rats). It is concluded that some of the modifications underlying habituation of the vestibuloocular reflex probably occur in portions of the neuronal reflex pathways that are downstream from the vestibular nuclei.Prof. Precht died on March 12, 1985     

Withdrawal reflex responses evoked by repetitive painful stimulation delivered on the sole of the foot during late stance: site,phase, and frequency modulation

The modulation of the lower limb nociceptive withdrawal reflex elicited during late stance by a stimulus train with frequencies of 15 and 30 Hz delivered to the mid-forefoot, arch of the foot, and heel was investigated. Stimulation was delivered at four moments of the gait cycle between heel-off and toe-off. Stimulation at 15 Hz induced larger kinematic responses at the knee and hip. Reduced plantarflexion and increased dorsiflexion, compared to control steps, were evoked at the ankle; these kinematic responses were site dependent with minimum responses evoked by stimulation at the heel. The dorsiflexion response was largest when stimulating at toe-off and was larger for stimulation at 15 Hz than at 30 Hz. The muscle reflex responses were site modulated in tibialis anterior with largest responses evoked by stimulation at the arch of the foot and mid-forefoot, and phase and frequency modulated in soleus. This study presents a detailed assessment of the lower limb nociceptive reflex modulation and provides results, which might have application in the rehabilitation of the hemiparetic gait.     

Corticospinal input in human gait: modulation of magnetically evoked motor responses

 Transcranial magnetic stimulation (TMS) of the motor cortex was applied during locomotion to investigate the significance of corticospinal input upon the gait pattern. Evoked motor responses (EMR) were studied in the electromyogram (EMG) of tibialis anterior (TA), gastrocnemius (GM) and, for reference, abductor digiti minimi (AD) muscles by applying below-threshold magnetic stimuli during treadmill walking in healthy adults. Averages of 15 stimuli introduced randomly at each of 16 phases of the stride cycle were analysed. Phase-dependent amplitude modulation of EMR was present in TA and GM which did not always parallel the gait-associated modulation of the EMG activity. No variation of onset latency of the EMR was observed. The net modulatory response was calculated by comparing EMR amplitudes during gait with EMR amplitudes obtained (at corresponding background EMG activities) during tonic voluntary muscle contraction. Large net responses in both muscles occurred prior to or during phasic changes of EMG activity in the locomotor pattern. This facilitation of EMR was significantly higher in leg flexor than extensor muscles, with maxima in TA prior to and during late swing phase. A comparison of this facilitation of TA EMR prior to swing phase and prior to a phasic voluntary foot dorsiflexion revealed a similar onset but an increased amount of early facilitation in the gait condition. The modulated facilitation of EMR during locomotion could in part be explained by spinal effects which are different under dynamic and static motor conditions. However, we suggest that changes in corticospinal excitability during gait are also reflected in this facilitation. This suggestion is based on: (1) the similar onset yet dissimilar size of facilitatory effects in TA EMR prior to the swing phase of the stride cycle and during a voluntary dynamic activation, (2) the inverse variation of EMR and EMG amplitudes during this phase, and (3) the occurrence of this inversion at stimulation strengths below motor threshold (motor threshold was determined during weak tonic contraction and EMR were facilitated during gait). It is hypothesized that the facilitation is phase linked to ensure postural stability and is most effective during the phases prior to and during rhythmical activation of the leg muscles resulting in anticipatory adjustment of the locomotor pattern. Received: 17 May 1996 / Accepted: 29 November 1996     

Time course of the hemodynamic responses to aortic depressor nerve stimulation in conscious spontaneously hypertensive rats

The time to reach the maximum response of arterial pressure, heart rate and vascular resistance (hindquarter and mesenteric) was measured in conscious male spontaneously hypertensive (SHR) and normotensive control rats (NCR; Wistar; 18-22 weeks) subjected to electrical stimulation of the aortic depressor nerve (ADN). The parameters of stimulation were 1 mA intensity and 2 ms pulse length applied for 5 s, using frequencies of 10, 30, and 90 Hz. The time to reach the hemodynamic responses at different frequencies of ADN stimulation was similar for SHR (N = 15) and NCR (N = 14); hypotension = NCR (4194 ± 336 to 3695 ± 463 ms) vs SHR (3475 ± 354 to 4494 ± 300 ms); bradycardia = NCR (1618 ± 152 to 1358 ± 185 ms) vs SHR (1911 ± 323 to 1852 ± 431 ms), and the fall in hindquarter vascular resistance = NCR (6054 ± 486 to 6550 ± 847 ms) vs SHR (4849 ± 918 to 4926 ± 646 ms); mesenteric = NCR (5574 ± 790 to 5752 ± 539 ms) vs SHR (5638 ± 648 to 6777 ± 624 ms). In addition, ADN stimulation produced baroreflex responses characterized by a faster cardiac effect followed by a vascular effect, which together contributed to the decrease in arterial pressure. Therefore, the results indicate that there is no alteration in the conduction of the electrical impulse after the site of baroreceptor mechanical transduction in the baroreflex pathway (central and/or efferent) in conscious SHR compared to NCR.     

Somatic noxious mechanical stimulation induces Fos expression in the postsynaptic dorsal column neurons in laminae III and IV of the rat spinal dorsal horn

This study was conducted to ascertain the possible expression of Fos-like immunoreactivity (Fos-LI) in the postsynaptic dorsal column (PSDC) neurons in response to noxious mechanical stimulation of the forepaw glabrous area of normal rats. For this purpose, Fos immunohistochemistry along with Fluoro-Gold (FG) retrograde tracing was utilized. After repeated noxious pinching of the forepaw glabrous area, there was a marked increase in number of Fos-LI neurons in the dorsal horn, including Rexed's laminae III and IV, at C5-T1 spinal cord segments ipsilateral to the stimulation. Between segments C5 and T1, about 40% of the Fos-LI neurons in laminae III and IV were distributed at segment C7. In the rats subjected to the noxious pinch coupled with FG injection into the right cuneate nucleus, PSDC neurons double labeled with Fos and FG were localized in the ipsilateral laminae III and IV extending from segment C5 to T1, with about 70% of them distributed at segments C6 and C7. At segment C6 or C7, double-labeled neurons made up about 10% of the PSDC neurons that projected their axons to the cuneate nucleus. Most of the double-labeled neurons appeared fusiform with their primary dendrites projected dorso-ventrally. The present results suggest that the morphologically distinct, subclasses of PSDC neurons in spinal laminae III and IV may contribute to the central transmission of mechanical nociceptive information through the dorsal column into the cuneate nucleus.     

Transcranial magnetic stimulation of the human brain: responses in muscles supplied by cranial nerves

Summary The present investigation demonstrates that time-varying magnetic fields induced over the skull elicit distinct types of responses in muscles supplied by the cranial nerves both on the ipsilateral and the contralateral side. When the center of the copper coil was positioned 4 cm lateral to the vertex on a line from the vertex to the external auditory meatus, bilateral responses in the masseter, orbicularis oculi, mentalis, and sternocleidomastoideus muscles with a delay of about 10 to 14 ms after the stimulus occurred. Similar to the transcranially evoked muscle responses in hand muscles, the responses in the cranial muscles can be influenced in latency and amplitude by background excitation. It is concluded that these responses are induced by excitation of the face-associated motor cortex followed by multiple I-waves in the corticonuclear tract with both ipsilateral and contralateral projections to the corresponding motoneurones. Additionally, at higher stimulation strengths short-latency ipsilateral responses in muscles supplied by the trigeminal, facial, and accessory nerves occurred which we suggest are induced by direct stimulation of the peripheral cranial nerves in their intracisternal course. The present study confirms the bilateral projection of corticonuclear tracts in awake unanesthetised human subjects which has been observed by electrical stimulation on the exposed cortex during surgical procedures already decades ago. The present investigation will serve as a basis for the assessment of pathophysiological mechanisms involving the corticonuclear system or the peripheral cranial nerves in their proximal parts in awake humans.Supported by the Deutsche Forschungsgemeinschaft     

Reflex responses evoked in the adrenal sympathetic nerve to electrical stimulation of somatic afferent nerves in the rat

The present study was initiated to determine the role of somatic A (myelinated) and C (unmyelinated) afferent fibers in both responses of increases and decreases in adrenal sympathetic nerve activities during repetitive mechanical pinching and brushing stimulations of the skin in anesthetized rats with central nervous system (CNS) intact. Accordingly, changes in adrenal sympathetic nerve activity resulting from repetitive and single shock electrical stimulation of various spinal afferent nerves, especially the 13th thoracic (Th13) spinal nerve and the sural nerve, were examined in urethane/chloralose-anesthetized rats. Repetitive electrical stimulation of A afferent fibers in Th13 spinal or sural nerve decreased the adrenal nerve activity similarly as brushing stimulation of skin of the lower chest or hindlimb did, while repetitive stimulation of A plus C afferent fibers of those nerves increased the adrenal nerve activity as pinching stimulation of those skins did. Single shock stimulation of spinal afferent nerves evoked various reflex components in the adrenal nerve: an initial depression of spontaneous activity (the early depression); the following reflex discharge due to activation of A afferent fibers (the A-reflex); a subsequent reflex discharge due to activation of C afferent fibers (the C-reflex); and following post-excitatory depressions. These reflexes seem to be mediated mainly via supraspinal pathways since they were abolished by spinal transection at the C1-2 level. Although the supraspinal A- and C-reflexes could be elicited from stimulation of a wide variety of spinal segmental afferent levels, the early depression was more prominent when afferents at spinal segments closer to the level of adrenal nerve outflow were excited. It is suggested that the decreased responses of the adrenal nerve during repetitive electrical stimulation of A afferent nerve fibers are attributable to summation of both the early depression and post-excitatory depression evoked by single shock stimulation, while the increased responses during repetitive stimulation of A plus C afferent fibers are attributable to summation of the C-reflex after single shock stimulation. In spinalized rats, repetitive stimulation of Th13 always increased the adrenal nerve activities regardless of whether A fibers alone or A plus C fibers were stimulated, just as brushing and pinching of the lower chest skin always increased them. The increased responses in spinal animals seem to be related to the fact that single electrical stimuli of Th13 produced A- and C-reflexes of spinal origin without clear depressions.     

Time course and effective sites for inhibition from midbrain periaqueductal gray of spinal dorsal horn neuronal responses to cutaneous stimuli in the cat

Summary Inhibition of spinal dorsal horn neuronal responses to noxious (50 °C) skin heating by stimulation of the midbrain periaqueductal gray (PAG) was quantitatively investigated in cats anesthetized with sodium pentobarbital and nitrous oxide. Systematic variation of the interval between onset of PAG stimulation (PAGS) and onset of noxious skin heating revealed that a marked reduction of spinal unit heat-evoked discharges occured immediately upon onset of PAGS, and ceased immediately at offset of PAGS with a post-stimulation excitatory rebound. Stimulation at sites in both ventral and dorsal PAG produced inhibition, the strength of which increased sometimes in a linear manner with increasing strength of PAGS. Thresholds for the generation of descending inhibition were higher in dorsal than ventral PAG. PAGS also inhibited spinal unit responses to non-noxious skin stimulation (brushing of hairs). Descending inhibition from PAG is considered as a possible mechanism for analgesia produced by stimulation of PAG and other brainstem structures.The work was supported by a grant from the Deutsche Forschungsgemeinschaft (Zi 110)     

Inferior colliculus in the rat: neuronal responses to stimulation of the auditory cortex

Responses to electrical stimulation of the auditory cortex (silver ball bipolar electrodes, single pulses, duration 0.2 ms, current 0.1-1.5 mA) were recorded in neurones in the inferior colliculus of rats anaesthetized with pentobarbital. Excitatory or inhibitory effects were obtained in 84 out of 162 recorded neurones. The majority of neurones responded with a short excitatory burst (with a latency from 3 to 15 ms); in some of them the initial excitation was followed by inhibition lasting from 30 to 150 ms. Few neurones only reacted to electrical stimulation by inhibition, which occurred 3-10 ms after the stimulus and lasted up to 300 ms. The inhibition either suppressed the spontaneous activity or the acoustically evoked response. Neurones reacting to stimulation of the auditory cortex were found mainly in the caudal and dorsal parts of the inferior colliculus.     

The role of the glycine sensitive area of the ventral medulla in cardiovascular responses to carotid chemoreceptor and peripheral nerve stimulation

The present study on cats anaesthetised with Althesin, which unlike more commonly used anaesthetics does not prevent reflex activation of the brain-stem defence areas, reaffirmed that carotid chemoreceptor stimulation and radial nerve stimulation can evoke the visceral components of the alerting stage of the defence response (visceral alerting response). This includes tachycardia, mesenteric vasoconstriction but vasodilatation in skeletal muscle which is not secondary to the hyperventilation. However, mild chemoreceptor stimulation which evoked but a weak hyperventilation elicited bradycardia and vasoconstriction is mesentery and in muscle i.e. a response comparable with that evoked by chemoreceptor stimulation under chloralose or barbiturate anaesthesia. This suggests that chemoreceptor stimulation can evoke two separate patterns of response, the visceral alerting response predominating when the defence areas are strongly activated.The efferent pathway from the defence areas is known to synapse in the glycine sensitive area of the ventral medulla which contains neurones whose activity seems to provide the main sympatho-excitatory drive for normal arterial pressure. Bilateral application of glycine to that area produces a pronounced fall in arterial pressure, apnoea and greatly attenuates the response to defence area stimulation, the vasoconstrictor components being abolished. In the present study bilaterally applied glycine abolished the muscle vasodilatation of the visceral alerting response evoked by chemoreceptor and radial nerve stimulation but both stimuli evoked vasoconstriction in mesenteric and muscle vasculature at least until arterial pressure was very low.It is proposed that both chemoreceptor and peripheral nerve stimulation can activate the defence areas to produce a visceral alerting response which is relayed via neurones of the glycine sensitive area. However, so long as neurones in that region continue to provide some tonic sympatho-excitatory drive both these peripheral inputs can activate sympathetic vasoconstrictor fibres via pathways which do not synapse in the glycine sensitive area. Such vasoconstriction may be part of the response normally seen when chemoreceptor stimulation fails to activate the defence areas.Supported by Medical Research Council UK     

Altered responses of human elbow flexors to peripheral-nerve and cortical stimulation during a sustained maximal voluntary contraction

 The short-latency electromyographic response evoked by transcranial magnetic stimulation (MEP) increases in size during fatigue, but the mechanisms are unclear. Because large changes occur in the muscle action potential, we tested whether changes in the response to stimulation of the peripheral motor nerve could fully account for the increase in the MEP. Subjects (n=8) performed sustained maximal voluntary contractions (MVCs) of the right elbow flexors for 2 min. During the contraction, the MEP and the response to supramaximal stimulation of motor-nerve fibres in the brachial plexus were alternately recorded. During the contraction, responses to motor-nerve stimulation increased in area by 87±35% (mean±SD) in the biceps brachii and 74±30% in the brachioradialis, but the area of the MEPs increased by 153±86% and 175±122%, respectively. Thus, the increase in the MEP was greater than the increase in the peripheral M-wave. The onset latency of the MEP in the biceps brachii increased by 0.7±0.6 ms (range: –0.2 to 1.9 ms) during the sustained contraction. A smaller increase occurred in response to peripheral nerve stimulation (0.3±0.3 ms; from –0.3 to 0.9 ms). In the contralateral elbow flexors, neither responses to transcranial magnetic stimulation nor responses to motor-nerve stimulation changed in size or latency. During the sustained contraction, the short silent period after stimulation of the peripheral nerve (48±5 ms in biceps brachii and 48±4 ms in brachioradialis) increased in duration by about 12 ms (to 61±12 ms and 60±9 ms, respectively), whereas the silent period following transcranial magnetic stimulation increased from 238±39 ms in biceps brachii and 243±34 ms in brachioradialis to 325±41 ms and 343±42 ms, respectively. During a sustained MVC, while the motor responses to peripheral and to cortical stimulation grow concurrently, growth of the MEP cannot be entirely accounted for by changes in the muscle action potential. Hence, some of the increase in MEP size during fatigue must reflect changes in the central nervous system. Increased latency of the MEPs and lengthening of the peripherally evoked silent period are consistent with decreased excitability of the alpha motoneurone pool. Thus, an increased response from the motor cortex to the magnetic stimulus remains a likely contributor to the increase in the size of the MEP in fatigue. Received: 11 September 1998 / Accepted: 28 January 1999