Evidence that low-threshold muscle afferents evoke long-latency stretch reflexes in human hand muscles.

1. The aim of the present study was to identify the type of spinal afferents involved in the generation of the long-latency response in intrinsic human hand muscles. Position-controlled extensions were imposed on the index finger or on the wrist of healthy subjects who were exerting a steady voluntary flexion force at the relevant joint. Averaged surface electromyographic (EMG) responses of the first dorsal interosseus muscle (FDI) or of the wrist flexors were evaluated with respect to latency and size. 2. Small transient angular displacements of the index finger (1 degree, as measured at the metacarpophalangeal joint), which are supposed to excite primary rather than secondary afferents, evoked two clearly discernible EMG responses with mean latencies of 32.3 ms (M1 response) and 54.7 ms (M2 response), respectively. The size of the M2 response exceeded the size of the M1 response by 60%. In the wrist flexors, transient stretch (1 degree) gave rise to a large M1 response (latency 22.8 ms) and a small, inconstent M2 response. 3. Small-amplitude vibration of the index finger elicited EMG responses in the FDI that were qualitatively and quantitatively similar to those seen in response to small transient stretches of the index finger. This was also true for fast ramp-and-hold stretches (stretch velocity 400 degrees/s, amplitude 5 degrees), whereas slow ramp-and-hold stretches (125 degrees/s, 5 degrees) elicited predominantly M2 responses. 4. In the FDI, the mechanical threshold of the M1 and M2 response to the transient angular displacement was approximately 0.15 degrees, with a tendency for the M2 response to appear at a lower threshold.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neuronal pathways from low-threshold hindlimb cutaneous afferents to motoneurons innervating trunk muscles in low-spinal cats

Postsynaptic potentials (PSPs) evoked in motoneurons innervating the back and abdominal muscles in the lumbar part of the body by stimulating hindlimb cutaneous afferents were investigated in unanesthetized decerebate and spinal cats. Various types of PSP: pure excitatory postsynaptic potential (EPSP), pure inhibitory postsynaptic potential (IPSP), and mixed PSP (i.e., EPSP followed by IPSP, EPSP/IPSP; and IPSP followed by EPSP, IPSP/EPSP) were observed. The weak stimulation at 2 times threshold (2T) produced predominantly the EPSP, while at 5T the incidence of IPSP or EPSP followed by IPSP was increased. In about 20-50% of the various groups of motoneurons, PSPs evoked by ipsi- and contralateral nerves were qualitatively and quantitatively similar. For the other motoneurons, PSPs evoked by ipsi- and contralateral nerves were markedly different with respect to magnitude and/or polarity. These findings suggest that, within each motoneuron pool, some neurons act to increase stiffness of the trunk or to move vertically in response to an increased activity of cutaneous afferents, while the other motoneurons act to produce lateral bending of the trunk.     

Reflex connections from forearm and hand afferents to shoulder girdle muscles in humans

Using surface electromyographic recordings from the trapezius and serratus anterior muscles and percutaneous electrical stimulation of the median, ulnar and radial nerves, the reflex connections from forearm and hand afferents to these shoulder girdle muscles have been investigated in normal human subjects. Stimulation of the median, ulnar and radial nerves at the shoulder, elbow and wrist evoked late, excitatory reflexes in the upper and lower parts of trapezius and in serratus anterior. These reflexes are not evoked by stimulation of cutaneous afferents alone, since there was no response to stimulation of the distal cutaneous branches of these three nerves. Measurements of the conduction velocity of afferents of the median, ulnar and radial nerve evoking these reflexes gave a mean conduction velocity of approximately 50 m/s. The lowest stimulus intensities at which these reflexes could be evoked were found to be 0.3 times motor threshold (MT). By taking into account the reflex latency, the length of the conduction path and that the reflex was mediated by low threshold, fast conducting afferents, it is proposed that group I muscle afferents from the forearm or hand evoke a supraspinal reflex to trapezius and serratus anterior. It appears that the functional significance of these reflexes is to aid in the stability of the shoulder girdle. Electronic Publication     

Functional properties of low-threshold mechanoreceptive afferents in the human labial mucosa

We used microneurography to investigate the functional properties of low-threshold mechanoreceptive afferents innervating the oral mucosa of the inside of the lower lip. Impulse responses were recorded from the inferior alveolar nerve of four human subjects. The threshold force and receptive field boundaries were identified for 19 single mechanoreceptive afferents using thin filaments (von Frey hairs) that applied known forces to the mucosa. Most of the receptive fields were located close to the corners of the mouth. Twelve of the afferents were slowly adapting (SA) and the remaining seven units were fast adapting (FA). Two types of slowly adapting responses were observed, SA I and SA II. Four of the six SA II units were spontaneously active. The geometric mean value of the receptive field sizes was 4.20 mm² for the SA I units, 5.65 mm² for the SA II units, and 5.60 mm² for the FA I units. None of the FA afferents showed response properties characteristic of Pacinian-corpuscle type afferents (FA II units). All afferents showed low force threshold between 0.06 and 1 mN. The properties of the mechanoreceptors supplying the human labial mucosa appear more similar to those of the vermilion and facial skin of the lower lip than those supplying the mucosa of the dorsal tongue.     

Long-latency responses in human thenar muscles mediated by fast conducting muscle and cutaneous afferents

Electrical stimulation of median nerve at the wrist near motor threshold evokes two distinct reflexes in voluntarily innervated thenar muscles. The first is the Hoffmann reflex (HR) and the second is the long-latency reflex (LLR). Stimulation of the motor branch of the median nerve at the thenar sub-threshold for motor fibers evokes the same pattern suggesting that group IA muscle afferents are mediating the LLR. Stimulation of pure cutaneous afferents evokes a LLR of similar latency without a preceding HR. Statistical analyses of the reflex latencies of HR and LLR are in favor of a fast conduction velocity for the cutaneous afferents involved. Hence for muscular and cutaneous LLR a long-lasting central conduction time has to be assumed.     

Excitatory amino acids mediate responses elicited in vitro by stimulation of cortical afferents to reticularis thalami neurons of the rat

The effects of the excitatory amino acids on the nucleus reticularis thalami were examined by intracellular recordings from rat thalamic slices. Non-N-methyl-D-aspartate receptor agonists and glutamate induced a membrane depolarization and a reduction in input resistance, while N-methyl-D-aspartate and aspartate induced a prolonged discharge, which in some neurons took the form of a burst firing associated with an apparent increase in membrane input resistance. Both the N-methyl-D-aspartate and the aspartate effects were blocked by D-2-amino-5-phosphonovalerate, while the effects of glutamate, kainate and quisqualate were not. The excitatory postsynaptic potential evoked by corticothalamic fiber stimulation shows two components: an early, short-lasting, 2-amino-5-phosphonovalerate-insensitive portion, and a late, 2-amino-5-phosphonovalerate-sensitive decay phase. It is suggested that glutamate acts in nucleus reticularis thalami cells preferentially on the non-N-methyl-D-aspartate receptors, while aspartate shows an N-methyl-D-aspartate-like effect. The two excitatory amino acids glutamate and aspartate play a determinant role in the modulation of thalamic activity driven by corticothalamic projection.     

Post-tetanic facilitation of vasomotor reflexes elicited by electrical stimulation of spinal afferents

Summary Somato-vegetative reflexes elicited by electrical stimulation of various spinal afferents and the effect of conditioning were investigated in immobilized cats, conscious or anaesthetized with chloralose or urethane. Test stimuli (1–2–4 imp/s) were applied before and after higher frequency (30 imp/s) conditioning stimulation on the same (homotopic conditioning) or different (heterotopic conditioning) nerves. Changes in reflex blood pressure responses and in renal sympathetic efferent activity were evaluated.In case of homotopic conditioning facilitation of reflexes was observed in anaesthetized cats. It manifested itself in reversal of depressor reactions into biphasic or pressor ones and in the appearance of marked reflex discharges in the sympathetic efferent activity, if the testing stimulations involved A-fibre afferents only. Pressor reflexes elicited by testing stimulations of A+C-afferents were enhanced; in the renal neurograms second bursts of discharges (C-reflexes) appeared or, if they had been present, were enlarged.Facilitation failed 1. if conditioning stimulation was not strong enough to excite C-fibre afferents; 2. in case of heterotopic stimulations; 3. in conscious animals independently of the site of conditioning. Depression of reflexes could be observed if heterotopic conditioning was performed in urethane anaesthesia or in conscious animals.It is concluded that post-tetanic facilitation of sympathetic reflexes may be due to enhanced excitability of the first spinal interneurons; controlling role of supraspinal influences is admitted.     

Reflex inhibition of the diaphragm by esophageal afferents

The electrical activity of the different parts of the diaphragm and of the inspiratory intercostals was recorded during esophageal distension in anesthetized, spontaneously breathing dogs. Distension of the esophagus always produced an immediate and remarkable inhibition of the diaphragm and an intercostal compensation. The phenomenon disappeared after vagotomy. The purpose of this reflex is probably to allow the passage of the bolus of food into the stomach while an adequate ventilation is maintained.     

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     

Early and late stretch responses of human foot muscles induced by perturbation of stance

In eight subjects standing on a movable platform, surface EMG activity was recorded from the foot muscles extensor digitorum brevis (EDB) and flexor digitorum brevis (FDB) and from the leg muscles soleus (Sol) and tibialis anterior (TA) during perturbations of upright stance. Perturbations inducing foot dorsiflexion (upward tilt and backward translation) evoked a short-latency response (SLR) and a medium-latency response (MLR) to stretch in the physiological extensors FDB and Sol, and a long-latency response (LLR) in the physiological flexors EDB and TA. Perturbations inducing plantarflexion (downward tilt and forward translation) evoked the MLR in EDB and TA, and the LLR in FDB and Sol. The latency of the FDB and Sol SLR was compared to that of the H and T reflexes evoked in the same muscles by electrical or mechanical stimulation, respectively. In both muscles, the T reflex and the SLR followed the H reflex at delays accounted for by the different stimulation mode, indicating that the SLR induced in both muscles by upward tilt and backward translation was a true autogenetic stretch reflex from spindle primaries. The time interval between the onset of SLR and of MLR was significantly greater for the FDB than the Sol muscle, suggesting that MLR is a spinal reflex travelling through slower peripheral afferent pathways than SLR. From these latency differences and from the distance between the muscles, we calculated in four subjects the conduction velocity of the afferent fibres presumably responsible for the MLR in FDB. This was about 29 m/s. LLRs were evoked in TA and EDB during upward tilt and backward translation, and in Sol and FDB during downward tilt, but not forward translation. LLRs did not adhere to a proximal-to-distal pattern, since these could appear earlier in the foot than in the leg muscles. All responses were modulated by perturbation type (tilt vs translation) and body posture (normal stance vs forward leaning). Both the large amplitude of the foot muscle responses and their temporal pattern indicate that the muscles acting on the toes play a major role in stabilising posture. Their action increases in amplitude and extends in time the foot-ground reaction force, thereby improving the efficiency of the superimposed action of the leg muscle responses.     

Identification of pathways mediating cardiovascular responses elicited by stimulation of the septum in the rat.

1. Experiments were done in rats anaesthetized with sodium pentobarbitone to localize pathways mediating the cardiovascular responses elicited by electrical stimulation of the septum. The major efferent projections from the septum were first identified anatomically by the Fink-Heimer II technique and these pathways were subsequently lesioned in acute experiments to establish their role in the mediation of the cardiovascular responses elicited by septal stimulation. 2. Electrical stimulation of histologically localized sites in the lateral septum elicited hypotension and bradycardia whereas stimulation of sites in the medial septum elicited hypertension and bradycardia. 3. Selective lesions of cardiovascular responsive sites in either the lateral or medial septum produced a pattern of degeneration essentially similar to that of previous anatomical studies, i.e. the main efferent projections were localized to the fornix, stria medullaris and the medial forebrain bundle. 4. Stimulation of the fornix did not elicit cardiovascular changes and lesions of this pathway did not alter cardiovascular responses to septal stimulation. 5. Stimulation of the stria medullaris elicited hypotension and bradycardia whereas stimulation of the medial forebrain bundle elicited hypertension and bradycardia. Ipsilateral lesions of the stria medullaris, signicicantly attenuated the hypotension and bradycardia elicited by stimulation of the lateral septum but did not affect the responses to stimulation of the medial septum. Bilateral lesions of the medial forebrain bundle in the region of the lateral hypothalamus abolished the hypertension but did not affect the bradycardia elicited by medial septum stimulation. Cardiovascular, reponses elicited by stimulation of the lateral septum were not affected by medial forebrain bundle lesions. 6. It is suggested that the cardiovascular responses elicited by stimulation of the lateral septum are mediated via the stria medullaris and that the hypertension elicited by stimulation of the medial septum is mediated via the medial forebrain bundle. On the other hand, the bradycardia elicited by stimulation of the medial septum is probably mediated by a pathway presently unknown.     

Reflex responses of motor units in human masseter muscle to electrical stimulation of the lip

Summary The reflex responses of single motor units in human masseter muscle to electrical stimulation of the lip were recorded. The subject maintained a contant mean level of pre-stimulus excitation of the parent motor neurone by biting in such a way that the unit fired at either 10 or 15 Hz during each trial. When firing at 10 Hz, most units were reflexly inhibited for up to 90 ms by electrical stimuli at intensities that were perceived to be mildly uncomfortable. In many units, the inhibition consisted of 2 phases which were separated from each other by a few spikes occurring about 30 ms after the stimulus. It was occasionally possible to evoke only the later phase (latency about 40 ms) with stimuli at intensities near the response threshold. In these instances, the inhibitory response became biphasic at higher stimulus intensities with the emergence of a shorter (10–15 ms) component. Still higher intensities caused the 2 phases of inhibition to merge, giving the appearance of a single, prolonged, inhibitory response. When the pre-stimulus firing frequency of the unit was changed from 10 Hz to 15 Hz, the inhibitory responses to the same stimuli were decreased, with the longer-latency component usually surviving beyond the shorter-latency phase. The pattern of reflex responses observed can be explained by a model based on information derived from intracellular recordings in animal experiments.