Journal of Neurophysiology

Dixon, D.; Atwood, H. L.

doi: N/Apmid: 2480994

Abstract 1. Pulsatile application of serotonin (5-HT) leads to facilitation of excitatory postsynaptic potentials (EPSPs) in crayfish "opener" neuromuscular preparations. The facilitation resulting from a single application of serotonin shows two phases: an early, rapidly decaying phase, and a less intense, long-lasting phase of 1- to 2-h duration. A previous study implicated the phosphatidylinositol system as an essential component in serotonin-induced facilitation, especially the early phase. The present study was conducted to determine the roles of the adenylate cyclase and phosphatidylinositol systems in both phases of serotonin-induced facilitation. 2. Relatively brief applications of agents known to affect the intracellular concentration of cAMP (forskolin, 1 microM; and IBMX, 100 microM) cause an increase in EPSP amplitude, which persists for 1-2 h. 3. The duration of the less intense, long-lasting phase of serotonin-induced facilitation is prolonged in the presence of 1 microM IBMX. This concentration of IBMX does not affect EPSP amplitude by itself. A membrane-permeant analog of cAMP (applied in concentrations less than or equal to 1 mM) is also not effective in altering EPSP amplitude. However, when dibutyryl cAMP is applied in the presence of 1 microM IBMX, EPSP amplitude is increased (60-80%). 4. Localized presynaptic injection of the "Walsh Inhibitor" (PKI), which inhibits cAMP-activated protein kinase, blocks the less intense, long-lasting phase of serotonin-induced facilitation at synapses near the site of injection. Normal facilitation develops at synapses within the same preparation remote from the site of injection. Distribution of the injected inhibitor within the axon can be visualized by tagging PKI with a fluorescent marker.(ABSTRACT TRUNCATED AT 250 WORDS) Copyright © 1989 the American Physiological Society

Akaike, N.; Inomata, N.; Yakushiji, T.

doi: N/Apmid: 2557392

Abstract 1. Kinetic properties of gamma-aminobutyric acid (GABA)-gated inward and outward anion currents were investigated in the frog sensory neurons perfused internally and externally with various anions with the use of a rapid concentration-jump (termed as 'concentration-clamp') technique. 2. Extracellular Br- ( Br-o) shifted the dose-response curves of GABA-induced inward anion currents to the left without affecting the maximum values, whereas Cl-o, I-o, No3-o, HCOO-o, and CH3COO-o altered the rate of desensitization differently without shifting the GABA dose-response curves, indicating that the kinetics of desensitization phase are affected differently by various extracellular anions. 3. CH3COO-o suppressed the maximum current of the dose-response curve of the GABA-induced inward ICl without affecting Kd. 4. Both activation and desensitization phases of GABA-induced ICl consisted of fast and slow components, respectively. Br-o, I-o, and NO3-o significantly prolonged the slow desensitization component, whereas both HCOO-o and CH3COO-o shortened it. The fast desensitization and the fast and slow activation components were also affected by these foreign anions. 5. GABA dose-response curves of inward currents carried by various intracellular anions (Cl-, Br-, NO3-, I-, SCN-, HCOO-, F-, CH3COO-, CH3CH2COO-, BrO3-, and ClO3-) while keeping a constant Cl-o had a constant Kd value but different saturating maximum currents. There were no marked differences among their current kinetics except in the case of SCN-, indicating that the current kinetics is not affected by replacing intracellular Cl- ( Cl-i) with various foreign anions. 6. The configuration and amplitude of GABA-gated outward anion currents at a constant Cl-i reflected the extracellular action of individual anions on the anion-binding site of GABA receptor associated with the anion-selective channel. 7. The relative conductances of the various anions, calculated from the maximum peak currents in dose-response curves of the GABA-induced inward anion currents at a constant Cl-o, was in the sequence: I- greater than Br- greater than or equal to NO3- greater than ClO3- greater than SCN- greater than or equal to Cl- greater than HCOO- greater than BrO3- greater than CH3COO- greater than F- greater than CH3CH2COO-.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1989 the American Physiological Society

Gardner, E. P.; Palmer, C. I.

doi: N/Apmid: 2600632

Abstract 1. Tactile discrimination of form requires motion of the hand across the object scanned. To dissociate lateral distortion of the skin from neuronal processing mechanisms involving multiple receptor classes and parallel central networks, we have simulated motion of bar patterns across the fingers and palm by the use of a computer-controlled grid of sequentially activated probes (OPTACON stimulator). Horizontal bar patterns have been swept across the hand at speeds of 30-120 mm/s to quantitatively characterize responses of cutaneous mechanoreceptive afferents recorded in the median and ulnar nerves. 2. Mechanoreceptors with phasic responses to pressure are activated by spatial patterns on the OPTACON, whereas those with tonic pressure responses are not; moving-bar patterns strongly excite both Meissner's afferents rapidly adapting (RA) mechanoreceptors and Pacinian corpuscles (PCs) but fail to excite slowly adapting (SA) afferents. OPTACON-type stimulators thus allow selective activation of phasic mechanoreceptor channels with spatially complex stimuli. 3. RA afferents respond in an all-or-none fashion to activation of two to five adjacent rows spanning 1-5 mm on the finger, with nearly identical latencies on all trials; response profiles are remarkable for their regularity and reproducibility. PCs have larger fields (4-13 rows) and stronger but more irregular responses than RAs. 4. Uniform sensitivity throughout the receptive field is a consistent feature of RA responses. Individual mechanoreceptor terminals appear to have equal access to the spike initiation zone and provide the same amplitude input as the fiber discharges 1 spike/pulse at each field location in 75% of the RAs tested. Uniform sensitivity allows each afferent to transmit a repetitive signal of the parameter of interest such as object speed, contour, or texture. 5. One-quarter of RAs fire two spikes to probe indentation and retraction at the field center. Such graded responses are usually observed in only one direction of motion, reflecting a preferred sequence of receptor activation rather than a specific location on the skin. PCs fire bursts of two to four spikes throughout most of their receptive fields; sensitivity is broadly distributed rather than peaked. Thus phasic mechanoreceptors fail to provide a precise signal of stimulus location; localization at the level of individual papillary ridges appears to be signaled by a population mechanism involving unique combinations of RA, SA, and PC afferents.(ABSTRACT TRUNCATED AT 250 WORDS) Copyright © 1989 the American Physiological Society

Raabe, W.

doi: N/Apmid: 2600634

Abstract 1. Glutamine is thought to be a precursor of the pool of glutamate that is used as synaptic transmitter. NH4+ inhibits glutaminase, the enzyme presumed to cleave glutamine into glutamate in synaptic terminals. Therefore a decrease by NH4+ of excitatory synaptic transmission in hippocampus was suggested to be due to the inability to utilize glutamine as a precursor for glutamate and subsequent transmitter depletion. This study reexamines the effects of NH4+ on excitatory synaptic transmission. 2. The effects of NH4+ on excitatory synaptic transmission from low-threshold afferent fibers, presumably Ia-afferent fibers, to motoneurons was investigated in the spinal cord of anesthetized cats in vivo. 3. Action potentials of low-threshold afferent fibers were recorded at the entry of the dorsal roots into the spinal cord. An extracellular electrode within a motoneuron nucleus recorded the action potential of low-threshold afferent fibers and the extracellular monosynaptic excitatory postsynaptic potential, i.e., the focal synaptic potential (FSP). This extracellular electrode also recorded the antidromic field potential (AFP) in response to ventral root stimulation. Electrodes on the ventral roots recorded the monosynaptic reflex (MSR) and the monosynaptic excitatory postsynaptic potential in motoneurons electrotonically conducted into the ventral roots (VR-EPSP). 4. Intravenous infusion of ammonium acetate (AA) reversibly decreased MSR, VR-EPSP, and FSP, i.e., decreased excitatory synaptic transmission. 5. The decrease of VR-EPSP and FSP was accompanied initially by a decrease of conduction and, eventually, a conduction block in presynaptic terminals of low-threshold afferent fibers. 6. The decreases of VR-EPSP and FSP were also accompanied by the transient appearance of a reflex discharge, triggered by VR-EPSPs of decreased amplitude, and changes of the AFP indicating increased invasion of motoneuron somata by antidromic action potentials. 7. It is suggested that NH4+ depolarizes intraspinal Ia-afferent fibers and motoneurons. This depolarization initially decreases and then blocks conduction of action potentials into the presynaptic terminals of Ia-afferent fibers. The conduction block prevents the release of excitatory transmitter and decreases excitatory synaptic transmission. 8. The suggested depolarizing action of NH4+ may be due to K+-like ionic properties of NH4+ and/or an inhibition of K+-uptake into astrocytes. 9. The conduction block in presynaptic terminals of low-threshold afferent fibers can fully explain the decrease of excitatory synaptic transmission by NH4+. Because of the conduction block in presynaptic terminals, this study does not permit a conclusion as to an inhibition by NH4+ fo the utilization of glutamine as a precursor for glutamate used as synaptic transmitter. Copyright © 1989 the American Physiological Society

Buchanan, T. S.; Rovai, G. P.; Rymer, W. Z.

doi: N/Apmid: 2600619

Abstract 1. We studied the patterns of electromyographic (EMG) activity in elbow muscles of 14 normal human subjects. The activity of five muscles that act in flexion-extension and forearm supination-pronation was simultaneously recorded during isometric voluntary torque generation, in which torques generated in a plane orthogonal to the long axis of the forearm were voluntarily coupled with torques generated about the long axis of the forearm (i.e., supination-pronation). 2. When forearm supination torques were superimposed on a background of elbow flexion torque, biceps brachii activity increased substantially, as expected; however, brachioradialis and brachialis EMG levels decreased modestly, a less predictable outcome. The pronator teres was also active during pure flexion and flexion coupled with mild supination (even though no pronation torque was required). This was presumably to offset inappropriate torque contributions of other muscles, such as the biceps brachii. 3. When forearm supination torque was superimposed on elbow extension torque, again the biceps brachii was strongly active. The pronator teres also became mildly active during extension with added pronation torque. These changes occurred despite the fact that both the pronator and biceps muscles induce elbow flexion. 4. In these same elbow extension tasks, triceps brachii activity was also modulated with both pronation or supination loads. It was most active during either supination or pronation loads, again despite the fact that it has no mechanical role in producing forearm supination-pronation torque. 5. Recordings of EMG activity during changes in forearm supination-pronation angle demonstrated that activation of the biceps brachii followed classic length-tension predictions, in that less EMG activity was required to achieve a given supination torque when the forearm was pronated (where biceps brachii is relatively longer). On the other hand, EMG activity of the pronator teres did not decrease when the pronator was lengthened. Triceps EMG was also more active when the forearm was supinated, despite its having no direct functional role in this movement. 6. Plots relating EMG activity in biceps brachii, brachialis, and brachioradialis at three different forearm positions revealed that there was a consistent positive near-linear relationship between brachialis and brachioradialis and that biceps brachii is often most active when brachioradialis and brachialis are least active. 7. We argue that, for the human elbow joint at least, fixed muscle synergies are rather uncommon and that relationships between muscle activities are situation dependent.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1989 the American Physiological Society

Price, D. D.; McHaffie, J. G.; Larson, M. A.

doi: N/Apmid: 2600624

Abstract 1. Psychophysical experiments were initiated to determine the possible influence of increasing stimulus size on perceived pain intensity. Six trained human subjects (5 male, 1 female) made visual analogue scale (VAS) ratings for pain-sensation intensity and unpleasantness in response to nociceptive thermal stimuli. Test stimuli consisted of 5-s duration heat pulses (45-50 degrees C in 1 degrees increments) delivered by one, two, or three contact thermal probes (1 cm2 each) applied to the medial aspect of the anterior forearm. 2. The area of skin receiving noxious thermal stimuli was changed by randomly varying the number of thermodes activated. The effects of varying the distance between the thermal probes also were evaluated. In the first series of experiments, thermal-probe separation was kept close to 0; in subsequent experimental series, the thermodes were separated by either 5 or 10 cm. 3. In each experimental series, considerable spatial summation occurred in both pain-sensation intensity and unpleasantness dimensions of pain. This summation occurred throughout the nociceptive thermal range of 45-50 degrees C and was larger at suprathreshold temperatures (greater than or equal to 47 degrees C) than those near threshold (less than or equal to 46 degrees C). Unlike spatial summation of perceived warmth, that of pain was not characterized by systematic changes in power-function exponents but as approximately upward parallel displacements in double-logarithmic coordinates. 4. Thermal-probe separation over a range of 0-10 cm had no effects on spatial summation of pain-sensation intensity or pain unpleasantness. In contrast, increasing thermal-probe separation increased the subjects' ability to discriminate differences in stimulus size and their ability to detect correctly the number of thermal probes activated. 5. Because affective VAS ratings of unpleasantness were linearly related to, but distinctly and systematically less than, VAS ratings of pain-sensation intensity, it was clear that subjects responded quite differently to these two pain dimensions. Affective judgements were not additionally influenced by thermal probe separation and hence by the ability to perceive stimulus size or number of thermal probes activated. 6. The results indicate that powerful spatial-summation mechanisms exist for heat-induced pain. Spatial summation of pain is likely to be subserved both by local integration mechanisms at the level of single spinothalamic-tract neurons and by recruitment of central nociceptive neurons, because spatial summation of pain occurred to approximately equal extents under conditions of thermode separations over a distance of at least 20 cm. Copyright © 1989 the American Physiological Society

Girard, P.; Bullier, J.

doi: N/Apmid: 2600626

Abstract 1. The presence of a direct lateral geniculate nucleus (LGN) input to area V2 raises the possibility that some neurons in this area remain active when area 17 is inactivated. It is also known that many neurons in area MT are visually responsive in the absence of input from area 17. Because MT sends a strong projection to V2, it appears likely that visual activity could be transferred to V2 through this feedback connection when the V1 afferents are disabled. For these reasons, we decided to reexamine the residual visual activity of neurons in V2 during inactivation of area 17. A circular region 16 mm in diameter on the opercular part of area 17 was cooled by a thermoelectric Peltier device, and single- and multiunit activity was recorded in the retinotopically corresponding region of area V2. 2. Because of the proximity of areas V1 and V2, it was necessary to make sure that neurons in V2 could not be directly blocked by cooling applied to V1. Temperature gradients within cortex were measured with a specially designed thermocouple at different heat flows imposed by the Peltier device. Gradients ranged between 2.3 and 5.5 degrees C/mm. Knowing the temperature gradients and the temperature of the cooling plate, it was possible to deduce the temperature at a given depth within cortex. With this method we measured the blocking temperatures of neurons in area 17, i.e., the temperature at which neurons completely ceased to respond to optimal visual stimulation. Blocking temperatures ranged between 4 and 18 degrees C, values that are substantially lower than those reported in previous papers. Knowing the blocking temperatures, it was possible to determine the cooling-plate temperature necessary to entirely block the region of V1 under the cooling plate. Using the temperature gradients, we then calculated the depth of recording for which V2 neurons could not be directly blocked by the cold. For this reason, all our recordings were made in or near the fundus of the lunate sulcus. 3. During cryoblocking of V1, we recorded 154 sites in penetrations normal to area V2. All these sites had receptive fields included within the visual-field region coded in the cooled zone. In addition, we recorded 55 sites in tangential penetrations aimed at traveling in V2 for long distances. Among these 209 sites, only 3 could be considered as unambiguously active when V1 was blocked. Two of these sites were located at or close to the V2-V3 border.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1989 the American Physiological Society

Enoka, R. M.; Robinson, G. A.; Kossev, A. R.

doi: N/Apmid: 2600629

Abstract 1. The activity of single motor units was recorded in the first dorsal interosseus muscle of human subjects while they performed an isometric ramp-and-hold maneuver. Motor-unit activity was characterized before and after fatigue by the use of a branched bipolar electrode that was positioned subcutaneously over the test muscle. Activity was characterized in terms of the forces of recruitment and derecruitment and the discharge pattern. The purpose was to determine, before and after fatigue, whether motor-unit activity was affected by the direction in which the force was exerted. 2. Regardless of the task during prefatigue trials, interimpulse intervals were 1) more variable during increases or decreases in force than when force was held constant at the target value (4-6% above the recruitment force), and 2) more clustered around an arbitrary central value than would be expected with a normal (Gaussian) distribution. Both effects were seen during the flexion and abduction tasks. The behavior of low-threshold motor units in first dorsal interosseus is thus largely unaffected by the direction of the force exerted by the index finger. The absence of a task (i.e., a direction of force) effect suggests that the resultant force vector about the metacarpophalangeal joint of the index finger is not coded in terms of discrete populations of motor units, but, rather, it is based on the net muscle activity about the joint. 3. Motor-unit behavior during and after fatigue showed that the relatively homogeneous behavior seen before fatigue could be severely disrupted. The fatiguing protocol involved the continuous repetition, to the endurance limit, of a 15-s ramp-and-hold maneuver in which the abduction target force was 50% of maximum and was held for 10-s epochs (ramps up and down were approximately 2 s each). Motor-unit threshold was assessed by the forces of recruitment and derecruitment associated with each cycle of the fatigue test. Changes in recruitment force during the protocol were either minimal or, when present, not systematic. In contrast, the derecruitment force of all units exhibited a marked and progressive increase over the course of the test. 4. After the fatigue test, when the initial threshold tasks were repeated, the behavior of most motor units changed. These changes included the derecruitment of previously active motor units, the recruitment of additional motor units, and an increased discharge variability of units that remained recruited. The variation in recruitment order seemed to be much greater than that reported previously for nonfatiguing conditions.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1989 the American Physiological Society

Akaike, N.; Kaneda, M.

doi: N/Apmid: 2600631

Abstract 1. Electrical and pharmacologic properties of glycine-induced currents were investigated in single hypothalamic neurons acutely isolated from young and adult rats by the use of a "concentration-clamp" technique, which allows both internal perfusion and rapid application of an external solution under single-electrode voltage-clamp. 2. The glycine-induced current reversed at the Cl- equilibrium potential (ECl), and a 10-fold decrease of extracellular Cl- with a large impermeable anion resulted in a 53 mV shift of the glycine reversal potential (EGly). 3. Glycine-induced Cl- currents (ICl) increased sigmoidally in a concentration-dependent manner with a Kd of 9 X 10(-5) M at a Hill coefficient of 1.8. Current inactivation occurred completely at all concentrations within 10 s. EGly remained unchanged during continuous application of glycine, suggesting that the inactivation process is because of desensitization. 4. The glycine-induced conductance exhibited a striking voltage dependency at membrane potentials more negative than -50 mV and reached a steady state value when hyperpolarized beyond -110 mV. 5. Both the activation and inactivation phases of glycine-induced ICl are described by double exponential (fast and slow components) functions with the concentrations used. All four time constants decreased with increasing glycine concentration. 6. The slow time constant of the current decay induced by glycine increased with depolarization and decreased with hyperpolarization, indicating that the rate of desensitization is considerably voltage dependent. The fast decay showed little voltage dependency. 7. Recovery of the glycine response after complete desensitization consisted of two components. 8. The blockade of the glycine response by strychnine and picrotoxin was noncompetitive. Copyright © 1989 the American Physiological Society

Holstege, G.; Blok, B. F.

doi: N/Apmid: 2600623

Abstract 1. The cutaneus trunci muscle (CTM) is a thin broad sheet of skeletal muscle just beneath the skin. It does not contain muscle spindles and receives its afferents from the overlying skin. Contraction of the muscle can easily be triggered by pinching the skin or, in the cat, by gentle displacement of the fur (CTM reflex). The afferent information of this reflex is conveyed via the cutaneous nerves, which are segmentally organized. In the cat, the CTM motoneurons are located in a circumscribed cell group in the ventrolateral part of the ventral horn of the C8 and T1 spinal segments. The CTM motor nucleus corresponds with "nucleus X" of Giovanelli Barilari and Kuypers and with "ventral motor nucleus" of Matsushita and Ueyama. 2. Relatively long ascending propriospinal pathways, originating in the thoracolumbar cord, exist between the cutaneous afferents and the CTM motor nucleus. Such pathways have been described physiologically, as well as anatomically. Our results, based on anterograde autoradiographic experiments with 3Hleucine injections in the C1, C2, C6, and C8 segments, suggest that propriospinal pathways to the CTM motor nucleus originating in the cervical cord do not exist, although these propriospinal projections are very strong to all other motoneuronal cell groups surrounding the CTM motor nucleus. 3. The present results also demonstrate specific supraspinal projections to the CTM motor nucleus originating in 1) the contralateral nucleus retroambiguous (NRA) and 2) the ipsilateral dorsolateral pontine tegmentum. These projections suggest that the CTM motor nucleus is not only involved in spinal reflexes, but also in other functions such as abdominal straining.(ABSTRACT TRUNCATED AT 250 WORDS) Copyright © 1989 the American Physiological Society