Journal of Neurophysiology

Aniss, A. M.; Gandevia, S. C.; Burke, D.

doi: N/Apmid: 1597720

Abstract 1. Reflex responses were elicited in muscles that act at the ankle by electrical stimulation of low-threshold afferents from the foot in human subjects who were reclining supine. During steady voluntary contractions, stimulus trains (5 pulses at 300 Hz) were delivered at two intensities to the sural nerve (1.2-4.0 times sensory threshold) or to the posterior tibial nerve (1.1-3.0 times motor threshold for the intrinsic muscles of the foot). Electromyographic (EMG) recordings were made from tibialis anterior (TA), peroneus longus (PL), soleus (SOL), medial gastrocnemius (MG), and lateral gastrocnemius (LG) muscles by the use of intramuscular wire electrodes. 2. As assessed by averages of rectified EMG, stimulation of the sural or posterior tibial nerves at nonpainful levels evoked a complex oscillation with onset latencies as early as 40 ms and lasting up to 200 ms in each muscle. The most common initial responses in TA were a decrease in EMG activity at an onset latency of 54 ms for sural stimuli, and an increase at an onset latency of 49 ms for posterior tibial stimuli. The response of PL to stimulation of the two nerves began with a strong facilitation of 44 ms (sural) and 49 ms (posterior tibial). With SOL, stimulation of both nerves produced early inhibition beginning at 45 and 50 ms, respectively. With both LG and MG, sural stimuli produced an early facilitation at 52-53 ms. However, posterior tibial stimuli produced different initial responses in these two muscles: facilitation in LG at 50 ms and inhibition in MG at 51 ms. 3. Perstimulus time histograms of the discharge of 61 single motor units revealed generally similar reflex responses as in multiunit EMG. However, different reflex components were not equally apparent in the responses of different single motor units: an individual motor unit could respond slightly differently with a change in stimulus intensity or background contraction level. The multiunit EMG record represents a global average that does not necessarily depict the precise pattern of all motor units contributing to the average. 4. When subjects stood erect without support and with eyes closed, reflex patterns were seen only in active muscles, and the patterns were similar to those in the reclining posture. 5. It is concluded that afferents from mechanoreceptors in the sole of the foot have multisynaptic reflex connections with the motoneuron pools innervating the muscles that act at the ankle. When the muscles are active in standing or walking, cutaneous feedback may play a role in modulating motoneuron output and thereby contribute to stabilization of stance and gait. Copyright © 1992 the American Physiological Society

Recanzone, G. H.; Merzenich, M. M.; Jenkins, W. M.; Grajski, K. A.; Dinse, H. R.

doi: N/Apmid: 1597696

Abstract 1. Adult owl monkeys were trained to detect differences in the frequency of a tactile flutter-vibration stimulus above a 20-Hz standard. All stimuli were delivered to a constant skin site restricted to a small part of a segment of one finger. The frequency-difference discrimination performance of all but one of these monkeys improved progressively with training. 2. The distributed responses of cortical neurons ("maps") of the hand surfaces were defined in detail in somatosensory cortical area 3b. Representations of trained hands were compared with those of the opposite, untrained hand, and to the area 3b representations of hands in a second set of monkeys that were stimulated tactually in the same manner while these monkeys were attending to auditory stimuli (passive stimulation controls). 3. The cortical representations of the trained hands were substantially more complex in topographic detail than the representations of unstimulated hands or of passively stimulated control hands. 4. In all well-trained monkeys the representations of the restricted skin location trained in the behavioral task were significantly (1.5 to greater than 3 times) greater in area than were the representations of equivalent skin locations on control digits. However, the overall extents of the representations of behaviorally stimulated fingers were not larger than those of control fingers in the same hemisphere, or in opposite hemisphere controls. 5. The receptive fields representing the trained skin were significantly larger than receptive fields representing control digits in all but one trained monkey. The largest receptive fields were centered in the zone of representation of the behaviorally engaged skin, but they were not limited to it. Large receptive fields were recorded in a 1- to 2-mm-wide zone in the area 3b maps of trained hands. 6. Receptive-field sizes were also statistically significantly larger on at least one adjacent, untrained digit when compared with the receptive fields recorded on the homologous digit of the opposite hand. 7. There was an increase in the percent overlaps of receptive fields in the cortical zone of representation of the trained skin. A significant number of receptive fields were centered on the behaviorally trained skin site. 8. The effects of increased topographic complexity, increased representation of the trained skin location, increased receptive-field size, and increased receptive-field overlap were not observed in the representations of the untrained hands in these same monkeys. Only modest increases in topographic complexity were recorded in the representations of passively stimulated hands, and no effects on receptive-field size or overlap were noted.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1992 the American Physiological Society

Ariel, M.; Tusa, R. J.

doi: N/Apmid: 1597702

Abstract 1. Eye movements were measured in three rhesus monkeys after monocular intravitreal injections of picrotoxin, a gamma-aminobutyric acid (GABA) antagonist. The effects of this drug were tested when the animals were in a completely dark room, when they performed a smooth pursuit task, and when they viewed either a stationary pattern or a full-field optokinetic pattern rotating horizontally. 2. Between 15 and 20 min after the injection, a sustained conjugate spontaneous nystagmus developed in the dark, with the slow-phase movement in the temporal-to-nasal direction with respect to the injected eye. Peak slow-phase velocity ranged from 15 to 45 degrees/s. The nystagmus persisted for at least 1 h but stopped by the next day. 3. In a well-lit room, the nystagmus was completely suppressed, even during monocular viewing with the injected eye. When the lights were turned off, the slow-phase velocity of the spontaneous nystagmus slowly increased to a steady-state level within 70-120 s. 4. Horizontal smooth pursuit eye movements to a 1 degree target light moving in front of the animal +/- 20 degrees to either side of center of gaze at constant speeds were normal. Target speeds ranging from 15 to 60 degrees/s for both monocular and binocular viewing conditions were used. Binocular and monocular optokinetic nystagmus (OKN) to a full-field drum rotating at a constant velocity (5-90 degrees/s) were also normal. The initial pursuit and steady-state components of OKN were measured, as well as the velocity-storage component (optokinetic after nystagmus, OKAN).(ABSTRACT TRUNCATED AT 250 WORDS) Copyright © 1992 the American Physiological Society

Recanzone, G. H.; Merzenich, M. M.; Schreiner, C. E.

doi: N/Apmid: 1597698

Abstract 1. Temporal response characteristics of neurons were sampled in fine spatial grain throughout the hand representations in cortical areas 3a and 3b in adult owl monkeys. These monkeys had been trained to detect small differences in tactile stimulus frequencies in the range of 20-30 Hz. Stimuli were presented to an invariant, restricted spot on a single digit. 2. The absolute numbers of cortical locations and the cortical area over which neurons showed entrained frequency-following responses to behaviorally important stimuli were significantly greater when stimulation was applied to the trained skin, as compared with stimulation on an adjacent control digit, or at corresponding skin sites in passively stimulated control animals. 3. Representational maps defined with sinusoidal stimuli were not identical to maps defined with just-visible tapping stimuli. Receptive-field/frequency-following response site mismatches were recorded in every trained monkey. Mismatches were less frequently recorded in the representations of control skin surfaces. 4. At cortical locations with entrained responses, neither the absolute firing rates of neurons nor the degree of the entrainment of the response were correlated with behavioral discrimination performance. 5. All area 3b cortical locations with entrained responses evoked by stimulation at trained or untrained skin sites were combined to create population peristimulus time and cycle histograms. In all cases, stimulation of the trained skin resulted in 1) larger-amplitude responses, 2) peak responses earlier in the stimulus cycle, and 3) temporally sharper responses, than did stimulation applied to control skin sites. 6. The sharpening of the response of cortical area 3b neurons relative to the period of the stimulus could be accounted for by a large subpopulation of neurons that had highly coherent responses. 7. Analysis of cycle histograms for area 3b neuron responses revealed that the decreased variance in the representation of each stimulus cycle could account for behaviorally measured frequency discrimination performance. A strong correlation between these temporal response distributions and the discriminative performances for stimuli applied at all studied skin surfaces was even stronger (r = 0.98) if only the rising phases of cycle histogram were considered in the analysis. 8. The responses of neurons in area 3a could not account for measured differences in frequency discrimination performance. 9. These representational changes did not occur in monkeys that were stimulated on the same schedule but were performing an auditory discrimination task during skin stimulation. 10. It is concluded that by behaviorally training adult owl monkeys to discriminate the temporal features of a tactile stimulus, distributed spatial and temporal response properties of cortical neurons are altered.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1992 the American Physiological Society

Totosy de Zepetnek, J. E.; Zung, H. V.; Erdebil, S.; Gordon, T.

doi: N/Apmid: 1597722

Abstract 1. Isolated and glycogen-depleted motor units (MUs) have been studied in normal and reinnervated tibialis anterior (TA) muscles of the rat to examine 1) the correspondence between physiological and histochemical classifications, 2) the extent to which unit properties cluster according to type, 3) the relation between unit force and fatigability, and 4) the extent to which reinnervated MUs recover their former properties. 2. MUs were isolated by ventral root dissection and stimulation in reinnervated and normal TA muscles, 3.5-8 mo after common peroneal (CP) nerve section and resuture and in age-matched control rats, respectively. The units were characterized physiologically for classification into four types: slow twitch (S), fast twitch, fatigue resistant (FR), fast twitch fatigue intermediate (FI), and fast twitch fatigue sensitive (FF). Four muscle fiber types were identified histochemically with the use of a modification of the techniques of Brooke and Kaiser, and Guth and Samaha to delineate fiber subtypes on the basis of the pH sensitivity of myofibrillar adenosine triphosphatase (ATPase). 3. Neither the time-to-peak twitch force development nor the profile of unfused tetanus ("sag test") was unambiguous in separating fast from slow MUs. However, all units with a time to peak greater than 22 ms were fatigue resistant, and this time was chosen to delineate fast from slow. The fast unit population was further subdivided on the basis of their fatigability. There is normally a small proportion of S units (6% S) that increased to 20% after reinnervation. Although the fast population was subdivided, there was a continuous distribution of fatigue indexes in normal and reinnervated muscles with the highest number of fast units falling into the FI category. The proportions of fast units were 28% FR, 45% FI, and 21% FF in normal muscles and 29% FR, 38% FI, and 13% FF in reinnervated muscles. 4. In normal muscles, delineation of fast and slow fibers and subdivision of fast fiber types on the basis of acid and alkali stability of myofibrillar ATPase provided a histochemical classification that showed 78% correspondence with physiological classification of the same identified units. In reinnervated muscles the correspondence between physiological and histochemical classifications was reduced to 72%. 5. The normal correlation between MU fatigability and isometric force in TA muscles was not seen in reinnervated muscles that contained more FR MUs. Mean fatigue index from normal units was significantly less at 0.55 +/- 0.03 (mean +/- SE) compared with 0.68 +/- 0.03 from reinnervated units.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1992 the American Physiological Society

Ewald, D. A.; Roper, S. D.

doi: N/Apmid: 1597715

Abstract 1. Taste cells in intact taste buds in slices of Necturus lingual epithelium were impaled with microelectrodes for intracellular recording. Two types of cells were investigated: taste receptor cells and basal cells. 2. Impaling cells in the apical end of taste buds resulted in intracellular records from taste receptor cells. Applying short pulses (100- to 200-ms duration) of 140 mM KCl solution to the apical pore elicited receptor potentials in the taste receptor cells. 3. Impaling cells in the base of the taste bud resulted in intracellular records from taste receptor cells and basal cells. KCl applied to the taste pore elicited responses in the basal region that varied greatly in both magnitude and time of onset. The latency of these responses (time of onset compared with the onset of the receptor potential) ranged from 0 to hundreds of milliseconds. 4. Impaled cells were identified by injecting Lucifer yellow after recording KCl responses for 21 cells. KCl responses recorded from identified basal cells all had latencies of greater than 75 ms. KCl responses from identified receptor cells all had latencies of less than 75 ms. 5. One explanation for the long latency of KCl responses recorded in basal cells is that the responses represent postsynaptic potentials. In agreement with this interpretation, long-latency responses, but not short-latency responses, were reversibly reduced by the Ca antagonist Cd (1 mM, 10- to 20-min bath exposure). 6. Long-latency responses also differed from short-latency responses in their voltage dependence. Short-latency responses had the same voltage dependence as apically recorded receptor potentials, increasing with hyperpolarization from resting potential with an extrapolated reversal potential near 0 mV. Long-latency responses were much less dependent on voltage in this range. 7. We measured the spread of exogenously applied KCl with potassium-sensitive electrodes. Long-latency responses were not generated by diffusion of applied KCl to the basal region of the taste bud. A small transient increase in extracellular potassium occurred at the base of the taste bud after chemostimulation at the apical pore. This increase was due to depolarization-evoked release of potassium from taste cells and did not cause the long-latency responses in basal cells. 8. We conclude that short-latency (less than 75 ms) responses recorded from cells situated in the bases of taste buds are electrotonically conducted receptor potentials generated at the apical region. Long-latency (greater than 75 ms) responses are consistent with recording postsynaptic responses in basal cells. Copyright © 1992 the American Physiological Society

Travers, J. B.; Jackson, L. M.

doi: N/Apmid: 1597706

Abstract 1. Thirty-five neurons in the hypoglossal nucleus (mXII) of the rat were characterized during licking and swallowing in response to fluid stimulation in an awake, freely moving preparation. Simultaneously recorded electromyographic (EMG) recordings from a subset of oropharyngeal muscles were obtained to delineate both the lick cycle and the occurrence of swallows. Most mXII neurons discharged with rhythmic bursts in phase with licking. Twenty-six of the 35 mXII neurons had bimodal interspike interval (ISI) histograms, reflecting rhythmic bursts and the absence of spontaneous activity. Three mXII cells with unimodal ISI histograms were rhythmically active during licking but had some spontaneous activity. Of the remaining six cells with unimodal ISI histograms, five had nonbursting modes of activity. 2. Phase relationships between neural and EMG activity during licking were determined by cross-correlation and compared with distributions of cross-correlations between lingual and masticatory EMG activity. A bimodal distribution of cross-correlations was obtained by cross-correlating EMG activity between lingual protrudor muscles genioglossus (GG) or geniohyoid (GH) and masticatory jaw-opener activity anterior digastric (AD) and cross-correlating lingual retractor activity styloglossus (STY) with anterior digastric EMG. A similar bimodal distribution of cross-correlations obtained between mXII neuron activity and AD contractions suggested that the majority of mXII neurons (30/35) could be classified as protrudor- or retractor-related. Neurons classified as protrudor-related cells were located ventrally in mXII; cells classified as retractor-related were more dorsally located, consistent with anatomic and physiological descriptions of the myotopic organization of mXII. 3. Ten mXII protrudor-related neurons responded with a mean of 4.9 +/- 2.2 (SD) action potentials per lick cycle and preceded the peak jaw-opening phase of licking by a mean of 22.3 ms. In contrast, the activity of 20 retractor-related mXII neurons lagged the jaw-opening phase of licking by a mean of 55.9 ms, with a mean of 5.5 +/- 3.4 (SD) action potentials occurring per lick cycle. Five other mXII neurons exhibited nonrhythmic activity during licking and could not be classified as protrudor- or retractor-related on the basis of cross-correlations with the AD. 4. The occurrence of a swallow decreased the licking frequency by 21%, corresponding to an increase of approximately 43 ms in the period between AD contractions.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1992 the American Physiological Society

Meyers, D. E.; Graf, R. A.; Cooke, I. M.

doi: N/Apmid: 1597714

Abstract 1. The X-organ sinus gland is a major peptidergic neurosecretory system in Crustacea, analogous to the vertebrate hypothalamoneurohypophyseal system. Neuronal somata isolated from the crab (Cardisoma carnifex) X-organ and maintained in primary culture in unconditioned, fully defined medium show immediate regenerative outgrowth. Outgrowth occurring as broad lamellipodia ("veiled") distinguishes neurons consistently showing crustacean hyperglycemic hormone immunoreactivity. Neurons that are immunoreactive against molt-inhibiting hormone and red pigment concentrating hormone antisera give rise to branched neurites ("branched"). 2. The whole-cell variation of the patch-clamp technique was used to study the electrophysiology of these two cell types 24-48 h after plating. Under current clamp, only veiled neurons fired overshooting action potentials either spontaneously or in response to depolarization. 3. Under voltage clamp, net current was predominantly outward. When solutions that suppressed outward current were used, only veiled neurons showed significant inward current. These included a tetrodotoxin (TTX)-sensitive Na current and a slow (time to peak 6-10 ms at 0 mV) Cd-sensitive Ca current (ICa) that was activated at potentials less than -30 mV, was maximal at 0 to +20 mV, and did not reverse at potentials up to +60 mV. 4. In TTX, the form of the Ca current I(V) curve was unchanged by changes of holding potential between -40 and -80 mV, and 75-100% of ICa was available from -40 mV. 5. ICa inactivated slowly and incompletely. Analysis with two-pulse regimes suggested that both inactivation and facilitation mechanisms were present. 6. Outward current was examined in the presence and absence of 0.5 mM Cd2+ (1 microM TTX was always present in the external medium). Cd2+ ions slightly reduced the peak outward current, usually by less than 10% (Vc = -10 to +20 mV; Vh = -80 mV). All additional observations were in the presence of TTX and Cd2+. 7. Both cell types expressed a 4-aminopyridine (4-AP)-sensitive transient current, analogous to IA, and a slower-rising (minimum time to peak 20 ms), sustained current that was partially sensitive to tetraethylammonium, analogous to IK. 8. The mean Vh at which IA was half inactivated was -46 mV, and the mean time constant for removal of inactivation was 46 ms.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1992 the American Physiological Society

Yue, G.; Cole, K. J.

doi: N/Apmid: 1597701

Abstract 1. This study addressed potential neural mechanisms of the strength increase that occur before muscle hypertrophy. In particular we examined whether such strength increases may result from training-induced changes in voluntary motor programs. We compared the maximal voluntary force production after a training program of repetitive maximal isometric muscle contractions with force output after a training program that did not involve repetitive activation of muscle; that is, after mental training. 2. Subjects trained their left hypothenar muscles for 4 wk, five sessions per week. One group produced repeated maximal isometric contractions of the abductor muscles of the fifth digit's metacarpophalangeal joint. A second group imagined producing these same, effortful isometric contractions. A third group did not train their fifth digit. Maximal abduction force, flexion/extension force and electrically evoked twitch force (abduction) of the fifth digit were measured along with maximal integrated electromyograms (EMG) of the hypothenar muscles from both hands before and after training. 3. Average abduction force of the left fifth digit increased 22% for the Imagining group and 30% for the Contraction group. The mean increase for the Control group was 3.7%. 4. The maximal abduction force of the right (untrained) fifth digit increased significantly in both the Imagining and Contraction groups after training (10 and 14%, respectively), but not in the Control group (2.3%). These results are consistent with previous studies of training effects on contralateral limbs. 5. The abduction twitch force evoked by supramaximal electrical stimulations of the ulnar nerve was unchanged in all three groups after training, consistent with an absence of muscle hypertrophy. The maximal force of the left great toe extensors for individual subjects remained unchanged after training, which argues against strength increases due to general increases in effort level. 6. Increases in abduction and flexion forces of the fifth digit were poorly correlated in subjects of both training groups. The fifth finger abduction force and the hypothenar integrated EMG increases were not well correlated in these subjects either. Together these results indicate that training-induced changes of synergist and antagonist muscle activation patterns may have contributed to force increases in some of the subjects. 7. Strength increases can be achieved without repeated muscle activation. These force gains appear to result from practice effects on central motor programming/planning. The results of these experiments add to existing evidence for the neural origin of strength increases that occur before muscle hypertrophy. Copyright © 1992 the American Physiological Society

Hasselmo, M. E.; Anderson, B. P.; Bower, J. M.

doi: N/Apmid: 1597709

Abstract 1. The effect of cholinergic modulation on associative memory function was studied in a computational model based on the physiology and anatomic structure of piriform cortex. Both the cholinergic suppression of intrinsic fiber synaptic transmission and the cholinergic changes in postsynaptic excitability described in the companion paper were examined. 2. Distributed input patterns representing odors were stored in the model with the use of a synaptic modification rule dependent on pre- and postsynaptic activity (i.e., Hebbian). Associative recall of these patterns was tested by presenting the model with degraded versions of the learned patterns and testing whether these degraded patterns evoked the same network response as the full learned input pattern. Storage was evaluated with the use of a performance measure designed to reflect how well degraded input patterns could be recognized as a particular learned input pattern. 3. When memory function was evaluated with a selective cholinergic suppression of intrinsic fiber synaptic transmission during learning, associative memory performance was greatly enhanced. Cholinergic suppression during learning prevents previously stored patterns from interfering with the storage of new patterns. 4. When memory function was evaluated with a cholinergic mediated enhancement in cell excitability during learning, the speed of learning increased, but so did the decay in performance due to interference during learning. 5. When suppression of intrinsic fiber synaptic transmission was coupled with an increase in cell excitability, the best memory performance was obtained. 6. These results provide a possible theoretical framework for linking the neuropharmacological effects of acetylcholine to behavioral evidence for a role of acetylcholine in memory function. This could help describe how memory deficits might arise from cholinergic dysfunction in diseases such as Alzheimer's dementia. Copyright © 1992 the American Physiological Society