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Biomedical subjects

J T Buchanan

Publications and source records attributed to J T Buchanan.

At least 19 recordsLinked to original sources
PubMed2001

Cryopreservation of heart cells from the eastern oyster.

Conditions were developed to cryopreserve cells from pronase-dissociated atria and ventricles of eastern oysters (Crassostrea virginica). The effect of three concentrations (5, 10, 15%) of the cryoprotectants (dimethyl sulfoxide, glycerol, and propylene glycol), three thawing temperatures (25, 45, 75 degrees C), and three cooling rates (slow, medium, fast) were compared. Cells were frozen at -80 degrees C and plunged in liquid nitrogen. Thawed cells were seeded in 96-well plates and primary cultures were evaluated after 3 d by measuring the metabolic activity using a tetrazolium compound, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, and by comparing the relative spreading of cells between treatments. The best conditions for freezing and thawing of cells for each cryoprotectant were selected and a final study was performed to compare cryoprotectants. For this final study, we measured the number of cells and their viability 3 d after thawing, in addition to determining cell metabolic activity and cell spreading. Primary cultures of cells fozen without cryoprotectant and of nonfrozen cells were used as controls in all studies. Atrial cells were best cryopreserved with glycerol at a concentration of 10%, a medium cooling rate, and thawing at 45 degrees C. After thawing, atrial cells showed 53+/-5% of the metabolic activity, 84+/-5% of the number, and 92+/-2% of the viability of nonfrozen cells. For ventricular cells, 10% glycerol, a medium cooling rate, and thawing at 25 degrees C yielded the best results. The thawed ventricular cells showed 83+/-5% of the metabolic activity, 91+/-5% of the number, and 96+/-2% of the viability of nonfrozen cells.

Animals↗
PubMed2001

Contributions of identifiable neurons and neuron classes to lamprey vertebrate neurobiology.

Among the advantages offered by the lamprey brainstem and spinal cord for studies of the structure and function of the nervous system is the unique identifiability of several pairs of reticulospinal neurons in the brainstem. These neurons have been exploited in investigations of the patterns of sensory input to these cells and the patterns of their outputs to spinal neurons, but no doubt these cells could be used much more effectively in exploring their roles in descending control of the spinal cord. The variability of cell positions of neurons in the spinal cord has precluded the recognition of unique spinal neurons. However, classes of nerve cells can be readily defined and characterized within the lamprey spinal cord and this has led to progress in understanding the cellular and synaptic mechanisms of locomotor activity. In addition, both the identifiable reticulospinal cells and the various spinal nerve cell classes and their known synaptic interactions have been used to demonstrate the degree and specificity of regeneration within the lamprey nervous system. The lack of uniquely identifiable cells within the lamprey spinal cord has hampered progress in these areas, especially in gaining a full understanding of the locomotor network and how neuromodulation of the network is accomplished.

Animals↗
PubMed2000

[Effect of serotonin on isolated cells with the various functionality from the lamprey spinal cord].

The differential actions of 5-hydroxytryptamine (5-HT) (100 microM) were investigated on isolated motoneurons, interneurons, and primary sensory neurons from the lamprey spinal cord using patch-clamp techniques. Application of 5-HT did not evoke membrane currents in any of the spinal neurons tested (n = 62). However, in most motoneurons and interneurons (15 of 18), 5-HT produced a small depolarization (2-6 mV), which was not accompanied by a change in input resistance. In the remaining motoneurons and interneurons (3 of 18), 5-HT induced a large depolarization (up to 10-20 mV) and a decrease in input resistance of 20-60%. In most sensory neurons (dorsal sensory cells, DSCs), 5-HT evoked a short-lasting, low-amplitude depolarization, followed by a long-lasting hyperpolarization of 2-7 mV. The DSCs showed no significant change in input resistance to 5-HT application (n = 8). Spike afterpolarization were also differentially modulated by 5-HT. In motoneurons and interneurons, 5-HT decreased the amplitude of the afterhyperpolarization following the action potential while increasing the amplitude of the after depolarization. In the DSCs, no significant effect of 5-HT on spike afterpolarization was observed. 5-HT differentially modulated the current induced by application of N-methyl-D-aspartate (NMDA). In motoneurons and interneurons, 5-HT enhanced NMDA-evoked current, while in DSCs, 5-HT decreased this current. These results demonstrate that 5-HT differentially modulates the activity of functionally different groups of spinal neurons. In motoneurons and interneurons, 5-HT enhances excitation by inducing depolarization and decreasing the afterhyperpolatization, while NMDA currents are enhanced. These effects facilitate the appearance of rhythmic discharges in these cells in the presence of NMDA. In primary dorsal sensory cells, 5-HT enhances inhibition by hyperpolarizing the cells and depressing NMDA currents. These differential effects are presumably mediated by different types of 5-HT receptors on these classes of spinal neurons.

Action Potentials↗
PubMed2000

[Serotonin modulates oscillation parameters of the membrane potential in isolated spinal neurons in the lamprey].

The 5-HT was shown to depolarize branch cells (supposedly motoneurones and interneurones) by 2-6 mV, inducing, however, no MP oscillations. In case the MP oscillations were present (induced by the NMDA, for instance), the 5-HT altered their parameters: increased the amplitude of all types of oscillations, frequency of irregular oscillations, and duration of the depolarising plateau with the AP discharges. This modulation of the induced oscillations may enhance activity of neuronal locomotor network and thus reinforce muscle contractions and increase the intensity of the animal's movements. Possible mechanisms of the receptor modulation, of the AP enhancement, and of the changes in locomotor rhythm parameters, are discussed.

Animals↗
PubMed1999

The roles of spinal interneurons and motoneurons in the lamprey locomotor network.

The isolated lamprey spinal cord offers a relatively simple and convenient adult preparation in which to investigate how nerve cells generate behavior and in particular the rhythmic motor patterns of locomotion. Nerve cell classes can be identified and their cellular and synaptic properties characterized, and a simple model based on demonstrated synaptic connectivity can account for major aspects of fictive swimming. Clearly, however, much remains to be learned. In particular, the properties of the spinal neurons have been shown to change during swimming activity but relatively little is known about how these changes occur or the effects that these changes have upon the activities of the network. In addition, much remains to be learned about the cell types and their synaptic interactions as demonstrated here with the newly discovered feedback connections from motoneurons, which have not been previously taken into account in modeling of the lamprey locomotor network.

Animals↗
PubMed1999

Physiological and morphological correlates of presynaptic inhibition in primary afferents of the lamprey spinal cord.

Patch-clamp recordings in a whole-cell mode were performed on dorsal sensory cells enzymatically isolated from the spinal cord of two lamprey species, Ichthyomyzon unicuspis and Lampetra fluviatilis. The voltage-activated currents through calcium channels were analysed. GABA and the specific GABA(B) receptor agonist baclofen reduced the peak amplitude of inward Ba2+ current, as a robust alternate charge carrier through voltage-dependent Ca2+ channels. These effects were dose-dependent and reversible. GABA(B) receptor antagonists, 2-hydroxysaclofen and delta-amino-n-valeric acid, blocked the reduction of Ba2+ currents by GABA and baclofen, while bicuculline, a GABA(A) receptor antagonist, had no blocking action. GABA and baclofen did not modify the dorsal sensory cell membrane conductance, indicating that they did not activate ligand-gated channels. However, GABA, but not baclofen, considerably increased membrane conductance and induced Cl- currents in isolated multipolar neurons (presumably interneurons and/or motoneurons). These findings suggest that GABA and baclofen action on lamprey dorsal sensory cells is mediated by GABA(B) receptors. We concluded that GABA-mediated presynaptic inhibition of lamprey dorsal sensory cell fibers results from GABA(B) receptor activation followed by a decrease of inward voltage-activated calcium currents. Appositions of GABA-immunoreactive boutons to horseradish peroxidase-labeled fibers from the dorsal root were observed at the ultrastructural level in the dorsal column using postembedding immunogold cytochemistry. It seems likely that these appositions represent the morphological substrate of dorsal sensory cell fiber presynaptic inhibition. In very rare cases, ultrastructural features were observed which could be interpreted as synaptic specializations between the GABA-immunoreactive boutons and the primary afferent fibers. The extrasynaptic action of GABA as a basis of presynaptic inhibition of this population of primary afferent neurons is discussed.

Afferent Pathways↗
PubMed1999

Commissural interneurons in rhythm generation and intersegmental coupling in the lamprey spinal cord.

Commissural interneurons in rhythm generation and intersegmental coupling in the lamprey spinal cord. To test the necessity of spinal commissural interneurons in the generation of the swim rhythm in lamprey, longitudinal midline cuts of the isolated spinal cord preparation were made. Fictive swimming was then induced by bath perfusion with an excitatory amino acid while recording ventral root activity. When the spinal cord preparation was cut completely along the midline into two lateral hemicords, the rhythmic activity of fictive swimming was lost, usually replaced with continuous ventral root spiking. The loss of the fictive swim rhythm was not due to nonspecific damage produced by the cut because rhythmic activity was present in split regions of spinal cord when the split region was still attached to intact cord. The quality of this persistent rhythmic activity, quantified with an autocorrelation method, declined with the distance of the split spinal segment from the remaining intact spinal cord. The deterioration of the rhythm was characterized by a lengthening of burst durations and a shortening of the interburst silent phases. This pattern of deterioration suggests a loss of rhythmic inhibitory inputs. The same pattern of rhythm deterioration was seen in preparations with the rostral end of the spinal cord cut compared with those with the caudal end cut. The results of this study indicate that commissural interneurons are necessary for the generation of the swimming rhythm in the lamprey spinal cord, and the characteristic loss of the silent interburst phases of the swimming rhythm is consistent with a loss of inhibitory commissural interneurons. The results also suggest that both descending and ascending commissural interneurons are important in the generation of the swimming rhythm. The swim rhythm that persists in the split cord while still attached to an intact portion of spinal cord is thus imposed by interneurons projecting from the intact region of cord into the split region. These projections are functionally short because rhythmic activity was lost within approximately five spinal segments from the intact region of spinal cord.

Animals↗
PubMed1999

Segmental distribution of common synaptic inputs to spinal motoneurons during fictive swimming in the lamprey.

These experiments were designed to measure the degree of shared synaptic inputs coming to pairs of myotomal motoneurons during swimming activity in the isolated spinal cord of the lamprey. In addition, the experiments measured the decrease in the degree of shared synaptic inputs with the distance between the motoneurons to assess the segmental distribution of these shared inputs. Intracellular microelectrode recordings of membrane potential were made simultaneously on pairs of myotomal motoneurons during swimming activity induced with an excitatory amino acid. The swim cycle oscillations of motoneuron membrane potentials were removed with a digital notch filter, thus leaving the fast synaptic activities that underlie these slower oscillations. Cross-correlations of the fast synaptic activities in two simultaneously recorded motoneurons were made to measure the degree of shared inputs. The cross-correlation was done on time windows restricted to one swim cycle or to part of a swim cycle, and 50 consecutive swim cycle cross-correlograms then were averaged. The peak coefficients of the cross-correlations exhibited a wide range, even for pairs of motoneurons located near one another (range = 0.06-0.74, for pairs located within 2 segments). This observation suggests that there may be different functional classes of myotomal motoneurons with inputs originating from different sets of premotor interneurons. In spite of this variability, the mean peak correlation coefficients of motoneuron pairs clearly decreased with the distance between them. With separations of more than five segments, there was little or no clear correlation between the motoneurons (range = 0.04-0.10). These results suggest that common synaptic inputs to motoneurons during fictive swimming originate from local premotor interneurons and that beyond five spinal segments, common premotor inputs are rare or weak to motoneurons. Thus the premotor signals originating from the locomotor network have relatively short distribution lengths, on the order of 5 segments of 120 total spinal segments.

Animals↗
PubMed1996

Lamprey spinal interneurons and their roles in swimming activity.

An isolated lamprey spinal cord generates rhythmic ventral root bursting that closely resembles swimming activity: ventral roots on opposite sides of the spinal cord burst in alternation, and rostral ventral roots lead caudal ventral roots. This rhythmic activity is induced by superfusion of the spinal cord with an excitatory amino acid and is called 'fictive' swimming. Three main types of spinal interneurons that are active during fictive swimming have been characterized: small excitatory interneurons with ipsilateral axons, large inhibitory interneurons with ipsilateral descending axons (lateral interneurons), and inhibitory commissural interneurons. The synaptic connectivities of these inter-neurons can account for the pattern of motoneuron excitation and inhibition occurring during fictive swimming, and it has been proposed that the synaptic interactions of these spinal neurons form the unit segmental oscillators of the swim network. Computer modeling has confirmed that this network can generate rhythmic activity resembling fictive swimming. The core of the model is the reciprocal inhibition between commissural interneurons on opposite sides of the cord. Evidence that the commissural interneurons are essential to rhythm generation comes from lesion studies in which the spinal cord was split down the midline and also from photo-ablation studies in which commissural interneurons were inactivated by illumination after retrograde labeling with a photo-toxic tracer. In both types of experiments, rhythmic activity of fictive swimming can be abolished, supporting the view that the commissural interneurons are necessary for rhythmogenesis.

Action Potentials↗
PubMed1995

Modulation of swimming in the lamprey, Petromyzon marinus, by serotonergic and dopaminergic drugs.

The effects of serotonergic and dopaminergic drugs on free swimming behavior in adult sea lampreys (Petromyzon marinus) were investigated using video image analysis. Injections of the serotonin precursor 5-hydroxy-L-tryptophan along with the serotonin reuptake blocker clomipramine into the visceral cavity of lampreys resulted in significant increases in the cycle period of swimming, but had no significant effects on the propagation time of the swim waves down the body (normalized to cycle period), or on the degree of body curvature. Injections of the dopamine agonist apomorphine resulted in significant decreases of cycle period and body curvature with no significant effects on the normalized wave propagation time. The effects on cycle period are consistent with previous findings using serotonin and apomorphine on swimming activity in the isolated spinal cord.

5-Hydroxytryptophan↗
PubMed1995

The neuronal network for locomotion in the lamprey spinal cord: evidence for the involvement of commissural interneurons.

The spinal cord of the lamprey, a primitive vertebrate, has been used as a model system for investigating the cellular basis of rhythmic locomotor activity. Three classes of interneurons have been characterized that are active during locomotor activity in the isolated spinal cord (ie fictive swimming). The identified synaptic interactions of these neurons form a network which has been proposed to underlie locomotor rhythmogenesis. Modeling studies confirmed that the network can produce oscillatory activity with phase relations among the neurons similar to those found in the spinal cord. Within the network, inhibitory commissural interneurons form reciprocal inhibitory connections and play a key role in rhythmogenesis. Several experiments have been done to test whether these cells participate in the generation of rhythmic activity in the spinal cord. First, midline lesions that sever the axons of commissural interneurons eliminate rhythmic ventral root bursting. Second, photo-ablation of commissural interneurons on one side of the spinal cord alters the symmetry of ventral root bursts, alters the cycle period, and can eliminate rhythmic bursting. Taken together, these experiments support the model that commissural interneurons are involved in rhythmogenesis in the lamprey spinal cord.

Animals↗
PubMed1995

Localization and interaction of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors of lamprey spinal neurons.

Small volumes of N-Methyl-D-Aspartate (NMDA) and non-NMDA excitatory amino acid receptor agonists were applied to localized regions of the dendritic trees of lamprey spinal neurons along their medial-lateral axis to obtain a spatial map of glutamate receptor distribution. Voltage clamp and frequency domain methods were used to obtain quantitative kinetic data of the voltage dependent ionic channels located both on the soma and on highly branched dendritic membranes. Pressure pulses of NMDA applied to the most peripheral regions of the dendritic tree elicited large somatic impedance increases, indicating that the most peripheral dendrites are well supplied with NMDA receptors. Experiments done with kainate did not elicit somatic responses to agonist applications on peripheral dendrites. The data obtained are consistent with the hypothesis that the activation of NMDA receptors by exogenous glutamate is significantly modified by the simultaneous activation of non-NMDA receptors, which shunts the NMDA response. The non-NMDA shunting hypothesis was tested by a combined application of kainate and NMDA to mimic the action of glutamate showing that the shunting effect of non-NMDA receptor activation virtually abolished the marked voltage dependency typical of NMDA receptor activation. These data were interpreted with a compartmental neuronal model having both NMDA and non-NMDA receptors.

Animals↗
PubMed1995

Activities of spinal neurons during brain stem-dependent fictive swimming in lamprey.

1. We made intracellular microelectrode recordings of membrane potential from spinal neurons during fictive swimming elicited by brief electrical shocks to the spinal cord in a brain stem-spinal cord preparation of the adult silver lamprey (Ichthyomyzon unicuspis). 2. We characterized membrane potential activities recorded during brain stem-dependent fictive swimming in five spinal cell types: myotomal motoneurons, lateral interneurons (inhibitory neurons with ipsilateral descending axons), CC interneurons (neurons with contralateral and caudal projecting axons), edge cells (intraspinal stretch receptors), and dorsal cells (primary mechanosensory neurons with cell bodies in the spinal cord). The membrane potential activities were compared with data from previous reports recorded during fictive swimming in the isolated spinal cord with fictive swimming induced by superfusion with D-glutamate. 3. Compared with the same cell types recorded during D-glutamate-induced fictive swimming in brain stem-dependent fictive swimming, the motoneurons and CC interneurons had significantly larger trough-to-peak amplitudes of membrane potential oscillations, whereas lateral interneurons were not significantly different in amplitude. The timings of the membrane potential oscillations and of cell spiking were not significantly different in the two preparations, with the exception that motoneurons in brain stem-dependent fictive swimming were significantly earlier by approximately 10% of a cycle. Edge cells had only weak or no oscillatory activities, and dorsal cells had no detectable input during brain stem-dependent fictive swimming. These findings are similar to those in D-glutamate-induced fictive swimming.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1995

Quantitative analysis of electrotonic structure and membrane properties of NMDA-activated lamprey spinal neurons.

Parameter optimization methods were used to quantitatively analyze frequency-domain-voltage-clamp data of NMDA-activated lamprey spinal neurons simultaneously over a wide range of membrane potentials. A neuronal cable model was used to explicitly take into account receptors located on the dendritic trees. The driving point membrane admittance was measured from the cell soma in response to a Fourier synthesized point voltage clamp stimulus. The data were fitted to an equivalent cable model consisting of a single lumped soma compartment coupled resistively to a series of equal dendritic compartments. The model contains voltage-dependent NMDA sensitive (INMDA), slow potassium (IK), and leakage (IL) currents. Both the passive cable properties and the voltage dependence of ion channel kinetics were estimated, including the electronic structure of the cell, the steady-state gating characteristics, and the time constants for particular voltage- and time-dependent ionic conductances. An alternate kinetic formulation was developed that consisted of steady-state values for the gating parameters and their time constants at half-activation values as well as slopes of these parameters at half-activation. This procedure allowed independent restrictions on the magnitude and slope of both the steady-state gating variable and its associated time constant. Quantitative estimates of the voltage-dependent membrane ion conductances and their kinetic parameters were used to solve the nonlinear equations describing dynamic responses. The model accurately predicts current clamp responses and is consistent with experimentally measured TTX-resistant NMDA-induced patterned activity. In summary, an analysis method is developed that provides a pragmatic approach to quantitatively describe a nonlinear neuronal system.

Animals↗
PubMed1994

Effects of strychnine on fictive swimming in the lamprey: evidence for glycinergic inhibition, discrepancies with model predictions, and novel modulatory rhythms.

1. Inhibitory postsynaptic potentials (ipsps) produced by two classes of interneurons, CC (Contralateral and caudal projecting) and lateral interneurons, were tested for strychnine sensitivity using paired intracellular recordings in the lamprey spinal cord. The ipsps were partially blocked by 0.2-0.5 microM strychnine and were completely blocked by 5 microM strychnine. Thus, the ipsps may be glycinergic. 2. These interneurons are key participants in a proposed circuit model for fictive swimming. A connectionist-type computer simulation of the model demonstrated that the cycle period of the network increased with decreasing ipsp strength. 3. Application of strychnine (0.1-0.5 microM) to the spinal cord during fictive swimming induced by an excitatory amino acid increased cycle period, consistent with previous reports, but at odds with stimulation predictions. 4. Strychnine also produced slow rhythmic modulation of fictive swimming (period = 12 s) which maintained left-right alternation and rostral-caudal coordination. Auto- and cross-correlation analyses revealed that the slow modulation was present in a weaker form in most control preparations during fictive swimming. 5. Since the proposed model for the swimming pattern generator in the lamprey spinal cord does not predict the observed speeding with strychnine, nor the slow modulatory rhythm, it appears to be deficient in its present formulation.

Animals↗
PubMed1993

Electrophysiological properties of identified classes of lamprey spinal neurons.

1. As part of a continuing analysis of the mechanisms of the central pattern generator underlying fictive swimming in lamprey, a systematic survey of electrophysiological properties of lamprey neurons was made in the in vitro spinal cord preparation with the use of intracellular current-clamp recordings. A total of 70 neurons was included in the study, representing 6 classes of spinal neurons. The classes were myotomal motoneurons, three classes of interneurons involved in fictive swimming [lateral interneurons, nerve cells with contralateral and caudal projecting axons (CC interneurons), and excitatory interneurons], and two classes of interneurons involved in sensory processes (edge cells and giant interneurons). The recordings were done in quiescent preparations. 2. There was little or no significant difference among the cell classes with regard to resting potential, threshold potential, action-potential amplitude, or action-potential duration. 3. The voltage versus current relationships for the cells were fairly linear near resting potential, although most cells showed a slight tendency to rectify with depolarization above resting potential. This tendency was strongest among edge cells and lateral interneurons and weakest among motoneurons and CC interneurons. The input resistances, membrane time constants, and rheo-bases for the cell classes showed significant differences among some classes. For example, CC interneurons and excitatory interneurons had significantly higher input resistances than the other cell classes. 4. The late afterhyperpolarization following the action potential tended to be larger in amplitude with an earlier peak and a longer duration in edge cells and giant interneurons than in the other cell classes. 5. All cells responded to depolarizing current injections by firing action potentials, and almost all cells fired action potentials throughout the 400-ms current pulse. The cells exhibited adaptation resulting in increasing interspike intervals during the current pulse. The adaptation, however, was insufficient to terminate firing before the end of the current pulse. The relationship between frequency of firing and input current was generally monotonic with a tendency to saturate at higher current levels. 6. The general conclusion from this study is that the spinal neurons that partake in fictive swimming (motoneurons, lateral interneurons, CC interneurons, and excitatory interneurons) are similar in their resting and action-potential mechanisms. Their most prominent differences are in size-related properties. The sensory-related interneurons, especially the edge cells and to some extent the giant interneurons, exhibited more pronounced differences in their resting and action-potential properties when compared with the other cell classes.

Afferent Pathways↗
PubMed1993

The effects of neurotransmitters on the integrative properties of spinal neurons in the lamprey.

1. The integrative behavior of lamprey central neurons was analyzed by white noise frequency domain methods and simulated with a minimal, non-linear neuronal model consisting of two voltage-dependent processes: (i) a depolarizing inwardly directed conductance carrying calcium and monovalent ions and (ii) a repolarizing outwardly directly conductance representing a generalized potassium conductance. In addition to normal properties, the effects of neurotransmitters were interpreted with the model. Specifically, N-methyl-D-aspartate (NMDA)-induced properties were simulated under conditions where the intrinsic voltage dependence of the potassium channels was constrained by properties of lamprey neurons. However, the NMDA channel kinetics were fixed by the single-channel properties of other neurons. The effects of focally applied neurotransmitters on the membrane properties of intact spinal cord neurons were quantitatively described with a reduced neuronal model that was also used to simulate transmitter-induced responses. In addition, transmitters were also released synaptically by KCl depolarization of projecting neurons. 2. Both synaptically released transmitters and focally applied putative excitatory or inhibitory transmitters directly applied to the spinal cord generally resulted in a decrease in the magnitude of the impedance function that was modeled by a decrease in membrane resistance (shunting effect). 3. Local application of the inhibitory neurotransmitters glycine or gamma-aminobutyric acid (GABA) led to small voltage responses when recorded near the resting potential. However, large decreases in the magnitude of the impedance function were observed in both current-clamp or voltage-clamp recording modes. 4. The excitatory amino acids quisqualate, kainate and glutamate evoked depolarizations in current clamp that activated intrinsic voltage-dependent conductances and obscured the direct effects of the transmitters. Under voltage-clamp conditions these transmitters caused a small decrease in the impedance magnitude that could be modeled by a shunt. 5. In contrast to the other excitatory amino acids, NMDA elicited large increases, rather than decreases, in both the magnitude and the phase lag of the impedance function. These changes were modeled by a negative conductance (a voltage-dependent conductance that produces an inward current). 6. The reduced neuron model provides an experimentally based description of the highly oscillatory and non-linear responses observed during NMDA activation of the spinal neurons involved in the pattern generation of locomotion. Simulations of sustained oscillatory behaviors consistent with experimental observations were carried out to illustrate the NMDA-induced integrative properties of central neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acids↗