Biomedical subjects

J C Rekling

Publications and source records attributed to J C Rekling.

17 recordsLinked to original sources

PubMed2006

The Edinger-Westphal nucleus of the juvenile rat contains transient- and repetitive-firing neurons.

Classically, the Edinger-Westphal nucleus is described as containing neurons controlling accommodation and pupillary constriction via projections to the ciliary ganglion. However, in several species including rat, some Edinger-Westphal neurons have ascending or descending CNS projections suggesting that the Edinger-Westphal nucleus might also have non-ocular functions. To further characterize the function of this nucleus we studied the electrophysiological properties of Edinger-Westphal neurons in a slice preparation from juvenile rats. The position of the Edinger-Westphal nucleus was determined using an immunohistochemical procedure directed at the peptide Urocortin, which is expressed in Edinger-Westphal neurons. Passive and active membrane responses were investigated and two different neuron types were identified. One type had a transient firing response to 400 ms depolarizing current pulses and one type had a repetitive firing response. Transient-firing neurons had an outward rectifying response inhibiting firing, possibly due to slowly inactivating I(D)-like potassium channels since low concentrations (200 microM) of the potassium channel blocker 4-aminopyridine elicited repetitive firing. In all neurons, low threshold Ca(2+) spikes were seen and these were blocked by nickel(II) chloride hexahydrate, suggesting that they are mediated via low voltage-activated Ca(2+) channels. Some biocytin-labeled neurons had axons or axonal collaterals projecting laterally or dorsally, suggesting possible non-ocular targets. In conclusion, the rat Edinger-Westphal nucleus contains two separate types of neurons with distinct electrophysiological properties.

4-Aminopyridine↗

PubMed2000

Electrical coupling and excitatory synaptic transmission between rhythmogenic respiratory neurons in the preBötzinger complex.

Breathing pattern is postulated to be generated by brainstem neurons. However, determination of the underlying cellular mechanisms, and in particular the synaptic interactions between respiratory neurons, has been difficult. Here we used dual recordings from two distinct populations of brainstem respiratory neurons, hypoglossal (XII) motoneurons, and rhythmogenic (type-1) neurons in the preBötzinger complex (preBötC), the hypothesized site for respiratory rhythm generation, to determine whether electrical and chemical transmission is present. Using an in vitro brainstem slice preparation from newborn mice, we found that intracellularly recorded pairs of XII motoneurons and pairs of preBötC inspiratory type-1 neurons showed bidirectional electrical coupling. Coupling strength was low (<0.10), and the current that passed between two neurons was heavily filtered (corner frequency, <10 Hz). Dual recordings also demonstrated unidirectional excitatory chemical transmission (EPSPs of approximately 3 mV) between type-1 neurons. These data indicate that respiratory motor output from the brainstem involves gap junction-mediated current transfer between motoneurons. Furthermore, bidirectional electrical coupling and unidirectional excitatory chemical transmission are present between type-1 neurons in the preBötC and may be important for generation or modulation of breathing rhythm.

Action Potentials↗

PubMed2000

Synaptic control of motoneuronal excitability.

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Action Potentials↗

PubMed1999

Modulation of respiratory frequency by peptidergic input to rhythmogenic neurons in the preBötzinger complex.

Neurokinin-1 receptor (NK1R) and mu-opioid receptor (muOR) agonists affected respiratory rhythm when injected directly into the preBötzinger Complex (preBötC), the hypothesized site for respiratory rhythmogenesis in mammals. These effects were mediated by actions on preBötC rhythmogenic neurons. The distribution of NK1R+ neurons anatomically defined the preBötC. Type 1 neurons in the preBötC, which have rhythmogenic properties, expressed both NK1Rs and muORs, whereas type 2 neurons expressed only NK1Rs. These findings suggest that the preBötC is a definable anatomic structure with unique physiological function and that a subpopulation of neurons expressing both NK1Rs and muORs generate respiratory rhythm and modulate respiratory frequency.

Animals↗

PubMed1998

Brainstem neurons projecting to the rostral ventral respiratory group (VRG) in the medulla oblongata of the rat revealed by co-application of NMDA and biocytin.

Groups of neurons in the medulla and pons are essential for the rhythm generation, pattern formation and modulation of respiration. The rostral Ventral Respiratory Group (rVRG) is thought to be a crucial area for rhythm generation. Here we co-applied biocytin and NMDA in the rVRG to label retrogradely brainstem neurons reciprocally connected to a population of inspiratory neurons in the rat rVRG. The procedure excited rVRG neurons in multi-unit recordings and led to a Golgi-like labelling of distant cells presumably excited by efferents from the rVRG. Injection of biocytin without NMDA did not label neurons in distant structures. Several brainstem ipsi- and contralateral structures were found to project to the rVRG, but three major respiratory-related structures, the nucleus of the solitary tract (NTS), the parabrachialis medialis and Kölliker-Fuse nuclei (PB/KF) and the caudal VRG, which are known to project bilaterally to the rVRG, were exclusively labelled ipsilaterally, suggesting an ipsilateral excitation of these structures by the rVRG. The pathways of efferent axons from labelled neurons in the rVRG were traced rostrally towards the pons and caudally to the spinal cord. Terminal axonal arborizations were seen in the same regions where retrogradely filled neurons were found as well as in a few other motor nuclei (the dorsal vagal motor nucleus and XII nucleus). Moreover, in the NTS and the PB/KF, efferent terminal varicosities were seen closely apposed to the soma and proximal dendrites of labelled neurons, suggesting monosynaptic connections between the rVRG and these nuclei.

Animals↗

PubMed1998

PreBötzinger complex and pacemaker neurons: hypothesized site and kernel for respiratory rhythm generation.

Identification of the sites and mechanisms underlying the generation of respiratory rhythm is of longstanding interest to physiologists and neurobiologists. Recently, with the development of novel experimental preparations, especially in vitro en bloc and slice preparations of rodent brainstem, progress has been made In particular, a site in the ventrolateral medulla, the preBötzinger Complex, is hypothesized to contain neuronal circuits generating respiratory rhythm. Lesions or disruption of synaptic transmission within the preBötzinger Complex, either in vivo or in vitro, can abolish respiratory activity. Furthermore, the persistence of respiratory rhythm following interference with postsynaptic inhibition and the subsequent discovery of neurons with endogenous bursting properties within the preBötzinger Complex have led to the hypothesis that rhythmogenesis results from synchronized activity of pacemaker or group-pacemaker neurons.

Animals↗

PubMed1997

Calcium-dependent plateau potentials in rostral ambiguus neurons in the newborn mouse brain stem in vitro.

Calcium-dependent plateau potentials in rostral ambiguus neurons in the newborn mouse brain stem in vitro. J. Neurophysiol. 78: 2483-2492, 1997. The nucleus ambiguus contains vagal and glossopharyngeal motoneurons and preganglionic neurons involved in respiration, swallowing, vocalization, and control of heart beat. Here we show that the rostral compact formation's ambiguus neurons, which control the esophageal phase of swallowing, display calcium-dependent plateau potentials in response to tetanic orthodromic stimulation or current injection. Whole cell recordings were made from visualized neurons in the rostral nucleus ambiguus using a slice preparation from the newborn mouse. Biocytin-labeling revealed dendritic trees with pronounced rostrocaudal orientations confined to the nucleus ambiguus, a morphological profile matching that of vagal motoneurons projecting to the esophagus. Single-stimulus orthodromic activation, using an electrode placed in the dorsomedial slice near the nucleus tractus solitarius, evoked single excitatory postsynaptic potentials (EPSPs) or short trains of EPSPs (500 ms to 1 s). However, tetanic stimulation (5 pulses, 10 Hz) induced voltage-dependent afterdepolarizations or long-lasting plateau potentials (>1 min) with a constant firing pattern. Depolarizing or hyperpolarizing current pulses elicited voltage-dependent afterdepolarizations or plateau potentials lasting a few seconds to several minutes. Constant spike activity accompanied the long-lasting plateau potentials, which ended spontaneously or could be terminated by weak hyperpolarizing current pulses. Current-induced afterdepolarizations and plateau potentials were dependent on extracellular and intracellular Ca2+, as they were blocked completely by extracellular Co2+, Cd2+, or intracellular bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA). Orthodromically induced afterdepolarizations and plateau potentials were blocked by intracellular BAPTA. Afterdepolarizations and plateau potentials were completely blocked by substitution of extracellular Na+ with choline. Afterdepolarizations persisted in tetrodotoxin. We conclude that rostral ambiguus neurons have a Ca2+-activated inward current carried by Na+. Synaptic activation of this conductance may generate prolonged spike activity in these neurons during the esophageal phase of swallowing.

Animals↗

PubMed1997

Bidirectional electrical coupling between inspiratory motoneurons in the newborn mouse nucleus ambiguus.

Some spinal and brain stem motoneurons are electrically coupled in the early postnatal period. To test whether respiratory motoneurons in the brain stem are electrically coupled, we performed single and dual whole cell patch recordings from presumptive motoneurons in the nucleus ambiguus in a rhythmically active brain stem slice from newborn mice. Two of eight (25%) biocytin-injected neurons showed dye-coupling and 4 of 11 (36%) of intracellularly recorded pairs of neurons showed evidence of bidirectional electrical coupling. Impulse activity in one cell elicited small spikelets in the other and hyperpolarization of one cell led to hyperpolarization of the other with a coupling ratio (DeltaV2:DeltaV1) of 0.03-0.14. We conclude that inspiratory ambiguus motoneurons in the newborn mouse brain stem are bidirectionally electrically coupled, which may serve to transmit or coordinate signals, chemical or electrical.

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PubMed1996

Electroresponsive properties and membrane potential trajectories of three types of inspiratory neurons in the newborn mouse brain stem in vitro.

1. The electrophysiological properties of inspiratory neurons were studied in a rhythmically active thick-slice preparation of the newborn mouse brain stem maintained in vitro. Whole cell patch recordings were performed from 60 inspiratory neurons within the rostral ventrolateral part of the slice with the aim of extending the classification of inspiratory neurons to include analysis of active membrane properties. 2. The slice generated a regular rhythmic motor output recorded as burst of action potentials on a XII nerve root with a peak to peak time of 11.5 +/- 3.4 s and a duration of 483 +/- 54 ms (means +/- SD, n = 50). Based on the electroresponsive properties and membrane potential trajectories throughout the respiratory cycle, three types of inspiratory neurons could be distinguished. 3. Type-1 neurons were spiking in the interval between the inspiratory potentials (n = 9) or silent with a resting membrane potential of -48.6 +/- 10.1 mV and an input resistance of 306 +/- 130 M omega (n = 15). The spike activity between the inspiratory potentials was burst-like with spikes riding on top of an underlying depolarization (n = 11) or regular with no evidence of bursting (n = 12). Hyperpolarization of the neurons below threshold for spike initiation did not reveal any underlying phasic synaptic activity, that could explain the bursting behavior. 4. Type-1 neurons showed delayed excitation after hyperpolarizing square current pulses or when the neurons were depolarized from a hyperpolarized level. This membrane behavior resembles the response seen in other CNS neurons expressing an IA. The response to 1-s long depolarizing pulses with a large current strength showed signs of activation of an active depolarizing membrane response leading to a transient reduction in the spike amplitude. The relationship between the membrane potential and the amplitude of square current pulses (Vm-I) showed a small upward rectification below -70 mV, and spike adaptation throughout a 1-s pulse had a largely linear time course. 5. Type-1 neurons depolarized and started to fire spikes 398 +/- 102 ms (n = 20) before the upstroke of the integrated XII nerve discharge. The inspiratory potential was followed by fast hyperpolarization, a short fast-repolarizing phase (1,040 +/- 102 ms, n = 5) and a longer slow-repolarizing phase (lasting until the next inspiratory discharge). 6. Type-2 neurons were spiking in the interval between the inspiratory potentials with no evidence of bursting behavior and had an input resistance of 296 +/- 212 M omega (n = 26). The response to hyperpolarizing pulses revealed an initial sag and postinhibitory rebound depolarization. This membrane behavior resembles the response seen in other CNS neurons expressing an Ih. The Vm-I relationship was linear at depolarized potentials and showed a marked upward rectification below -60 mV. Spike trains elicited by 1-s long pulses showed a pronounced early and late adaptation. 7. Type-2 neurons depolarized and started to fire spikes 171 +/- 87 ms (n = 23) before the upstroke of the integrated XII nerve discharge. The inspiratory potential had a variable amplitude from cell to cell and was followed by a short hyperpolarization in the cells displaying a large amplitude inspiratory potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1996

Thyrotropin-releasing hormone (TRH) depolarizes a subset of inspiratory neurons in the newborn mouse brain stem in vitro.

1. To extend the classification of respiratory neurons based on active membrane properties and discharge patterns to include responses to respiratory modulators, we have studied the effect of thyrotropin-releasing hormone (TRH, 1-5 microM) on the spontaneous respiratory-related neural activity in a thick brain stem slice preparation from the newborn mouse. The action of TRH on the respiratory output from the slice was investigated by recordings from the XII nerve. Cellular responses to TRH were investigated using whole cell recordings from hypoglossal motoneurons and three types of inspiratory neurons located in the rostral ventrolateral part of the slice. 2. Bath-applied TRH (1 microM) decreased the time between inspiratory discharges recorded on the XII nerve from 12.3 +/- 3.3 s to 4.9 +/- 1.1 s (n = 28; means +/- SD), i.e., caused an approximate threefold increase in the respiratory frequency. The coefficient of variation of the time between the inspiratory discharges decreased by one-half. Thus the respiratory output became more stable in response to TRH. The duration of the inspiratory discharges increased from 474 +/- 108 ms to 679 +/- 114 ms, and the amplitude decreased by 24%. An increase in the interdischarge noise on the XII nerve was recorded in the early phase of the TRH application. 3. Anatomically identified hypoglossal motoneurons (7 cells) responded to bath applied TRH with a depolarization eliciting spikes between the inspiratory potentials. The depolarization was accompanied by an increase in spontaneous excitatory synaptic activity that disappeared late during the TRH application. The duration of the inspiratory potentials was increased, indicating that the hypoglossal motoneurons received a longer duration synaptic input from the respiratory rhythm generator. 4. Type-1 inspiratory neurons showed a prolonged depolarization (3 cells), a transient depolarization (2 cells), or no change in membrane potential (2 cells) during 10 min of continued superfusion with a TRH-containing solution. The duration of the inspiratory potentials was increased during the TRH superfusion. With tetrodoxin (TTX, 1 microM) present in the superfusing solution TRH induced a prolonged depolarization (3 cells) or a transient depolarization (1 cell), demonstrating that type-1 inspiratory neurons are depolarized postsynaptically by TRH. The input resistance was not changed during the depolarizing response to TRH. 5. Type-2 inspiratory neurons showed a transient depolarization (7 cells) in response to bath-applied TRH. The duration of the inspiratory potentials was increased markedly during TRH. The transient depolarization was not the result of a postsynaptic action of TRH, because type-2 neurons (9 cells) showed no depolarization to TRH with TTX present in the superfusing solution. 6. Type-3 inspiratory neurons showed a transient depolarization (4 cells) with a partial recovery of the membrane potential late during the TRH application. The duration of the inspiratory potentials increased markedly during TRH. Four cells showed a transient depolarization with an increase in input resistance during TRH with TTX present in the superfusing solution. Thus type-3 neurons are depolarized postsynaptically by TRH. 7. We conclude that TRH increases the frequency of the respiratory rhythm in newborn mice through an action at the level of the brain stem.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Effects of the pyrethroid insecticide, deltamethrin, on respiratory modulated hypoglossal motoneurons in a brain stem slice from newborn mice.

We have studied the action of deltamethrin on respiratory modulated hypoglossal motoneurons in a brain stem slice from newborn mice. Deltamethrin depolarized the hypoglossal motoneurons, increased the background synaptic noise and reduced the frequency and amplitude of current elicited action potentials. Deltamethrin transiently increased the frequency of the respiratory rhythm. Inspiratory potentials in hypoglossal motoneurons were decreased in amplitude and increased in duration. In conclusion, deltamethrin perturbs the respiratory output from the hypoglossal nucleus through postsynaptic actions on hypoglossal motoneurons and by affecting the inspiratory synaptic drive.

Animals↗

PubMed1993

Effects of met-enkephalin on GABAergic spontaneous miniature IPSPs in organotypic slice cultures of the rat hippocampus.

The action of met-enkephalin on GABAergic spontaneous miniature IPSPs (smIPSPs) was investigated in CA1 neurons from hippocampal slice cultures. In the presence of excitatory amino acid blockers (2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline, DL-2-amino-5-phosphonovaleric acid) and TTX, a continuous high-frequency bombardment of smIPSPs was recorded. The smIPSPs were blocked by the GABAA antagonist bicuculline. The occurrence of the smIPSPs was random and their amplitude distribution was skewed toward larger smIPSPs. Met-enkephalin (10-20 microM) reversibly reduced the frequency and changed the amplitude distribution of the smIPSPs. The proportion of "large" smIPSPs was reduced, but a loss of "small" smIPSPs also contributed to the reduction in smIPSP frequency. The selective mu-receptor agonist DAGO mimicked the effect of met-enkephalin and naloxone blocked the effect of DAGO. Hyperpolarization of the neuronal membranes, produced by reducing the extracellular K+ concentration, did not reduce the frequency of the smIPSPs, nor did it block the effect of DAGO. Reduction of the extracellular concentration of Ca2+ combined with an increase in extracellular Mg2+ or the addition of Cd2+ did not reduce the smIPSP frequency, nor did it block the effect of DAGO. These results suggest that CA1 pyramidal cells of hippocampal organotypic cultures are tonically inhibited by spontaneous release of GABA, through a release mechanism that is independent of propagated sodium action potentials. Met-enkephalin and DAGO reduce the tonic inhibition by reducing the frequency of the smIPSPs, through a direct action on the presynaptic GABAergic terminals. The effect was probably not mediated by hyperpolarization of the presynaptic membrane or by modulation of presynaptic Ca2+ currents.

Animals↗

PubMed1992

Interaction between thyrotropin-releasing hormone (TRH) and NMDA-receptor-mediated responses in hypoglossal motoneurones.

The effect of thyrotropin-releasing hormone (TRH) on the responses to excitatory amino acids was investigated in hypoglossal motoneurones in an in vitro preparation of the brainstem from guinea pigs using current clamp and discontinuous single electrode voltage clamp (dSEVC). Bath application of 20-50 microM TRH markedly potentiated the response to iontophoretically applied NMDA, whereas no potentiation of the response to glutamate, aspartate or quisqualic acid was seen. Voltage clamp experiments showed that TRH did not increase the current flowing through NMDA channels, thus a direct modulatory role of TRH on NMDA channels was not a likely explanation of the potentiation. Voltage clamp studies of the current-voltage relationship showed that the potentiation of the response to NMDA and lack of potentiation of the response to quisqualic acid was a result of an interaction between the actions of TRH and the amino acids on the electroresponsive profile of the membrane. Endogenous NMDA receptor activation was produced by tetanic stimulation of the reticular formation dorsolaterally to the hypoglossal nucleus, evoking large APV sensitive EPSPs in the presence of CNQX, a non-NMDA blocker. The amplitude and duration of these potentials were increased at more positive membrane potentials in response to TRH. It is concluded that TRH can act as a neuromodulator-potentiating the response to NMDA receptor activation-simply by changing the electroresponsive properties of the membrane.

2-Amino-5-phosphonovalerate↗

PubMed1990

Excitatory effects of thyrotropin-releasing hormone (TRH) in hypoglossal motoneurons.

The effect of thyrotropin-releasing hormone (TRH) was studied in 30 hypoglossal motoneurons from brainstem slices of guinea pigs. Bath application of TRH resulted in an increase of the spontaneous excitatory synaptic activity, depolarization of the neurons, increase of the input resistance and change of the duration of the falling phase of excitatory postsynaptic potentials. The depolarizing response and membrane conductance change was the result of a direct postsynaptic action of TRH, possibly mediated by a reduction of a potassium conductance.

Action Potentials↗

PubMed1990

The effect of two lipophilic gamma-aminobutyric acid uptake blockers in CA1 of the rat hippocampal slice.

1. Drugs that increase inhibitory synaptic transmission in the central nervous system may be valuable tools in the treatment of seizures. Theoretically, substances that block the uptake of inhibitory transmitters such as gamma-aminobutyric acid (GABA) into intracellular compartments should also increase inhibition and therefore have potential value as antiepileptic drugs. However, most of these substances penetrate the blood-brain barrier poorly and have therefore until now had limited value. NO-05-0328 and NO-05-0329 are two new lipophilic GABA uptake inhibitors that readily enter the CNS from the blood. 2. We have investigated the effect of these two uptake inhibitors on the responses to exogenous GABA and on GABA-mediated inhibitory synaptic potentials in pyramidal neurones of the CA1 region in the rat hippocampal slice. 3. We found that both drugs increased the amplitude and duration of responses to exogenous GABA. Furthermore, the inhibitory synaptic potentials increased in amplitude. This increase was seen in both early and late phases of the synaptic potentials. We conclude that NO-05-0328 and NO-05-0329, at least in vitro, are more effective than older GABA uptake inhibitors such as nipecotic acid and they therefore deserve consideration for clinical use.

Animals↗

PubMed1989

Evidence for a persistent sodium conductance in neurons from the nucleus prepositus hypoglossi.

Intracellular recordings were made from 39 neurons in a slice preparation of the prepositus hypoglossi nucleus from guinea pigs. Morphological characteristics were confirmed by dying neurons with Lucifer yellow. The neurons were spontaneously active, firing in the range of 8-50 spikes/s. Spike duration was short (0.32 ms) and the spikes were followed by fast and slow afterhyperpolarizations. The current vs frequency relationship was linear during steady state firing, but showed dual firing ranges corresponding to the first, third and fifth interspike interval. The instantaneous frequency of the first few interspike intervals could reach 500 spikes/s. Depolarizing and hyperpolarizing responses to square pulses displayed initial sag and rebound responses sensitive to extracellular Cs+, pharmacologically classifying the responses as a result of a Q-like current. Substitution of Ca2+ in the medium with the inorganic calcium blockers Mn2+ or Co2+ resulted in oscillatory firing, depolarizing excursions being sensitive to tetrodotoxin (TTX). Mn2+ or Co2+ in combination with extracellular Cs+ elicited TTX-sensitive plateau potentials, blocked in Na+ -free solution. In conclusion, the prepositus neurons displayed spontaneous activity in the slice preparation and active membrane properties above as well as below the threshold of the action potential. In addition, the prepositus neurons possess a persistent sodium conductance that can be uncovered by inorganic calcium blockers. It may be involved in sustaining the spontaneous discharge.

Action Potentials↗

PubMed1989

Electrophysiological properties of hypoglossal motoneurons of guinea-pigs studied in vitro.

Intracellular recordings were made from the hypoglossal nuclear complex in brain slices from guinea-pigs. Retrograde transport of horseradish peroxidase from the tongue confirmed the identity of the visually identified hypoglossal nucleus. Eighteen neurons were stained by intracellular electrophoresis of Lucifer Yellow through the recording pipette. Two types of neurons were encountered, motoneurons with maximal discharge rates of 90 Hz and another type with maximal discharge rates of 250 Hz. Motoneurons were prevalent in the hypoglossal nucleus and the other type prevailed in the adjoining nucleus prepositus hypoglossi. In both nuclei the two types were mixed. Antidromic spikes elicited from hypoglossal root fibres had initial segment and somatodendritic components. Electrical stimulation of the reticular matter dorsolateral to the hypoglossal nucleus elicited excitatory postsynaptic potentials and strychnine sensitive inhibitory postsynaptic potentials. Motoneurons responded to depolarizing current pulses with a train of spikes. The initial spike interval was much shorter than the rest and fast adaptation occurred over three to four intervals. Slow adaptation was most prominent when the neuron was depolarized and discharged at a high rate. High threshold calcium spikes were evoked by depolarizing pulses when sodium spikes were blocked by tetrodotoxin and the potassium conductance reduced by tetraethylammonium bromide. Motoneurons discharged in a single range, inflections on the frequency-current plot being absent. Spikes and spike trains evoked by depolarizing pulses were followed by afterhyperpolarizations with fast and slow parts. The fast phase was eliminated by tetraethylammonium bromide, possibly because the delayed rectifier was involved. A calcium dependent potassium conductance was probably involved in the slow phase, because it was sensitive to inorganic calcium blockers. The amplitude of the afterhyperpolarization following trains of spikes depended on the frequency of the preceding spikes. At constant frequency, the amplitude depended, in addition, on the strength of stimuli arising from different hyperpolarized potentials. Afterdepolarizing potentials were absent. Lissajous plots of double ramp current stimulation showed anomalous rectification between resting potential and spike threshold. The rectification was sensitive to inorganic calcium blockers. Subthreshold responses showed initial sags and rebound responses in all healthy cells and these were eliminated by caesium. Barium, substituted for calcium, unleashed a depolarizing plateau potential sensitive to tetrodotoxin, indicating the presence of a persistent sodium conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗