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Biomedical subjects

B Walmsley

Publications and source records attributed to B Walmsley.

At least 19 recordsLinked to original sources
PubMed1995

Interpretation of 'quantal' peaks in distributions of evoked synaptic transmission at central synapses.

At some synaptic connections in the central nervous system, amplitude distributions of evoked synaptic currents exhibit surprisingly sharp and regularly spaced peaks. At these connections, detailed analysis of the peaks has led to the proposal that the 'quantal' synaptic current displays very little variability, not only at a release site, but also between release sites. In this study the latter hypothesis has been tested using simulations of evoked transmission. In contrast with previous conclusions, these simulations demonstrate that the experimental observation of regularly spaced peaks in amplitude distributions of synaptic currents is compatible with large underlying differences in the synaptic current amplitudes between release sites. The simulations also reveal that quantal analysis based entirely on the observation and analysis of regularly spaced peaks in evoked synaptic current amplitude distributions, cannot be used with confidence to estimate presynaptic release probabilities, 'quantal' current amplitudes at each release site, or the total number of available release sites. This problem may be a confounding factor in determining whether pre- or postsynaptic changes underlie alterations in synaptic efficacy, such as occurs during long term potentiation.

Animals↗
PubMed1995

Counting quanta: direct measurements of transmitter release at a central synapse.

Contradictory hypotheses regarding the nature of synaptic transmission in the CNS have arisen from indirect methods of quantal analysis. In this study, we directly count the quanta released following nerve stimulation to examine synaptic transmission at a fast glutamatergic synapse in the mammalian auditory brainstem. Our results demonstrate the relationship between spontaneous and nerve-evoked synaptic events, indicate that asynchronous transmitter release governs the time course of evoked transmission, and show that the stochastic quantal release process, as originally proposed at the neuromuscular junction, is highly conserved at this central synapse.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1995

Receptors underlying excitatory synaptic transmission in slices of the rat anteroventral cochlear nucleus.

1. The anteroventral cochlear nucleus (AVCN) contains two principal cell types that receive input from the auditory nerve. Stellate cells receive conventional synapses on their dendrites, and bushy cells of the AVCN receive axosomatic input via large, calyceal terminals (the end bulbs of Held). We have used whole cell patch-clamp recording techniques to study excitatory postsynaptic currents (EPSCs) in these two principal cells of the rat AVCN. 2. EPSCs evoked in stellate cells by stimulation of the auditory nerve were graded with stimulus strength, indicating a high degree of convergence of input to these cells. At depolarized membrane potentials, EPSCs evoked in stellate neurons had a dual-component time course. The slow component was blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV), and the fast component was abolished by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 3. EPSCs evoked in bushy cells by auditory nerve stimulation were large (50 nS average conductance) and all-or-none at the threshold stimulus level. At -70 mV, the time course of the EPSC was very brief (average time constant of decay 700 microseconds at room temperature). Membrane depolarization revealed a slow component to the EPSC. The fast and slow components were mediated by non-NMDA and NMDA receptors, respectively. The switch-off of end bulb NMDA EPSCs by voltage jumps to the EPSC reversal potential was very rapid, suggesting that the NMDA component arises from sites on or close to the soma. 4. Miniature EPSCs, recorded in the presence of tetrodotoxin (TTX) at depolarized potentials, also had a dual-component time course. The fast and slow components of the miniature EPSCs were blocked by CNQX and APV, respectively. This result indicates that NMDA and non-NMDA receptors can be co-localized at the same, presumably end bulb, release sites. 5. The relative contribution of the slow, NMDA component to the end bulb EPSC declined significantly with age (postnatal days 11-22). 6. These results indicate that both NMDA and non-NMDA receptors underlie excitatory synaptic transmission in the AVCN of young rats. The end bulb synapse onto bushy cells generates a non-NMDA receptor-mediated EPSC with very fast kinetics. NMDA receptors can also mediate synaptic transmission at the end bulb synapse, but their contribution becomes less as the auditory system matures. This finding suggests that NMDA receptors may play an important role in the development of this synapse.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1995

Serial E-M and simulation study of presynaptic inhibition along a group Ia collateral in the spinal cord.

1. A muscle spindle primary afferent (group Ia) was physiologically identified and labeled intracellularly with the use of horseradish peroxidase (HRP) in the cat lumbar spinal cord. Serial-section electron microscopy (EM) was used to examine and reconstruct an entire axon collateral and its branches within Clarke's column. In the present study the existence and location of presynaptic contacts on Ia afferent boutons along these collateral branches were determined from examination of the serial-section electron-micrographs. 2. Of 36 Ia boutons examined in serial sections along the branches of the same collateral, 3 presynaptic contacts were found. Two of these contacts were made with Ia boutons in a complex nodal region consisting of two unmyelinated side branches exhibiting a total of six Ia boutons. The other presynaptic contact was made with a Ia bouton in a nodal region consisting of two Ia boutons connected by a thin unmyelinated bridge. 3. Computer simulations, based directly on the serial-section-reconstructions, were used to investigate the possible effects of these presynaptic contacts on membrane potential and on a propagating action potential along the Ia collateral. The effect of a presynaptic contact was modeled by a sustained gamma-aminobutyric acid-A (GABAA)-activated chloride conductance. 4. The simulation results indicated that the effect of a presynaptic contact on membrane potential and action-potential amplitude is likely to extend beyond the contacted bouton to other boutons occurring along the short unmyelinated branches arising from the same node.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1994

An in vivo pharmacological study of single group Ia fibre contacts with motoneurones in the cat spinal cord.

1. Direct experimental evidence was obtained on the spatial distribution of active synaptic contacts from single Ia muscle afferents on the dendrites of lumbosacral motoneurones in anaesthetized cats. 2. An extracellular micropipette was used to pressure eject the AMPA/kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) in close proximity to the intracellular recording site, in order to create an extracellular concentration gradient of the antagonist. The effect of antagonist ejection on the time course and amplitude of excitatory postsynaptic potentials (EPSPs) evoked in motoneurones by impulses in single group Ia fibres was examined. 3. Pressure ejection of NBQX resulted in a complete block of the monosynaptic group Ia EPSP in two cells, and a significant reduction to 23-57% of control EPSP peak amplitudes in a further six cells (mean, 27%; n = 8). These effects were not associated with changes in membrane potential or membrane time constant. 4. The reduction in amplitude of these single group Ia fibre EPSPs following ejection of NBQX was usually accompanied by a pronounced slowing in the time course of the EPSPs. On average, the EPSP rise times and half-widths were increased by 269 and 37%, respectively. This is most probably due to a considerable spatial spread of the synaptic contacts along the dendrites of motoneurones, with the most proximal synaptic contacts (producing the briefest synaptic potentials) subjected to a greater reduction in amplitude due to a higher local antagonist concentration. 5. An equivalent dendritic cable model of the motoneurone was used to interpret the observed changes in the time course of single fibre EPSPs. The time course of control single fibre EPSPs examined in the present study could be well matched using the cable model and assuming a single location for synaptic input. The observation of a slowed EPSP time course following antagonist ejection indicated that this assumption was not correct and that there was in fact considerable spatial spread in the synaptic contacts arising from these single afferent fibres. These results provide direct evidence that spatial spread of synaptic input may not be detected using the time course of a synaptic potential in conjunction with a neuronal cable model of the postsynaptic cell.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1991

A serial section electron microscope study of an identified Ia afferent collateral in the cat spinal cord.

Serial section electron microscopy has been used to examine a horseradish peroxidase (HRP)-labelled group Ia afferent collateral from its entry point in the grey matter to its termination in Clarke's column of the cat spinal cord. A wide range of geometries and myelination patterns were identified along the collateral, including 1) nodes specialized to exhibit a single synaptic bouton, 2) nodes specialized to exhibit two or more synaptic boutons connected by fine, unmyelinated lengths of the collateral, 3) terminal heminodes, along which boutons were separated by unmyelinated branches, and 4) complex arrangements along which myelinated and unmyelinated branches gave rise to one or more boutons. Thirty-six synaptic boutons of varied shape and size were exhibited by this collateral. Previous studies have shown that the geometry, branching, and myelination pattern of an axon play an important role in determining the amplitude and duration of an action potential propagating along that axon. In turn, the amplitude and duration of a presynaptic action potential influence the efficacy of transmitter release. The varied axonal geometries and myelination patterns observed in the present study provide further evidence in support of our previous proposal that there may be considerable nonuniformity in the efficacy of synaptic transmission among release sites arising from the same primary afferent fiber.

Afferent Pathways↗
PubMed1991

Calcium facilitation of group Ia EPSPs evoked in cat spinal motoneurones in vivo.

The extracellular environment of motoneurones in the cat spinal cord in vivo was altered by means of local perfusion of the central canal. Intracellular recordings were made to determine the effects of raised extracellular Ca2+ or Mg2+ concentration on the monosynaptic afferent excitatory postsynaptic potential (EPSP). Raised extracellular Mg2+ concentration reversibly reduced the EPSP amplitude, whereas raised extracellular Ca2+ concentration produced extremely large increases in the monosynaptic EPSP amplitude, up to almost an order of magnitude. In some cases, a reduction in amplitude of the EPSP and a delay in its onset were also observed, following raised extracellular Ca2+ concentration. This effect was thought to be due to a divalent cation block of the presynaptic action potential. A major conclusion from this study is that group Ia afferent terminals have a much greater transmitter release capacity than suggested by previous studies at this connection.

Animals↗
PubMed1991

The effects of Ca2+, Mg2+ and kynurenate on primary afferent synaptic potentials evoked in cat spinal cord neurones in vivo.

1. A technique was developed for perfusing the central canal of the cat spinal cord over a defined region to alter the extracellular environment and examine the effects of various ions and pharmacological agents on synaptic transmission in vivo. 2. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by hindlimb muscle nerve stimulation were recorded intracellularly from dorsal spinocerebellar tract (DSCT) neurones in Clarke's column, in close proximity to the central canal. 3. The effects of central canal perfusion of solutions containing Ca2+, Mg2+ and kynurenate on the monosynaptic afferent EPSP were examined. 4. Perfusion of the central canal with solutions containing a high Mg2+ concentration completely and reversibly blocked the monosynaptic EPSP, while perfusion with solutions containing a high Ca2+ concentration produced up to a fourfold increase in the peak amplitude of the EPSP. This large increase in the EPSP indicates that the pool of quanta available for release is considerably greater than estimated from previous quantal analysis studies at this synaptic connection. 5. Perfusion of the central canal with kynurenate, an antagonist at excitatory amino acid receptors, resulted in a complete block of the monosynaptic EPSP in DSCT neurones. This provides direct evidence that an excitatory amino acid, such as glutamate, is released from primary muscle afferent terminals in Clarke's column of the cat spinal cord in vivo.

Animals↗
PubMed1990

Location and morphology of dorsal spinocerebellar tract neurons that receive monosynaptic afferent input from ankle extensor muscles in cat hindlimb.

1. The present experiments were carried out to investigate the morphology and somatotopic location of dorsal spinocerebellar tract (DSCT) neurons that receive monosynaptic group 1 afferent input from hindlimb ankle extensor muscles in the cat. 2. Intracellular recordings were obtained from DSCT neurons throughout the rostrocaudal extent of the L3 dorsal root entry zone of the spinal cord. DSCT neurons, physiologically identified as receiving monosynaptic group I input from the ankle extensor muscles, were injected with horseradish peroxidase (HRP) and subsequently reconstructed under the light microscope. 3. In contrast to previous HRP studies of DSCT neurons, these cells were found to have extremely extensive and complex dendritic trees, that often extend beyond the region of Clarke's column. Dendrites were found to extend into the white matter of the dorsal columns, and/or into the spinal gray matter in a ventrolateral direction. The large dendritic spread of DSCT neurons was found to occupy up to 60% or more of the cross-sectional area of Clarke's column. 4. DSCT neurons receiving monosynaptic group I input from the single functional group of ankle extensor muscles were not found to be confined within a specific transverse region of Clarke's column, in contrast to a previous proposal. Instead, these cells could be found throughout Clarke's column. 5. The present results demonstrate that DSCT neurons, physiologically identified as receiving group I muscle afferent input, exhibit dendritic trees that are considerably more extensive and morphologically complex than indicated by previous studies. In addition, the present results do not support a previous proposal of a strict somatotopic arrangement for DSCT neurons and their dendritic envelopes within Clarke's column in the transverse plane.

Afferent Pathways↗
PubMed1989

Synaptic potentials evoked in cat dorsal spinocerebellar tract neurones by impulses in single group I muscle afferents.

1. Excitatory postsynaptic potentials (EPSPs) evoked by impulses in single group I muscle afferents were recorded intracellularly in dorsal spinocerebellar tract (DSCT) neurones in the spinal cords of anaesthetized cats. 2. In the same experiments, electrotonic membrane properties of DSCT neurones were measured using the voltage response of each cell to a brief intracellular current pulse. 3. Single group I fibre EPSPs were found to exhibit a large range of amplitudes, from 210 microV to 3.4 mV. All of these EPSPs exhibited uniformly rapid rise times, in contrast to the wide range of time courses exhibited by group I a EPSPs recorded in motoneurones. 4. Electrotonic analysis of DSCT neurones indicated that the time constants of these cells ranged from 5.9 to 18.2 ms, with an average value of 10.9 ms. 5. Current pulse responses of the majority (approximately three-quarters) of DSCT neurones were well described by a simple cable model. Equivalent dendritic cable lengths were calculated for DSCT neurones and found to have an average value of 1.0 space constants, which is considerably less than that calculated for motoneurones. 6. Application of the simple cable model of DSCT neurones demonstrated that the rapid rise-times of single group I EPSPs can be explained by a substantial somatic input to these cells. However, in addition to this strong somatic component, there may also be a contribution from dendritic synapses which prolong the initial decay phase of these EPSPs. The final decay of single fibre EPSPs in DSCT neurones is explained simply by the passive membrane time constant of these cells.

Action Potentials↗
PubMed1989

Effects of spatial and temporal dispersion of synaptic input on the time course of synaptic potentials.

1. As part of the ongoing studies on the time course of single-fiber synaptic potentials recorded in spinal neurons, a theoretical analysis of the effects of spatial and temporal dispersion of synaptic input to a neuronal cable model was undertaken. 2. Results were obtained using a simple R-C soma, equivalent dendritic cylinder cable model of a neuron. Synaptic input was represented by a current injection at various points on the dendritic cable. 3. Spatial dispersion of multiple inputs to the cable model generally produced somatic transients with smooth time courses that could be closely matched by a transient generated at a single input location, usually with a different current time course. 4. Temporal dispersion, representing nonsynchronous activation of multiple synaptic contacts at the same electrotonic location, generally resulted in somatic transients with an increased rise-time and a corresponding small increase in the half-width. The somatic transient generated by these temporally dispersed inputs could usually be well matched by a single input at a different location. 5. Addition of temporal dispersion to a spatially dispersed input produced variable results in which the rise-times and half-widths of somatically recorded transients could be either increased or decreased. 6. It is concluded that a detailed knowledge of both the spatial and temporal properties of synaptic input is essential to the interpretation of single-fiber synaptic potentials. Previous results on the amplitude and time course of single-fiber synaptic potentials recorded in spinal neurons are discussed in light of the present observations.

Animals↗
PubMed1988

Spinocerebellar neurones in the guinea pig--a morphological study.

The morphology and distribution of spinocerebellar neurones were examined in the guinea pig. Horseradish peroxidase was injected into the cerebellum, and after a survival time of 72 h retrogradely labelled cells were examined in the spinal cord. The distribution of spinocerebellar cells was similar to that previously demonstrated in the rat. Three major groups of neurones were distinguished: the central cervical nucleus (C1-C2), Clarke's column (T2-L3) and spinal border cells (L3-L6). Neurones in the central cervical nucleus were multipolar, had mean equivalent diameters of about 24 microns, and their axons ascended on the contralateral side of the spinal cord. Neurones in Clarke's column were spindle-shaped, approximately 25 X 35 microns, and their axons ascended on the ipsilateral side of the spinal cord. Spinal border cells were multipolar, with mean equivalent diameters of about 35 microns; their axons were predominantly crossed.

Animals↗
PubMed1988

Nonuniform release probabilities underlie quantal synaptic transmission at a mammalian excitatory central synapse.

1. Excitatory postsynaptic potentials (EPSPs) evoked by impulses in single group I muscle afferents were recorded in dorsal spinocerebellar tract (DSCT) neurons in the spinal cords of anesthetized cats. Fluctuations in the amplitude of these single-fiber EPSPs were determined from measurements of EPSP peak amplitude and contaminating noise (800-4600 trials). 2. In a previous study at this connection, we found that these single-fiber EPSPs fluctuated in amplitude between approximately equal, or quantal, increments. However, these quantal fluctuations could not be described by simple binomial statistics (39). In the present study we have applied further analysis procedures to the same single-fiber EPSPs to formulate a more appropriate probabilistic model of transmission at this connection. 3. In the first stage we have demonstrated that each single-fiber EPSP is composed of the sum of a number (3-30) of uniform quantal events, and that there is extremely little variability in the amplitude of the single quantal event. 4. In a further procedure, we have demonstrated that these quantal fluctuations can be described by a compound binomial model in which each underlying quantal event is associated with a particular, but independent, release probability. The results of this analysis indicate that the probability of transmitter release varies considerably between release sites at this connection. (The use of such a compound binomial model reemphasized previous warnings concerning the interpretation of the results of all statistical models of quantal release. Problems regarding the non-unique nature of N, the total population of quantal events, and other such difficulties are discussed.) 5. A model of transmission at this connection is proposed, in which there are a number of "active" release sites, exhibiting generally high release probabilities, and a number of "reserve" release sites, with zero, or close to zero, release probability. The physiological consequences of such a scheme are discussed.

Afferent Pathways↗
PubMed1987

A preparation for patch clamp studies of labelled, identified neurones from guinea pig spinal cord.

Details are described of techniques which allow the isolation of labelled, identified neurones suitable for patch clamp recording from the guinea pig spinal cord. Fluorescent labels, injected into either the hindlimb muscles or the cerebellum, are retrogradely transported to motoneurones or dorsal spinocerebellar tract neurones respectively. Cells are then enzymatically dissociated from spinal cord slices and identified using fluorescence microscopy. Patch clamp or whole cell recordings are then made.

Afferent Pathways↗
PubMed1987

Intracellular recording of identified dorsal spinocerebellar tract neurones from guinea pig spinal cord in vitro.

Recent studies on the monosynaptic connection between primary afferent fibres and dorsal spinocerebellar tract (DSCT) neurones in the spinal cord of anaesthetized cats have been undertaken to investigate the mechanisms of excitatory synaptic transmission in the mammalian central nervous system. The need to extend these observations to a study of the effects of changes in the extracellular environment of DSCT neurones prompted us to develop the in vitro preparation described in this paper. We have developed an isolated spinal cord preparation from young guinea pigs, in which DSCT neurones can be identified and recorded from intracellularly. The spinal cord can be maintained in vitro for at least 7 hours. This preparation is sufficiently stable to allow intracellular penetration of DSCT cells in excess of 2 h, with resting membrane potentials of -65 mV obtainable. Reconstruction of neurones stained with horseradish peroxidase confirmed their location in Clarke's column. The dendritic morphology of the reconstructed DSCT neurones was found to be similar to DSCT neurones described in the cat spinal cord. Since the motor system of the guinea pig is quite advanced at birth, this preparation should provide a valid extension to in vivo results on neuronal membrane properties and synaptic potentials in DSCT neurones.

Afferent Pathways↗
PubMed1987

The probabilistic nature of synaptic transmission at a mammalian excitatory central synapse.

The synaptic connection between single group I afferents and dorsal spinocerebellar tract (DSCT) neurons in the cat spinal cord has been studied in an attempt to gain insight into the mechanisms of excitatory synaptic transmission in the mammalian CNS. Fluctuations in the amplitude of single group I fiber EPSPs in DSCT neurons were examined using a numerical deconvolution procedure to reduce the effects of contaminating noise. In general, it was found that single fiber EPSPs fluctuate in peak amplitude between discrete levels separated by equal or quantal increments. Many previous studies have proposed simple binomial statistics as a general model of quantal synaptic transmission. In the present study we show that simple binomial statistics do not describe the fluctuations in amplitude of single group I fiber EPSPs in DSCT neurons. It is suggested that nonuniformities in the probability of transmitter release from release site to release site explain the failure of the binomial model to describe the EPSP fluctuation pattern at this synapse. Nonuniform quantal transmission is proposed as a more adequate description of excitatory synaptic transmission in the mammalian CNS.

Animals↗
PubMed1987

Ultrastructural evidence related to presynaptic inhibition of primary muscle afferents in Clarke's column of the cat.

As part of an investigation on excitatory synaptic transmission in the mammalian CNS, we have examined ultrastructural details of the synaptic connection between primary afferent fibers and dorsal spinocerebellar tract (DSCT) neurons in Clarke's column of the cat spinal cord. Single primary muscle afferents (group Ia and Ib) and DSCT neurons were identified and stained intracellularly with HRP. The terminations of these afferent fibers were examined in serial sections under the EM. Five of 6 Ib boutons and 1 of 14 Ia boutons were contacted by small presynaptic boutons. An example was illustrated in which only 1 out of 7 boutons arising from the same Ia fiber and contacting the same postsynaptic DSCT neuron was contacted by a presynaptic bouton. It is likely that the presynaptic contacts are responsible for presynaptic inhibition of synaptic transmission between primary afferents and DSCT neurons. We have proposed that the observed differences in presynaptic contacts from bouton to bouton may be one of the causes of a nonuniformity in the probability of transmitter release between release sites at this connection.

Animals↗