What's in store for imaging workstations?
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Arteriosclerosis↗
Biomedical subjects
M Merickel
Publications and source records attributed to M Merickel.
Evidence for an alteration of the tonotopic map in the gerbil cochlea during development.
We have investigated developmental alterations in the tonotopic projection of the gerbil lateral superior olive. Single neurons were characterized in the frequency domain and the recording site marked with fast green. Transverse tissue sections from the auditory brainstem of each animal were visualized with a video-equipped microscope, and the image was digitized for subsequent alignment. The three-dimensional display indicated little variation in the rostrocaudal axis, allowing us to collapse the data into a two-dimensional tonotopic map. The tonotopic map was found to change with age such that the characteristic frequency of neurons in a given anatomical location became successively higher during development. These results are consistent with the hypothesis that the place code gradually shifts in the developing cochlea.
Acoustic Stimulation↗
Electrophysiology of human muscle in culture.
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Cell Differentiation↗
Cultured muscle from myotonic muscular dystrophy patients: altered membrane electrical properties.
Myotonic muscular dystrophy (MyD) is an inherited human disease involving skeletal muscle as well as many other organ systems. We have approached the study of this disorder by growing normal and diseased human muscle in a primary tissue culture system and investigating some of the electrical properties of the resulting myotubes. The most distinctive abnormality noted in MyD myotubes was an increased tendency to fire repetitive action potentials. A decreased action potential afterhyperpolarization amplitude and the presence of depolarizing afterpotentials were also noted, as were a decreased resting membrane potential, decreased action potential amplitude and overshoot, and decreased outward-going rectification. Although the ionic basis of these abnormal properties in vitro is not clearly defined, changes in the slow outward-going potassium current offer the best explanation. Furthermore, MyD cell culture offers a valuable model for critical analysis of the molecular mechanisms underlying MyD deficits.
Action Potentials↗
Investigation of burst generation by the electrically coupled cyberchron network in the snail Helisoma using a single-electrode voltage clamp.
This paper describes the results of investigating burst generation by the cyberchron network in the snail Helisoma. The cyberchron network is composed of aproximately 20 electrically coupled neurons and controls the feeding behavior of the snail. The electrical coupling between network members has made it particularly difficult to distinguish between the importance and involvement of single-cell and network properties in burst generation by this system. The present investigations utilized the new single-electrode voltage clamp to examine the membrane properties and network interactions of the cyberchron neurons: (1) A slow outward current is activated by moderately large depolarizing commands (-40 to 0 mV) and does not undergo inactivation decay (i.e., decline in magnitude) during a command potential step maintained for 10 sec or more. The lack of inactivation of the outward current in cyberchron neurons appears to be due to the dominating role of a Ca-dependent K current. (2) There are two functionally distinct classes of cyberchrons--current generator cyberchrons and follower cyberchrons. (3) Primary current generator cyberchrons have membrane properties similar to endogenous bursting neurons (e.g., persistent inward Ca current and negative resistance region in I-V plot) and appear to provide the main driving and timing current for the rest of the network. (4) The vast majority of cyberchrons are secondary current generator cyberchrons with membrane properties which exhibit inward-going rectification and appear to burst as a result of regenerative excitation with one another and the primary current generator cyberchrons. (5) The second class of cyberchrons are driven by the electrical synaptic input from the current generator cyberchrons, do not exhibit inward-going rectification, and are called follower cyberchrons. (6) Burst termination is due to activation of a slow outward tail current in most cyberchrons during the burst (probably Ca-activated K current) which causes a hyperpolarization in individual cyberchrons, terminating the burst. (7) Decay of the outward tail current causes the cyberchrons to depolarize, which activates the persistent inward Ca current in the primary current generator cyberchrons, starting the burst cycle anew.
Action Potentials↗
Design of a single electrode voltage clamp.
The detailed schematic diagrams and construction techniques are presented for a single microelectrode voltage clamp. The devices is used to study the membrane processes of small cells not able to be penetrated with the traditional two microelectrode system. The technique utilizes the same microelectrode alternately for current injection and membrane potential sampling on a time-sharing basis controlled by electronic switching circuitry. Current is injected in pulses and the membrane potential is sampled after an individual current pulse discharges from the microelectrode capacitance to the true membrane potential. The device can either measure the voltage response to an injected current waveform (current injection mode) or the membrane currents generated during a controlled change in membrane potential (voltage clamp mode). In voltage clamp mode, the membrane potential reaches steady-state within 2 msec (maximum time) in response to a 40 mV step command. The single electrode voltage clamp is potentially very important to the investigation of slow current processes within electrically excitable cells too small to be previously studied with traditional voltage clamp technology.
Animals↗
Single cell isolation: a way to examine network interactions.
A procedure for isolating identified, small neurons from snail ganglia is described. The technique allows a particular neuron, previously identified by morphological and electrophysiological characteristics, to be marked and then isolated from the ganglia. This procedure was developed to permit the detailed comparison of the electrical characteristics of a neuron before and after isolation from an intact system. An earlier description has appeared. The cell somata is marked intracellularly by the iontophoretic injection of Procion navy blue H3RS which visually differentiates the cell from other cells in the ganglion. The ganglion is then treated with a trypsin-haluronidase solution to soften the ganglion sheath, which is then removed. The cells are gently shaken to isolate them from the ganglion and then examined electrophysiologically. A comparison of membrane properties, such as action potential height, duration and rate of rise and decay was made before and after all treatments were applied to assess deleterious effect. An analysis of network properties, such as burst duration, number of spikes per burst and presynaptic activity was also performed after each phase of the procedure. No significant differences were noted after dye injection, enzyme treatment, and where appropriate, after isolation. An increase in input resistance and corresponding decrease in the slope of the steady state current--voltage plot (I--V plot) were observed after isolation of a cell. These were expected results of removing the 'load' (i.e. axon or electrical coupling) from the cell soma. This method may be applied to many other systems to study the effects of network interactions on the properties of a single cell and should therefore facilitate the analysis of neuronal networks as well as single cell properties.
Animals↗
Burst generation by electrically coupled network in the snail helisoma: analysis using computer simulation.
The effectiveness of electrical coupling between neurons as a mechanism for mediating single and repetitive bursts is investigated here using computer simulation. The cyberchron network in the snail Helisoma generates repetitive bursts controlling the animal's feeding behavior and served as the basic model for the simulation studies described in this paper. The action potential properties of individual neurons were modeled by the Rall equations describing generalized action potentials. Several properties of electrical coupling and its role in burst generation were demonstrated, including: (1) A neuron in an electrically coupled network can generate action potentials at a higher frequency than an isolated neuron with similar membrane properties due to the loading through the electrical junctions. However, the ability of electrically coupled neurons to generate high-frequency bursts of action potentials requires a concomitantly greater amount of driving current to overcome the junctional loading. (2) Temporal and spatial summation of synaptic input onto a neuron is maintained at its most effective level because the postsynaptic current is integrated across the long postsynaptic membrane time constant. (3) Initial simulations concentrated on a pair of electrically coupled neurons which were below threshold. Stimulation of one of the two neurons with a short pulse resulted in a reverberation or regenerative excitation between the two neurons. The reverberation terminated after a number of action potentials dependent on the specific model parameters. Similar results were obtained with a network containing a greater number (20) of model neurons if approximately one-half of the neurons were stimulated simultaneously. However, none of the cases studied produced more than a single discrete burst. (4) Simulations were also conducted on 20-neuron networks containing two subpopulations of model neurons differing in their values of coupling resistance and excitability. Some networks of this type required stimulation of only one cell to make the two subpopulations of model neurons reverberate with one another. Such simulations suggest the possibility that 'preferred' input pathways involving a small number of neurons would be capable of 'turning on' the activity of the entire network.
Animals↗
Centrally programmed feeding in Helisoma: identification and chracteristics of an electrically coupled premotor neuron network.
(1) The oscillatory network underlying centrally programmed feeding in the fresh water pulmonate, Helisoma trivolvis, was studied using intracellular recording and staining techniques. These premotor neurons have been termed cyberchron neurons. (2) Intracellular staining with Procoin yellow has allowed the construction of a soma map and tentative identification of axonal projections of the cyberchron neurons. (3) Cyberchron neurons form a tightly electrically coupled network. Coupling coefficients range from 0.15 to 0.5, and electrotonic junctions allow the passage of Procion dye from cell to cell. Electrical synapses act as low pass filters, and allow spatial and temporal summation. (4) Burst generation within the network is the result of network interaction manifest as regeneration positive feedback from neuron to neuron via attenuating electrical synapses. (5) Decreased coupling between cyberchron neurons during and immediately following a burst is observed, and is discussed as a possible mechanism for burst termination.
Action Potentials↗