William N. Goolsby

Effects of anisotropy on the development of cardiac arrhythmias associated with focal activity

Abstract. The anisotropy that normally exists in the myocardium may be either enhanced in peri-infarction zones by loss of lateral cell connections or reduced by redistribution of gap junctions. To test how the degree of anisotropy affects the development of ectopic focal activity, we carried out computer simulations in which a model of an ectopic focus is incorporated as the central element of a two-dimensional sheet of ventricular cells. At low values of intercellular coupling conductance (Gc), the focus region is spontaneously active, but the limited intercellular current flow inhibits propagation. At high Gc, automaticity is suppressed by the loading effects of the surrounding cells. At intermediate Gc, the ectopic activity may propagate into the sheet. In the case of isotropic coupling, the minimum size of the focus region for propagation to occur (in terms of number of collaborating cells within the focus) is as small as approximately ten cells, and this number decreases with increasing anisotropy. Thus, the presence of anisotropy facilitates the development of ectopic focal activity. We conclude that the remodeling that occurs in peri-infarction zones may create a substrate that either facilitates (enhanced anisotropy) or inhibits (reduced anisotropy) the development of cardiac arrhythmias associated with ectopic focal activity.

Experimental Model for an Ectopic Focus Coupled to Ventricular Cells

BACKGROUND We used a mathematical model of a sinoatrial nodal cell (SAN model) electrically coupled to real ventricular cells (VCs) to investigate action potential conduction from an automatic focus. METHODS AND RESULTS Since input resistance of a VC is less than that of an SAN cell, coupling of the SAN model, with a size factor of 1, to a VC produced either (1) spontaneous pacing at the slower rate of the SAN model but without driving (activation) of the VC for lower values of coupling conductance (Gj) or (2) inhibition of pacing of the SAN model by electrical coupling to the VC for higher values of Gj. When the SAN model was adjusted in size to be 3 to 5 times larger than a sinoatrial nodal cell, thus making effective SAN model capacitance 3 to 5 times larger and input resistance 3 to 5 times smaller, the SAN model propagated activity to the coupled VC for Gj above a critical value. When the VC was paced at 1 Hz, the coupled cell pair demonstrated a stable rhythm of alternating cycle lengths and alternating conduction directions. By increasing pacing frequency to 2 Hz, we converted this rhythm to a regular 2-Hz frequency in which each action potential originated in the VC. More complex periodic interactions were observed at intermediate cycle lengths and lower or higher values of Gj. CONCLUSIONS The phenomena we observed demonstrate the critical role of the size of an automatic focus as well as the coupling in the propagation of activity from the focus into surrounding myocardium.

Optically-Induced Neuronal Activity Is Sufficient to Promote Functional Motor Axon Regeneration In Vivo.

Peripheral nerve injuries are common, and functional recovery is very poor. Beyond surgical repair of the nerve, there are currently no treatment options for these patients. In experimental models of nerve injury, interventions (such as exercise and electrical stimulation) that increase neuronal activity of the injured neurons effectively enhance axon regeneration. Here, we utilized optogenetics to determine whether increased activity alone is sufficient to promote motor axon regeneration. In thy-1-ChR2/YFP transgenic mice in which a subset of motoneurons express the light-sensitive cation channel, channelrhodopsin (ChR2), we activated axons in the sciatic nerve using blue light immediately prior to transection and surgical repair of the sciatic nerve. At four weeks post-injury, direct muscle EMG responses evoked with both optical and electrical stimuli as well as the ratio of these optical/electrical evoked EMG responses were significantly greater in mice that received optical treatment. Thus, significantly more ChR2+ axons successfully re-innervated the gastrocnemius muscle in mice that received optical treatment. Sections of the gastrocnemius muscles were reacted with antibodies to Synaptic Vesicle Protein 2 (SV2) to quantify the number of re-occupied motor endplates. The number of SV2+ endplates was greater in mice that received optical treatment. The number of retrogradely-labeled motoneurons following intramuscular injection of cholera toxin subunit B (conjugated to Alexa Fluor 555) was greater in mice that received optical treatment. Thus, the acute (1 hour), one-time optical treatment resulted in robust, long-lasting effects compared to untreated animals as well as untreated axons (ChR2-). We conclude that neuronal activation is sufficient to promote motor axon regeneration, and this regenerative effect is specific to the activated neurons.

Use of electric field sensors for recording respiration, heart rate, and stereotyped motor behaviors in the rodent home cage

BACKGROUND Numerous environmental and genetic factors can contribute significantly to behavioral and cardiorespiratory variability observed experimentally. Affordable technologies that allow for noninvasive home cage capture of physio-behavioral variables should enhance understanding of inter-animal variability including after experimental interventions. NEW METHOD We assessed whether EPIC electric field sensors (Plessey Semiconductors) embedded within or attached externally to a rodents home cage could accurately record respiration, heart rate, and motor behaviors. COMPARISON WITH EXISTING METHODS Current systems for quantification of behavioral variables require expensive specialty equipment, while measures of respiratory and heart rate are often provided by surgically implanted or chronically affixed devices. RESULTS Sensors accurately encoded imposed sinusoidal changes in electric field tested at frequencies ranging from 0.5-100Hz. Mini-metronome arm movements were easily detected, but response magnitude was highly distance dependent. Sensors accurately reported respiration during whole-body plethysmography. In anesthetized rodents, PVC tube-embedded sensors provided accurate mechanical detection of both respiratory and heart rate. Comparable success was seen in naturally behaving animals at rest or sleeping when sensors were attached externally. Video-verified motor behaviors (sniffing, grooming, chewing, and rearing) were detectable and largely separable by their characteristic voltage fluctuations. Larger movement-related events had comparably larger voltage dynamics that easily allowed for a broad approximation of overall motor activity. Spectrograms were used to quickly depict characteristic frequencies in long-lasting recordings, while filtering and thresholding software allowed for detection and quantification of movement-related physio-behavioral events. CONCLUSIONS EPIC electric field sensors provide a means for affordable non-contact home cage detection of physio-behavioral variables.

Modulation of propagation from an ectopic focus by electrical load and by extracellular potassium concentration

Cardiac arrhythmias may be due to spontaneous automatic activity from an ectopic focus. We have developed a technique in which a computer model of an ectopic focus (represented by an SA Node cell model), running in real time, can be coupled by a variable conductance, G/sub c/, to a real ventricular cell. We used this technique to investigate the effects of G/sub c/, cell size, and elevated potassium on the ability of an ectopic focus to successfully drive the ventricular cell. For the hybrid cell pair there are three possible outcomes in the steady state: (1) pacing of the SAN model cell but not driving of the ventricular cell, (2) cessation of pacing, or (3) successful pacing of the SAN model cell and driving of the ventricular cell. Elevation of potassium concentration increases both the lower and upper bound of the values of G/sub c/ which define the successful pacing and driving range.

MEMBRANE NOISE AND EXCITABILITY

MACROCHAN is a program that simulates the macroscopic behavior of biological membranes from the microscopic properties of ion channels. We illustrate the use of this program by constructing nerve and heart action potentials from simple, stochastic models of K, Na, and Ca channels. MACROCHAN is available on request.

Slow Breathing Can Be Operantly Conditioned in the Rat and May Reduce Sensitivity to Experimental Stressors

In humans, exercises involving slowed respiratory rate (SRR) counter autonomic sympathetic bias and reduce responses to stressors, including in individuals with various degrees of autonomic dysfunction. In the rat, we examined whether operant conditioning could lead to reductions in respiratory rate (RR) and performed preliminary studies to assess whether conditioned SRR was sufficient to decrease physiological and behavioral responsiveness to stressors. RR was continuously monitored during 20 2-h sessions using whole body plethysmography. SRR conditioned, but not yoked control rats, were able to turn off aversive visual stimulation (intermittent bright light) by slowing their breathing below a preset target of 80 breaths/min. SRR conditioned rats greatly increased the incidence of breaths below the target RR over training, with average resting RR decreasing from 92 to 81 breaths/min. These effects were significant as a group and vs. yoked controls. Preliminary studies in a subset of conditioned rats revealed behavioral changes suggestive of reduced reactivity to stressful and nociceptive stimuli. In these same rats, intermittent sessions without visual reinforcement and a post-training priming stressor (acute restraint) demonstrated that conditioned rats retained reduced RR vs. controls in the absence of conditioning. In conclusion, we present the first successful attempt to operantly condition reduced RR in an animal model. Although further studies are needed to clarify the physio-behavioral concomitants of slowed breathing, the developed model may aid subsequent neurophysiological inquiries on the role of slow breathing in stress reduction.

Propagation of the cardiac action potential from an ectopic focus into a two-dimensional sheet of ventricular cells

We have carried out computer simulations in which a model of an ectopic focus is incorporated as the central element of a two-dimensional sheet of ventricular cells in which the coupling conductances may be different in the X and Y directions and a specific region of lack of coupling conductance may serve as a resistive barrier. We determined the critical size of the central element for successful propagation of its action potential into the sheet and found that this critical size was decreased when anisotropy was present compared to the isotropic case and was further decreased when the central site of stimulation was close to the resistive barrier. We conclude that the normal existence of anisotropy and enhancement of the degree of anisotropy under pathophysiological conditions may play a facilitating role in the development of ectopic foci which may lead to cardiac arrhythmias.