The effect of proton pump inhibitors on the human microbiota.

Proton pump inhibitors (PPIs) are commonly used to treat acid-related diseases, most notably gastroesophageal reflux disease. PPIs are designed to shut down the gastric proton pump (H+/K+-ATPase) of parietal cells, thereby raising the pH of the stomach. While effective, a number of side effects have been associated with PPI use. Naturally occurring bacteria, some of which are acid-producing and contain ATPase enzymes, have also been found within the stomach, upper gastrointestinal tract, and oral cavity. Likewise, a number of fungi are known to inhabit the human body; some of these fungi contain H+-ATPase enzymes. Recent literature has suggested that PPIs may be inadvertently affecting these bacteria and fungi in two different ways: 1) PPIs may directly target the proton pumps of the bacteria and fungi, and/or 2) PPIs may indirectly affect the microenvironment of the flora via changes in pH. These unintended interactions are exasperated by the systemic distribution of PPIs throughout the body and may potentially lead to some of the side effects observed with PPI use. Herein we summarize what is currently known about the interactions between the PPIs and the natural human microbiota.

Tryptophan pyrrolase induced in human liver by hydrocortisone: effect on excretion of kynurenine.

Administration of hydrocortisone causes two- to fourfold increase in the level of activity of tryptophan pyrrolase in human liver, as measured in needle-biopsy specimens. Correlation of the higher levels of the enzyme with the amounts of urinary kynurenine suggests that the tryptophan pyrrolase level, which is regulated by adrenocortical hormones, may be the important variable in the increased excretion of tryptophan metabolites that accompanies various diseases.

Prolonged ethanol consumption increases testosterone metabolism in the liver.

Male alcoholics often suffer from features of hypogonadism related to abnormal metabolism of sex steroids. Since the activity of testosterone reductases is rate limiting for testosterone metabolism in the liver, the effect of prolonged ethanol consumption by rats and human volunteers on the activities of these microsomal and cytosolic enzymes was studied. In rats, long-term ethanol ingestion doubled microsomal 5alpha-testosterone reductase activity, a major pathway for testosterone metabolism, while in human volunteers the activity was increased two- to fivefold. These changes may play a role in the altered androgenic activity of the chronic alcoholic.

Forehead reduction and orbital contouring in facial feminisation surgery for transgender females.

Forehead reduction and orbital contouring form a considerable component of the procedures available to feminise the face in transgender females. In this paper I shall discuss the history and development of techniques to reduce bossing of the forehead and contour the orbits, and describe their classification, assessment, surgical approach, and complications.

Are vaccine strain, type or administration protocol risk factors for canine parvovirus vaccine failure?

Canine parvovirus (CPV) is a highly contagious and worldwide cause of serious and often fatal disease in dogs, despite the widespread availability of vaccines. Which vaccine-related factors are associated with vaccination failure is largely unknown, and there are no reports from Australia. In this study - the first national population-level CPV study of its kind ever conducted - we analysed data on 594 cases of apparent CPV vaccination failure reported from an Australian national surveillance system to determine whether vaccine strain, type or administration protocol are risk factors for vaccination failures. The strain of CPV used in vaccine manufacture was not significantly associated with vaccination failure in clinical practice. The vaccine type (killed versus attenuated vaccine) for puppies diagnosed with CPV was associated with a lower mean age at time of vaccination (P=0.0495). The age at administration of the last CPV vaccination a puppy received prior to presenting with disease was a significant (P=0.0334) risk factor for vaccination failure, irrespective of whether the vaccine was marketed for a 10-week or 12-week or greater vaccination finish protocol. There was also a strong negative correlation between age at last vaccination prior to disease and vaccination failure (P<0.0001): the later a puppy received this last vaccination, the lower the risk of vaccination failure. This supports the hypothesis that the use of final vaccination in puppies at less than 16 weeks of age predisposes to vaccination failure and warrants a final age for vaccination recommendation to be at least 16 weeks for all canine parvovirus vaccines, especially in outbreak situations. The large number of cases identified in this study confirms that CPV vaccination failure is occurring in Australia. Veterinarians should consider CPV as a differential diagnosis in cases with appropriate clinical presentation, regardless of the reported vaccination status of the dog.

The rat ovarian phospholipase A2 system: gene expression, cellular localization, activity characterization, and interleukin-1 dependence.

We have previously demonstrated that interleukin-1 beta (IL-1 beta), a putative intermediary in the ovulatory process, is a potent stimulator of ovarian PG biosynthesis. In this communication, we examine the possibility that this IL-1 effect reflects in part the induction of arachidonic acid mobilization by phospholipase A2 (PLA2). Molecular probing of whole ovarian material revealed the immature rat ovary to be a site of modest secretory PLA2 (sPLA2) gene expression. However, no change in ovarian sPLA2 gene expression was noted during the periovulatory period. Comparable probing for cytosolic PLA2 (cPLA2) failed to disclose a quantifiable signal. However, in situ hybridization localized both sPLA2 and cPLA2 (sPLA2 > cPLA2) transcripts to the granulosa cell layer of the ovarian follicle. Treatment of cultured whole ovarian dispersates with IL-1 beta produced significant (P < 0.01) increments in the steady state levels of transcripts corresponding to both sPLA2 (1.7-fold increase) and cPLA2 (5-fold increase), an effect reversed by an IL-1 receptor antagonist, suggesting mediation via a specific IL-1 receptor. Treatment with cycloheximide, a protein synthesis inhibitor, resulted in significant attenuation of the ability of IL-1 beta to up-regulate sPLA2 and cPLA2 gene expression as well as medium PLA2 activity. Treatment with aminoguanidine, an inhibitor of inducible nitric oxide synthase, led to augmentation of the ability of IL-1 beta to up-regulate sPLA2 and cPLA2 gene expression as well as medium PLA2 activity. Total cellular PLA2 activity proved time, cell density, and calcium dependent, with an optimal pH of 8.0-9.0 and K(m) values in the low micromolar range (2-5 microM). Our observations 1) establish the rat ovary as a site of sPLA2 and cPLA2 gene expression, 2) localize the corresponding transcripts to the granulosa cell layer, and 3) establish IL-1 beta as an up-regulatory agent for ovarian sPLA2 and cPLA2 gene expression as well as for ovarian PLA2 activity. These findings also indicate that the IL-1 effect is 1) receptor mediated, 2) contingent in part upon de novo protein biosynthesis, and 3) inhibited by nitric oxide. These observations support the proposition that PLA2 may be a key component in the IL-1-stimulated biosynthesis of ovarian PGs.

Rat ovarian prostaglandin endoperoxide synthase-1 and -2: periovulatory expression of granulosa cell-based interleukin-1-dependent enzymes.

This laboratory has previously shown that interleukin-1 (IL-1), a putative intermediary in the ovulatory process, is capable of up-regulating PG biosynthesis by cultured whole ovarian dispersates from immature rats. In part, this phenomenon was attributable to the stimulation of ovarian phospholipase A2 activity. In this communication we examine the possibility that the PG-promoting property of IL-1 is also due to the up-regulation of PG endoperoxide synthase (PGS), the rate-limiting step in prostanoid biosynthesis. The in vivo expression of ovarian PGS-2 transcripts in the course of a simulated estrous cycle rose abruptly to a peak (35-fold increase over the control value; P < 0.05) 8-12 h after hCG administration (i.e. before or during projected ovulation). PGS-1 transcripts, in turn, were not significantly altered during the periovulatory period. Treatment of cultured whole ovarian dispersates with IL-1beta resulted in dose- and time-dependent up-regulation of PGS-2 transcripts (as well as of immunoreactive PGS-2 protein and PGE2 accumulation), characterized by an ED50 of 2 ng/ml and a maximal (72-fold) increase at 10 ng/ml. Although treatment with IL-1beta also led to an increase in PGS-1 transcripts and immunoreactive PGS-1 protein, the relative magnitude of the effect was markedly reduced compared with that of PGS-2. Cotreatment with an IL-1 receptor antagonist completely reversed the IL-1 effects, thereby suggesting mediation via the IL-1 receptor. The ability of IL-1 to up-regulate PGS-2 transcripts proved relatively specific, in that other cellular regulators (insulin-like growth factor I, activin A, endothelin-1, transforming growth factor-alpha, tumor necrosis factor-alpha, vascular endothelial growth factor, leukemia inhibitor factor, hepatocyte growth factor, or keratinocyte growth factor) were not effective. The optimal IL-1 effect required heterologous contact-dependent coculturing of granulosa and thecal-interstitial cells. Taken together, these observations 1) reaffirm (by molecular probing) the granulosa cell as the primary site of ovarian PGS-1 and PGS-2 expression, 2) document an increase in ovarian PGS-2 transcripts before ovulation, and 3) reveal a marked dependence of ovarian PGS (2 >> 1) transcripts, proteins, and activity on IL-1. The effects of IL-1 proved relatively specific, contingent upon somatic cell-cell cooperation, dose and time dependent, and IL-1 receptor mediated. These results are compatible with the proposition that the PG-promoting property of IL-1 is due, in large measure, to the activation of ovarian PGS transcription and translation. The ability of IL-1 to up-regulate ovarian PGS, an obligatory component of ovulation, is in keeping with the idea that IL-1 may constitute an intermediary in the ovulatory process.

Specific glucocorticoid receptor in the iris--ciliary body of the rabbit.

The cytoplasm of the iris--ciliary body of the rabbit contains a receptor capable of specifically binding dexamethasone. This binding protein has a high affinity for dexamethasone (average KD = 2.0 X 10(-8) M), a low capacity (average 4.8 X 10(-13) mol of steroid bound per milligram of protein), and extreme heat sensitivity; it exhibits a pattern of competition virtually identical to that obtained with glucocorticoid receptors from other tissues and shows characteristic physicochemical behavior in various salt concentrations. The demonstration of a specific dexamethasone receptor in the iris--ciliary body provides the first direct biochemical evidence that these tissues may function as a target organ for glucocorticoids.

The rat intraovarian interleukin (IL)-1 system: cellular localization, cyclic variation and hormonal regulation of IL-1beta and of the type I and type II IL-1 receptors.

An increasing body of evidence supports the possibility that intraovarian interleukin (IL)-1 plays an intermediary role in the periovulatory cascade. To gain further insight into the intraovarian IL-1 hypothesis, we studied the cellular localization cyclic variation and hormonal regulation of IL-1beta, as well as of the type I and type II IL-1 receptors (IL-1R) in immature rats. In situ hybridization localized IL-1beta and type I IL-1R transcripts to the granulosa cell compartment, the innermost layers of the theca interna and to the oocyte of the untreated immature ovary. Molecular probing of whole ovarian material in the course of a simulated estrous cycle revealed a progressive preovulatory increase in IL-1beta and type I IL-1R transcripts to an in vivo peak at the time of ovulation (3.0- and 2.5-fold increases over untreated controls; P < 0.05). Comparable efforts to localize and probe for type II IL-IR transcripts failed to elicit a detectable signal. The basal in vitro expression pattern of IL-1beta and type II IL-1R transcripts by whole ovarian dispersates revealed an early (4 h) spontaneous increase to a peak (2.1- and 5.8-fold increases over time 0: P < 0.05) followed by a gradual decline to a 48 h nadir. Treatment of whole ovarian dispersates with the IL-1 receptor antagonist (IL-1RA) or with IL-1beta failed to alter the initial (4 h) burst of IL-1beta or of type II IL-1R expression thereby suggesting IL-1-independence. Treatment with hCG proved equally ineffective. However, longer-term treatment of whole ovarian dispersates with IL-1beta produced a significant secondary increase (5.9-fold over time 0; P < 0.05) in IL-1beta (but not type II IL-1R) transcripts by 48 h. This IL-1 effect was completely blocked by co-treatment with IL-1RA thereby suggesting mediation via a specific IL-1 receptor. Qualitatively comparable but quantitatively reduced results obtained for isolated granulosa cells. The basal in vitro expression pattern of type I IL-1R transcripts by whole ovarian dispersates revealed a progressive spontaneous increase (3.1-fold increase overall) over the 48 h culture. Treatment with IL-1beta produced a significant (P < 0.05) increase (5-fold) in type I IL-1R transcripts by 48 h, an effect which was completely blocked by co-treatment with IL-1RA. Taken together, these observations: (1) localize IL-1beta and its type I receptor to granulosa cells, the innermost layers of the theca interna and to the oocyte; (2) confirm their periovulatory in vivo expression pattern; (3) document their expression by untreated cultured whole ovarian dispersates; and (4) demonstrate their in vitro responsiveness to receptor-mediated/IL-1-driven autocrine amplification. The type II IL-1R was undetectable in vivo, its in vitro expression pattern proving IL-1- and hCG-independent. The periovulatory expression pattern of IL-1beta and its receptor (type I) is compatible with the notion that the intraovarian IL-1 system may play an intermediary role in the ovulatory process.

Spatial tuning curves along the chick basilar papilla in normal and sound-exposed ears.

Intense sound exposure destroys chick short hair cells and damages the tectorial membrane. Within a few days postexposure, signs of repair appear resulting in nearly complete structural recovery of the inner ear. Tectorial membrane repair, however, is incomplete, leaving a permanent defect on the sensory surface. The consequences of this defect on cochlear function, and particularly frequency analysis, are unclear. The present study organizes the sound-induced discharge activity of cochlear nerve units to describe the distribution of neural activity along the tonotopic axis of the basilar papilla. The distribution of this activity is compared in 12-day postexposed and age-matched control groups. Spontaneous activity, tuning curves, and rate-intensity functions were measured in each unit. Discharge activity at 60 frequency and intensity combinations was identified in the tuning curves of hundreds of units. Activity at each of these criterion frequency/intensity combinations was plotted against the unit's characteristic frequency to construct spatial tuning curves (STCs). The STCs depict tone-driven cochlear nerve activity along the length of the papilla. Tuning sharpness, low- and high- frequency slopes, and the maximum response were quantified for each STC. The sharpness of tuning increased with increasing criterion frequency. However, within a frequency, increasing sound intensity yielded more broadly tuned STCs. Also, the high-frequency slope was consistently steeper than the low-frequency slope. The STCs of exposed ears exhibited slightly less frequency selectivity than control ears across all frequencies and larger maximum responses for STCs with criterion frequencies spanning the tectorial membrane defect. When rate-intensity types were segregated, differences were observed in the STCs between saturating and sloping-up units. We propose that STC shape may be determined by global mechanical events, as well as localized tuning and nonlinear processes associated with individual hair cells. The results indicated that 12 days after intense sound exposure, global and local contributions to spatially distributed neural activity are restored.

Identification of chlamydia in cervical smears by immunofluorescence: technic, sensitivity, and specificity.

Immunofluorescent staining was compared to cell culture for the diagnosis of Chlamydia trachomatis infections in the female genital tract. By culture, the incidence in the 496 patients in the study was 12.3%. Immunofluorescence demonstrated organisms in 90% of the culture-positive cases, with a specificity of 99.3%. This method appears to be a successful replacement for culture for general screening.

Transmural surgical gown pressure measurements in the operating theater.

Transmural gown pressures encountered when the surgeon comes into contact with a patient were measured in the operating theater. The surgical gown industry has assumed these pressures to be less than 5 psi in testing the efficacy of the gown and drape barrier material to impede bacterial transmission through its pores. In this study, pressure-sensitive contact film and resistive strain gauge recordings made from the surgeon's abdominal region and forearms indicated peak contact pressures in excess of 60 psi. This report indicates a need to reassess the basis of test utilization in evaluating barrier materials used in gowns and drapes.

Comparison of scrape, swab, and cytobrush samples for the diagnosis of cervical chlamydial infection by immunofluorescence.

An endocervical swab, cytologic scraper, and endocervical cytobrush were used to prepare simultaneous full-slide smears for immunofluorescence for the diagnosis of cervical chlamydial infection. The cytobrush produced the best sample, with significantly higher numbers of organisms. Most of the false negative results were obtained with the cytologic scraper. The optimum technique appears to be to use a cytobrush sample; however, a swab should be used with pregnant patients.

Effect of progestins on androgen delta4-3-ketosteroid-5alpha-A-ring reductase system in rat oral mucosa.

Rat oral mucosa microsomal delta4-3-ketosteroid-5alpha-A-ring reductase enzyme system, reducing testosterone and 4-androstenedione, was found to be inducible by systemic administration of medroxyprogesterone acetate (MPA). MPA, when used in a mixture with testosterone and/or 4-androstenedione in vitro, acted as a competitive inhibitor of the reduction of these substrates.

Follicular atresia as an apoptotic process: atresia-associated increase in the ovarian expression of the putative apoptotic marker sulfated glycoprotein-2.

OBJECTIVE: To evaluate the possibility that morphologically confirmed/hypophysectomy-induced ovarian follicular atresia, a putative apoptotic process, is coupled to alterations in the steady-state levels of ovarian sulfated glycoprotein-2 (SGP-2) transcripts. METHODS: Hypophysectomy-induced follicular atresia in immature rats, morphologically confirmed at the light and electron microscopic levels, was correlated with alterations in the steady-state levels of ovarian SGP-2 transcripts as assessed by a solution hybridization/RNase protection assay. Cellular localization was accomplished by in situ hybridization technology. RESULTS: Hypophysectomy of the 24-day-old immature rat, an established precipitant of follicular atresia, led (3 days later) to a significant (P < .05) increase (up to 3.3-fold) in the relative abundance of densitometrically quantified ovarian SGP-2 transcripts compared with age-matched intact controls. Detailed time-course analysis after hypophysectomy revealed significantly (P < .05) increased ovarian SGP-2 mRNA expression as early as 2 days after hypophysectomy; no further increments were noted on days 4 or 8. Light microscopic analysis of comparable ovarian material 4 days after hypophysectomy revealed increased numbers of atretic follicles displaying large numbers of degenerating granulosa cells. Electron microscopic analysis of the degenerating cells of atretic follicles (from hypophysectomized rats) disclosed nuclear condensation and cytoplasmic shrinkage as well as apoptotic bodies at all levels of the granulosa cell layer. In situ hybridization established the granulosa cell of the intact untreated rat as the somatic cell concerned with SGP-2 gene expression. In turn, hypophysectomy led to an increase in SGP-2 expression at the level of the theca-interstitial cell, an effect prevented by the concurrent provision of pregnant mare serum gonadotropin (PMSG). The hypophysectomy-induced increase in ovarian SGP-2 transcripts was similarly reversed (54% inhibition by day 27) by the concomitant provision of FSH, an established antiatretic principle. The delayed administration (day 26) of a single dose of PMSG to rats hypophysectomized on day 24 eliminated the hypophysectomy-induced increase in ovarian SGP-2 transcripts as assessed on day 28. Qualitatively similar but quantitatively more pronounced increments in ovarian SGP-2 gene expression were obtained when atresia was induced by hypophysectomy of PMSG-primed immature rats. CONCLUSIONS: These observations establish the immature rat ovary as a site of SGP-2 gene expression and reveal hypophysectomy-induced follicular atresia to result in the up-regulation of ovarian (specifically, theca-interstitial) SGP-2 gene expression, an effect prevented by the concurrent provision of FSH or PMSG. To the extent that SGP-2 is an acceptable apoptotic marker, the present findings support the hypothesis that ovarian follicular atresia may be an apoptotic process.

Detection and in vivo hormonal regulation of rat ovarian type I and type II interleukin-I receptor mRNAs: increased expression during the periovulatory period.

OBJECTIVE: To study the expression, localization, and in vivo hormonal regulation of type I and type II interleukin-1 (IL-1) receptors in the rat ovary. METHODS: Segments of the cDNAs for rat type I and type II IL-1 receptors were cloned and used as probes in RNase protection assays and in situ hybridization. Tissues obtained from immature rats and hormonally treated rat ovaries were examined. RESULTS: Type I IL-1 receptor (IL-1R(1)) was ubiquitously expressed in rat tissues, including granulosa cells prepared from immature ovaries, whereas type II IL-1 receptor (IL-1R(2)) expression was restricted to macrophages, thymus, and lung. Hypophysectomy and subsequent treatment with FSH and/or diethylstilbestrol did not alter significantly the abundance of IL-1R(1) transcripts in the whole ovary. However, the relative amount of ovarian IL-1R(1) transcripts increased 7.3-fold 6 hours after the administration of hCG to pregnant mare serum gonadotropin-primed immature rats. During this time, IL-1R(1) mRNA was localized primarily in the granulosa cells. The increased expression of IL-1R(1) persisted 24 hours after hCG administration but declined to baseline by 48 hours. Ovarian expression of IL-1R(2) mRNA was observed only before ovulation in amounts that were approximately 70-fold lower than IL-1R(1). CONCLUSION: The increased intraovarian expression of IL-1R(1) in granulosa cells during the periovulatory period implies that this cell type has a heightened receptivity to IL-1 and provides further indirect evidence that this cytokine is involved in the ovulatory process.

Radiation-induced increase in the release of amino acids by isolated, perfused skeletal muscle.

Local exposure of the hindquarter of the rat to 15 Gy of gamma-radiation resulted, 4-6 h after irradiation, in an increased release of amino acids by the isolated, perfused hindquarter preparation, 70 per cent of which is skeletal muscle. This increase in release involves not only alanine and glutamine which are synthesized to a large extent de novo in muscle, but also those amino acids which are not metabolized by muscle and, therefore, released in proportion to their occurrence in muscle proteins. Because metabolic parameters and content of energy-rich phosphate compounds in muscle remain unchanged, it is unlikely that general cellular damage is the underlying cause of the radiation-induced increase in amino acid release. The findings strongly favour the hypothesis that the increased availability of amino acids results from enhanced protein breakdown in skeletal muscle which has its onset shortly after irradiation. This radiation-induced disturbance in protein metabolism might be one of the pathogenetic factors in the aetiology of radiation myopathy.

Modification of effects of radiation on thymidine kinase.

Thymidine kinase (TdR-K) and the incorporation of iododeoxyuridine (IUdR) into DNA of murine bone marrow cells are acutely and temporarily inhibited by low doses (0.01 Gy) of whole-body gamma-radiation with a maximal effect at 4 h after exposure and full recovery at 10 h. The inhibitory effect was totally abolished by whole-body exposure to a strong static magnetic field of 1.4T. The present investigation was designed to gain insight into the mechanism(s) underlying the inhibition of TdR-K activity and the incorporation of 125I-UdR by challenging the system with various pharmacological and biochemical means. To this end the response of TdR-K and 125I-UdR incorporation into DNA to the administration of actinomycin-D, cycloheximide, cysteamine, misonidazole and procaine hydrochloride as well as to dietary manipulations, i.e. vitamin E deficiency and enrichment of the diet by soya oil, and to changes in the glutathione levels were investigated in bone marrow cells of irradiated and sham-irradiated mice. Furthermore, the effect of various NaHCO3 concentrations on optimizing the radiation-induced inhibition of TdR-K was investigated under conditions of radiation, vitamin E deficiency and enrichment of the diet by soya oil. The data point to the involvement of the cellular radical-detoxification system in changing the activity of TdR-K, especially on the basis of the concurrent increase of glutathione concentration and decrease in TdR-K activity.

Congenital airway abnormalities in patients requiring hospitalization.

OBJECTIVE: To determine the cause of congenital airway abnormalities in pediatric patients requiring hospitalization for their respiratory status. DESIGN AND SETTING: Case series in a tertiary care center. PATIENTS: A 5-year retrospective chart review was conducted at our institution. A total of 174 patients were identified who required hospitalization for their respiratory status as a result of a congenital airway abnormality. RESULTS: Of the 174 patients, 114 (65.5%) were male and 60 (34.5%) were female. Eighty patients (47%) presented within the first 3 months of life. Forty-six patients (26%) were born prematurely, and 49 patients (28%) were diagnosed as having gastroesophageal reflux. The majority of patients (139 [80%]) had multiple presenting symptoms or signs. Stridor was the most common (129 [74%]), followed by accessory respiratory effort, cyanosis, apnea, and failure to thrive. Diagnosis was made at the time of surgical evaluation in 91% of the patients, with the remaining diagnoses made using radiological findings and/or clinical evaluation. Sixty-five patients (37%) had multiple sites of airway abnormalities; laryngeal abnormalities were noted almost 3 times as often as tracheal abnormalities (161 vs 62, respectively). Of the laryngeal abnormalities, laryngomalacia was the most common, followed by glottic web, subglottic stenosis, vocal-cord paralysis, and subglottic hemangioma. Tracheomalacia was the most common tracheal abnormality, followed by external compression and tracheal stenosis. Thirty-three patients (19%) required tracheotomy for management of recurrent respiratory decompensation. CONCLUSIONS: While congenital airway abnormalities are usually self-limited, those patients requiring hospitalization represent a group with a more severe respiratory status who have a greater chance of requiring tracheotomy. The recognizable percentage of patients with gastroesophageal reflux and prematurity accounts for comorbid factors in the need for hospitalization for respiratory issues related to congenital airway abnormalities.

Point source nerve bundle stimulation: effects of fiber diameter and depth on simulated excitation.

Excitation response of different diameter myelinated nerve fibers situated at various depths within a cylindrical nerve bundle from the applied field of a point source electrode are analytically evaluated. For the potential field calculation, the fiber bundle is considered to be immersed in an infinite isotropic conductive medium and is idealized as an infinitely extending cylinder represented as an anisotropic bidomain (where electrical coupling from interstitial to intracellular space is included). Myelinated nerve fiber excitation is determined from a core-conductor nerve model, whose nodal currents are described by the Frankenhaeuser-Huxley kinetics and the aforementioned field providing the applied potentials. Stimulation level necessary for a nerve fiber to reach threshold is quantified in response to four descriptions of the volume conductor: the isotropic homogeneous case, the monodomain case, the bidomain case, and the "modified monodomain" case (where axial current is considered to flow through a parallel combination of longitudinal interstitial and intracellular resistive pathways, i.e., "complete" current redistribution). Model results indicate the importance of a bidomain representation of the nerve bundle, and provide insight into the relationship between the physical medium and the physiological properties of nerve fiber excitation.