The fast-slow continuum in mammalian life history: an empirical reevaluation

Many life-history traits co-vary across species, even when body size differences are controlled for. This phenomenon has led to the concept of a "fast-slow continuum," which has been influential in both empirical and theoretical studies of life-history evolution. We present a comparative analysis of mammalian life histories showing that, for mammals at least, there is not a single fast-slow continuum. Rather, both across and within mammalian clades, the speed of life varies along at least two largely independent axes when body size effects are removed. One axis reflects how species balance offspring size against offspring number, while the other describes the timing of reproductive bouts.

Success of a nurse led community based genitourinary medicine clinic for young people in Liverpool: review of the first year

OBJECTIVES

To assess the outcome and workload of a community based, nurse led comprehensive sexual health and contraceptive service for clients aged less than 25.

METHODS

Review of appointment diaries and clinic records of clients who attended the Brook genitourinary medicine (GUM) clinic. The workload, case mix, and achievement of national targets in the first year of the service were compared with those for the same age group of clients attending the nearby hospital based GUM clinic. A limited client satisfaction questionnaire was carried out 8 months after the clinic opened.

RESULTS

1061/1700 (62.4%) clients (185, 17.4% male) attended booked appointments. Chlamydia trachomatis was detected in 16.1% of women and 20.5% of men at Brook (p<0.05), where 22.6% of women and 50% of men had at least one sexually transmitted infection (p<0.001). HIV testing was offered to 98.5% of clients. 60.7% of all identified contacts attended a clinic for testing and/or treatment. Client responses to a questionnaire about the service were very favourable. Only 2.3% of Brook GUM clients needed referral to a physician.

CONCLUSIONS

Nurse led community based GUM services, such as the one provided at the Merseyside Brook Centre, appeal to young people and our success should encourage others to consider similar ventures.

Finding, confirming, and managing gonorrhoea in a population screened for chlamydia using the Gen-Probe Aptima Combo2 assay

OBJECTIVES

To identify the prevalence of Neisseria gonorrhoeae (NG) within a population screened for Chlamydia trachomatis (CT). To monitor confirmatory microscopy, culture, and partner findings following reactive Aptima Combo2 assay (AC2) gonorrhoea screening tests.

METHODS

Between June and December 2004, all gonorrhoea screening tests performed using AC2 for clients taking part in the Liverpool Chlamydia Screening Programme were monitored. Clients with AC2 NG reactive results were referred to a local genitourinary medicine (GUM) department for confirmatory microscopy, culture, treatment, and partner follow up.

RESULTS

47 (1%) of 4680 women and eight (1.7%) of 473 men had AC2 reactive gonorrhoea screening tests. Of those clients who agreed to follow up and were tested before any treatment, supportive evidence for a gonorrhoea diagnosis was found in 37 (97%) of 38 women and all five men. In the population opportunistically screened for chlamydia, CT prevalence rates were 12% for women and 15.7% for men. Although both women and men showed a higher relative risk for NG if chlamydia positive, of the 47 women who were reactive for NG by AC2, 55% (26) were negative for chlamydia.

CONCLUSIONS

Sexually transmitted infections are rising in England and reduction of gonorrhoea rates is an objective of the Department of Health Sexual Health and HIV Strategy. AC2 tests provide an acceptable and accurate means of testing for gonorrhoea in an asymptomatic population in the community. AC2 had a higher positive predictive value than might be suggested by previous clinical trials in this low prevalence population. Although antibiotic sensitivity must be monitored, AC2 testing may offer a more acceptable alternative to microscopy and culture for NG in some populations.

Characteristics of Type 1 diabetes of over 50 years duration (the Golden Years Cohort)

BACKGROUND

Type 1 diabetes mellitus is associated with high levels of premature morbidity and mortality. Prolonged survival is possible, however, and some patients appear to be protected from the long-term complications of this condition.

METHODS

Diabetes UK awards medals to patients who have had Type 1 diabetes for 50 years or more. By examining medal-holders, we have established the clinical and biochemical features of a group of 400 subjects (54% male) with Type 1 diabetes of long duration.

RESULTS

Mean age of the subjects was 68.9 years and mean age-at-onset of diabetes 13.7 years. Features of long duration diabetes in this cohort include normal body mass (mean BMI 25.0 kg m-2), low insulin dose (mean 0.52 units kg-2) and greatly elevated HDL-cholesterol (mean 1.84 mmol/l). Mean HbA1c was 7.6% (normal range 3.8-5.0%) and no patient had a normal HbA1c at the time of venesection. As a group, they have long-lived parents and consume moderate amounts of alcohol. Medical contact has often been sporadic. A significant proportion (29%) were taking anti-hypertensive medication. Screening for micro- and macroalbuminuria was positive in 35.7%.

CONCLUSIONS

Patients with long-duration (> 50 years) Type 1 diabetes are relatively protected from clinical diabetic nephropathy and large vessel disease; our data are consistent with protection possibly being genetically determined in part via elevated HDL-cholesterol levels. An abnormal urinary albumin/creatinine ratio is common in these patients, despite their low risk of significant renal deterioration; this may have implications for microalbuminuria screening programmes.

Proprioceptive feedback is reduced during adaptation to a visuomotor transformation: preliminary findings

Adapting movements in relation to visual feedback is a ubiquitous characteristic of sensorimotor control and involves the integration of multiple sources of sensory information. We recorded sensory feedback from muscle spindle afferents during visuomotor adaptation while subjects performed an aiming task to investigate whether the activity of the muscle spindles was modulated by the fusimotor system under these learning conditions. None of the muscle spindles showed an increase in activity, rather in 83% of the trials the firing rates were decreased. These preliminary results suggest that the CNS reduces the sensory signals arising from muscle spindles perhaps as a means of resolving the conflict between visual and proprioceptive feedback during the task.

Directional tuning of human forearm muscle afferents during voluntary wrist movements

1. Single unit activity was recorded with the microneurography technique from sixteen spindle afferents and one Golgi tendon organ afferent originating from the forearm extensor muscles. Impulse rates were studied while subjects performed unobstructed aiming movements at the wrist in eight different directions 45 deg apart. In addition, similar imposed movements were performed while the subject was instructed to remain relaxed. Movement amplitudes were about 5 deg and the speed 10-30 deg x s(-1). Joint movements were translated to movements of a cursor on a monitor to provide visual feedback. 2. Individual spindle afferents modulated their activity over a number of targets, i.e. were broadly tuned, during these aiming movements. The preferred direction for a spindle afferent was the same during both passive and active movements, indicating that the fusimotor effects associated with active contractions had little or no effect on the direction of tuning. 3. The direction of tuning of individual spindle afferents could be predicted from the biomechanically inferred length changes of the parent muscle. Thus spindle afferents responded as stretch receptors, i.e. impulse rates increased with lengthening and decreased with shortening, in active as well as passive movements. 4. Spindles from muscles, which continuously counteracted gravity exhibited a stretch response and directional tuning during the phase of movement alone whereas their position sensitivity was poor. In contrast, spindle afferents from the muscles that had no or minimal antigravity role were directionally tuned during both the dynamic and the static phase of the aiming task and their position sensitivity was substantially higher. 5. In spite of the limited data base from three extensor muscles it could be demonstrated that wrist joint position was remarkably well encoded in the ensemble muscle spindle data. In some cases the ensemble muscle spindle data encoded the instantaneous trajectory of movement as well.

Extinction

In the life of any species, extinction is the final evolutionary process. It is a common one at present, as the world is entering a major extinction crisis. The pattern of extinction and threat is very non-random, with some taxa being more vulnerable than others. Explaining why some taxa are affected and some escape is a major goal of conservation biology. More ambitiously, a predictive model could, in principle, be built by integrating comparable studies of past and present extinctions. We review progress towards both explanatory and predictive frameworks, comparing correlates of extinction in different groups at different times. Progress towards explanatory models for the current crisis is promising, at least in some well-studied taxa, but the development of a truly predictive model is hampered by the formidable difficulties of integrating studies of present and past extinctions.

Extinction

In the life of any species, extinction is the final evolutionary process. It is a common one at present, as the world is entering a major extinction crisis. The pattern of extinction and threat is very non-random, with some taxa being more vulnerable than others. Explaining why some taxa are affected and some escape is a major goal of conservation biology. More ambitiously, a predictive model could, in principle, be built by integrating comparable studies of past and present extinctions. We review progress towards both explanatory and predictive frameworks, comparing correlates of extinction in different groups at different times. Progress towards explanatory models for the current crisis is promising, at least in some well-studied taxa, but the development of a truly predictive model is hampered by the formidable difficulties of integrating studies of present and past extinctions.

Control of the wrist joint in humans

As one considers changes in motor activity from lower mammals to higher primates, one of the major changes one observes lies in the cortical control of forelimb muscles. There has been a shift from disynaptic control of spinal motoneurons in, for example, the cat, to a greater and greater percentage of monosynaptic control of hand and forelimb motoneurons in the primate. In spite of the species and evolutionary changes in the synaptic connections of the corticospinal tract, it appears that the interneurons identified in the cat are retained in the monkey and human. These interneurons, under the influence of descending pathways, modulate the output of motoneuron pools. Perhaps the control of these interneurons has also changed towards finer control of movement, as has been suggested by recent studies in the monkey. Whether in cat or human, the recruitment pattern for motor units is the same; the change from disynaptic to monosynaptic connections has not changed the recruitment pattern of muscles. Differences in the recruitment patterns of muscles may lie in the finer control of inputs to motoneurons in the primate. This review seeks to integrate the current knowledge of the mechanisms involved in the motor control of the wrist joint and especially in the recruitment patterns of the muscles. These motor control mechanisms include the biomechanics of the wrist joint, recruitment patterns of wrist muscles, interneurons and spinal cord circuits in the cervical regions mediating the output of spinal motoneurons, and the supraspinal control of these muscles.

Dendritic L-type calcium currents in mouse spinal motoneurons: implications for bistability

The intrinsic properties of mammalian spinal motoneurons provide them with the capability to produce high rates of sustained firing in response to transient inputs (bistability). Even though it has been suggested that a persistent dendritic calcium current is responsible for the depolarizing drive underlying this firing property, such a current has not been demonstrated in these cells. In this study, calcium currents are recorded from functionally mature mouse spinal motoneurons using somatic whole-cell patch-clamp techniques. Under these conditions a component of the current demonstrated kinetics consistent with a current originating at a site spatially segregated from the soma. In response to step commands this component was seen as a late-onset, low amplitude persistent current whilst in response to depolarizing-repolarizing ramp commands a low voltage clockwise current hysteresis was recorded. Simulations using a neuromorphic motoneuron model could reproduce these currents only if a noninactivating calcium conductance was placed in the dendritic compartments. Pharmacological studies demonstrated that both the late-onset and hysteretic currents demonstrated sensitivity to both dihydropyridines and the L-channel activator FPL-64176. Furthermore, the alpha1D subunits of L-type calcium channels were immunohistochemically demonstrated on motoneuronal dendrites. It is concluded that there are dendritically located L-type channels in mammalian motoneurons capable of mediating a persistent depolarizing drive to the soma and which probably mediate the bistable behaviour of these cells.

Two-drug combinations that increase apoptosis and modulate bak and bcl-X(L) expression in human colon tumor cell lines transduced with herpes simplex virus thymidine kinase

Herpes simplex virus thymidine kinase (HSV-TK) and ganciclovir (GCV) gene therapy can induce apoptosis in tumor cells that are normally resistant to this type of cell death, although the cellular mechanisms by which this occurs remain to be elucidated. Human colon tumor cell lines expressing HSV-TK were treated with GCV or four other inducers of apoptosis: butyrate, camptothecin (CPT), Taxol (paclitaxel), or 7-hydroxystaurosporine (UCN-01). Over a 2-4 day treatment period with GCV or the other four drugs, protein levels of the apoptosis agonist Bak increased 1.5- to 3-fold, whereas a corresponding decrease in the levels of the apoptosis antagonist, Bcl-X(L), was observed in butyrate-, CPT-, and 7-hydroxystaurosporine (UCN-01)-treated cells. GCV and paclitaxel treatments resulted in increased levels of Bcl-X(L). In two-drug combinations with GCV plus one of the four other drugs, increased tumor cell killing was found with GCV plus UCN-01 or with some GCV/butyrate combinations; the other two tested combinations were largely antagonistic. The GCV/UCN-01 and GCV/butyrate combinations resulted in increased Bak and decreased Bcl-X(L) protein levels, while the GCV/CPT and GCV/paclitaxel combinations resulted in increased levels of both proteins. The results highlight the potential for new combination therapies of HSV-TK/GCV and chemotherapeutic drugs that result in increased tumor cell apoptosis for future treatments of colon cancer.

Uneven mapping of magnocellular and parvocellular projections from the lateral geniculate nucleus to the striate cortex in the macaque monkey

Central vision is substantially over represented in the lateral geniculate nucleus (dLGN) and striate cortex. The over representation could be accompanied by a selective expansion of central vision in parvocellular dLGN, in which case the ratio of parvocellular to magnocellular inputs to striate cortex should change with retinal eccentricity. To test this, sample ratios were determined from counts of neurons in dLGN labelled retrogradely with WGA-HRP from striate cortex at the cortical representations of various eccentricities. Parvocellular to magnocellular ratios decreased from a mean of 35:1 at the fovea to 5:1 at 15 degrees eccentricity. Furthermore, they exceeded the ratio of P beta to P alpha ganglion cells in central retina, but not in peripheral retina, showing that the uneven P to M ratio in the LGN does not merely mirror the distribution of ganglion cells in the retina. This provides direct evidence for selective over representation of central vision in parvocellular dLGN.

Comparison of the depression of H-reflexes following previous activation in upper and lower limb muscles in human subjects

When conditioning-testing (C-T) stimuli are applied to Ia afferents to elicit H-reflexes, the test reflex is abolished immediately following the conditioning reflex. As the C-T interval is increased, the test response slowly begins to recover, taking several hundred milliseconds to attain control values. The time course of this recovery is known as the H-reflex recovery curve. H-reflex recovery curves were compared using surface EMG and single motor unit activities in lower limb soleus and upper limb flexor carpi radialis (FCR) muscles in seven healthy human subjects. Under rest conditions, the recovery of H-reflexes and single motor unit activity was slow for soleus; the recovery was not complete even in 1 s. In comparison, the recovery was very fast for FCR motor units, occurring in 200-300 ms. The effects of rate of stimulation (0.1-10.0 imp/s) were also examined on the magnitude of H-reflex responses. The reflex response declined with increasing rate of stimulation, the decline being slightly greater in soleus than in FCR. When these phenomena were examined with voluntary facilitation of the spinal cord, the time of recovery shortened and the effect of stimulus rate also diminished. Changes with background facilitation were greater in FCR than in soleus. The differences between the two muscles are attributed mainly to differences in presynaptic inhibition in the two spinal segments, and/or to the differences in dynamics of the transmitter release in terminals of Ia afferents synapsing with slow soleus motoneurons and those synapsing with the fast FCR motoneurons.

A comparison of human motoneuron data to simulated data using cat motoneuron models

The response of repetitively firing human motoneurons to a composite excitatory input was evaluated. It was clearly shown that the response of the motoneurons to the transient input decreased with an increase in the background firing rate of the cell. The current model of repetitively firing human motoneurons could not account for this experimental result. Therefore, a compartmental modelling approach was used to simulate the repetitive firing properties of anaesthetised cat motoneurons under current clamp conditions. The modelled motoneurons were used in simulations similar to the experimental paradigms where the response to a composite excitatory input was evaluated at different background firing rates. The motoneuron models also showed a decrease in response to the excitatory input at faster background firing rates. The results suggest that human motoneurons are more comparable to motoneurons in the anaesthetised cat preparation than formerly thought. The results also demonstrate that the apparent efficacy of a synaptic input may be modulated by changes in background firing rate of the postsynaptic neuron.

Lack of bystander killing in herpes simplex virus thymidine kinase-transduced colon cell lines due to deficient connexin43 gap junction formation

The efficacy of herpes simplex virus thymidine kinase (HSV-TK) gene therapy for colorectal carcinoma has been investigated in an in vitro system. The magnitude and the mechanism of the HSV-TK bystander effect was determined in three human colon tumor cell lines: HCT-116, HCT-8, and HT-29. Each HSV-TK(+) cell line was generated by stable transduction with a bicistronic retroviral vector containing the HSV-TK and neomycin resistance (neo) genes; each exhibited an IC50 for GCV of < or =4 microM. When GCV was added to HSV-TK(+) cells mixed with parental cells or known bystander-positive cell lines, no bystander killing was evident in the HT-29 or HCT-8 cells. Western blots detected the expression of the gap junction protein connexin43 (Cx43) in HCT-8 and HT-29 cells; however, immunolocalization studies indicated predominantly cytoplasmic staining of Cx43 and no cell surface staining in these cell lines. Stable transfection of HCT-8 and HT-29 cells with Cx43 resulted in increased levels of Cx43 expression with the same subcellular distribution as before, yet there was again no apparent bystander killing. In contrast, Cx43 expression was localized to the cell surface in the bystander-positive colon tumor cell line HCT-116. These results demonstrate that expression and proper surface localization of Cx43 gap junctions are necessary components of the bystander effect in human colon tumor cells. They also indicate that future combination gene therapy approaches using coexpression of HSV-TK and Cx43 genes may not be applicable to all tumor systems.

An advanced demultiplexing system for physiological stimulation

A CMOS very large scale integration (VLSI) chip has been designed and built to implement a scheme developed for multiplexing/demultiplexing the signals required to operate an intracortical stimulating electrode array. Because the use of radio telemetry in a proposed system utilizing this chip may impose limits upon the rate of data transmission to the chip, the scheme described herein was used to reduce the amount of digital information which must be sent to control a large quantity (up to several hundred) of stimulating electrodes. By incorporating multiple current sources on chip, many channels may be stimulated simultaneously. By incorporating on-chip timers, control over pulse timing is assigned to the chip, reducing by up to fourfold the amount of control data which must be sent. By incorporating on-chip RAM, information associated with the desired stimulus amplitude and pulse timing can be stored on chip. In this manner, it is necessary to send control information to the chip only when the information changes, rather than at the stimulus repeat rate for each channel. This further reduces the data rate by a factor of five to ten times or more. The architecture described here, implemented as an eight-channel stimulator, is scalable to a 625-channel stimulator while keeping data transmission rates under 2 Mbps.

Making our offices universally accessible: guidelines for physicians

OBJECTIVE

To develop recommendations for office-based physicians who wish to make their offices accessible to all patients.

OPTIONS

Include taking steps to make offices more accessible, or not; offices may be accessible to varying degrees.

OUTCOMES

Outcomes of accessibility involve patient-care, economic, ethical and legal issues. Stakeholders in these outcomes include patients, physicians, government and society.

EVIDENCE

Data were obtained from a series of searches of MEDLINE, CINAHL and Healthstar (previously Health) databases for articles on disability and family medicine, primary (health) care and family practice, and on access and offices, and health services accessibility, and from a telephone survey of 50 stakeholders.

VALUES

A high value was placed on services to persons with disabilities and on stakeholder input. Universal accessibility was valued as an overall goal; improved accessibility was also highly valued.

BENEFITS, HARMS AND COSTS

Benefits to patients include improved access to care as guaranteed by the Canada Health Act and in keeping with provincial Human Rights Codes. Benefits to physicians include contact with a broader patient population and freedom from fear of litigation. Costs of improved accessibility vary depending on individual circumstances and on whether an office is being built or renovated; some improvement costs are minimal.

RECOMMENDATIONS

All physicians should take measures to improve practice accessibility. Improved access should be considered in each of the following areas: transportation and entrance to the facility, entrance to the office, waiting rooms, rest rooms, examination rooms, general building features and other features.

VALIDATION

No similar guidelines exist. To assess the content validity of these guidelines, the authors had a draft document reviewed by 18 stakeholders. All specific recommendations met the minimum criterion of adherence to current legislation, including national and provincial building codes. The specific recommendations are endorsed by the Canadian Paraplegic Association (national and Ontario offices), the DisAbled Women's Network (Ontario) and the Centre for Independent Living (Toronto).

SPONSORS

Development of these guidelines was supported in part by the Department of Family and Community Medicine, Toronto Hospital, Toronto, Ont.

Computer simulation of the responses of human motoneurons to composite 1A EPSPS: effects of background firing rate

Two compartmental models of spinal alpha motoneurons were constructed to explore the relationship between background firing rate and response to an excitatory input. The results of these simulations were compared with previous results obtained from human motoneurons and discussed in relation to the current model for repetitively firing human motoneurons. The morphologies and cable parameters of the models were based on two type-identified cat motoneurons previously reported in the literature. Each model included five voltage-dependent channels that were modeled using Hodgkin-Huxley formalism. These included fast Na+ and K+ channels in the initial segment and fast Na+ and K+ channels as well as a slow K+ channel in the soma compartment. The density and rate factors for the slow K+ channel were varied until the models could reproduce single spike AHP parameters for type-identified motoneurons in the cat. Excitatory synaptic conductances were distributed along the equivalent dendrites with the same density described for la synapses from muscle spindles to type-identified cat motoneurons. Simultaneous activation of all synapses on the dendrite resulted in a large compound excitatory postsynaptic potential (EPSP). Brief depolarizing pulses injected into a compartment of the equivalent dendrite resulted in pulse potentials (PPs), which resembled the compound EPSPs. The effects of compound EPSPs and PPs on firing probability of the two motoneuron models were examined during rhythmic firing. Peristimulus time histograms, constructed between the stimulus and the spikes of the model motoneuron, showed excitatory peaks whose integrated time course approximated the time course of the underlying EPSP or PP as has been shown in cat motoneurons. The excitatory peaks were quantified in terms of response probability, and the relationship between background firing rate and response probability was explored. As in real human motoneurons, the models exhibited an inverse relationship between response probability and background firing rate. The biophysical properties responsible for the relationship between response probability and firing rate included the shapes of the membrane voltage trajectories between spikes and nonlinear changes in PP amplitude during the interspike interval at different firing rates. The results from these simulations suggest that the relationship between response probability and background firing rate is an intrinsic feature of motoneurons. The similarity of the results from the models, which were based on the properties of cat motoneurons, and those from human motoneurons suggests that the biophysical properties governing rhythmic firing in human motoneurons are similar to those of the cat.

Comparison of peripheral Ia and corticomotoneuronal composite EPSPs in human motoneurons

The effects of excitatory inputs arising from Ia afferent and corticomotoneuronal volleys on repetitively firing flexor carpi radialis (FCR) motoneurons were compared in normal human subjects. Peripheral (Ia) volleys were produced by transcutaneous electrical stimulation of the median nerve and by mechanical taps to the FCR tendon. Transcranial magnetic stimulation (TMS) was used to activate the corticomotoneuronal pathway. The duration of the excitatory response peaks measured from peri-stimulus time histograms (PSTHs) and the time course of the response trajectories were both taken to reflect the shapes of the underlying composite excitatory postsynaptic potentials (EPSP)s acting upon that motoneuron. The duration of excitatory response peaks for the H-reflex and the first sub-peak (SP1) of the motor unit's response to TMS were similar and were typically less than those arising from tendon taps. The response trajectories, which measure the excitability of the motoneuron during different phases of the afterhyperpolarization, overlapped for H-reflex and SP1 responses, but were different for tendon tap inputs. Our results indicate that the SP1 response of a motoneuron to TMS input and its response to near-synchronous Ia afferent activation are mediated by composite EPSPs with similar rise times. We suggest that a similar spatial distribution of synaptic boutons for both Ia and corticomotoneuronal input to motoneurons innervating FCR is likely.