Irritable bowel syndrome (IBS) is a common and multifactorial functional gastrointestinal (GI) disorder characterized by altered gut-brain communication. Due to the complexity of gut-brain interactions, the aim of this thesis was to enhance the understanding of associations between GI symptom severity and measures from multiple levels along the gut-brain axis in IBS patients.In this thesis, various indications of altered gut-brain interactions were demonstrated: I. IBS patients with anxiety or depression reported more severe GI and non-GI symptoms than patients without psychological distress. Visceral hypersensitivity, aberrant function of the autonomic nervous system, GI-specific anxiety, and non-GI somatic symptoms differentiated between patients with and without psychological distress. II. Overall, modest associations were discovered among neurophysiological factors, and between neurophysiological factors and the severity of IBS symptoms. The most important combination of neurophysiology measures for GI symptom severity in IBS patients were extracted through a computerized method and were found to be visceral hypersensitivity and psychological distress. III. Alterations in a wide range of psychological measures were common in IBS. A strong cumulative effect of psychological alterations on the severity of GI symptoms was found. IV. Central sensitization was frequent in IBS patients, but the severity of central sensitization and GI symptoms were only modestly associated in IBS, suggesting that the presence, rather than the level, of central sensitization is of importance for GI symptoms in IBS.In conclusion, the results from this thesis support the current view of IBS being a disorder of gut-brain interaction, where both peripheral and central factors contribute to this multifactorial disease. Complex associations between psychological and neurophysiology measures, and the severity of GI symptoms were demonstrated in the studies included in this thesis.

Habitat choice by juvenile cod (Gadus morhua L.) on sandy bottoms with different vegetation types was studied in laboratory. The experiment was conducted day and night in flowthrough tanks on two different size-classes of cod (7-13 and 17-28 cm TL). Four habitats, typical of shallow soft bottoms on the Swedish west coast: Fucus vesiculosus, Zostera marina, Cladophora sp. and bare sand, were set up pair-wise in six combinations. The main difference between habitats in this study was vegetation structure, since all parameters except vegetation type was considered equal for both sides of the experimental tanks and natural prey was eliminated. The results showed a difference in habitat utilization by juvenile cod between day (light) and night (dark). During day time the fishes showed a significant preference for vegetation, while nocturnally no significant choice of habitat was made. Both size-classes preferred Fucus, considered the most complex habitat in this study, when this was available. The smaller size-class seemed to be able to utilize the other vegetation types as well, always preferring vegetation over sand. Larger juvenile cod, on the other hand, appeared to be restricted to Fucus. This difference in habitat choice by the two size-classes might be due to a greater dependence on shelter from predation by the smaller juveniles, causing them to associate more strongly with vegetation. The larger juveniles avoided Cladophora, since they might have difficulties in entering the compact structure of this filamentous algae. Availability of vegetation at day time, as a predation refuge, as well as of open sandy areas for feeding during night, thus seems to be important for juvenile cod. It is concluded that eutrophication-induced changes in habitat structure, such as increased dominance by filamentous algae, could alter the availability of predation refuges and foraging habitats for juvenile cod.

We assessed the effect of four voluntary breathing conditions on maximal voluntary isometric force of large muscle groups. Ten subjects performed maximum voluntary isometric contractions (MVIC) of knee flexion and extension, shoulder abduction and adduction, and elbow flexion and extension under all breathing conditions: normal breathing, forced inhalation, forced exhalation, and the Valsalva maneuver (VM). Forced exhalation significantly increased peak force during shoulder adduction, elbow extension and knee extension MVIC tasks (p=0.001, p=0.024, p=0.002, respectively); the peak force during the Valsalva maneuver was not different from forced exhalation for all tested muscle groups. No voluntary breathing condition appeared to influence the peak force during knee flexion, elbow flexion and shoulder abduction MVIC tasks. The results demonstrate that voluntary breathing imposes a significant impact on isometric muscle strength. Given increased cardiovascular risks associated with the Valsalva maneuver, it is highly recommended that forced exhalation should be used during exercises at maximal levels, especially in repetitive repetitions.

The study investigated squeezing reaction time (RT) in response to a visual cue during rhythmic voluntary breathing at 0.6 Hz paced by a metronome, breath holding, or at rest in 13 healthy subjects. Rhythmic voluntary breathing slowed down RT, only in the expiratory phase with accompanied changes in the length of respiratory phases, while breath-holding reduced RT. The prolonged RT during voluntary expiratory phases and the absence of changes in RT during voluntary inspiratory phases are most likely related to disproportionally increased cognitive demands during the expiratory phase of voluntary breathing. The absence of changes in RT during voluntary inspiration is likely to be compensated by respiratory-motor facilitation mechanisms in this phase. Shortened RT during breath holding is possibly associated with increased attention.

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