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Non-invasive Method of Assessing Mechanisms that Control Breathing Essay Sample

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Introduction of TOPIC

A Copy of the Central Command to move may contribute to the increase in breathing at the start of

exercise. A study to evaluate such a notion.

 

PREVIOUS RESEARCH

According to Wasserman, Shipp and Davies (1981), no precise role of neural and chemical drives to breathe has been wholly established.

Krogh and Lindhard (1913) were the key pioneers who paved the way for further research looking at feed forward as a concept for breathing. They proposed that signals, possibly from the motor corner of the brain, controlled respiration and thereby increased ventilation. More modern research allows us to see that it has been difficult to conduct definitive research in conscious man to determine the existence of a cortical component.

This theory has been tested using muscle spindle vibration, a method described by Goodwin & McCloskey (1972).  During bicep contraction of 50% of maximum, they used a vibrator to activate a muscle spindle reflex.  This reflex would cause a contraction, meaning less central command would be needed to maintain 50% contraction.  In these experiments, Ventilation was seen to decrease during vibration.

For the intention of this assignment we are looking at central command as a mechanism to control breathing.   Central command is a common mechanism involved in an exercise presser reflex. This refers to activation of the cardiovascular centre by descending central neural pathways which are involved in initiation of muscular activity (Krogh & Lindhard, 1913; Goodwin et al. 1972; Hobbs, 1982; Mitchell, 1985). Both central command and reflex neural mechanisms have been proposed to explain cardiovascular and respiratory responses that occur during exercise.

According to Mitchell (1985) both central command and reflex neural mechanisms are shown to affect the same neural circuits. The research proposed that central command and reflex neural mechanisms are capable of functioning either in combination with one another or independently. To support this, it has been shown that either decreasing (Goodwin et al. 1972) or increasing (Leonard et al. 1985) central command results in correspondingly decreased or increased cardiovascular responses to isometric exercise when the muscle is kept at the same tension.

To support this idea, research by the likes of Gallagher et al. (2001) selectively increased the influence of central command during steady state static and dynamic leg exercise using partial neuromuscular blockade. Such research was done in the hope of seeing how it would affect cardiovascular and respiratory rhythm. By blocking the muscle they were able to see the direct effect of central command on respiratory and cardiovascular control. Similarly, Querry et al. (2001) increased central command activation while simultaneously reducing exercise presser reflex input at the same time as activation of static and dynamic arm exercise occurred.

They encouraged partial neural blockade with lidocaine. Augmentations in central command in both studies resulted in increases in heart rate (HR) and mean arterial blood pressure (MAP). Further support for central command comes from research by Eldridge, Milhorn and Waldrop (1981) who have shown in decorticated cats that stimulation of an area of the hypothalamus is capable of inducing both locomotion and breathing.

Seals & Victor, (1991); Rowell, (1994) found that central command produces different effects on heart rate and muscle sympathetic nerve activity during sustained isometric exercise in humans. Based on this observation it may prove difficult to establish a clear link between central command and the respiratory system.

 

HYPOTHESIS

The purpose of the experiment was to establish the role of central command in the exercise-induced contraction in the bicep femoral and the triceps. The experiments were designed to show whether elements of the command descending from the higher command (independent variable) provide a stimulus for respiratory and cardiovascular control (dependant variable). The study was a repeat of the Goodwin and McCloskey (1972) paper. It was valid to repeat this study as its former statistical results were not recorded. In this paper we simply tried to confirm that central command has a direct link with respiration.

SUMMARY

Six subjects performed 30% maximal voluntary contraction (MVC) in the bicep, femoral and the tricep, with the manipulation of central command. De Gail, Lance and Nielson, (1966) stated that vibration can elicit a reflex in the muscle without central command. According to Lind et al(1966) cardiovascular and respiratory responses to isometric exercise are best seen at tensions over 10% of MVC. It was therefore important that the subject made a contraction via central command of 20%. 10% of the contraction could be achieved reflexly by vibration.

Vibration was applied to the bicep tendon while subjects performed sustained isometric contractions with either the bicep or the tricep. When the bicep was contracted and the vibration was applied there was activation of muscle spindle afferents.  It was presumed that less central command would be required to maintain the level of sustained contraction. The reverse was the case when the triceps were stimulated, that is to say that more central command was required as the activation of muscle spindle afferents in the bicep contributes to a reflex inhibition.

A clear established result was determined between central command, heart rate, and pulmonary ventilation, in and of the fact that a decrease in central command causes less of an increase in heart rate, blood pressure, and pulmonary ventilation. Similarly, blood pressure, heart rate and pulmonary ventilation all increased when the central command increased.

It was also concluded that there was irradiation of cardiovascular and respiratory control centres by the descending central command during voluntary muscular contractions in man.

METHOD

For this experiment we assigned each member of our group to certain differing tasks. One person operated the computer and analysis. Another individual was set the task of operating the equipment, in order to ensure that the subject was as comfortable as possible, and they operated muscle spindle stimulation (vibration) as well. Another group member co-ordinated the team effort.

The Participants – They were five healthy subjects (3 men and 2 women: age, 20.9+/-0.4) from our group who were assigned to this study, and acted as participants. All participants were non-smokers and undertook regular exercise. They were, however, all trained to varying degrees.

Procedures and control measures – After the participant was informed about how the study would work, a heart rate monitor was attached. The subject was then seated in an exercise chair and their arm was placed in a dynometer which was linked to the computer where analysis could be undertaken. Various precautions were taken to ensure the experiment was accurate. For instance, the angle of the elbow was always kept at 90 degrees, and subjects were also instructed to confine their efforts in any experiment to the biceps or triceps and were instructed to avoid using shoulder or trunk movements to alter their leverage. Similarly, subjects were asked to ensure their elbow was always firmly in contact with the support.

The subjects breathed down a branta flow meter which contained a three way valve for this experiment and they breathed through a low resistanc

e, small dead-space valve (Cunningham, Johnson and Lloyd, 1956) connected to a mouth piece. The flow

meter was calibrated before each trial was conducted. We set the calibration to 3 litres to ensure that the measurement recorded was accurate.

The inspired room air and the expired gas was recorded on a computer using spike 3 software. The flow meter recorded the frequency, flow and volume (l) of breaths. (UNITS). Heart rate was also measured throughout the experiments (polar heart rate monitor) at 30 second intervals. We had the option of using an ECG monitor in this experiment. Unfortunately, though, there was a problem with this at the time of the experiment so we opted to use a polar heart rate monitor.

‘For valid and accurate analyses of the expired breath sample great care is necessary to account for factors such as gas transit time to the analyser and variation of water vapour pressure and temperature’ (Wasserman et al., 1986). Calibration of the gas analyser was done using a 3L calibrator, ten pumps were used therefore 30L, and this gas was then sucked out again (30L if calibrated correctly).

The computer analyses system was calibrated by pumping an amount of gas with 5.1%CO2 through the analyser, and the same with a 2%CO2 gas sample. Each concentration pumped through three times to minimise error. The sensitivity of the range of gas samples breathed per person was measured by carrying out 3 sets of 2 trials. These trials involved the same setup as in the protocol. Trial 1 was 15 seconds of hyperventilation followed by a forced expiration, whereas Trial 2 was 15 seconds of breath holding followed by a forced expiration. The variability inevitably found in the subjects was accounted for by repeated measurings. This minimised human error and the anomalies that can result from single measurements.

Equipment Used- A physiotherapy vibrator (Vibrator Massager, Pifco Ltd) was used to induce muscle contraction by activation of the patellar tendon reflex. In the present investigation the oscillating vibration was applied to the patellar tendon while the subjects performed sustained isometric contractions of the bicep femoral or tricep at 30% MVC. Initially to establish 30% MVC the subjects were asked to make several contractions at 100% MVC for 2-3 seconds. 30% MVC was then calculated from this.

            Initially in the exercise protocol a subject was asked to maintain a constant tension of 30% of their MVC with either the bicep or the tricep. This was achieved by showing the subject a display of tension on the screen while they were informed to make an alignment with the marker of the required tension. After the MVC trials the subject was familiarised with the experimental protocol, the physiological measurements to be obtained and the aim of the study.

The experiment was split into two studies:

Study 1 (control condition)

In the first part of the experiment subjects were asked to produce a static 30% MVC, in the absence of vibration. The subject in turn would be required to call forth a greater central command to achieve the required tension.

Study 2 (vibration condition)

The physiotherapy vibrator, oscillating at 100Hz was applied to the tendon of either the exercising muscle or its antagonist. In Study 1 the bicep was the muscle being stimulated by the physiotherapy vibrator while measurements were also taken from the bicep. The subject was asked to contract until the force which was produced matched 30% on the display screen.

The subject made the contraction in both studies for 2 minutes and a rest interval occurred before and after the contraction for a 1 minute period, this last to ensure that the subject had recovered before their next contraction, thus reducing the effect of fatigue. Goodwin and McCloskey (1972) stated that a fatigued muscle may require a greater amount of central command to maintain a given tension than it would normally. They investigated this further by allowing subjects a short rest interval between each contraction. When fatigued in this way, subjects showed a greater than normal response in blood pressure, heart rate and ventilation at a set level of intensity.

By removing the vibrator we were able to see a fall in the tension on the oscilloscope. This showed that the vibrator was causing a partial contraction. The effect, however, was usually short lived due to the fact that the subjects often increased their contraction when they could see a change on the display screen.

 

Random control measures

            We set the experiment up in a way that meant that new subjects would not learn from previous subjects as to how much they should contract_______________________________________________________________ ETC.  Study 1 is the control condition whereas Study 2 is the vibration condition.

Participant Study Study
1 1 2
2 2 1
3 1 2
4 1 2
5 2 1

 Statistical Analysis – a paired t test was used to compare the minute ventilation values for each subject in each condition, or whether there was vibration or non vibration, in other words.  Data gained was entered into the SPSS v11.5 and pair sampled T-tests were carried out to determine whether significant differences existed between the two conditions. Significance for all tests was set at P<0.05

DIAGRAM OF THE SETUP USED IN THE EXPERIMENT

 

Physiotherapy vibrator-

Branta flowmeter & 3 way valve

Biceps strain gauge measuring system (3L calibration syringe)

Stop watch

Computer and CED1401 interface with Spike 2 respiratory analysis system

Polar Heart rate monitor

MS excel programs for calculating metabolic rate from Douglas Bag experiments via & indirect calorimetry

RESULTS

Initially we conducted the experiment on five subjects. When testing one of the subjects we noticed that the valve system was not registering the breaths. In terms of the effect of vibration on minute ventilation (ml/min) the results were found to be insignificant, at the level of <.536 due to this anomaly. The diagram below illustrates this point more clearly. As can be seen there is a point where no breaths are being registered:

Fig 1

As a group we decided to eliminate this result from our findings. The below diagram shows another subject breathing. This highlights the problem we were experiencing with the equipment as shown in the fig 1.

Fig 2

 

The graph shows the minute ventilation (ml/min) against the condition and compares the results for a control condition against the vibration condition in four subjects. The results were found to be significant to the level of <.041. A correlation of 0.959 was measured.

Condition 1 is control and 2 is vibration
1
2

In the first part of the experiment, the control condition, subjects conducted a contraction voluntarily. In the second part of the study, on the other hand, vibration provided some of the given tension. The subject would provide a given tension in conjunction. This point is discussed in more detail in the methods section.

The reduction of central command due to an increase in the recruitment of muscle fibres (vibration) reduced the pressor, heart rate, and ventilator response. A few subjects experienced that less effort was required. More often, however, subjects found that effort was neither easier nor harder, while one subject found that the effort was more difficult with vibration. According to the Goodwin and McCloskey (1972) paper this may be due to the increased mental concentration required. What is clear from the above graph is that with vibration there was a decrease in ventilation.

The graph below shows the stages of exercise for four subjects both in a vibrating and control condition against minute volume (ml/min).

Subject Number

In the above graph we can see that minute volume (l/min) was highest in the exercise condition for the control subjects.

DISCUSSION

The present studies indicate that elements of the motor command lead to an increase in breathing, blood pressure, heart rate and ventilation during voluntary muscular contraction.

Ideal experiment to test this?

Any adjustments we made, any particular way the equipment had to be used?

Any limitations in the results?

Any further experiments we now wish we did that are feasible?

Respiratory Coursework Practical

The present experimental results

REFERENCES

Cunningham, D.J.C., Johnson , W.G.H & Lloyd, B.B (1956) A modified ‘Cormack’ respiratory valve

De Gail P, Lance JW & Neilson PD (1966). Differential effects on tonic and phasic reflex mechanisms produced by vibration of muscles in man. J Neurol Neurosurg Psychiat 29, 1-11

Goodwin GM, McCloskey DI, Mitchell JH. (1972) Cardiovascular and respiratory responses to changes in central command during isometric exercise at constant muscle tension. J Physiology. 226(1):173-90.

Krogh, A. & Lindhard, J. (1913) The regulation of respiration and circulation during the initial stages of muscular work, J. Physiology 47 112-136

Leonard B, Mitchell JH, Mizuno M, Rube N, Saltin B, Secher NH. Partial neuromuscular blockade and cardiovascular responses to static exercise in man. J Physiology. Feb;359:365-79.

Lind, A.R., McNicol, G.W & Donald, K.W. (1966). Circulatory adjustments to sustained (static) muscular activity. In Proceedings of the International Symposium on Physical Activity in Health and Disease. Pp. 36-63

McCloskey, D.I & Mitchell, J.H (1972). Reflex cardiovascular and respiratory responses originating in exercising muscle. J. Physiology Jul;224(1):173-86.

Querry, R. G., Smith, S. A., Stromstad, M., Ide, K., Raven, P. B. & Secher, N. H. (2001). Neural blockade during exercise augments central command’s contribution to carotid baroreflex resetting. American Journal of Physiology – Heart and Circulatory Physiology 280, H1635-1644

Papelier, Y., Escrourrou, P., Gauthier, J. P. & Rowell, L. B. (1994). Carotid baroreflex control of blood pressure and heart rate in men during dynamic exercise. Journal of Applied Physiology 77, 502-506

  1. Ogoh, W. L. Wasmund, D. M. Keller, A. O-Yurvati, K. M. Gallagher, J. H. Mitchell and P. B. Raven (2002) Role of central command in carotid baroreflex resetting in humans during static exercise. Journal of Physiology 543.1, pp. 349-364

Seals, D. R. & Victor, R. G. (1991). Regulation of muscle sympathetic nerve activity during exercise in humans. Exercise and Sport Sciences Reviews 19, 313-349

Waldrop, T. G., Eldridge, F. L., Iwamoto, G. A. & Mitchell, J. H. (1996). Central neural control of respiration and circulation during exercise. In Handbook of Physiology, section 12, Exercise Regulation and Integration of Multiple Systems, ed. ROWELL, L. B. & SHEPARD, J., pp. 333-380. Oxford University Press, New York

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