The scope of the work involves understanding the basic principles in cardiac physiology such as the force of contraction and the use of electrocardiogram (ECG) and the manner in which neurotransmitters enable the functioning of the heart.
The principal findings from the study include: an increase in heart rate when there is an increase in temperature, an increase in the Degree of Stretch as a result of an increase in Contractile Force (CF), there was an increase in the systolic force when there was an increase in the amount of blood that flowed into the heart in the same manner as described in the Starling’s law of the heart. It was also found that some drugs result into an increase in heart rate while others result into a decrease.
There are a number of factors that affect the heart rate. Studies that examine the functioning of the heart have indicated that the heart of a frog has a unique characteristic of the capacity to continue pumping even if it has been separated from the from thus enabling the study of a number of cardiac functions. The heart is composed of special tissue called cardiac muscles that have the ability to contract and expand significantly, thus enabling pumping of blood to different parts of the body (Miller et al. 284). Cardiac muscles have similar characteristics to skeletal muscles, but are distinct in terms of its special properties such as rhythmicity. The heart has specialized muscle cells called pacemakers that depolarize and polarize in a rhythmical manner so that electric signals are spread to different parts of the heart. Consequently, an intrinsic regular heart beat is achieved. Despite lack of external stimulation required to maintain heart beat, there exists a continuous input to the heart from the sympathetic nervous system. There are also a number of neurotransmitters that affect the responses of cardiac muscles, thus resulting into an increase or a decrease in heart rate. The neurotransmitters are able to affect the heart rate by causing a change to the rate of spontaneous depolarization of the pacemaker cells of the heart located in the sinoartrial (SA) and atrioventricular (AV) nodes in the heart of a mammal.
The changes in the forces of contraction and expansion of the heart have been studied by the use of an electrocardiogram (ECG) that enables recording of the actual force when a particular factor is change to determine the relationship between heart rate and that factor.
This study involved the use of the ECG to determine the force of contraction of the heart when different factors are changed. The factors that were used include: temperature, drugs, and the establishment whether the Heart rate occurred according to Starling’s Law of the Heart. Different measures were used to measure the heart rate such as beats per minute (BPM) and Newton. The use of electrocardiograph enables recording of the resulting electrical activity of the heart muscles that result into pulsating electrical waves that travel towards the skin. Full ECG set-up is composed of at least four electrodes that are placed on the chest or at the locations of the right arm, left arm, right leg, or left leg. The resulting force in the heart can be recorded in the force transducer and the relationship with other factors can be determined and presented graphically.
The relevance of Starling’s law of the heart has also been an area of interest in the study of the heart. Due to the importance of understanding the pumping force of the heart in greater details, there has been the need to determine the manner in which the pumping force changes with the change in volume that enters the heart. This experiment enables understanding of this relationship by comparing the volumes at different times and the corresponding pumping force of the heart.
It was found that there was an increase in the pumping force with the increase in time until 5.75th second when there was a reduction in heart rate with the increase in time.
Figure 1. Force and Cardiogram pulse rates during the experiment
It was also found that an increase in temperature did not have an impact on the pumping force of the heart because the actual pumping force remained constant. There was also no change in ECG reading when the temperature of the heart was increased. However, there was an increase in heart rate when temperature was increased until the 42nd second when there was a sharp increase in heart rate after which there was a decrease with increase in time.
Figure 2. Relationship between a change in temperature and heart rate
It was also found that when the degree of stretch was increased, there was a corresponding increase in the contractile force of the heart. This observation enabled understanding the relevance of Sarling’s Law of the heart. The relationship between the pumping force and the Stretch is illustrated in the figure below.
Figure 3. Relationship between Degree of Stretch and the Contractile force (N) of the heart
The investigation of the effect of drugs on heart rate involved the use of three drugs namely: Acetylcholine, Epinephrine, Pilocarpine, and a combination of Atropine and Acetylcholine. It was found that Acetylcholine resulted into a negative change in heart rate by 35.1%, Epinephrine resulted into a positive percentage change of 37.8%, Pilocarpine resulted into a negative change in heart rate by -68.4%, and a Combination of Atropine and Acetylcholine resulted into 0.0% change in heart rate. These results are as illustrated in the figure below.
Figure 4. The Effects of Drugs on the percentage change in heart rate
The study also examined the relationship between the Contractile force (N) and Stretch. It was found that the higher the strecth, the higher the contractile force. This supports Starling’s Law of the heart which states that the heart’s systolic force is directly proprotional to the distolic expansion with the outcome that under normal physiologocal processes the pumping action of the heart through the right atruum occurs without letting any blood in the veins.
Another finding from the study is that
From the findings of the study, it was found that the heart pumping force continued to increase with an increase in time. This is because the heart is composed of muscles that have the capability to initiate its pumping action independently. It was also found that the heart rate increased with an increase in temperature. Higher temperatures result into increase in heart rate because of the increase in kinetic capability resulting from higher temperature.
The study also resulted into the finding that some drugs had a positive impact on heart rate by increase the heart rate such as Epinephrine, other have a negative impact by reducing the heart rate such as Pilocarpine, while others do not have any impact on change in heart rate such as a combination of Atropine and Acetylcholine. The use of Epinpherine results into a narrowing impact on blood vessels and causes an increase in the force with which the heart pumps blood in order to reach different parts of the body (Wright et al. 148). Pilocarpine results into a reduction in heart rate by effecting the Acetylcholine receptors. Pilocarpine constitutes a muscarinic receptor that is agonist and results into an increase in heart rate. Atropine is a muscarinic receptor that acts in an antagonistic manner by competing with acetylcholine during binding onto the receptors which implies blocking the acetylcholine produced in the central nervous system (CNS) and enables the heart rate to increase.
Miller, M. L., et al. «Changes in ambient temperature differentially alter the thermoregulatory, cardiac and locomotor stimulant effects of 4-methylmethcathinone (mephedrone).» Drug and alcohol dependence 127.1 (2013): 248-253.
Wright Jr, M. Jerry, et al. «Effect of ambient temperature on the thermoregulatory and locomotor stimulant effects of 4-methylmethcathinone in Wistar and Sprague-Dawley rats.» PloS one 7.8 (2012): e44652.