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The operation of the heart and smooth muscles primarily depend electrical activity. However, unlike in other muscular activities where an operation mainly relies on external influences, the function of the heart and smooth muscles is spontaneous. For instance, the cells within the sinoatrial node for instance are considered as pacemakers. From the studies that have been conducted, it has emerged that these cells have no true resting potential (Ruch and Patton, 2002). In this respect therefore, their operation resulting in the generation of spontaneous energy within regular intervals. On the other hand, smooth muscles of the body also highly depend on electrical activity to conduct their operations. The results that were conducted on early studies asserted that the spontaneous electrical activity of the smooth muscle originated from within the smooth muscle cells. However, modern research has revealed that this action originate from pacemaker cells. The energy that is generated from this activity assists in supporting the various functions that smooth muscles are involved in such as gut activity. This paper will thus critically analyze the spontaneous function of the heart and smooth muscles. The aim of this analysis will be to compare the spontaneous activity of the heart and smooth muscles and hence determine the differences that might be present between them. To focus on the spontaneous activity of the heart, this paper will expound on the anatomy of the heart and critically examine the electrical activation process of the heart. On the other hand, a critical analysis of the smooth muscles will be conducted to determine the origin of its spontaneous activity. From these analyses, the similarities and differences will be determined.
Spontaneous Activity of the Heart
The heart is considered to be the most powerful organ in the body of a living organism. In the human body, the heart is located within the chest cavity between the lungs and above the diaphragm. To effectively perform its activity of circulating the blood around the body, the heart comprises of a series of vessels. This includes the superior and inferior vena cava, pulmonary artery, pulmonary vein, as well as the aorta. These vessels play a critical part in transporting blood to and from the heart. It is the combination of the heart and these vessels that make up the circulatory system.
For the heart to function efficiently, it needs to pump the blood at high pressure. This ensures that the blood reaches the various parts of the body at the right pressure to facilitate the process oxygenation and deoxygenation. Thus, to achieve this function, the heart is made up of muscular walls that rhythmically contract and relax to push blood in an out of the heart at a high pressure. The walls of the heart are made up of cardiac muscles that are referred to as myocardium (Fozzard et al., 2001). These muscles have striations that are very similar to that of the skeletal muscles. Consequently, the heart comprises of four chambers. The anterior part of the heart comprises of the right and left atria while the posterior segment of the heart comprise of the right and left ventricles. This arrangement plays a significant role in supporting the conduction of electrical impulses within the heart. The walls of the septum and the left ventricle are usually thicker as compared to the walls of the right ventricle. This arrangement is also essential since the left ventricle supports the systemic circulation that is characterized with high pressure and the right ventricle supports pulmonary circulation. The heart also has cardiac muscles. These muscles are oriented in a spiral nature. These muscles are also divided into four different groups. The first two groups of these muscles are wound within the outside of these ventricles. The third group on the other hand is wound around the ventricles. The final group of these muscles winds around the left ventricle only (Malmivuo, & Plonsey, 1995, p. 119). The arrangement of cardiac muscular cells in a spiral manner has a lot of importance especially in the electrical conduction of impulses within the heart.
According to Fozzard et al. (2001), the electrical activation of the heart cells (myocytes) is triggered by the movement of sodium ions across the cell membrane. While this activation process is similar to that of the nerve cell and skeletal cells, the duration of an impulse within the myocytes takes much longer. It is approximated to take double the length of time as compared to the nerve of skeletal cells. Cardiac depolarization is usually followed by a plateau phase and then repolarization takes place as a result of the movement of potassium ions out of the cell membrane (Malmivuo, & Plonsey, 1995, p. 121). Electrical activation of the heart muscle results in the contraction of the heart. However, this process occurs a while later after the heart has been activated. From the studies that have been conducted, it has emerged that an electrical impulse that originates from one cell can be propagated to another cell (Fozzard et al., 2001). Consequently, the propagation of this impulse can occur in any direction. On the other hand however, the propagation of impulses within the smooth muscle cells is usually unilateral resulting in movements in a specific direction. At this point however, it is critical to note that cardiac cells have the ability to conduct impulses in any direction except at the boundary between atria and ventricles. At this point, the activation wave is prevented from crossing over due to the presence of a non-conducting barrier that is made of fibrous. However, this wave can only cross the aria-ventricle boundary only along a special conducting system (Ruch and Patton, 2002).
The spontaneous activity of the heart is controlled by the sinus node (SA). SA is located at the right atrium within the superior vena cava. It comprises of a series of specialized muscle cells. In humans, SA is shaped like a crescent. These cells are self-excitatory pacemaker cells. In humans, these cells are active resulting in an action potential at an approximate rate of seventy pulses per minute (Ruch and Patton, 2002). The impulse that has been generated at the SA is propagated throughout the atria. However, as it has been stated earlier, this impulse cannot crossover to the ventricles due to the presence of a fibrous barrier. In a normal heart, pulse from the atria to the ventricles is permitted by the atrioventricular node (AN). This node is located at the boundary of the atria and ventricles. The AN has an intrinsic rate of fifty pulse per minute. Consequently, the AN is triggered by a high frequency pulse allowing the pulse to move from the atria to the ventricles in the direction of propagation. At this point, it is essential to state that the AN can be triggered by any pulse that passes through it and not just the ones that originate from the atria.
The movement of the pulse from the AN to the ventricles is supported by specialized conduction system referred to as the bundle of His. This bundle divides into two branches that extend to the two sides of the septum resulting into the right and left bundle branches (Malmivuo, & Plonsey, 1995, p. 122). Furthermore, these bundles divide into fibers that extend into the ventricular walls. The velocity of the pulse from the atria to the ventricles is usually low. However, once it has passed through the AN, the velocity of the impulse is greatly increase as it extends to the ventricular walls. As the impulse flows through the inner side of the ventricular walls, it passes through various activation sites that form a wave from which moves through the ventricular mass towards the outer wall (Malmivuo, & Plonsey 1995, p. 122). This phenomenon usually results to cell-to-cell activation (Malmivuo, & Plonsey 1995, p. 122). Finally after every segment of the ventricle has been depolarized, repolarization then takes place. Unlike depolarization, repolarization does not have a propagating effect (Malmivuo, & Plonsey 1995, p. 122). However, the action of this impulse is much shorter in the epicardium as compared to the endocardium.
Spontaneous Activity of the Smooth Muscle
The smooth muscle can be defined as an involuntary striated muscle. This muscle exhibit a wide range of activities ranging from rapid contractions that are exhibited in the ureter lymphatics to slow contractions such as in the urethra (Sergeant et al., 2001). Rapid contractions that result from smooth muscle contractions are mainly involved in the propulsion of fluids within various systems in the body. On the other hand, slow contractions that originate from these muscles mainly tend to support resistance of flow. According to Sergeant et al. (2001), ionic channel dynamics result in the spontaneous contractions of the smooth muscles. Consequently, the spontaneous activity of these muscles results in two types of electrical events. These are:
- Slow waves – highly dependent on membrane potential
- Spikes – highly dependent on voltage
In this respect therefore, many researchers have concluded that the spontaneous activity of the smooth muscle is triggered by metabolism and is moderated by the concentration of calcium that is present within a membrane.
Recent research has however revealed that different types of smooth muscles contain ICC-like cells (Klemm et al., 1999). These cells are mainly found in the blood vessels, lymphatics, fallopian tubes and so on. The cells that are found within areas such as the lymphatics are believed to play a pacemaker role. This function is considered to be atypical to smooth muscle cells (Klemm et al., 1999). However, the same concept does not apply for the cells that are found in areas such as the arteries and the uterus.
(Author and year) went further ahead to give a detailed description of ICC-like cells that act as pacemakers in non-gastrointestinal tract. They described them as excitable and non-contractile. Consequently, they asserted that these cells high contents of vitamins and lack myosin filaments. Furthermore, these cells comprise of calcium activated chloride current. Like myocytes, ICC-like cells also act as pacemakers resulting in polarization and depolarization phases that are relatively similar to that of the heart muscles. However, unlike in the heart where current stimulation was triggered by sodium ions, electrical activity in the smooth muscle in triggered by calcium ions.
From the studies that have been conducted, it has emerged that pacemaker activity in the urethra highly relies on the release of calcium ions from the intracellular stores as well as the movement of extracellular calcium ions (citation). The release of calcium ions that have been stored activate the following two membranes:
- Chloride ions – referred to as above
- Outward current – mediated by large conduction of calcium ions (BK Channels)
The activation of the above two membranes result in the generation of spontaneous transient inwards and outwards currents (STICs and STOCs) respectively. The fact that Chloride channels do not highly rely on voltage and BK channels are strongly voltage dependent explains the reason why the currents that originate from these two opposing channels do not even each other out (citation). Thus, the spontaneous elevation of calcium ions results in the depolarization from a resting potential. In rabbits, for instance, the spontaneous electrical activity of the urethra usually develops from the oscillation of calcium ions from within the intracellular stores.
By critically analyzing the spontaneous activity of the heart and smooth muscles, it is evident that there are a number of similarities and differences. In the myocytes and the smooth muscles, the spontaneous activities are highly dependent on electrical activation hence resulting in depolarization and repolarization. Consequently, this electrical impulse is transmitted from one cell to the other. In the smooth muscle, this current is only propagated in one direction while in the heart, this impulse can be propagated in any direction. However, the spontaneous activities of the heart and the smooth muscles originate from specialized cells (mycocytes and ICC-like cells) that act as pacemakers regulating the intensity and frequencies of the impulses that are generated.
Fozzard, HA, Haber, E, Jennings, RB, Katz, AM and Morgan, HI, 2001, The Heart and Cardiovascular System, Raven Press, New York.
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Klemm, MF, Exintaris, B, and Lang, RJ, 1999, ‘Identification of the cells underlying pacemaker activity in the guinea-pig upper urinary tract’, Journal of Physiology, vol. 519 no. 1, pp. 867–884.
Malmivuo, J & Plonsey, R 1995, Bioelectromagnetism – Principles and Applications of Bioelectric and Biomagnetic Fields, Oxford University Press, New York.
Ruch, TC and Patton, HD, 2002, Physiology and Biophysics, W. B. Saunders, Philadelphia.
Sergeant, GP, Hollywood, MA, McCloskey, KD, McHale, NG, and, Thornbury, KD, 2001, ‘Role of IP3 in modulation of spontaneous activity in pacemaker cells of rabbit urethra’, American Journal of Physiology, vol. 280 no. 1, pp. 1349-1356.