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Regulation of heart rate is influenced by several factors.

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Regulation of heart rate is influenced by several factors We know that the heart is capable of beating independently of its bodily control systems. However, in order to adapt its rate to the changing needs of the body, it is subject to the most careful regulation by the nervous system. Additional factors such as hormones, fluctuations in body temperature, and concentrations of various ions also influence heart rate. Autonomic Nervous System The heart is innervated by both components of the autonomic nervous system. Parasympathetic fibres decrease heart rate, whereas sympathetic fibres increase heart rate. Parasympathetic innervation originates in the cardiac inhibitory centre in the medulla oblongata of the brain stem and is conveyed to the heart by way of the vagus nerve (Cranial Nerve X). Both the SA and AV nodes are richly supplied with vagal fibres. There is a minor distribution of vagal fibres to muscle of the atria and ventricles. When these parasympathetic fibres are stimulated they release acetylcholine, which slows the heart rate. ...read more.


The heart rate may nearly triple, and the strength of contraction may nearly double, under the influence of maximal sympathetic stimulation. Various parts of the circulatory system relay messages (e.g., regarding blood pressure) to the cardiac centres, which respond by sending messages to the heart via the vagus nerves. In this manner the cardiac centres are responsible for maintaining a balance between the inhibitory effects of the parasympathetic nerves and the stimulatory effects of the sympathetic nerves. When the parasympathetic messages decrease, the sympathetic nerves are able to function in an unopposed manner and thereby increase the heart rate. For example, severance of vagal nerve fibres results in an increased heart rate. Hormonal influence Under conditions of stress, adrenaline and noradrenaline are released from the tissues of the adrenal medulla into the general circulation. Each of these hormones produces an increase in heart rate. Thyroid hormones, thyroxin (T4) and triiodothyronine (T3), also accelerate the heart rate and this is most likely due to a direct effect of these substances on the heart. The strength of heart contraction is also modulated by thyroid hormones. ...read more.


Ordinarily the concentrations of these ions are kept within appropriate limits and thus do not affect the heart adversely. However, in instances where their concentration becomes excessive or deficient, cardiac function may be seriously affected. An excess of potassium ions in the extracellular environment markedly reduces the heart rate as well as the strength of contraction. On the other hand, spastic contraction of the heart results from the presence of excess calcium ions. This typically results from the direct effects of calcium ions upon the contractile process of cardiac muscle. A marked reduction in the calcium ion concentration has effects similar to those observed with high potassium levels. Excessive levels of sodium ions result in depression of cardiac function, which is thought to stem from their competition with calcium ions at some critical site during the contractile process. At the other extreme, a deficiency of sodium ions in the extracellular environment leads to the development of a potentially lethal condition called cardiac fibrillation. In this situation, the cardiac muscle contracts at an extremely high rate and in an uncoordinated fashion such that little or no blood is actually pumped by the heart. ...read more.

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