Cardiac Muscles 2

The human heart starts developing very early in embryonic life and even before it is completely formed it starts beating by about 22 days after the fertilization has occurred. The heart has to begin working very early as in an early embryo, growth is at a furious rate. Every 4 hours the early embryo doubles in mass and the cells need the nutrients and oxygen.

The hearts of all vertebrates like - fish, frog, lizards, birds, mammals are myogenic, that is, they work without any stimulus coming from the nervous system. External stimuli change the heart rate but do not initiate the heart beats. For some invertebrates like insects this is not the case. Their hearts are neurogenic. That is unless there is a stimulus form the CNS their heart does not carryout its pumping activity.

In humans a mature cardiac muscle cell (fiber) is cylindrical in shape and is about 7 times as long as it is broad. Its length is approximately 100 micrometers (microns) and width about 15 micrometers (microns). The heart muscle cells contain numerous mitochondria and are resistant to fatigue. The heart muscles may not be able to contract as powerfully as skeletal muscles do but they do not easily get tired. Perhaps about 40% of the volume of cytoplasm of these cells is of mitochondria. In case of skeletal muscles the mitochondria would occupy about 2% volume. Auricular cardiac muscle cells are somewhat smaller in size than the ventricular cardiac muscle cells.

Between the right and left auricular cardiac muscle cells the difference is - more auriculin (ANF) granules are found in the right auricle. (for ANF details kindly refer Cardiac Muscles - I, the previous article in this series)
Cardiac muscle cells are of two basic types - the contractile cells and impulse generating non-contractile cells. The impulse generating cells of the heart make it possible for the atria (auricles ) and ventricles to contract in a rhythmic manner.

These impulse generating cells of Sinu-Atrial Node are smaller than the contractile cells. Also they contain less numbers of myofibrils.The SA Node in each species has its own specific rate at which it initiates the contraction of heart and sets the heart beats. For humans it is about 72 times a minute. For an elephant about 35 per minute and for a dog about 90. For very small animals like a mouse 500 times and a humming bird's heart beats over 1000 times a minute. By and large, larger the animal lesser is the heart rate.

In an embryo the heart rate is low and as the embryo grows the heart rate increases. Later as the animal becomes old there is reduction in the number of times the heart beats.

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