The heart has muscles. The muscles are only in a single layer of heart i.e. in myocardium. From outside inward the layers of heart are - 1. fibrous pericardium, 2. serous pericardium ( 2.1 parietal and 2.2 visceral layer i.e. the 2.2 epicardium), 3. myocardium and 4 .endocardium. The myocardium is the thickest layer of human heart.
The muscle in the myocardium is neither of smooth nor of skeletal type. It is a special type of muscle found only in the heart and is called cardiac muscle. The cardiac muscle is striated and has sarcomeres but is involuntary. In other words you can not make your heart stop or start or go slow or fast just by wishing or ordering it to do so. The cardiac muscle has cylindrical fibers arranged parallel to each other. But the bundles of muscle fibers branch and are joined to each other forming a three dimensional network. Take a close look at cardiac muscle - http://webanatomy.net/anatomy/cardiac_mu... Ends of cardiac muscle fibers are thrown into many folds and form intercalated discs.
These are straight or steplike dark lines at interfaces of cardiac muscle fibers. Cardiac muscle cells show tiny granules. These are the precursors of a hormone. Till recently no one everthought that heart would be involved in making a hormone but now it is clear that heart does produce a hormone. Especially right auricle (and to a smaller extent even left auricle ) produces a hormone called ANF (Auriculo Natriuretic Factor ) also called as auriculin or atriopeptin. This hormone is a peptide i.e. holds a few amino acids together by peptide bonds and it acts on kidneys to promote excretion of sodium and water.
The heart muscle scan relax between periods of contractions. They have long refractory period.
The cardiac muscle contract rhythmically about 75 times a minute. A little slower when a man is sleeping and much faster when a man is running, digging etc. The heart muscles contract on their own and do not need any external prodding in the form of a stimulus. Nobody knows how this spontaneous contraction takes place.
The heart has natural pacemaking activity. The natural pacemakers of heart are three groups of specialized cardiac muscles.
They are the - sinuauricular node, auriculoventricular node and the Bundle of his. The SAN i.e. the sinuauricular node located in wall of right atrium or right auricle is the prime pacemaker. Have a look at the S.A. Node at http://sprojects.mmi.mcgill.ca/cardiophy... The S.A.Node generates the contraction events 75 times a minute. The AVV i.e the auriculoventricular node can also generate the contractions of heart at a lesser rate of about 50 times a minute or so. The Bundle of His can generate the contractions of heart at an ven lesser rate of about 30 times a minute.
These are straight or steplike dark lines at interfaces of cardiac muscle fibers. Cardiac muscle cells show tiny granules. These are the precursors of a hormone. Till recently no one everthought that heart would be involved in making a hormone but now it is clear that heart does produce a hormone. Especially right auricle (and to a smaller extent even left auricle ) produces a hormone called ANF (Auriculo Natriuretic Factor ) also called as auriculin or atriopeptin. This hormone is a peptide i.e. holds a few amino acids together by peptide bonds and it acts on kidneys to promote excretion of sodium and water.
The heart muscle scan relax between periods of contractions. They have long refractory period.
The cardiac muscle contract rhythmically about 75 times a minute. A little slower when a man is sleeping and much faster when a man is running, digging etc. The heart muscles contract on their own and do not need any external prodding in the form of a stimulus. Nobody knows how this spontaneous contraction takes place.
The heart has natural pacemaking activity. The natural pacemakers of heart are three groups of specialized cardiac muscles.
They are the - sinuauricular node, auriculoventricular node and the Bundle of his. The SAN i.e. the sinuauricular node located in wall of right atrium or right auricle is the prime pacemaker. Have a look at the S.A. Node at http://sprojects.mmi.mcgill.ca/cardiophy... The S.A.Node generates the contraction events 75 times a minute. The AVV i.e the auriculoventricular node can also generate the contractions of heart at a lesser rate of about 50 times a minute or so. The Bundle of His can generate the contractions of heart at an ven lesser rate of about 30 times a minute.
The copyright of the article Cardiac Muscles- I in Human Anatomy is owned by Narayan Dattatray Wadadekar. Permission to republish Cardiac Muscles- I in print or online must be granted by the author in writing.
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