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By the end of this section, you will be able to:
  • Describe the structure of cardiac muscle
  • Identify and describe the components of the conducting system that distributes electrical impulses through the heart
  • Compare the effect of ion movement on membrane potential of cardiac conductive and contractile cells
  • Relate characteristics of an electrocardiogram to events in the cardiac cycle
  • Identify blocks that can interrupt the cardiac cycle

Recall that cardiac muscle shares a few characteristics with both skeletal muscle and smooth muscle, but it has some unique properties of its own. Not the least of these exceptional properties is its ability to initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger the contractile mechanism. This property is known as autorhythmicity    . Neither smooth nor skeletal muscle can do this. Even though cardiac muscle has autorhythmicity, heart rate is modulated by the endocrine and nervous systems.

There are two major types of cardiac muscle cells: myocardial contractile cells and myocardial conducting cells. The myocardial contractile cells    constitute the bulk (99 percent) of the cells in the atria and ventricles. Contractile cells conduct impulses and are responsible for contractions that pump blood through the body. The myocardial conducting cells    (1 percent of the cells) form the conduction system of the heart. Except for Purkinje cells, they are generally much smaller than the contractile cells and have few of the myofibrils or filaments needed for contraction. Their function is similar in many respects to neurons, although they are specialized muscle cells. Myocardial conduction cells initiate and propagate the action potential (the electrical impulse) that travels throughout the heart and triggers the contractions that propel the blood.

Structure of cardiac muscle

Compared to the giant cylinders of skeletal muscle, cardiac muscle cells, or cardiomyocytes, are considerably shorter with much smaller diameters. Cardiac muscle also demonstrates striations, the alternating pattern of dark A bands and light I bands attributed to the precise arrangement of the myofilaments and fibrils that are organized in sarcomeres along the length of the cell ( [link] a ). These contractile elements are virtually identical to skeletal muscle. T (transverse) tubules penetrate from the surface plasma membrane, the sarcolemma, to the interior of the cell, allowing the electrical impulse to reach the interior. The T tubules are only found at the Z discs, whereas in skeletal muscle, they are found at the junction of the A and I bands. Therefore, there are one-half as many T tubules in cardiac muscle as in skeletal muscle. In addition, the sarcoplasmic reticulum stores few calcium ions, so most of the calcium ions must come from outside the cells. The result is a slower onset of contraction. Mitochondria are plentiful, providing energy for the contractions of the heart. Typically, cardiomyocytes have a single, central nucleus, but two or more nuclei may be found in some cells.

Questions & Answers

what are the functions of nervous tissue?
Christa Reply
what the name of the largest nerve in the body and the disease associated with it
ORIATO Reply
 sciatic nerve disease - sciatica
Fath
what is body fluid compartment
Zinny Reply
what is negative feedback
Zinny
What is the importance of anatomical position?
Charles
it helps in the knowledge of the position of the various internal organs which can be very useful in surgery or diagnosis
mirabelle
Can I go to nursing school?
Kaye Reply
Any student from Philippines here?
Kaye
no
Hussain
what ur class
Hussain
no class
Kaye
no
up. student from Phil.😊
Aila
present
Silyn
here from Philippines
Myline
I'm country Bangladesh
Kawsar
my country is INDIA
Mosin
Me, I am a student from the Philippines
janyl
me im from phil...
Kristine
Pwede po mag ask?
janyl
What's the difference of radiologist and radtech?
janyl
Kaye:Can I go to nursing school? Yes Po you can..there are Alot of nursing/pre-med schools in our country.that offers scholarships
janyl
I'm not sure po but you can try and enroll in Up diliman😊
janyl
Nursing Nursing is a profession that involves health care and health awareness activities for the general public. This profession emphasizes the importance of restoring the health and well-being of a patient or person personally, family or socially. A person involved in this profession, skilled
Kawsar
I'm suggesting you to try up diliman because of their holistic and superior education..
janyl
What is the difference between physiology and anatomy?
Kawsar
Physiology is the study of normal body functions & its mechanisms. Anatomy is the study of normal structure.
Fath
thanks
Kawsar
Thankyousomuch to all of you💖
Kaye
what is the difference between anatomy and physiology?
Grace Reply
I don't know let's tell me about
syed
Hi
Philip
anatomy is about structure, physiology is about function. Human physiology is the scientific study of the chemistry and physics of the structures of the body 
Moira
hello
Kaye
hi
Kawsar
hi
Twevs
good morning
Kawsar
Are you a nursing student?
Twevs
yes soon
Kaye
hello
Brenda
Can you upload anatomy of skeletal system with structure of all bony parts?
Fath Reply
any students from kazakhstan
Sruthi Reply
What kind of tests are there to identify the RBCs in blood?
Amjad Reply
what kinds of people can use Cefixime
abul Reply
especially women where there is uterine healing
Moses
what is a cell
Diana Reply
Hey guys
abul
***medlineplus.gov/genetics/understanding/basics/cell/#:~:text=Cells%20are%20the%20basic%20building,and%20carry%20out%20specialized%20functions.&text=Cells%20have%20many%20parts%2C%20each%20with%20a%20different%20function.
Donna
that should help you diana
Donna
cell is basic structural and functional unit of life
Sabika
Cell is the basic structural , functional & biological unit of all known organisms.
Fath
list the body fluid compactment
Zinny Reply
how many Bone's are the in human being
Rose Reply
A normal human body have 206 bones
Charles
206
Zinny
206
juma
In child 300 and in adult 206
rahul
Explain the Kreb cycle
Jerry Reply
what is thymus?
Jakia Reply
The thymus is a specialized primary lymphoid organ of the immune system.  thymus is located in the upper front part of the chest, in the anterior superior mediastinum, behind the sternum, and in front of the heart.
Analyn
if you are shot with a bullet and it penetrate your lung, which thoracic body cavity will be affected, and which layer of serous membrane would damage first?
Mnm Reply
hello
Abila
heart and cell membrane
Romantic
Anterior thoracic body cavity will be affected and parietal layer of serous membrane will he damaged.
ok thank you sir
Romantic
plura
Ishwar
hi
irshad
hi
Romantic

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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