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By the end of this section, you will be able to:
  • Describe intercalated discs and gap junctions
  • Describe a desmosome

Cardiac muscle tissue is only found in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Similar to skeletal muscle, cardiac muscle is striated and organized into sarcomeres, possessing the same banding organization as skeletal muscle ( [link] ). However, cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibers also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs. An intercalated disc    allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.

Cardiac muscle tissue

This image is a micrograph of cardiac muscle cells.
Cardiac muscle tissue is only found in the heart. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

View the University of Michigan WebScope at (External Link) to explore the tissue sample in greater detail.

Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium. The remainder of the intercalated disc is composed of desmosomes. A desmosome    is a cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting ( [link] ).

Cardiac muscle

This image shows the structure of the cardiac muscle. A small image of the heart is shown on the top left of the figure and then a magnified view of the cardiac muscle is shown, with the nucleus and the cardiac muscle fiber labeled. A further magnification shows the structure of the intercalated discs with the desmosome and gap junction.
Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes.

Contractions of the heart (heartbeats) are controlled by specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.

The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity    ; they do this at set intervals which determine heart rate. Because they are connected with gap junctions to surrounding muscle fibers and the specialized fibers of the heart’s conduction system, the pacemaker cells are able to transfer the depolarization to the other cardiac muscle fibers in a manner that allows the heart to contract in a coordinated manner.

Another feature of cardiac muscle is its relatively long action potentials in its fibers, having a sustained depolarization “plateau.” The plateau is produced by Ca ++ entry though voltage-gated calcium channels in the sarcolemma of cardiac muscle fibers. This sustained depolarization (and Ca ++ entry) provides for a longer contraction than is produced by an action potential in skeletal muscle. Unlike skeletal muscle, a large percentage of the Ca ++ that initiates contraction in cardiac muscles comes from outside the cell rather than from the SR.

Chapter review

Cardiac muscle is striated muscle that is present only in the heart. Cardiac muscle fibers have a single nucleus, are branched, and joined to one another by intercalated discs that contain gap junctions for depolarization between cells and desmosomes to hold the fibers together when the heart contracts. Contraction in each cardiac muscle fiber is triggered by Ca ++ ions in a similar manner as skeletal muscle, but here the Ca ++ ions come from SR and through voltage-gated calcium channels in the sarcolemma. Pacemaker cells stimulate the spontaneous contraction of cardiac muscle as a functional unit, called a syncytium.

Questions & Answers

what are the layers of heart..?
Subhadatta Reply
epicardium , myocardium ,endocardium are three layers of heart
what is gestation
the period of development in mammals, from fertilisation of the ovum till birth
what's lactation?
secretion of milk from female breast
different between artery and veins?
arteries caries oxygenated blood while veins caries deoxygenated blood except for pulmonary artery and pulmonary vein
can you guys HELP me with anatomy and physiology
is this Mbbs first year course?
hi guys how are you today?
hingi Reply
fine and you
I would like to know that my self
Diana Reply
I need HELP
can you guys help
what do you need?
what help do you need
and how should you be helped
A _____ Propagation of the action potential. this is where the AP will travel down adjacent membranes on the neuron to transmit a message from one neuron to another. B. ______ K+ is the most prominent ion inside the cell and it is at resting membrane potential. C. _______ Repolarization occur;
C. ______ Repolarization occur; allowing potassium ions to leak out of the cell changing the membrane permeability. D. ______ A stimulus changes the permeability of a patch of the membrane. this allow sodium ions to rush into the cell causing a change in the polarity of the membrane.
E._____ the sodium- potassium pump pumps out sodium in potassium ions to restore the initial ionic conditions.
is this course for first year Mbbs?
Zohaib Reply
yes...its for first year
what does Mbbs means please?
what is clinical anatomy
Namweene Reply
study of clinical structures
physiology of neurotransmission in ans
Pradip Reply
what about umbilical cord
Subhadatta Reply
nutrition passing tube
it is a connection for transfer of hormones,blood,food,etc between child and mother during pregnancy.
What the difference between anatomy and psychology
milika Reply
hahaha Anatomy vs. Physiology. Students of anatomy learn about the structure of body parts, while those enrolled in physiologystudy the functions and relationships of body parts. While these two fields are often paired together in the same class or program title, they may also be offered separately.
what is gross anatomy?
Vasco Reply
is the macroscopic form of anatomy where organs will be seen without the use of microscope
what is anatomy
Vasco Reply
is the structure of the part of the body
when did we expect heat period
Kabiru Reply
when is pregnancy expected after ovulation
Minimum blood pressure
Hasnain Reply
were can anatomy work
capella Reply
in hospitality
what is normal flora
Cris Reply
it's one kind of bacteria
the smallest blood vessel is capillary
Richard Reply
How does deoxygenated blood become oxygenated during blood circulation?
in lungs through alveoli ...

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