Cardiac Muscle在线视频

Welcome to Clinical Histology.

I am Dr. Xie from SUMC.

We will talk about cardiac muscle

in this session.

By the end of this session,

you will be able to

the characteristic features

of cardiac muscles,

and you will also be able to

explain the structures

and functions of the highly specialized

cell-to-cell junctions in the heart,

the intercalated discs.

Cardiac muscle is an involuntary

and striated muscle

that is found only in the heart.

Highly coordinated contractions of

cardiac muscles

allow your heart to pump

and keep blood circulating

throughout the body.

On your left

is H&E staining of cardiac muscle

cut longitudinally;

and on the other side is also

a cardiac muscle,

but in cross section.

As we can see in both sections,

as indicated by the yellow arrow heads,

each cardiac muscle cell possesses

a single,

large, round and centrally placed nucleus.

In some instances,

you may find two nuclei

in a cardiac muscle cell.

You may recall from our

previous session,

the skeletal muscle cell has many

peripheral nuclei,

which is different from the cardiac muscle.

Unlike skeletal muscles,

cardiac muscles are extensively branched

as evident in the longitudinal section.

If you look closely at the same section

on your left,

you could also find striations

across the cardiac muscle fibers.

This is similar to that of

skeletal muscle fibers.

I hope that you have noticed the thin,

typically dark-staining lines

in the longitudinal section,

indicated by the green arrow head.

It is a highly specialized

cellular end-to-end junction,

known as intercalated discs.

The intercalated discs run

perpendicularly to the direction

of muscle fibers

and represent

the interface between adjacent cells.

Under electron microscopy,

the intercalated disc

appears more complex.

At low magnification,

as indicated by the yellow arrow heads

on the left,

the intercalated disc

is a convoluted electron dense structure.

At high magnification,

the intercalated disc

appears more irregular and step-like,

with both vertical and parallel regions

to the direction of the muscle fiber,

as shown in the diagram on the far right.

Vertical regions of these irregular,

step-like discs

contain many desmosomes

and fascia adherens junctions,

which together provide

strong intercellular adhesion

so the cells do not pull apart

during fiber contraction.

The parallel regions of each

intercalated disc

run in the same direction

to the myofibrils

and are rich in gap junctions,

which allow action potentials

to spread between cardiac cells

by permitting

cations to flow from

one cardiac muscle cell to the next.

These regions serve as

"electrical synapses",

facilitating rapid impulse conduction,

and coordinated contraction

of the entire heart.

Let's move on

and take a look at other organelles

and features of the cardiac muscles.

As we mentioned above,

similar to that of skeletal muscle,

cardiac muscle is also striated,

and the thin and thick filaments

are organized into sarcomeres as well.

You can easily find out

the dark-colored A-band

and light-colored I-band

in alternating pattern.

This pattern

is the same as that of skeletal muscle

because of the similar organization of thin

and thick myofilaments in the sarcomere.

In this electron micrograph,

the Z discs are indicated

by the red arrow heads.

As you may recall,

the region of the myofibril between

two successive

Z discs is a sarcomere.

Now

I would like to draw your attention

to the yellow arrow heads

in the micrograph.

Please note that the intercalated discs

are overlying the Z discs

at the interface of adjacent cells.

Cardiomyocytes also contain T-tubules.

However,

in cardiac muscle fibers,

the sarcoplasmic reticulum

is less well-organized

compared to skeletal muscle fibers.

Therefore,

a single T-tubule

usually only combines with

one terminal cisterna

of the sarcoplasmic reticulum

to form dyads rather than triads,

which are found in skeletal muscle fibers.

Mitochondria are abundant

in cardiac muscles.

occupy up to 40% of the cell volume,

higher than that of skeletal muscles.

The plentiful mitochondria provide

a large amount of energy

to cardiac muscle cells,

fulfilling the energy demands

of muscle contraction.

In general,

cardiac muscle cell

contracts in a similar way

to that of skeletal muscles,

although with some important differences,

such as the properties of action potential.

The cardiac action potential

is long in duration

and complex in mechanism.

For the detailed mechanisms

of cardiac contraction,

I refer you to the physiology books

and our online clinical physiology sessions.

The structural features

we discussed above

endow the cardiac muscles

with their distinctive properties of

contraction.

Finally,

I'd like to emphasize

an endocrine function

of the atrial muscle cells.

These cardiac muscle cells house

secretory granules

containing a peptide hormone,

atrial natriuretic peptide (ANP),

that functions to lower blood pressure

by acting on target cells in the kidney.

Diseases

which compromise the pumping ability

of the heart muscles

are called cardiomyopathies.

Cardiac hypertrophy

is one of the most common types of

cardiomyopathies.

In cardiac hypertrophy,

the number of cardiac muscle cells

do not change.

However,

they increase their size and modify their

extracellular matrix,

resulting in abnormal enlargement

or thickening of cardiac muscle fibers.

On your left

is the longitudinal section of

normal cardiac muscles

from a young accident victim.

On your right

we have the cardiac muscle from a patient

with hypertrophic heart failure,

showing enlarged nuclei,

longer and thicker muscle fibers,

Please note that

both slides are at the same magnification.

At present

there is no cure for cardiac hypertrophy.

However,

we can use medications early

to control underlying causes

such as hypertension,

and treat symptoms once they develop.

Now

let's have a brief review of

today's session.

Generally,

we have dealt with the key structures

and functions of cardiac muscles.

Cardiac muscles possess some

distinct features,

including branched muscle fibers,

centrally-located nucleus

and dyads formed by the T-tubule

and terminal cisterna from only

one side of its neighboring

sarcoplasmic reticulum.

The organization

and constitution of sarcomere

are similar in both kinds of

striated muscles,

the skeletal muscle and cardiac muscles.

Another unique

characteristic of cardiac muscle

is the presence of intercalated discs.

Can you identify the intercalated discs

in both light and electron microscope?

Can you describe

the functions of desmosomes

and gap junctions

in the intercalated discs?

That's all for today.

Thank you for joining us!

See you next time!