Cardiac and smooth muscle
Date
@November 8, 2022
Session
Lecture
Week
Week 8
Table of contents
Objectives
Review
Cardiac muscle fibers and cardiomyocytes
Ultrastructure
Regulation
Intercalated discs (disks)
Regeneration
Smooth muscle
Objectives
Describe the organization of cardiac muscle fibers
Describe the ultrastructure of cardiomyocytes
Describe the components of the intercalated discs and their function
Describe the organization of smooth muscle fibers
Describe the process of smooth muscle contraction
Compare the three muscle types in terms of structure and mechanism of
contraction
Review
Contraction is an all or nothing phenomenon, strength determined by frequency of
contraction and number of motor units contracting.
Cardiac and smooth muscle
1
Cardiac muscle fibers and cardiomyocytes
Fiber is not homogeneous in its structure. The myofibers branch in the heart, they
are not end to end (one long fiber), they branch.
Cardiac and smooth muscle
2
There is perinuclear space at the poles of the nucleus, where we find organelles. A
lot of glycogen is there. This is not present in skeletal muscle. Mitochondria is the
only organelle that is spread out throughout the cell.
Cardiac myocytes have central nuclei compared to the eccentric nuclei of skeletal
muscle (which are pushed to the edge). There is stippling due to myofibrils, but they
are not as clear as the stippling in skeletal muscles. There is connective tissue
covering (LCT) covering each myocyte called the endomysium. This is the only
connective tissue in the heart: there is no perimysium or epimysium. This means
there is no fascicle organization.
Sometimes binucleated cells. Dark lines seen in a longitudinal sections represent
intercalated disks. They are not perfectly straight, they are stepped. They have two
components: one going across the end of the cell (transverse component).
Longitudinal component parallel to long end of the cell.
Endomysium contains blood supply in the heart. Note there is no blood vessels in
the endomysium of the skeletal muscle, this is because they have their blood
vessels in the other layers that are not present in cardiac muscle.
Ultrastructure
Cardiac and smooth muscle
3
Skeletal:
Cardiac:
T tubules are usually in the A-I junction
of the sarcomere. Sarcoplasmic
reticulum surrounds all myofibrils
completely. Scattered mitochondria.
Recall the amount of glycogen and
myoglobin affect the fiber type. Nucleus
in the periphery.
Contains t tubules, sarcoplasmic
reticulum that is not as well developed
as the skeletal. Terminal cisternae are
occasional formations along the t tubule
and are not as present as skeletal.
Skeletal muscle have triads (t tubules
sandwiched between SR), while the
cardiac has a diad (t tubule and
terminal cisternae) or just t tubule.
Functionally they are the same (calcium
regulation). Not enough calcium to
make everything contract normally. T
tubules store the calcium, which is a big
difference compared to skeletal muscle.
Myofibrils are not divided up because
the SR is not as well developed as in
the skeletal muscle. Myofibrils bond
with one another and branch. Finally,
the t tubules come in at the z line, not
Cardiac and smooth muscle
4
the A-I junction. Lots of mitochondria
(bigger?).
Regulation
Autonomic regulation. Both systems based in the atria. The sinoatrial node is near
the vena cava. Atrial ventricular node too. Both have sympathetic and
parasympathetic regulation. Some innervation goes to individual cardiac myocytes,
but unlike skeletal muscle, not every cell is innervated.
Intercalated discs (disks)
Irregular, not a straight line. They hold cells together, conduct electric signal. Can
generate their own signal. One contracting cell causes the adjacent cell to contract.
Cardiac and smooth muscle
5
Intercalated discs can act as z lines at the end of the cell.
Regeneration
Only regenerates at about 1% per year! They can undergo hypertrophy, which is not
always good. If there is peripheral resistance, the heart compensates by becoming
bigger, but then this means it is inefficient in pumping blood. Cardio does make the
heart heavier in athletes, but we are not sure. It may make it more efficient (stroke
volume).
Smooth muscle
Found around hollow organs, blood vessels, and many other places. They are
involuntary, a single cell is a myofibril. They are fusiform in shape, not connected to
each other by specialized adherens junctions, but are often connected by gap
junctions to coordinate electrical signals with one another (not all of them have it!).
Some cells are innervated, and the cells connected to it contract due to gap
junctions. Cells can be individually innervated too (either works). Cytoplasm is highly
eosinophilic because of actin and myosin.
Cardiac and smooth muscle
6
Smooth muscle can grow in size (hypertrophy), and/or in number (hyperplasia)
during pregnancy in the uterus. When it contracts, the nucleus morphology becomes
like a corkscrew
Actin and myosin not organized in sarcomeres, so there is no banding, but bundles
of myosin interact with actin that is attached to dense bodies. Dense bodies have
alpha actinin, which is the molecule they are attached to.
Once the light chain is phosphorylated, the heavy chain can bind actin.
Caveolae is where the calcium channels are.
Cardiac and smooth muscle
7