• Myofibers – branching; syncytial cardiomyocytes
• Cardiomyocyte: 1-2 central nucleus;
• Myofibrils – poorly defined (possess myofilaments)
• Have glycogen granules, especially at either pole of
• T-tubules and SR (dyads at Z-disks)
• Diad – formed by T-tubule and SR cistern
• Intercalated disks – gap junctions
• Contraction – spontaneous & rhythmic (autonomic
• Do not regenerate – repair of cardiac tissue forms
fibrous CT (scar) by fibroblasts.
The transversely oriented parts of the intercalated disk consist of a fascia adherens and numerous desmosomes. The
longitudinal parts (arrows) contain gap junctions. Mitochondria (M) are numerous. Fibrils of reticular fibers are seen between
the two cells. x18,000. (Junquiera, 2005).
• Contact between cells is accomplished by interdigitation in the transverse region; contact is broad and flat in the
longitudinal plane (LP). A, A band; I, I band; Z, Z line. (Redrawn and reproduced, with permission, from Marshall JM:
The heart. In: Medical Physiology, 13th ed, Vol 2. Mountcastle VB [editor]. Mosby, 1974. Based on the results of
Fawcett DW, McNutt NS: J Cell Biol 1969;42:1, modified from Poche R, Lindner E: Zellforsch Mikrosk Anat
• Transverse portion:
1. Cardiac myofiber are connected by z. adherens in
Z-lines (actin filament)
2. Desmosomes (macula adherens) connects the SR
3. Gap junctions connects the underlying myofibrils
• Lateral portion: Desmosomes and larger gap
junctions (promotes coupling and contraction) –
• Sarcomere units and SR (sarcoplasmic reticulum) cisterns and T-tubules (diad) near Z-disks.
• Excitation in cardiac muscles
• Modified cardiac cells in the Bundle of His
• Rich in glycogen and mitochondria
• Impulse conducting system:
• SA node AV node AV Bundle of His
• Bundles of Purkinje fibers are
present in areas of connective
tissue between areas of
"normal" cardiac muscle
tissue and beneath the
• Purkinje fibers appear as a
chain of light blue profiles
with a red rim.
• Relax: calcium leaks into the sarcoplasm at a slow
rate -> automatic rhythm
• Calcium reaches calcium channels passing through T-
tubules and sarcolemma
• Calcium stored in the SR is released through
• Calcium binds to Troponin C contraction
• found in the walls of hollow organs
– gastrointestinal tract, the bladder, and the uterus
– in the vasculature, the ureters, the bronchioles
– the muscles of the eye
• The functions:
– to produce motility (e.g., to propel chyme along the
gastrointestinal tract or to propel urine along the
– to maintain tension (e.g., smooth muscle in the walls
of blood vessels).
• specialised for continuous contractions of
relatively low force, producing diffuse
movements resulting in contraction of the
whole muscle mass.
• elongated, spindle-shaped cells
(which may be branched occasionally).
• no cross striations
• generally much shorter than skeletal muscle
• contain only one nucleus which is elongated and
• When contracted:
– nucleus appear to be spiral-shaped.
Smooth muscle fibers
• bound together in irregular, branching fasciculi -- the
functional contractile units.
• individual muscle fibres are arranged roughly parallel to
– the thickest part of one cell lying against the thin
parts of adjacent cells.
– Gap junctions (nexus) are areas of close opposition
(~2 nm) between plasma membranes of separate
• supporting collagenous tissue between individual muscle
fiber and between fasciculi.
• In many tubular
• smooth muscles are
disposed in layers
• the cells of one layer arranged at right angles to those of
the adjacent layer.
This arrangement permits a wave of contraction to pass
down the tube, propelling the contents forward.
With minimal amount of
– contains clumps of
large cells with pale
longitudinal outer smooth muscle layer
inner circular layer
of smooth muscle
• Thin filament: actin
• anchored either to the plasma membrane or to cytoplasmic
structures known as dense bodies, which are functionally
similar to the Z lines in skeletal muscle fibers.
• Tropomyosin; NO TROPONIN
• calmodulin, a calcium-binding protein
• Thick filaments: myosin
of smooth muscle
- arranged in a criss-cross
lattice inserted around the
No regular alignment of
filaments into sarcomeres
• No T tubule:
• Caveolae - surface vesicles in
individual cells which transmit
action potentials similar to T
Smooth muscle contraction
• Controlled by the autonomic nervous system,
hormones, autocrine/paracrine agents, and
other local chemical signals.
• Calcium is the signal for contraction in smooth
• Myosin interacts with actin only when its light
chain is phosphorylated.
Smooth muscle contraction
• Influx of Ca2+
• Because smooth muscle does not contain
binds to calmodulin
- calmodulin complex activates the
enzyme myosin light chain kinase
– Result: Phosphorylation of myosin and thereby
• Phosphorylated myosin has a high affinity for actin
• Crossbridges form between myosin and actin.
• Junquiera LC, Carneiro J. 2005. BASIC
HISTOLOGY : TEXT AND ATLAS 11th
McGraw-Hill’s ACCESS MEDICINE.
• Young B. 2009. WHEATER’S FUNCTIONAL
Edition. UK: Churchill
Livingstone. Distributor: Phils: C & E