MUSCLE TISSUE II
Smooth muscle. Ultrastructure of the myofibril. Muscle contraction
Institute of Histology and Embryology
Author: doc.MUDr. Tomáš Kučera, Ph.D.
Subject: General Histology and General Embryology, code B82241
Datum: 8.11.2013
1
Smooth muscle tissue
Basic structural and functional unit:
- smooth muscle cell – fusiform shape 5-10m in diameter, 20500 m in length, rod-like nucleus with several nucleoli
- does not display cross-striations
Smooth muscle tissue
smooth muscle cell
- surrounded by external lamina, network of reticular fibrils,
interconnected via gap junctions
- mitochondria in the perinuclear region, ribosomes, GER,
Golgi complex, sarcoplasmic reticulum is less developed
- dense bodies, thin myofilaments, thick myofilaments,
intermediate filaments
- synthetic capabilities: collagens, elastin, proteoglycans
Arrows:dense bodies
Smooth muscle tissue
• Visceral smooth muscle
- Walls of tubular organs (intestine, vessels, urinary
excretory passages and genital ducts)
• Mosaique smooth muscle
- iris
Ultrastructure of a myofibril
Myofibrils in the skeletal muscle fiber
H
nucleus
A
Z
I
M
Thin myofilaments (actin myofilaments) - double-helix of F-actin,
tropomyosin a troponin (a complex of 3 subunits): TnC binds Ca2+, TnT binds tropomyosin,
TnI inhibits interaction between actin and myosin)
Figure from Junqueira, Carneiro, Basic Histology, 2003
G-actin
(a binding site
for myosin)
Tropomyosin (a double helix from 2
polypeptide chains)
TnI TnC
TnT
F actin
Troponin
Tropomyosin
Tropomyosin stabilizes thin myofilaments
Myosin myofilament
Thick myofilament (myosin myofilament) contains myosin II
Myosin molecule is composed of two heavy chains and two pairs of
light chains
Figures from Ross, Pawlina, Histology, 2006
Flexible junction
Myosin light chains
Binding
site for
actin
heavy chains
tail (double helix from 2 polypeptide chains)
head
Binding
site for
ATP
ATP-ase
activity
Myosin myofilament – in skeletal muscle
M-linie
bipolar filament
Myomesin and C protein maintain regular arrangement of myofilaments in sarcomere
SARCOMERE – arrangement of myofilament (Figure from: Junqueira´s Basic Histology, Mescher, 2010)
Thin filaments are attached on one side to Z line (α-actinin), with their opposite end they extend
to A-band. Accessory proteins: nebulin and tropomodulin
Thick filaments occupy the whole A-band. They are bound to M line via myomesin and through
titin they are anchored to Z line.
sarcomere
thin filaments
Z line
I band
titin
M
thick filaments
A band
Z line
I band
Arrangement of myofilaments in the sarcomere is provided
by accessory proteins. Titin attaches thick myofilaments to Z-line, αactinin attaches thick myofilaments do Z line, nebulin twists around thin
myofilaments helps to attach them to Z-line, tropomodulin regulates the
length of thin filaments
Schéma: Ross, Pawlina, Histology, 2006
MECHANISM OF MUSCLE CONTRACTION
Length of myofilaments does not change
during contraction
Motor end-plate
(efferent nerve ending in the skeletal muscle)
Impulse transduction
MP
Release of acetylcholine, Ach binds to
receptors at the sarcolemma (postsynaptic
membrane).
Change in ion permeability leads to
depolarization of sarcolemma, which spreads
along the membrane of T-tubules.
Membrane depolarization of T tubules opens
voltage gated Ca2+ ion channels of
sarcoplasmic reticulum membranes.
Ca2+ ions released from sarcoplasmic
reticulum enter cytosol and bind to TnC subunit
of troponin.
Spatial configuration of troponin is changed
upon Ca2+ binding to TnC, binding sites for
myosin on actin (active sites) are demasked
and myosin heads bind to actin.
Actin myofilament
Myosin myofilament
From Junqueira´s Basic Histology, Mescher, 2010
MECHANISM OF MUSCLE CONTRACTION- II
ATP
End of muscle contraction
Myosin head has the binding site for
ATP. After the myosin head binds to
actin (co-factor of the myosin ATP-ase),
rapid cleavage follows ATP→ADP + Pi
The energy released leads to flexion of
the myosin head, which causes shifting
of actin myofilament (sliding of the
myofilament) towards the center of the
sarcomere. Next the myosin head is
disengaged and binds again to the
next actin molecule.
This cyclic process: binding, flexion,
detachment of the myosin head
repeats many times and leads to a
complete insertion of actin filaments
and shortening of the sarcomere (as
a consequence the muscle fiber or
cardiomyocytes contract as well)
End of contraction (repolarization of
sarcolemma) – transport of Ca2+ back
to sarcoplasmic reticulum, tropomyosin
covers again myosin binding sites on G
actin and actin myofilaments return to
their original position.
From Junqueira´s Basic Histology, Mescher, 2010
Change of the sarcomere length during contraction
Mechanism of smooth muscle
cell contraction
Stimulus:
Parasympathetic
and sympathetic
innervation
Hormones
Contractile apparatus of the smooth muscle cell
Relaxed cell
Sarcolemma
Dense body
IMF (desmin,
vimentin)
Actin filament
Myosin
filament
Dense
bodies
Myofilaments
Calmodulin – Ca2+ binding protein
Nucleus
Contracted cell
Myosin
filament
Tropomyosin
Actin
Dense bodies contain actin-binding protein (alpha-actinin)
Ross, Pawlina, Histology, 2003
Regeneration of the skeletal
muscle
• satellite cells
• proliferation and fusion – recapitulation of
embryonic myogenesis
• myoblasts → myotubes
Muscle atrophy
• Aging
• Neurogenic
• Nutrition
• Inactivity
Skeletal muscle fiber
Energy source: fast – ATP and
phosphocreatine
Fatty acids, glucose
anaerobic glycolysis → lactate
Types of fibers: can be differentiated
morphologically, histochemically and
biochemically
Types of skeletal muscle fibers
Type I – slow oxidative fibers, slow-twitch, fatigue resistant motor units
contains high amount of myoglobin, metabolize fatty acids, slow and
continuous contraction
- limb muscles, long muscles of the back
Type IIA – fast oxidative glycolytic fibers, fast-twitch, fatigue resistant motor
units
– many mitochondria, high amount of myoglobin, capable of anaaerobic
glycolysis, fast and short contractions
Type II B – fast glycolytic fibers, fast-twitch, fatigue prone motor units
Less myoglobin, few mitochondria
- high anaerobic enzyme activity, large amoun of glycogen stores
- rapid contraction, precise movements
Types of skeletal muscle fibers
Histochemical demonstration of
Type I and Type IIB fibers
(NADH-TR in the mitochondria)