Muscle tissue
Muscle fibers Rich capillary network Fibrous connective support tissue
Donley et al., 2004
Muscle types
Skeletal muscle40% of body massMultinucleate cells –very long!Muscle has cross-striations
Voluntary (higher brain) controlContracts rapidlyTerminal cell type
Cardiac muscle
Cross-striations Intercalated discs- intercellular
communication Involuntary contraction-speed and strength
regulated by endocrine and nervous systems
Myogenic contraction
Smooth muscle
Non-striated Individual cells Visceral smooth muscle- forms sheets Slow contracting Involuntary contraction
2002 by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter
Muscle tissue nomenclature
Cytoplasm = sarcoplasm ER = sarcoplasmic reticulum Cell membrane = sarcolemma
Skeletal muscle
Muscle cells fuse together during development (up to 30 cm long)
Nuclei are usually peripheral
2002 by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter
Muscle sheath
EpimysiumDense connective tissue that surrounds entire
muscle Perimysium
Thin connective tissue layer that surrounds fiber bundles
EndomysiumSurrounds each muscle fiber (cell)Mainly reticular fibers and basal lamina
Architecture of a skeletal muscle fiber Muscle fibers are composed of myofibrils Myofibrils are composed of repeating units called
sarcomeres Sarcomeres are the smallest functional unit of
skeletal muscle Sarcomeres are composed of overlapping thick and
thin filaments Thin filaments are composed of actin and associated
proteins Thick filaments are composed of myosin
Thick filaments-Myosin II
Thin filaments- Actin and associated proteins
M line-Creatine kinase
Sarcomere regions
I Band, isotropic- similar polarization characteristics throughout
A Band, anisotropic- different polarization characteristics throughout
H Band- ‘heller’ or bright Z line, ‘Zwischenscheibe’ or between M line, ‘mittel’ or middle
Insect flight muscle
2002 by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter
Figure 10—10. Electron micrograph of skeletal muscle of a tadpole. Note the sarcomere with its A, I, and H bands and Z line.
Sliding filament theory of muscle contraction
© 2000 by W. H. Freeman and Company
Molecular characteristics of contractile filaments Myosin II Each thick filament contains more than
200 myosin molecules
Myosin head ATPase activity Actin binding Molecular hinge
Actin thin filament 2 strands of F actin formed from globular or G
actin Tropomyosin- double stranded long filament
that wraps around F actin Troponin
TnI- inhibits myosin-actin interactionTnT- binds to tropomyosinTnC- binds to calcium
See it now!!!!
Troponin
Troponin•T, tropomyosin binding site•C, calcium binding site•I, inhibitory site
Mechanism of contraction1. Calcium binds to TnC2. Conformational change in troponin exposes
myosin binding site on actin3. Myosin binds to actin4. ATP cleavage causes myosin to bend5. This causes actin filament to move pass
myosin6. A new ATP binds to myosin head. This causes
head to de-attach and swing back on hinge
Thick and thin filaments
Actin (alpha helix) – thin filament
Myosin – thick filament
Z-disc
Thick and thin filaments
Excitation contraction coupling
Depolarization of sarcolemma causes release of calcium within muscle fiber
Application of electrical stimulus to area of I band produces sarcomere shortening
How do changes in membrane potential translate into an increase in intracellular calcium?
T-tubules
Invaginations of sarcolemma that surround myofibril at every sarcomere in the region of the A-I band
Adjacent to each T-tubule is a sarcoplasmic reticulum, which stores calcium
T-tubules
Action potential travels along sarcolemma, down T-tubule
This causes calcium to be released from sarcoplasmic reticulum
T-tubules in fish muscle
Long live the Triad!!!- SR, T-tubule, SR
The triad
the triad is a diagnostic feature of skeletal muscle
It is composed of two lateral cisternae on either side of a T-tubule.
It is not found in cardiac or smooth muscle
Neuromuscular junction
© 2000 by W. H. Freeman and Company.
Lodish et al., © 2000 by W. H. Freeman and Company, [Adapted from M. J. Berridge, 1993, Nature 361:315.]
Calcium channel inhibitors- ryanodine blocker
AmlodipineNorvasc
Skeletal Muscle Innervation
One muscle fiber innervated by one motor axon
Motor unit- motor axon and the muscle fibers that it innervates
Graded contraction depends upon # motor units recruited
Modulators of the neuromuscular synapse Strychnine- antagonist at the glycine
inhibitory synapse (activation of glycine receptors inhibits neurons by opening Cl- channels).
Nicotine – derive from the dried leaves of the tobacco plant Nicotinia tabacum
Muscarine -from the poisonous red mushroom Amanita muscaria
Physostigmine-inhibits Ach-esterase at neuromuscular junction
Calabar bean (Physostigma venenosum)
Acetylcholinesterase inhibitors
Organophosphate insecticides such as parathion and malathion
Organophosphate nerve gases such as sarin.
Acetylcholinesterase inhibitors
Organophosphate pesticides and physostigmine inhibit Ach-esterase by forming a covalent bond in the Ach binding pocket of the enzyme
Much slower dissociation from enzyme than Ach, which binds for 30-40 s. Physostigmine and organophosphates may remain bound to enzyme for 30-40 min or much longer.
Toxins from Clostridium bacteria that affect presynaptic motor terminals:•Tetanus toxin•Botulinum toxin cleave the vesicle SNARE protein,
synaptobrevin
Tetanus toxin
Poisoning with the tetanus toxin from Clostridium tetani bacteria inhibits the secretion of inhibitory neurotransmitters (GABA, glycine) controlling motor neurons in ventral spinal cord.
This produces a disinhibition of skeletal motor neurons, leading to overexcitement and hypercontraction of skeletal muscle fibers.
Botulinum toxin
Inhibits synaptobrevin in ACh neurons innervating skeletal muscle, preventing ACh release.
Cardiac muscle
Striated, one or two nuclei per cell Nucleus located in the center of cell Intercalated discs
Zonula adherens Desmosomes Gap junctions
40% of cytoplasm is mitochondria (compared to 10% in skeletal muscle).
Figure 10—27. Electron micrograph of a longitudinal section of heart muscle. Note the striation pattern and the alternation of myofibrils and mitochondria rich in cristae. Note the sarcoplasmic reticulum (SR), which is the specialized calcium-storing smooth endoplasmic reticulum. x30,000.
Figure 10—22. Drawing of a section of heart muscle, showing central nuclei, cross-striation, and intercalated disks.
Figure 10—23. Photomicrograph of cardiac muscle. Note the cross-striation and the intercalated disks (arrowheads). Pararosaniline—toluidine blue (PT) stain. High magnification.
Figure 10—24. Longitudinal section of portions of 2 cardiac muscle cells. The transversely oriented parts of the intercalated disk consist of a fascia adherens and numerous desmosomes. The longitudinal parts (arrows) contain gap junctions.
Figure 16-75. Effect on the heart of a subtle mutation in cardiac myosin. Left, normal heart from a 6-day old mouse pup. Right, heart from a pup with a point mutation in both copies of its cardiac myosin gene, changing Arg 403 to Gln. Both atria are greatly enlarged (hypertrophic), and the mice die within a few weeks of birth. (From D. Fatkin et al., J. Clin. Invest. 103:147, 1999.)
2002 by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter
Smooth muscle
Found in visceral organs, arteries and veins, iris of the eye
Each cell is surrounded by reticular fibers and a basal lamina
These harnesses help to convey force started by smooth muscle contraction
Smooth muscle
No T-tubules, SR is poorly developed Actin and myosin are no organized into
lattice network. Excitation-contraction coupling-
Calcium binds to calmodulinCauses phosphorylation of myosin light chain
kinaseNo tropomyosin
© 2000 by Geoffrey M. Cooper
Contraction in smooth muscle is controlled by phosphorylation of myosin
Figure 10—29. Photomicrographs of smooth muscle cells in cross section (upper) and in longitudinal section (lower). Note the centrally located nuclei. In many cells the nuclei were not included in the section. PT stain. Medium magnification.
Figure 10—30. Drawing of a segment of smooth muscle. All cells are surrounded by a net of reticular fibers. In cross section, these cells show various diameters.
Figure 10—33. Smooth muscle cells relaxed and contracted. Cytoplasmic filaments insert on dense bodies located in the cell membrane and deep in the cytoplasm. Contraction of these filaments decreases the size of the cell and promotes the contraction of the whole muscle. During the contraction the cell nucleus is deformed.
Smooth muscle lacks troponin but has tropomyosin. Another protein, Caldesmon, has been implicated in smooth muscle contraction.•Low Ca2+ - caldsemon binds to actin-TM and prevents myosin from binding•High Ca2+ - Ca2+-CaM complex binds to caldesmon, releasing it from actin