1. Muscle

2. Movement with muscles
2/20
movement is one of the most prominent characteristics of animal life
it can be either amoeboid, or more complicated using flagella, cilia or muscles
Galenus (2.c. BC) – “animal spirit” is flowing from the nerves into the muscles causing swelling and shortening
spiral shortening of proteins was the supposed mechanism until the 50’s
new research techniques such as EM helped to elucidate the exact mechanism
muscles can be either smooth or striated
two subtypes of striated muscles are skeletal and heart muscle
mechanism of contraction is identical in all muscle types

3. Structure of the skeletal muscle
3/20
Structure of the skeletal muscle
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

4. Ultrastructure of the striated muscle
4/20
Ultrastructure of the striated muscle
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

5. Sarcomeres in cross-section
5/20
Sarcomeres in cross-section
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

6. Structure of the thin filament
6/20
Structure of the thin filament
G-actin: globular, 5.5 nm spheres
polymerized to “necklace” – two necklaces form a helical structure – F-actin
F-actins (length about 1000 nm, width 8 nm) are anchored to z-discs (-actinin)
in the groove of the F-actin tropomyosin (40 nm) troponin complexes are found
tropomyosin-troponin regulates actin-myosin interaction
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

7. The thick filament the thick filament is built up of myosin molecules
7/20
The thick filament
the thick filament is built up of myosin molecules
myosin molecules consist of two heavy chains (length 150 nm, width 2 nm) and 3-4 (species dependent) light chains
heavy chains form -helices twisted around each other bearing globular heads at the end
myosin molecules associate to form the thick filament (length 1600 nm, width 12 nm)
head regions are arranged into “crowns” of three heads at intervals of 14.3 nm along the thick filament
successive crowns are rotated by 40° resulting in a thick filament with 9 rows of heads along its length

8. Structure of the myosin filament
8/20
Structure of the myosin filament
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-4, 6.

9. Sliding filament theory
9/20
Sliding filament theory
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig
during contraction A-band is unchanged, I-band shortens
length of actin and myosin filaments is unchanged
H.E. Huxley and A.F. Huxley independently described the sliding filament theory: actin and myosin are moving along each other
best proof is the length-tension curve, longer overlap stronger contraction 
sliding is caused by the movement of cross-bridges connecting filaments
contraction is initiated by Ca++ ions released from the SR
excitation propagating on the sarcolemma is conducted to the SR by T-tubules invaginating at the level of z-disks

10. Tubules in the muscle fiber
10/20
Tubules in the muscle fiber
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

11. Connection of T-tubules and SR
11/20
Connection of T-tubules and SR
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

12. Release of Ca++ ions
12/20
AP – spreads from the sarcolemma to the T-tubule – conformational change of the voltage-dependent dihydropyridin receptor – displacement or conformational change of the ryanodin receptor – Ca++ release
half of the ryanodin receptors are free and are opened by the Ca++ ions - trigger Ca++
restoration by Ca++-pump
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

13. Mechanism of sliding
13/20
released Ca++ binds to the troponin complex, myosin binding site on actin is freed 
cross-bridge cycle runs until Ca++ level is high
one cycle 10 nm displacement
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

14. Energetics of the contraction
14/20
Energetics of the contraction
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

15. Types of muscle fibers tonic fibers slow-twitch (type I) fibers
15/20
Types of muscle fibers
tonic fibers
postural muscles in amphibians, reptiles and birds
muscle spindles and extraocular muscles in mammals
no AP, motor axon forms repeated synapses
slow shortening – effective isometric contraction
slow-twitch (type I) fibers
mammalian postural muscles
slow shortening, slow fatigue – high myoglobin content, large number of mitochondria, rich blood supply – red muscle
fast-twitch oxidative (type IIa) fibers
specialized for rapid, repetitive movements – flight muscles of migratory birds
many mitochondria, relatively resistant to fatigue
fast-twitch glycolytic (type IIb) fibers
very fast contraction, quick fatigue
few mitochondria, relies on glycolysis
breast muscles of domestic fowl – white muscle

16. 16/20
Motor unit
skeletal muscles in vertebrates are innervated by spinal or brainstem motoneurons – “final common pathway”
one fiber is innervated by only one motoneuron
one motoneuron might innervate several fibers (usually about 100) – motor unit
1:1 synaptic transmission - 1 AP, 1 twitch
regulation of tension
AP frequency - tetanic contraction
recruitment – involvement of additional motor units
depending on the task, different types of fibers are activated – one motor unit always consists of fibers of the same type
type of muscle fibers can change, it depends on the innervation and the use – swapping of axons, change in type; difference between the muscles of a heavyweight lifter and a basketball player

17. Heart muscle
17/20
many differences, many similarities compared to skeletal muscles
pacemaker properties – myogenic generation of excitation
diffuse, modulatory innervation
individual cells with one nucleus
electrical synapses - functional syncytium
AP has plateau, long refractory period
voltage-dependent L-type Ca++-channels on T-tubules - entering Ca++ triggers Ca++ release from SR
Ca++ elimination: Ca++-pump (SR), Na+/Ca++ antiporter (cell membrane) - digitalis: inhibition of the Na/K pump - hypopolarization and increased Ca++ level
-adrenoceptor: IP3 - Ca++ release from SR
-adrenoceptor: cAMP - Ca++ influx through the membrane

18. Structure of the heart muscle
18/20
Structure of the heart muscle
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

19. Smooth muscle I. not striated
19/20
Smooth muscle I.
not striated
actin filaments are anchored to the plasma membrane or to the dense bodies in the plasma
myosin filaments in parallel
single-unit smooth muscle
myogenic contraction
electrical synapses – synchronous contraction
contracts when stretched - basal myogenic tone
innervation modulates a few cells only through varicosities
in the wall of internal organs (gut, uterus, bladder, etc.)
multi-unit smooth muscle
neurogenic contraction
individual cells innervated by individual varicosities
e.g. pupil, blood vessels

20. Smooth muscle II.
20/20
activation by pacemaker cells, hormones, mediators released from varicosities
no fast Na+-channel
AP is not necessarily generated; it might have plateau if present
contraction is initiated by the increased level of Ca++ ions
Ca++ influx through voltage/ligand-dependent channels, release from the SR (less developed)
instead of troponin-tropomyosin, caldesmon blocks the myosin binding site on actin – freed by Ca-calmodulin, or phosphorylation (PKC)
phosphorylation of myosin light chain (LC-kinase – activated by Ca-calmodulin) also induces contraction
light chain phosphorylation at another site by PKC - relaxation

21. End of text

22. Length-tension relation
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig

23. Role of the troponin complex
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig