Muscle

Movement with muscles

• 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

2/20

Structure of the skeletal muscle

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-1.

3/20

Ultrastructure of the striated muscle

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-2.

4/20

Sarcomeres in cross-section

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-3.

5/20

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. 10-5.

6/20

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

7/20

Structure of the myosin filament

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-4, 6.

8/20

Sliding filament theory

• 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

9/20

Tubules in the muscle fiber

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-21.

10/20

Connection of T-tubules and SR

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-25.

11/20

Release of Ca++ ions

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-4.

• 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

12/20

Mechanism of sliding• 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. 10-11.

13/20

Energetics of the contraction

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-29.

14/20

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

15/20

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

16/20

Heart muscle• 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

17/20

Structure of the heart muscle

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-50.

18/20

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

19/20

Smooth muscle II.• 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

20/20

End of text

Length-tension relation

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-8.

Role of the troponin complex

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 10-16.