1. Muscle Contraction

2. (contracting organelle) Myosin – thick filament
Connective Tissue
Structure
surrounds
Gross muscle
organ
Epimysium
Fascicle
(bundles of cells)
tissues
surrounds
Perimysium
Muscle fiber
(muscle cell)
Endomysium
surrounds
cells
Myofibril
(contracting organelle)
organelles
Myofilaments
Actin – thin filament
Myosin – thick filament
molecules

3. Molecular Structure of Myofilaments

5. Myosin
rod like tail or axis ending in 2 globular heads - “crossbridges”
- heads contain ATPase (enzyme)
tail
heads

6. Actin
globular subunits - G actin are found in long strands, like a double strand of twisted beads

7. Actin
tropomyosin spirals around beads, to add strength and stiffen, thin ribbon
troponin molecules bond actin to tropomyosin, has binding sites that will join with myosin crossbridge
Binding sites

9. Sarcoplasmic Reticulum

10. Sarcoplasmic Reticulum
smooth endoplasmic reticulum of muscle cells
storage area for Ca ions
At H zones and A - I junctions tubes fuse and form lateral channels - terminal cisternae which feed into transverse tubules (T tubules) at each Z line.
T tubules receive nerve stimuli and provide a pathway for oxygen, glucose, and Ca ions.
(because continuous with membrane, nerve impulse allowed deep into muscle)

12. Mechanism of Contraction
Mechanism of Contraction when muscle cells contract their sarcomeres shorten (Z to Z distance decreases)

13. Sliding Filament Theory of Muscle Contraction 1954 Hugh Huxley
In absence of Ca, myosin - binding sites on actin are physically blocked by tropomyosin.
When Ca ions are released through T tubules, they bond with troponin.
The resulting change in molecular conformation physically moves away tropomyosin from binding sites.

14. Exposure of binding sites leads to the following events in rapid succession:
crossbridges on myosin heads bond to troponin binding sites
as heads bind, they change shape to a bent shape, which causes heads to pull on actin sliding it to the center at the same time, ADP released from head (energy supplied the movement)

15. Exposure of binding sites leads to the following events in rapid succession:
As a new ATP molecule binds to myosin head, the bridge is released from actin.
Hydrolysis of ATP to ADP + P (ATPase) provides energy needed to cock myosin heads to high energy position (potential energy).

16. Now, myosin head is ready to repeat action further down thin filament
Now, myosin head is ready to repeat action further down thin filament. The cross bridges walk along the actin.
The contraction ends when Ca ions are withdrawn from sarcoplasm (lack of nerve stimulus).

19. Relaxed Sarcomere
Contracted Sarcomere

20. Rigor Mortis BUT in absence of ATP synthesis (shortly after
Injured and dying cells are
unable to exclude Ca, so
crossbridge binding is promoted.
BUT in absence of ATP
synthesis (shortly after
breathing stops) detachment is impossible causing rigor mortis.
Muscles begin to stiffen hr. after death.
Peak about 12 hr. and dissipates over next hr. due to breakdown of tissue.

21. Regulation of Contraction
In order for a nerve impulse to trigger a muscle response, the impulse must travel over the gap (neuromuscular junction) where two cells meet. (neuron and muscle)
In the gap (extracellular space = synapse) are sacs containing the neurotransmitter - acetylcholine (Ach).
Regulation of Contraction

23. Muscle Twitch studies done in vitro (outside the body)
myogram - graphic recording of contractile activity - traces twitch
muscle twitch - muscle responds to a single brief stimulus (contracts and relaxes)
- may be strong or weak
1. latent period - time between stimulus and response
2. contraction
3. relaxation - no more contractile force, tension decreases
(in vivo (inside body) - longer and smoother)

25. Motor Units
The strength of the muscle twitch depends on the number of motor units involved.
motor unit - a motor neuron and all the muscle fibers it supplies
Muscles with fine control have small units. ex: fingers, eyes
Large weight bearing muscles have less precise movement and larger motor units. ex: hip

26. As impulses continue at faster rate all evidence of relaxation disappears and muscle has a smooth sustained contraction. - tetanus
Prolonged tetanus will lead to fatigue.

27. Types of Muscle Contractions
Muscles do not always shorten during a contraction.
contraction - active process of generating force within a muscle by cross bridge activity
muscle tension - force exerted by a contracting muscle on some object
load - wt/ resistance to movement exerted by the object on the muscle
muscle tension and load are opposing forces
to move a load, the muscle tension must be greater than the load

28. isotonic contractions - “equal tension”
muscle shortens to move the load
most common
best for developing strength
ex: muscles moving the leg

29. isometric contractions - “equal distance”
result in an increase if tension but no shortening because the load is too great
ex: trying to lift a piano single handedly, muscles maintaining posture
In the body most
movement is a
combination because
muscles work
together.