1. Physiology of skeletal muscle contraction – events at the myofilaments
Resting sarcomere
Active-site exposure
ADP
Myosin head
ADP +
Sarcoplasm P + P
Troponin
Ca2+
Ca2+
Tropomyosin
Actin
Active site
ADP
ADP P + P +
Calcium (Ca+2 ) gates in the SR open, allowing Ca+2 to diffuse into the sarcoplasm
Calcium will bind to troponin (on the thin myofilament), causing it to change its shape.
This then pulls tropomyosin away from the active sites of actin molecules.
Figure 7-5
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

2. Physiology of skeletal muscle contraction – events at the myofilaments
Resting sarcomere
Active-site exposure
Cross-bridge formation
ADP
Myosin head
ADP + P +
Sarcoplasm P
Troponin
ADP + P
Ca2+
Ca2+
Ca2+
ADP
Tropomyosin
Actin
Active site
Ca2+ +
ADP
ADP P P + P +
Myosin heads are “energized” by the presence of ADP + PO43- at the ATP binding site (energy is released as phosphate bond of ATP breaks)
Once the active sites are exposed, the energized myosin heads hook into actin molecules forming cross-bridges
Figure 7-5
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3. The ADP & phosphate group are released from the myosin head
Physiology of skeletal muscle contraction – events at the myofilaments
Resting sarcomere
Active-site exposure
Cross-bridge formation
ADP
Myosin head
ADP + +
Sarcoplasm P P
Troponin
ADP + P
Ca2+
Ca2+
Ca2+
ADP
Tropomyosin
Actin
Active site
Ca2+ P +
ADP
ADP P + P +
Pivoting of myosin head
Using the stored energy, the attached myosin heads pivot toward the center of the sarcomere
The ADP & phosphate group are released from the myosin head
ADP + P
Ca2+
Ca2+
ADP + P
Figure 7-5
5 of 7
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

4. Physiology of skeletal muscle contraction – events at the myofilaments
Resting sarcomere
Active-site exposure
Cross-bridge formation
ADP
Myosin head
ADP + +
Sarcoplasm P P
Troponin
ADP + P
Ca2+
Ca2+
Ca2+
ADP
Tropomyosin
Actin
Active site
Ca2+ P +
ADP
ADP P + P +
A new molecule of ATP binds to the myosin head, causing the cross bridge to detach from the actin strand
The myosin head will get re-energized as the ATP  ADP+P
Cross bridge detachment
Pivoting of myosin head
ATP
ADP + P
Ca2+
Ca2+
Ca2+
Ca2+
ATP
ADP + P
As long as the active sites are still exposed, the myosin head can bind again to the next active site

5. Physiology of skeletal muscle contraction – events at the myofilaments
Resting sarcomere
Active-site exposure
Cross-bridge formation
ADP
Myosin head
ADP + P +
Sarcoplasm P
Troponin
ADP + P
Ca2+
Ca2+
Ca2+
ADP
Tropomyosin
Actin
Active site
Ca2+ P +
ADP
ADP P + P +
Myosin reactivation
Cross bridge detachment
Pivoting of myosin head
ADP
ATP
ADP + P + P
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
ADP P +
ATP
ADP + P

6. Muscle fatigue:
Prolonged and strong muscle contraction lead to muscle fatigue. This results from:
Depletion of glycogen stores in the muscle (inability of the contractile and metabolic processes to continue supplying the same work).
Diminished transmission in the NMJ.
Interruption of blood supply or flow results in nutrient and O2 deficiency.

7. Hypertrophy – “stressing” a muscle (i. e
Hypertrophy – “stressing” a muscle (i.e. exercise) causes more myofilaments/myofibrils to be produced within muscle fibers; allows for more “cross bridges” resulting in more force (strength) as well as larger size
There are always some motor units active, even when at rest. This creates a resting tension known as muscle tone, which helps stabilize bones & joints, & prevents atrophy
Play IP Contraction of motor units p. 3-7

8. Muscle Hypertrophy Muscle Atrophy Muscle growth from heavy training
increases diameter of muscle fibers
increases number of myofibrils
increases mitochondria, glycogen reserves
Muscle Atrophy
Lack of muscle activity
reduces muscle size, tone, and power

9. Tension Production Tension of a Single Muscle Fiber
The all–or–none principal:
as a whole, a muscle fiber is either contracted or relaxed
Tension of a Single Muscle Fiber
Depends on:
the number of pivoting cross-bridges
the fiber’s resting length at the time of stimulation
the frequency of stimulation

10. Frequency of Stimulation
A single neural stimulation produces:
a single contraction or twitch
which lasts about 7–100 msec
Sustained muscular contractions:
require many repeated stimuli

11. 3 Phases of Twitch Latent period before contraction:
the action potential moves through sarcolemma
causing Ca2+ release
Contraction phase:
calcium ions bind
tension builds to peak
Relaxation phase:
Ca2+ levels fall
active sites are covered
tension falls to resting levels

12. Smooth Muscle Fusiform cells One nucleus per cell Nonstriated
Involuntary
Slow, wave-like contractions

13. Smooth Muscle Grouped into sheets in walls of hollow organs
Longitudinal layer – muscle fibers run parallel to organ’s long axis
Circular layer – muscle fibers run around circumference of the organ
Both layers participate in peristalsis

14. Smooth Muscle Contraction: Mechanism

15. Smooth Muscle Relaxation: Mechanism

16. Cardiac muscle tissue
Makes up myocardium of heart
Unconsciously (involuntarily) controlled
Microscopically appears striated
Cells are short, branching & have a single nucleus
Cells connect to each other at intercalated discs