1. Fiber Types Twitch durations vary from 10 to 200 msecs. This variation
is due to the rate of myosin ATPase activity.
Fig
Slow Oxidative: “slow twitch” fiber - sustained effort
low power, fatigue resistant
mostly aerobic metabolism (mitochondria)
Fast Glycolytic: “fast twitch” fiber - bursts of power
high power, fatigue easily
can use anaerobic metabolism
(glycogen  glucose  lactic acid)

2. Motor Units All of the fibers in a motor unit are the same type;
Fig. 11.5
All of the fibers in a motor unit are the same type;
the pattern of stimulation from the motor neuron determines whether a fiber will be fast or slow twitch.

3. Anatomical muscles are mixtures of fiber types.
The proportion of different fiber types varies.
from Guyton,
Medical Physiology
The proportion varies with
1) function: posture vs. movement
heredity and early (pre- and postnatal) development
e.g., quadriceps of world class athletes
marathoners: 82% slow twitch
sprinters: 63% fast twitch

4. Summation to Tetanus  4 x > twitch tension
Fig Ganong
 4 x > twitch tension
Fig
When the time between stimuli is shortened, tension increases above twitch tension, because of temporal and wave summation. Sustained summation produces tetanic tension, or tetanus.

5. Isometric and Isotonic Contraction
isotonic (“same tension”)
muscle shortens against a load
load is moved
WORK IS PERFORMED
classic isotonic contraction =
isotonic concentric contraction
isometric (same length)
muscle length remains fixed
tension (force) produced
PROVIDES STIFFNESS
Muscle contractions in the body are a combination of isometric and isotonic contractions.
Fig. 3-9 Ganong
cf. Fig S

6. Smooth Muscle Small, spindle shaped fibers One nucleus per cell Smooth
no visible cross striations
dynamic (nonpermanent) sarcomeres
Visceral smooth muscle
The cells are interconnected by gap junctions, and function as a “single unit,” as a functional syncytium.
Fig
e.g., smooth muscle of GI tract,
vascular smooth muscle

7. Smooth muscle Has tone (tension) Excitable Slow Efficient
never totally relaxed or contracted
stimulation increases tone
inhibition decreases tone
Excitable
responds electrically to stimulation
Slow
Efficient
low ATP use
latching
Fig

8. Comparison to Skeletal Muscle
Similarities
Contraction by sliding actin and myosin filaments
Contraction is regulated by cytosolic Ca++.
Ca++ is stored in and released from the SR
Differences
Extracellular Ca++ is required for contraction.
via voltage-gated calcium channels
e.g., calcium channel blockers cause vasodilation
How much Ca++ comes from extracellular vs. intracellular (SR) sources?
varies widely
Myosin-based Ca++ regulatory system

9. Myosin-based Ca++ regulation of smooth muscle contraction
1) Ca++ activates
calmodulin.
2) Ca-Calmodulin activates myosin light chain kinase (MLCK).
3) MLCK activates
myosin.
from Alberts, et al.
Molecular Biology
of the Cell , 3rd ed.

10. End of Exam 2 Material