fiber types twitch durations vary from 10 to 200 msecs. this variation is due to the rate of myosin...
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Fiber Types
Twitch durations vary from 10 to 200 msecs. This variationis due to the rate of myosin ATPase activity.
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)
Fig. 11.19
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.
Fig. 11.5
Motor Units
The proportion varies with1) function: posture vs. movement2) heredity and early (pre- and postnatal) development
e.g., quadriceps of world class athletesmarathoners: 82% slow twitchsprinters: 63% fast twitch
Anatomical muscles are mixtures of fiber types.The proportion of different fiber types varies.
from Guyton,Medical Physiology
Summation to Tetanus
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.
Fig. 3-10 Ganong
Fig. 11.14
4 x > twitch tension
Isometric and Isotonic Contraction
isometric (same length) muscle length remains fixed tension (force) producedPROVIDES STIFFNESS
isotonic (“same tension”) muscle shortens against a load load is movedWORK IS PERFORMED
Muscle contractions in the body are a combination of isometric and isotonic contractions.
Fig. 3-9 Ganong
cf. Fig. 11.15 S
classic isotonic contraction =isotonic concentric contraction
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.
e.g., smooth muscle of GI tract, vascular smooth muscle
Fig. 11.20
Smooth muscle• Has tone (tension)– never totally relaxed or
contracted– stimulation increases tone– inhibition decreases tone
• Excitable– responds electrically to
stimulation
• Slow• Efficient
– low ATP use– latching
Fig. 11.23
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
Myosin-based Ca++ regulation of smooth muscle contraction
1) Ca++ activatescalmodulin.
2) Ca-Calmodulin activates myosin light chain kinase (MLCK).
3) MLCK activatesmyosin.
from Alberts, et al. Molecular Biology of the Cell , 3rd ed.
End of Exam 2 Material