Fiber Types Twitch durations vary from 10 to 200 msecs. This variation is due to the rate of myosin ATPase activity. Fig. 11.19 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)

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.

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

Summation to Tetanus  4 x > twitch tension Fig. 3-10 Ganong  4 x > twitch tension Fig. 11.14 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.

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. 11.15 S

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. 11.20 e.g., smooth muscle of GI tract, vascular smooth muscle

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. 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++ 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.

End of Exam 2 Material