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