Skeletal Muscle Classification of muscle fiber types Sarcomeres

Skeletal Muscle • Classification of muscle fiber types • Sarcomere’s length and muscle tension during contraction

Skeletal muscle fibers are classified • According to contraction speed and resistance to fatigue during repeated stimulation. • 1. Fast-twitch glycolytic fibers • 2. Fast-twitch oxidative fibers • 3. Slow-twitch (oxidative) fibers

Fast-twitch fibers • Fast speed of contraction • Fast myosin ATPase isoform

Fast-twitch fibers • Twitch has a short duration • Ca++ is quickly removed from the cytosol during relaxation • Fast Ca++ ATPase on SR • Where? Hands (typing, piano), Eyelids (blinking).

Slow-twitch fibers • For posture, standing, walking • Lift heavy loads • Contractions last a long time

Fast glycolytic fibers • Fatigue quickly • Anaerobic glycolysis to generate ATP • acidosis

Slow-twitch fibers • Fatigue slowly • Oxidative phosphorylation for ATP production • Blood vessels bring oxygen • Myoglobin inside the fibers. It has a high binding affinity of oxygen. • Small diameter of fibers, short distance for oxygen diffusion

Fast-twitch glycolytic fibers • Fewer blood vessels • Larger muscle fibers • Quickly run out of oxygen

Fast twitch oxidative • Have some myoglobin • Intermediate size • Use oxidative and glycolytic metabolism

Sarcomere resting length • The length of a sarcomere in the body • This is called the optimal length • A contraction of optimal force occurs when the sarcomere is at optimal length

The sliding filament theory of contraction predicts optimal length for muscle tension.

Skeletal muscle physiology • Muscle contraction in the body – Summation of muscle twitches – Motor units – Muscle contraction and loads • Friday: ch. 14 Cardiavascular Physiology (After ch. 14, we’ll cover smooth muscle located at the end of ch. 12)

A twitch is a single contraction/relaxation cycle

The muscle does not relax completely before the second stimulus occurs, resulting In a more powerful contraction: summation Stimulus

Motor unit • One somatic motor unit and the muscle fibers that it innervates

A muscle may have many motor units of many fiber types.

Recruitment of motor unit activity during muscle activity • Each motor neuron has its own threshold potential for firing action potentials • Each motor unit has its own threshold • A muscle’s activity varies with the number of motor units that are recruited • Recruitment is controlled by the nervous system

Recruitment • Weak stimulus activates low threshold neurons. These regulate slow fatigue resistant fibers. • Stronger stimuli fatigue resistant oxidative fast twitch fibers • Very strong stimuli glycolytic fast twitch fibers

Isotonic contraction – any contraction that creates a force and moves a load

Isometric contraction - Create force without movement

How can a muscle generate force without changing its length? • Each muscle elastic elements: – Tendons – Intracellular cytoskeletal proteins with elastic properties – Contractile proteins themselves can stretch • These are included in the term: series elastic elements.