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 Stimulus 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.