1. 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)

2. A twitch is a single contraction/relaxation cycle

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

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

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

9. 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

10. 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

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

12. Isometric contraction
- Create force without movement

13. 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.