Section 5, Chapter 9 Muscular System

Chapter 9, Section 5

Muscular Responses

Muscle Response • Muscle contraction can be observed by removing a single skeletal muscle fiber and connecting it to a device (myograph) that senses and records changes in the overall length of the muscle fiber.

Threshold stimulus • minimum strength that elicits a muscle fiber contraction muscle

Twitch • Single contractile response to a stimulus • All-or-none response

Myograph

Recording of a Muscle Contraction

Myogram of a twitch • Latent period – brief delay between stimulation and contraction <2 milliseconds in humans • Period of contraction • Period of relaxation

Summation Series of twitches (a) • Muscle completely relaxes before next stimulation • Each contraction occurs with equal force.

Figure 9.17a

Summation (b) • Stimulus occurs before muscle relaxes completely •Multiple stimuli increases force of contraction Figure 9.17b

Summation Tetanic Contraction (c) • Muscle is stimulated at a high frequency •Contractions fuse together and cannot be distinguished • Results in a maximal sustained contraction without relaxation

Figure 9.17c

Recruitment of Motor Units Recruitment – progressive activation of motor units to increase the force of a muscle contraction.

• Motor Unit- A motor neuron plus all of the fibers it controls.

•A whole muscle is composed of many motor units.

• As a general rule, motor units are recruited in order of their size • Small motor units are stimulated with light activities, but additional motor units are recruited with higher intensity activity.

• As intensity of stimulation increases, recruitment of motor units continues until all motor units are activated

Sustained Contractions The central nervous system can increase the strength of contractions in 2 ways: 1.

Recruitment • Smaller motor units are recruited first, followed by larger motor units.

• The result is a sustained contraction of increasing strength.

2.

Increased firing rate • A high frequency of action potentials results in tetanic contractions.

• Contributes to smooth muscle movements, instead of jerky contractions.

Both mechanisms occur together

Muscle tone – continuous state of partial contraction • response to repeated nerve impulses from spinal cord

Types of Contractions • Isotonic – muscle contracts and changes length • Concentric – shortening of muscle (a) • Eccentric – lengthening of muscle (b) • Isometric – muscle contracts but does not change length (c) •Stabilizes posture and holds body upright

Figure 9.18. muscle contractions

Fast Twitch and Slow Twitch Muscle Fibers Fast & Slow twitch refers to the contraction speed, and to whether muscle fibers produce ATP oxidatively (by aerobic respiration) or glycolytically (by glycolysis)

• Slow-twitch fibers (Type I) • Always oxidative and resistant to fatigue • Contain myoglobin for oxygen storage “red fibers” • Many mitochondria for aerobic respiration • Good blood supply

Fast Twitch and Slow Twitch Muscle Fibers

Fast-twitch glycolytic fibers (type IIa) • White fibers with less myoglobin • Poor blood supply • Faster contractions than red fibers, but fatigue quickly

Fast-twitch fatigue resistant fibers (type IIb) • Also called intermediate fibers • Fast-twitch fibers plus substantial oxidative capacity • Intermediate amounts of myoglobin • resistant to fatigue

Fast Twitch and Slow Twitch Muscle Fibers Migrating birds have abundant slow-twitch fibers for flying long distances, which is why their flesh is dark.

Chickens that can only flap around the barnyard have abundant fast-twitch muscles and mostly white flesh.

End of Chapter 9, Section 5