section 5, chapter 9 muscular system
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Chapter 9, Section 5
Muscular Responses
• 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.
Muscle Response
Threshold stimulus• minimum strength that elicits a
muscle fiber contraction
Twitch
• Single contractile response to a stimulus
• All-or-none response
muscle
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
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
Summation
Figure 9.17c
Recruitment of Motor Units
• 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
Recruitment – progressive activation of motor
units to increase the force of a muscle contraction.
• 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.• 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 glycolytic fibers (type IIa)
• White fibers with less myoglobin
• Poor blood supply
Fast Twitch and Slow Twitch Muscle Fibers
• 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
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