2. Muscle Response
A muscle contraction can be observed by removing a single skeletal
muscle and connecting it to a device (myograph) that senses and
records changes in the overall length of the muscle fiber.
A threshold stimulus is the minimum
stimulus that elicits a muscle fiber contraction
all-or-none response
A threshold stimulus will cause the muscle
fiber to contract fully and completely.
A stronger stimulus does not produce a
stronger contraction!
The muscle fiber will not contract at all if
the stimulus is less than threshold.
muscle
subthreshold stimulus
Myograph
3. Recording of a Muscle Contraction
A twitch is a single contractile response to a stimulus
A twitch can be divided into three periods.
1. Latent period
brief delay between the stimulus
and the muscle contraction
The latent period is less than 2
milliseconds in humans
2. Period of contraction
3. Period of relaxation
4. Summation
If the muscle is stimulated again before
it has completely relaxed, then the force
of the next contraction increases.
i.e. stimulating the muscle at a rapid
frequency increases the force of
contraction. This is called summation
series of twitches
Figure 9.17b
If the muscle is allowed to relax
completely before each stimulus than the
muscle will contract with the same force.
Figure 9.17a
summation
5. Summation
Tetanic Contraction (c)
If the muscle is stimulated at a high frequency the contractions fuse
together and cannot be distinguished.
A tetanic contraction results in a maximal sustained contraction without
relaxation
Figure 9.17c
6. Recruitment of Motor Units
all-or-none response
A muscle that is stimulated with threshold potential contracts
completely and fully.
A stronger stimulus does not produce a stronger contraction!
Instead, the strength of a muscle is increased by recruitment of
additional motor units.
7. Recruitment of Motor Units
Recruitment – progressive activation of motor units to
increase the force of a muscle contraction.
Recall that a motor unit is a motor neuron plus all of the fibers it controls.
• Muscles are 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 the intensity of stimulation increases,
recruitment of motor units continues until
all motor units are activated.
8. 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 summation of the muscle
contractions.
•
If the frequency is too high, however, it may produce tetanic contractions, in which
case the muscle does not relax.
Muscle tone is produced because some muscles are in a continuous
state of partial contraction in response to repeated nerve impulses from
the spinal cord.
9. 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)
Isometric contractions stabilizes posture and holds the
body upright
Figure 9.18. muscle contractions
10. 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”
• Also have many mitochondria for aerobic respiration
• Good blood supply
11. Slow-twitch fibers (Type I)
Slow-twitch fibers are best suited for
endurance exercise over a long period with
little force.
Slow-twitch fibers rely on aerobic
respiration for energy (ATP) and are
resistant to fatigue.
Slow-Twitch fibers contain abundant
myoglobin for oxygen storage “red fibers” and
mitochondria to carry out aerobic respiration.
Because of their oxygen demands, slow-twitch
fibers have a good blood supply.
12. Fast twitch muscle fibers – two types
Fast-twitch glycolytic fibers contract
rapidly and with great force, but they
fatigue quickly.
They are best suited for rapid
contractions over a short
duration.
Fast-twitch glycolytic fibers (type IIa)
contain very little mitochondria and
myoglobin and are “white fibers”
13. Fast twitch muscle fibers – two types
Fast-twitch intermediate or fast oxidative
fibers contain intermediate amounts of
myoglobin.
They contract rapidly but also have the
capacity to generate energy through aerobic
respiration.
14. Fast twitch and slow twitch muscle fibers
Migrating birds have abundant slowtwitch 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
