section 5, chapter 9

muscular responses

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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

Muscle Responsem

uscle

MyographThe muscle fiber will not contract at all if the stimulus is less than threshold.

all-or-none responseA threshold stimulus will cause the muscle fiber to contract fully and completely.

A stronger stimulus does not produce a stronger contraction!

subthreshold stimulus

Recording of a Muscle Contraction

2. Period of contraction

3. Period of relaxation

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

Summation

If the muscle is allowed to relax completely before each stimulus than the muscle will contract with the same force.

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

Figure 9.17a series of twitches

summationFigure 9.17b

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

Summation

Figure 9.17c

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.

Recruitment of Motor Units

Recruitment of Motor Units

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.

Recruitment – progressive activation of motor units to

increase the force of a muscle contraction.

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

Sustained Contractions

The central nervous system can increase thestrength 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.

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

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

Slow-twitch fibers (Type I)

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.

Slow-twitch fibers are best suited for endurance exercise over a long period with little force.

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”

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.

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

Fast twitch and slow twitch muscle fibers