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