HUMAN BIOLOGY

112 Chapter 6

What are energy sources for muscle fibers?

• Muscle fibers may sometimes use creatine phosphate, glucose,
  and/or glycolysis alone to form ATP.

takE-homE mEssagE

muscle fatigue Physiologi-
cal state in which a skeletal
muscle cannot contract.

oxygen debt State in which
muscles require more ATP
than aerobic cellular respira-
tion can provide.

Ways muscle Cells

get Energy

n A whole muscle may contract weakly, strongly, or

somewhere in between, depending on how many muscle

fibers are stimulated to contract and how often.

several factors determine the characteristics

of a muscle contraction

A motor neuron supplies a number of fibers in a muscle.

The motor neuron and the muscle fibers it synapses with

form a motor unit (Figure 6.13). The number of fibers in

a motor unit depends on how precise the muscle control

must be. For instance, motor units in the bulky, power-

ful thigh muscles may include hundreds of thousands of

fibers. In contrast, we need much more precise control over

the tiny muscles that move the eye. In these muscles, motor

units have only a few hundred muscle fibers.

A muscle contraction may last a long time or only a few

thousandths of a second. When a motor neuron fires, all

the fibers in its motor unit contract briefly. This response

is a muscle twitch (Figure 6.14A). If a new nerve impulse

arrives before a twitch ends, the muscle twitches again.

Repeated stimulation of a motor unit in a short period of

time makes all the twitches run together. The result is a

sustained contraction called tetanus (Figure 6.14B). Our

muscles normally contract in this way, which generates

three or four times the force of a single twitch.

A skeletal muscle contains a large number of muscle

fibers, but not all of them contract at the same time. If a

muscle is contracting only weakly—say, as your forearm

muscles do when you pick up a pencil—it is because the

nervous system is activating only a few of the muscle’s

motor units. In stronger contractions (when you heft a

stack of books) more motor units are stimulated. Even

when a muscle is relaxed, however, some of its motor units

are contracted. This steady, low-

level contracted state is called

muscle tone. It helps maintain

muscles in general good health

and is important in stabilizing

the skeleton’s movable joints.

Muscle tension is the force

that a contracting muscle exerts

on an object, such as a bone.

Opposing this force is a load,

either the weight of an object or

gravity’s pull on the muscle. A

stimulated muscle shortens only

when muscle tension exceeds

the opposing forces.

You have probably heard of

isotonic and isometric exercise.

Isotonic means “same or steady

n When a resting muscle is ordered to contract, the demand

for ATP in the muscle cell skyrockets.

n Links to how cells make ATP 3.14, Other energy sources 3.16

A resting muscle fiber has a small

amount of stored ATP and much

more of a substance called creatine

phosphate. This substance is gener-

ated in the fiber from natural stores

of the amino acid creatine. When

the fiber is stimulated to contract,

a fast reaction transfers phosphate

from creatine phosphate to ADP, to form more ATP. This

reaction can fuel contractions until a slower ATP-forming

pathway, such as aerobic cellular respiration, can start up

(Figure 6.12).

Normally, most of the ATP for muscle contraction comes

from the oxygen-using reactions of cellular respiration. If

you exercise hard, however, your respiratory and circulatory

systems may not be able to deliver enough oxygen for aero-

bic cellular respiration in some muscles. Then, glycolysis

(which does not use oxygen) will contribute more of the ATP

being formed. Muscle cells rely on glycolysis until there is

too little stored glycogen to provide glucose or until muscle

fatigue sets in. This is a state in which a muscle can no lon-

ger contract. One cause of fatigue may be an oxygen debt

that results when muscles need more ATP than aerobic cel-

lular respiration can deliver. They then switch to glycolysis,

which produces lactic acid. Along with the already low ATP

supply, the rising acidity hampers the contraction of muscle

cells. It also causes the “burn” you might feel while working

out. Deep, rapid breathing helps repay the oxygen debt.

Figure 6.12 Animated! Three metabolic pathways can form
ATP in active muscle cells. (© Cengage Learning)

properties of

Whole muscles

Pathway 1

Phosphate Transferred

from Creatine Phosphate

Pathway 2

oxygen

creatine

glucose from bloodstream and

from glycogen breakdown in cells

ADP + Pi

Relaxation

Aerobic Respiration

Pathway 3

Glycolysis Alone

Contraction

ATP

6.5 6.6

motor unit Unit consisting

of a motor neuron and the

muscle fibers it controls.

muscle tension The force

that a contracting muscle

exerts on an object.

muscle tone A steady, low-

level state of contraction of

a skeletal muscle.

muscle twitch One con-

traction of a motor unit.

tetanus Sustained muscle

contraction that devel-

ops when motor units are

repeatedly stimulated in

a short period of time, so

that individual twitches are

combined.

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