HUMAN BIOLOGY

106 Chapter 6

bursa Fluid-filled sac
around a tendon that helps
reduce friction as the ten-
don moves.

insertion End of a muscle
attached to the bone that
moves the most when the
muscle contracts.

myofibrils Threadlike
structures in a muscle fiber
that contain the units of
contraction.

origin End of a muscle that
is attached to a bone that
stays relatively motionless
during a movement.

the structure and Function of skeletal muscles

Bones and skeletal muscles work like

a system of levers

You have more than 600 skeletal muscles, and each one

helps produce some kind of body movement. In general,

one end of a muscle, called the origin, is attached to a bone

that stays relatively motionless during a movement. The

other end of the muscle, called the insertion, is attached

to the bone that moves the most (Figure 6.5). In effect, the

skeleton and the muscles attached to it are like a system of

levers in which bones (rigid rods) move near joints (fixed

points). When a skeletal muscle contracts, it pulls on the

bones it attaches to. Because muscles attach very close to

most joints, a muscle only has to contract a short distance

to produce a major movement.

many movements involve pairs

or groups of skeletal muscles

Many skeletal muscles are arranged as pairs or groups.

Some work in opposition (that is, antagonistically) so that

the action of one opposes or reverses the action of the other.

Figure 6.5 shows an antagonistic muscle pair, the biceps

and triceps of the arm. Try extending your right arm in

front of you, then place your left hand over the biceps in

the upper arm and slowly “bend the elbow.” Can you feel

the biceps contract? When the biceps relaxes and its partner

(the triceps) contracts, your arm straightens. This kind of

coordinated action comes partly from reciprocal innervation

by nerves from the spinal cord. When one muscle group is

stimulated, no signals are sent to the opposing group, so it

does not contract.

n Muscle cells produce force by contracting. After a muscle

contracts, it can relax and lengthen. skeletal muscles attach

to and interact with bones.

n Links to Metabolism 3.13, Connective tissue 4.2, Muscle

tissue 4.3

a whole skeletal muscle consists

of bundled muscle fibers

A skeletal muscle contains bundles of muscle fibers (Fig-

ure 6.3). Each fiber contains threadlike myofibrils (myo-

refers to skeletal muscle). As you will read in Section 6.4,

these structures contain the units that contract a muscle

fiber. There may be hundreds, even

thousands, of fibers in a muscle, all

bundled together by connective tis-

sue that extends past them to form

tendons. You may remember from

Chapter 5 that a tendon is a strap of

dense connective tissue that attaches

a muscle to bone or to another mus-

cle. Tendons make joints more stable

by helping keep the adjoining bones

prop erly aligned. Tendons often rub

against bones, but they slide inside

fluid-filled sacs that help reduce

the friction (Figure 6.4). Each sac is

called a bursa (plural: bursae). In

some cases a bursa is elongated into

a tendon sheath that folds around a tendon. Your knees,

wrists, and finger joints all have tendon sheaths.

Figure 6.3 in skeletal muscle, the muscle fibers are bundled together

inside a wrapping of connective tissue. (From Frances Sienkiewicz Sizer; Eleanor

Noss Whitney, Nutrition: Concepts and Controversies, © 2002 Cengage Learning)

Figure 6.4 A bursa encloses lubricating fluid that

prevents friction when the attached bone moves.

The bursa is called a tendon sheath when it wraps all

the way around a tendon. (© Cengage Learning)

muscle’s outer sheath

(connective tissue)

two bundles of muscle

fibers (each has its own

connective tissue sheath)

one muscle fiber

one myofibril

muscle

tendon

(attached

to bone)

tendon

sheath

bone

fluid

6.2

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