The Muscular System

Chapter 8 (new book); Chapter 6 (old book)

Simple Facts Muscles are bundles of cells and

fibers. Muscles work in very simple

ways. They tighten up, contract, and relax.

You have two sets of muscles attached to many of your bones which allow them to move.

There are 630 active muscles in your body and they act in groups.

Muscles can only pull. They never push.

Muscles come from the Latin word mus meaning “little mouse.”

Skeletal Muscle Skeletal Muscles Fibers are packaged into the body’s organs

called skeletal muscles that are attached to the body skeleton. They range up to one foot in length and can be seen with the

naked eye. They are fragile, yet they can exert tremendous power. They

do not rip apart because there are thousands of fibers bundled together by connective tissue which provides strength and support to the muscle.

Skeletal muscles are known as striated muscle because its fibers appear to be striped.

They are voluntary muscles.

Muscle Coverings Each muscle fiber is enclosed in a delicate connective

tissue sheath called an endomysium. They are then wrapped by a more coarse fibrous

membrane called a perimysium to form a bundle of fibers called a fascicle.

Fascicles are bound together by an even tougher “overcoat” of connective tissue called epimysium, and they blend into strong cordlike tendons.

Muscle Coverings Diagram

Microscopic Anatomy of Skeletal Muscle The sarcolemma is the plasma membrane in muscle cells. Myofibrils are long ribbon like organelles that push aside the

nuclei. Alternating light and dark bands along the length of the

myofibrils give the muscle cell as a whole its striped appearance.

Myofibrils are actually chains of tiny contractile units called sarcomeres.

The larger thick filaments, also called myosin filaments are made mostly of bundles molecules of the protein myosin.

Cross bridges link the thick and thin filaments together during contraction.

The thin filaments are composed of the contractile protein called actin.

The sarcoplasmic reticulum (SR) is a specialized smooth endoplasmic reticulum that surrounds every myofibril.

Microscopic Anatomy of Skeletal Muscle Diagram

Smooth Muscle

Has no striations and is involuntary. It is found in the walls of organs such as the stomach, urinary

bladder, and respiratory passages. They are spindle-shaped and have a single nucleus, and are

arranged in sheets or layers. There are two layers, one running circularly and the other

running longitudinally. As they alternately contract and relax, they change the size

and shape of the organ. Smooth muscles handle movement of food through the

digestive tract and emptying the bowels and bladder.

Cardiac Muscle

Found only in the heart, and is striated and involuntary. The cardiac fibers are cushioned by small amounts of soft

connective tissue and are arranged in spiral or figure 8-shaped bundles.

Cardiac muscle fibers are branching cells joined by special junctions called intercalated disks.

Cardiac muscle usually contracts at a fairly steady rate set by the heart’s “in-house” pacemaker, but the heart can also be stimulated by the nervous system to shift into “high gear” for short periods.

Muscle Type Comparison

Muscle Movements The type of movement depends on the mobility of the joint

and where the muscle is located in relation to the joint. The most common types of body movements are:

Flexion—Typical of hinge joints but it is also common at ball-and-socket joints (bending over at the hips).

Extension—opposite of flexion; increases the angle, or the distance, between two bones or parts of the body.

Muscle Movements cont… Abduction—moving a limb away from the

midline, or median plane, of the body. Adduction—movement of a limb toward the

body midline.

Muscle Movements cont. Rotation—movement of a bone around its longitudinal

axis. Circumduction—combination of flexion, extension,

abduction, and adduction; moving a part of the body so that its end follows a circular path

Note: If the difference between circumduction and rotation is confusing for you, try this: With your arm pointed straight ahead of you, trace a large circle in the air with your finger (using your whole arm) - this is circumduction of the shoulder. Now, with the same arm, pretend as though you are turning a screwdriver with your elbow straight - this is rotation of the shoulder.

Muscle Movements cont… Pronation—turning the hand so that the palm is facing

downward or posteriorly Supination—turning the hand so that the palm is facing

upward or anteriorly

Muscle Movements cont…

Inversion—turning the sole of the foot so that is faces medially.

Eversion—turning the sole of the foot laterally.

Muscle Movements cont… Dorsiflexion—movement at the ankle that moves the

instep of the foot up and dorsally toward the shin. Plantar flexion—straightens the ankle joint, causing

the toes to point downward.

Muscle Movements cont.

elevation—raising a part depression—lowering a part

Muscle Types When several muscles are contracting at the same

time, the muscle that has the major responsibility for causing a particular movement is called the prime mover.

Muscles that oppose or reverse a movement are antagonists.

Antagonist. The triceps brachii extends the forearm at the elbow while the biceps brachii, its antagonist, flexes the elbow.

Muscle Types cont…

Synergists help prime movers by producing the same movement or by reducing undesirable or unnecessary movement.

Fixators are specialized synergists. They hold a bone still or stabilize the origin of a

prime mover so all the tension can be used to move the insertion bone.