336
UNIT TWO
9.6
CARDIAC MUSCLE (PAGE 301)
1. Cardiac muscle contracts for a longer time than
skeletal muscle because transverse tubules supply
extra calcium ions.
2. Intercalated discs connect the ends of adjacent cardiac
muscle cells and hold the cells together.
3. A network of F
bers contracts as a unit and responds
to stimulation in an all-or-none manner.
4. Cardiac muscle is self-exciting, rhythmic, and remains
refractory until a contraction is completed.
9.7
SKELETAL MUSCLE ACTIONS (PAGE 301)
1. Body movement
a. Bones and muscles function together as levers.
b. A lever consists of a rod, a fulcrum (pivot), a
resistance, and a force that supplies energy.
c. Parts of a F rst-class lever are arranged resistance–
fulcrum–force; of a second-class lever, fulcrum–
resistance–force; and of a third-class lever,
resistance–force–fulcrum.
2. Origin and insertion
a. The movable end of attachment of a skeletal
muscle to a bone is its insertion, and the
immovable end is its origin.
b. Some muscles have more than one origin or
insertion.
3. Interaction of skeletal muscles
a. Skeletal muscles function in groups.
b. A prime mover is responsible for most of a
movement; synergists aid prime movers; antagonists
can resist the movement of a prime mover.
c. Smooth movements depend upon antagonists
giving way to the actions of prime movers.
9.8
MAJOR SKELETAL MUSCLES (PAGE 305)
Muscle names often describe sizes, shapes, locations,
actions, number of attachments, or direction of F
bers.
1. Muscles of facial expression
a. These muscles lie beneath the skin of the face
and scalp and are used to communicate feelings
through facial expression.
b. They include the epicranius, orbicularis oculi,
orbicularis oris, buccinator, zygomaticus major,
zygomaticus minor, and platysma.
2. Muscles of mastication
a. These muscles are attached to the mandible and
are used in chewing.
b. They include the masseter, temporalis, medial
pterygoid, and lateral pterygoid.
3. Muscles that move the head and vertebral column
a. Muscles in the neck and back move the head.
b. They include the sternocleidomastoid, splenius
capitis, semispinalis capitis, quadratus lumborum,
and erector spinae.
4. Muscles that move the pectoral girdle
a. Most of these muscles connect the scapula to
nearby bones and are closely associated with
muscles that move the arm.
b. They include the trapezius, rhomboid major,
rhomboid minor, levator scapulae, serratus
anterior, and pectoralis minor.
4. Recruitment of motor units
a. Muscles whose motor units have few muscle F
bers
produce F
ner movements.
b. Motor units respond in an all-or-none manner.
c. At low intensity of stimulation, relatively few
motor units contract.
d. At increasing intensities of stimulation, other
motor units are recruited until the muscle contracts
with maximal tension.
5. Sustained contractions
a. Tetanic contractions are common in everyday
activities.
b. Even when a whole muscle appears at rest, some
of its F
bers undergo sustained contraction. This is
called muscle tone.
6. Types of contractions
a. One type of isotonic contraction occurs when a
muscle contracts and its ends are pulled closer
together. Because the muscle shortens, it is called a
concentric contraction.
b. In another type of isotonic contraction, the force a
muscle generates is less than that required to move
or lift an object. This lengthening contraction is an
eccentric contraction.
c. When a muscle contracts but its attachments do
not move, the contraction is isometric.
d. Most body movements involve both isometric and
isotonic contractions.
7. ±ast-and-slow twitch muscle F
bers
a. The speed of contraction is related to a muscle’s
speciF
c function.
b. Slow-contracting, or red, muscles can generate
ATP fast enough to keep up with ATP breakdown
and can contract for long periods.
c. ±ast-contracting, or white, muscles have reduced
ability to carry on the aerobic reactions of cellular
respiration and tend to fatigue rapidly.
9.5
SMOOTH MUSCLES (PAGE 300)
The contractile mechanisms of smooth and cardiac
muscles are similar to those of skeletal muscle.
1. Smooth muscle F
bers
a. Smooth muscle cells contain F
laments of myosin
and actin.
b. They lack transverse tubules, and the sarcoplasmic
reticula are not well developed.
c. Types include multiunit smooth muscle and
visceral smooth muscle.
d. Visceral smooth muscle displays rhythmicity.
e. Peristalsis aids movement of material through
hollow organs.
2. Smooth muscle contraction
a. In smooth muscles, calmodulin binds to calcium
ions and activates the contraction mechanism.
b. Both acetylcholine and norepinephrine are
neurotransmitters for smooth muscles.
c. Hormones and stretching affect smooth muscle
contractions.
d. With a given amount of energy, smooth muscle
can maintain a contraction longer than skeletal
muscle.
e. Smooth muscles can change length without
changing tautness.
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