tion of variable numbers of muscle fibers in a
muscle.- Tendons attach muscles to bone; the origin is the
more stationary bone, the insertion is the more
movable bone. A tendon merges with the fascia of
a muscle and the periosteum of a bone; all are made
of fibrous connective tissue.
Muscle Arrangements
- Antagonistic muscles have opposite functions. A
muscle pulls when it contracts, but exerts no force
when it relaxes and it cannot push. When one mus-
cle pulls a bone in one direction, another muscle is
needed to pull the bone in the other direction (see
also Table 7–2 and Fig. 7–1). - Synergistic muscles have the same function and
alternate as the prime mover depending on the
position of the bone to be moved. Synergists also
stabilize a joint to make a more precise movement
possible. - The frontal lobes of the cerebrum generate the
impulses necessary for contraction of skeletal mus-
cles. The cerebellum regulates coordination.
Muscle Tone—the state of slight contraction
present in muscles
- Alternate fibers contract to prevent muscle fatigue;
regulated by the cerebellum. - Good tone helps maintain posture, produces 25%
of body heat (at rest), and improves coordination. - Isotonic exercise involves contraction with move-
ment; improves tone and strength and improves
cardiovascular and respiratory efficiency (aerobic
exercise).
- Concentric contraction—muscle exerts force
while shortening. - Eccentric contraction—muscle exerts force
while lengthening.
- Isometric exercise involves contraction without
movement; improves tone and strength but is not
aerobic.
Muscle Sense—proprioception: knowing
where our muscles are without looking
at them
- Permits us to perform everyday activities without
having to concentrate on muscle position. - Stretch receptors (proprioceptors) in muscles
respond to stretching and generate impulses that
the brain interprets as a mental “picture” of where
the muscles are. Parietal lobes: conscious muscle
sense; cerebellum: unconscious muscle sense used
to promote coordination.Energy Sources for Muscle Contraction- ATP is the direct source; the ATP stored in muscles
lasts only a few seconds. - Creatine phosphate is a secondary energy source; is
broken down to creatine phosphate energy.
The energy is used to synthesize more ATP. Some
creatine is converted to creatinine, which must be
excreted by the kidneys. Most creatine is used for
the resynthesis of creatine phosphate. - Glycogen is the most abundant energy source and
is first broken down to glucose. Glucose is broken
down in cell respiration:
Glucose O 2 →CO 2 H 2 O ATP heat
ATP is used for contraction; heat contributes to
body temperature; H 2 O becomes part of intracellu-
lar fluid; CO 2 is eventually exhaled. - Oxygen is essential for the completion of cell res-
piration. Hemoglobin in red blood cells carries
oxygen to muscles; myoglobin stores oxygen in
muscles; both of these proteins contain iron, which
enables them to bond to oxygen. - Oxygen debt (recovery oxygen uptake): Muscle
fibers run out of oxygen during strenuous exercise,
and glucose is converted to lactic acid, which causes
fatigue. Breathing rate remains high after exercise
to deliver more oxygen to the liver, which converts
lactic acid to pyruvic acid, a simple carbohydrate
(ATP required).
Muscle Fiber—microscopic structure- Neuromuscular junction: axon terminal and sar-
colemma; the synapse is the space between. The
axon terminal contains acetylcholine (a neurotrans-
mitter), and the sarcolemma contains cholinesterase
(an inactivator) (see Fig. 7–2). - Sarcomeres are the contracting units of a muscle
fiber. Myosin and actin filaments are the contract-
ing proteins of sarcomeres. Troponin and tropo-
myosin are proteins that inhibit the sliding of
myosin and actin when the muscle fiber is relaxed
(see Figs. 7–3 and 7–5). - The sarcoplasmic reticulum surrounds the sarco-
meres and is a reservoir for calcium ions. - Polarization (resting potential): When the muscle
fiber is relaxed, the sarcolemma has a () charge
160 The Muscular System