CHAPTER 6
MUSCULAR SYSTEM
(1)
Produce
movement
(2)
Maintain
posture
(3)
Stabilize
joints
(4)
Generate heat—is a byproduct of muscle activity; as ATP is used to
power muscle contraction nearly ¾ of its energy is given off as heat
-The ability of muscles to shorten depends
on two types of myofilaments(p. 70) 1)Actin,
2)Myosin
-Three basic muscle types are found in the
body(contrast the three)Table 6.1, also characteristics and locations are in
lab manual
(1)
Skeletal
Muscle-
(2)
Cardiac
(3)
Smooth
-Muscle cells = Muscle fibers****
-Contraction of muscles is due to the
movement of microfilaments
-All muscles share some terminology
Prefix myo refers to muscle;
prefix mys also refers to muscle; prefix sarco pertains to flesh
-Connective tissue wrappings of skeletal
muscle; Fig. 6.1
(1)
Endomysium-around each single muscle fiber
(2)
Perimysium-around fascicle (bundle) of fibers
(3)
Epimysium-covers the entire skeletal muscle; becomes tendon or aponeuroses
(4)
Fascia-on
the outside of the epimysium
-Microscopic Anatomy of Skeletal Muscle
(Fig. 6.3)
-Skeletal muscles have the properties of
Irritability and Contractility (shorten)
-Cells are multinucleated; nuclei are just beneath the sarcolemma
-Sarcolemma-plasma membrane of
muscle fibers
-Sarcoplasmic reticulum-specialized
smooth endoplasmic reticulum; stores Ca++ ions
-Myofibril; are bundles of
myofilaments; myofibrils are aligned to give distinct bands
-Sarcomere-contractile unit of a muscle fiber; structural and functional unit of skeletal muscle; Z disc, etc.
-Organization of the sarcomere; thick myofilaments are the myosin
filaments; thin myofilaments are the actin filaments
I bands; A bands (alternating I and A bands causes a muscle fiber to be striated)
STIMULATION
AND CONTRACTION OF SINGLE SKELETAL MUSCLE CELLS (FIBERS); skeletal muscle
cells must be stimulated by nerves in order to contract
Terms:
Motor
Unit-one
motor neuron and all the skeletal muscle cells it stimulates; Fig. 6.4
Neuromuscular
Junction-association
site (near contact) of nerve ending and muscle cell; Fig. 6.5
Synaptic
Cleft-the
gap between terminal end of motor neuron and muscle cell membrane(motor end
plate)
Neurotransmitter-chemical released by nerve
upon arrival of nerve impulse at the axonal terminal (it is Acetylcholine[Ach]
where nerve meets muscle fiber)
Propagating
action potential ( impulse)-Fig.-7.9
THE
SLIDING FILAMENT THEORY OF MUSCLE CONTRACTION; Fig . 6.7 and 6.8
CONTRACTION
OF A SKELETAL MUSCLE
-Muscle fiber contraction is “all
or none”
-Within a skeletal muscle, not all fibers
may be stimulated during the same interval
-Different combinations of muscle fiber
contractions may give differing responses
-Graded responses-different degrees
of skeletal muscle shortening
-Types of Graded Responses: Fig. 6.9
(1)
Twitch-single,
brief contraction; not a normal muscle function
(2)
Tetanus(summing of contractions); one contraction is immediately followed by
another; the muscle does not completely return to a resting state; the effects are added
(3)
Unfused(incomplete) tetanus-some relaxation occurs between contraction; the
results are summed
(4)
Fused (complete) tetanus; no evidence of relaxation before the following contractions;
the result is a sustained muscle contraction
MUSCLE
RESPONSE TO STRONG STIMULI
-Muscle force depends upon the number of
fibers stimulated; the more fibers contracting results in greater muscle
tension; muscles continue to contract
unless they run out of energy(ATP)
-Direct Phosphorylation-from creatine phosphate stored in muscle (CP supplies depleted in about 20 seconds)
-Aerobic Respiration-series of
metabolic pathways that occur in the mitochondria; glucose is broken down to
CO2 and water, releasing energy; this is a slower reaction that requires
continuous oxygen
-Anaerobic Glycolysis-reaction that
breaks down glucose without oxygen; glucose is broken down to pyruvic acid to
produce some ATP; pyruvic acid is converted to lactic acid which produces
muscle fatigue (this reaction is not efficient, but fast and requires huge
amounts of glucose)
MUSCLE
FATIGUE AND OXYGEN DEBT
-When a muscle is fatigued, it is unable
to contract
-The common reason for muscle fatigue is
oxygen debt; oxygen must be “repaid” to tissue to remove oxygen debt and oxygen
is required to get rid of accumulated lactic acid
-When a muscle has increasing amounts of
lactic acid and less ATP due to the lack of O2 is is unable to contract
Oxygen
Debt-the
difference between the resting rate of O2 consumption and the elevated rate
following exercise
TYPES
OF MUSCLE CONTRACTIONS
-Isotonic contractions-myofilaments
are able to slide past each other during contractions and the muscle shortens,
movements occurs(example: weight lifting)
-Isometric contractions-tension in
the muscles increases; muscle is unable to shorten (examples: pushing against a
brick wall or trying to pick up something too heavy)
MUSCLE
TONE
-Some fibers are contracted even in a
relaxed muscle; different fibers
contract at different times to provide muscle tone (this process is under
involuntary control)
-muscle tone is a state of
sustained partial muscular contraction that is vital for the health of muscles
MUSCLES
AND BODY MOVEMENTS
-Movement is attained due to a muscle
moving an attached bone;Fig. 6.12
-Muscles are attached to at least two
points
(1)Origin-attachment to an
immovable bone(part of muscle that does not move)
(2)Insertion-attachment to a
moveable bone(the part of muscle that moves)
EFFECTS
OF EXERCISE ON MUSCLE
-Increase in muscle size
-Increase in muscle strength
-Increase in muscle efficiency
-Muscle becomes more fatigue resistant
1. A nerve impulse reaches the end of a motor
neuron, triggering the release
of the neurotransmitter acetylcholine.
2. Acetylcholine diffuses rapidly across the
synaptic cleft of the
neuromuscular junction and binds to
acetylcholine receptors on the
motor endplate of the muscle fiber. (cell)
3. Stimulation of acetylcholine receptors
initiates an impulse that travels
along the sarcolemma (membrane is depolarized
by the in rushing of Na+
ions) and down into the fiber (cell)
sarcoplasm to the sarcoplasmic
reticulum where Ca++ ions are
released.
4. The Ca++ ions bind to the regulatory
proteins on the Actin myofilaments.
The regulatory proteins change both their
shape and their position on the
Actin myofilaments. This action exposes the myosin binding sites
on the
Actin myofilaments where the Myosin heads
(crossbridges) attach.
5. The crossbridges (on the myosin myofilament)
attach to their binding
sites and ATP is used to help slide the
Actin myofilament closer to the
center of the sarcomere and the
sarcomeres become shorter. (Z lines are
closer together)
6. The use of ATP to slide the Actin
myofilaments changes ATP to ADP
and then another phosphate is added to
ADP to form ATP and this
recocks the crossbridges to continue to
help move the Actin
myofilaments.
7. When the muscle contraction has subsided,
the Ca++ ions retreat back
into the sarcoplasmic reticulum and the
sarcolemma is back at its
resting state. (Caused by the out-rushing
of K+ ions and the Na+--K+
pump.