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The Muscular System
Three types of muscles…Smooth Cardiac Skeletal
This chapter focuses on skeletal muscle
Smooth muscle…walls of the
viscera (organs), blood vessels, bronchioles
INVOLUNTARY muscle
NONSTRIATED (no stripes)
The contraction of smooth muscle enables the viscera to perform their functions
Smooth muscle
Cardiac muscleFound only in the HEART Striated involuntary Cardiac muscle cells are long branching cells that fit together tightly at junctions called INTERCALATED DISCS
Intercalated discs promote rapid conduction of electrical signals throughout the heart
Cardiac muscle
Skeletal muscleGenerally attached to bone VOLUNTARY muscle Cells are long cylinders
Composed of proteins arranged to make the muscle appear striped, or STRIATED
Skeletal muscles: –produce movement –maintain body posture – stabilize joints –produce HEAT to maintain body
temperature
Skeletal MusclePrecursor cell: Myoblast
Fuses together with other myoblasts to form one large cell with many nuclei called a muscle fiber(myotube)
Skeletal muscleMany muscle fibers make a muscle Have the ability to shorten or contract
Skeletal Muscle CellsSarcolemma: plasma membrane in
muscles Myofibrils: long ribbon-like
organelles Sarcomeres: chains of contractile
units
Sarcomere: functional unit of muscle
Sarcomere
Sliding Filament Model of Muscle Contraction
Thin and thick filaments slide past each other to shorten sarcomere. Actin: thin filaments Myosin: thick filaments
How muscles contract…When muscles contract, they shorten
Muscles shorten because the sarcomeres shorten
The sarcomeres shorten because the actin and myosin filaments slide past each other
How does the sarcomere shorten?When stimulated, the myosin heads
make contact with the actin, forming temporary connections called CROSSBRIDGES
The crossbridges pull the actin to the center of the sarcomere
The actin slides past the myosin
Crossbridges in action…
Remember….The sarcomeres shorten not because the actin and myosin proteins shrink or shrivel up, but because the proteins slide past one another
The role of calcium and ATP ATP and CALCIUM play important
roles in the contraction and relaxation of muscle
ATP helps the myosin heads form and break the crossbridges with the actin
However, ATP can only perform its role if calcium is present
Calcium is stored in the sarcoplasmic reticulum (ER of muscle cells)
When the muscle is stimulated, calcium is released from the sarcoplasmic reticulum and causes the actin, myosin, and ATP to interact
Muscle Contraction controlled by the Nervous system
Skeletal muscle contraction can take place only if the muscle is first stimulated by a nerve
This type of nerve is called a MOTOR or SOMATIC NERVE
The area where the motor nerve meets the muscle is called the NEUROMUSCULAR JUNCTION (NMJ)
The NMJ
What happens at the NMJ?The stimulated nerve causes an electrical
signal, or nerve impulse, to move along the nerve toward the nerve ending
Stored in pouches at the nerve ending is a NEUROTRANSMITTER called ACETYLCHOLINE (ACh)
The ACh is released into the NMJ space The ACh diffuses across the space and
binds to receptor sites on the muscle membrane The ACh stimulates the receptors and
causes an electrical signal to develop along the muscle membrane
The electrical signal stimulates the sarcoplasmic reticulum to release calcium The calcium allows for the interaction
of actin, myosin, and ATP – the muscle contracts
Energy SourcesCreatine phosphate phosphorylates
ADP ADP ! ATP (no oxygen used)
Aerobic respiration: constant oxygen and glucose supply
Anaerobic glycolysis and lactic acid fermentation No oxygen used Not much energy produced this way but much faster than aerobic respiration
Lactic acid buildup: muscle fatigue and soreness
Muscle Fatigue and Oxygen DebtFatigue: muscle unable to contract
even though it is being stimulated.
pH denatures ATPase and excess H+ ions interfere with Ca2+ ions
Muscle Fatigue and Oxygen DebtOxygen debt occurs during
prolonged muscle activity Amount of work and length of
workout depends on a muscle’s blood supply
Muscle Fatigue and Oxygen DebtOxygen debt from vigorous activity
must be paid back Rapid deep breathing
Responses of a whole muscle..The sliding filament hypothesis explains
the contraction and relaxation of a single muscle fiber
A whole skeletal muscle, however, is composed of thousands of muscle fibers
The contraction of a whole muscle differs from that of a single muscle fiber
A single muscle fiber contracts in an ALL-OR-NOTHING response – the fiber contracts as strongly as possible
It never partially contracts
A whole muscle, however, is capable of contracting partially
It can contract weakly or very strongly
Lifting a pencil may require the contraction of several hundred muscle fibers, in an all-or-nothing manner
But only a few fibers are contracting
Lifting a 100 lb weight, however, requires contraction of thousands of fibers, all contracting in an all-or-nothing manner
The greater muscle force is achieved by using, or recruiting, additional fibers
This process is called RECRUITMENT
The strength of skeletal muscle contraction can be varied by RECRUITMENT of additional muscle fibers
Classification of MusclesSlow twitch –More mitochondria, more blood supply
(dark meat) –Slower contraction for endurance
Classification of MusclesFast twitch –Specialized for rapid, forceful
contractions –Short term contractions – Less blood supply (white meat)
Most muscles are a mixture of both Genetically determined Rigorous training can alter the ratio
Types of Muscle ContractionIsotonic: actin and myosin filaments slide past each other and the muscle shortens
Types of Muscle ContractionIsometric: filaments try to slide but
muscle is pitted against an immovable object
Muscle tone: continuous partial contraction
If nerve supply to muscle is destroyed, muscle is no longer stimulated, loses tone & becomes paralyzed, flaccid and atrophied.
Muscle attachment…Muscles form attachments to other
structures:
a tendon attaches muscle to bone
a flat, sheet-like fascia, called APONEUROSIS connects muscle to muscle, or muscle to bone
Tendon Ligament
aponeurosis
Origin and insertion….These terms refer to the sites of
muscle attachment:
ORIGIN of the muscle attaches to the stationary (immovable) bone
INSERTION attaches to the moveable bone
Although most movement is accomplished through the cooperation of groups of muscles, a single muscle is generally responsible for most of the movement
The “chief muscle” is called the PRIME MOVER
Assisting the prime mover are “helper muscles” called SYNERGISTS
Synergists cooperate with other muscles
In contrast, ANTAGONISTS are muscles that oppose the action of another muscle
Movement occurs from the contraction and relaxation of alternating muscles
When one contracts (prime mover), the other must relax (antagonist)
Concentric movement: contraction Eccentric movement: relaxation – Muscle exert
force even as they lengthen
Naming skeletal muscles…There are 656 muscles in the human body!!
The names of various muscles are generally based on one or more of the following characteristics:
–Size –Shape –Direction of fibers – Location –Number of origins –Origin and insertion –Muscle action
SizeVastus – huge Maximus – large Longus – long Minimus – small Brevis - short
ShapeDeltoid – triangular Latissimus – wide Trapezius – trapezoid Rhomboideus – rhomboid Teres – round
Fiber direction
Fibers are lined up in several directions:
Rectus – straight Oblique – diagonal Transverse – across Circularis - circular
locationThe names of muscles often reflect
their location:
Pectoralis – chest Gluteus – buttock Brachii – arm Supra – above Infra – below Sub – underneath Lateralis - lateral
Number of originsMuscle may be named according to
the number of sites to which it is anchored:
Biceps – 2 sites Triceps – 3 sites Quadriceps – 4 sites
Muscle actionAbductor muscle – moves
the limb away from the midline of the body
Adductor muscle – moves the limb toward the midline
Common Body MovementsFlexion: movement in (generally)
sagittal plane that decreases the angle of the joint and brings bones closer together Extension: opposite of flexion;
creates distance between bones or parts of body –Hyperextension: greater than 180
degrees
Common Body MovementsRotation: movement of a bone around
its longitudinal axis
Common Body MovementsAbduction: moving a limb away
(usually on the frontal plane) from the midline Fanning of fingers or toes also Adduction: opposite of abduction
Common Body MovementsCircumduction: combination of
flexion, extension, abduction, and adduction (circular movement)
Special Movements (occur only at certain joints
Dorsiflexion: lifting the foot so that superior surface approaches the shin (standing on heels)
Special Movements (occur only at certain joints
Plantar flexion: pointing the toes
Special Movements (occur only at certain joints
Inversion: turn sole medially Eversion: turn sole laterally
Special Movements (occur only at certain joints
Supination: forearm rotates laterally so that palms face forward Pronation: forearm rotates medially
so palms face posteriorly
Special Movements (occur only at certain joints
Opposition: move thumb to touch the tips of the other fingers on the same hand
Five Golden Rules on Muscle Activity
All muscles cross at least one joint. The bulk of the muscle lies proximal
to the joint crossed. All muscles have at least two
attachments: origin and insertion. Muscles can only pull, they never
push. During contraction, the muscle
insertion moves toward the origin.
Exercise Inactivity: muscle weakness and
wasting. Use it or lose it!
ExerciseAerobic or endurance exercise –Stronger, flexible muscles with greater
resistance to fatigue – Increased blood supply – Increased number of mitochondria – Improved digestion and elimination,
neuromuscular coordination, stronger skeleton, increased heart stroke volume, increased lung capacity
–No increase in muscle size
ExerciseResistance (isometric) – Increased muscle size and strength
What is wrong with “bulking up”?
Nothing, as long as it is done through weight lifting and exercise Increase in muscle size is due to
increase in actomyosin within the cell Bulking up with the use of
steroids is dangerous!
Steroids are believed to cause liver cancer, atrophy (shrinking) of the testicles in males, hypertension, and severe psychotic mood swings
American College of Sports Medicine advises resistance training with non-maximal
weights for adolescents
Optimal bone mass Reduce obesity Decrease sports
injuries
Homeostatic ImbalancesSprain: Ligament stretches or tears
Homeostatic ImbalancesStrain: injury to muscle or tendon
RICERest Ice Compression elevation
Homeostatic ImbalancesMuscular dystrophy: muscles enlarge due to fat and connective tissue deposit, but muscle fibers degenerate and atrophy Sex-linked, more common in males
Homeostatic ImbalancesAmyotrophic lateral sclerosis (ALS) Lou Gehrig’s disease Loss of control of voluntary muscles
due to destruction of nerves that innervate them.