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Chapter 7
The Muscular System
Function of Muscles
 Produce movement
 Maintain posture
 Stabilize joints
 Generate heat
 Guards openings
Fun Facts about Muscles
• There are 656 muscles in the human body.
• It takes 17 muscles to smile and 43 to frown.
• The tongue is the strongest muscle in the
body.
• The gluteus maximus (the buttocks) is the
largest muscle in the body.
• The sartorius muscle is the longest muscle in
the body.
The Muscular System
Three types of muscle tissue :
 Skeletal
1) Attach to the bony skeleton
2) Have longest fibers, fibers are striated (striped)
3) Controlled voluntarily
 Smooth
1) Linings of organs and cavities
2) Fibers are non-striated
3) Controlled involuntarily, function constantly
 Cardiac
1) Found only in the heart
2) Fibers are striated
3) Controlled involuntarily
Smooth Muscle Characteristics
 Has no striations
 Single nucleus
 Found mainly in
the walls of hollow
organs
Smooth Muscle
Cardiac Muscle Characteristics
 Has striations
 Usually has a
single nucleus
 Joined to another
muscle cell at an
intercalated disc
 Found only in the
heart
Cardiac Muscle
Functional Characteristics of Muscles
 Four properties of muscle:
1) Excitability: ability to receive and
respond to a stimulus
2) Contractility: ability to shorten with
force when stimulated
3) Extensibility: ability to be stretched
or extended
4) Elasticity: ability of a muscle fiber to
return to its original or resting length
Skeletal Muscle Organization
Skeletal Muscle
Muscle fascicle (bundle)
Muscle fiber (cell)
Myofibril
Sarcomere (Contractile unit of muscle fiber)
Thick and thin filaments
Anatomy of a Skeletal Muscle
• Consists of many bundles, called fascicles, of fibers covered by
connective tissue
• Each muscle contains at least one artery and one or two veins
… why?
• Most attach to bones by tendons (connective tissue extensions)
1) insertion: bone that moves during contraction
2) origin: bone that stays still during contraction
• Some muscles attach directly to bone by their connective
tissue coverings
Connective Tissue Wrappings of
Skeletal Muscle
 Epimysium –
covers skeletal
muscle
 Perimysium –
covers fascicle
(bundle) of
fibers
 Endomysium –
around single
muscle fiber
Microscopic Anatomy of a Skeletal Muscle
• Muscle fiber: muscle cell with many nuclei
• Sarcolemma: plasma membrane of a muscle cell
• Sarcoplasm: cytoplasm of muscle cell, contains large
amounts of glycogen and oxygen-binding protein
• Each muscle fiber contains large numbers of myofibrils in the
sarcoplasm, with mitochondria packed around them
• Myofibrils: contractile portion of the muscle cell, made up of
units called sarcomeres
• Sarcomeres contain two types of filaments:
1) thick: contain the protein myosin
2) thin: contain the protein actin
Microscopic Anatomy of the Muscle
Muscle Contraction: Sliding Filament Theory
 At rest, there is a bare zone that lacks
actin filaments (no filament overlap)
 To start contraction, calcium unlocks
active sites on actin
 Myosin heads bind to actin active sites –
connections are called cross-bridges
 Myosin pulls the actin microfilaments
toward the bare zone - muscle shortens
 Myosin detaches, then reattaches to
another active site, shortening the muscle
further
Muscle Contraction: Sliding Filament Theory
Sliding Filament Animation
Muscle Contraction: Rigor Mortis
• After death, there is an influx of calcium
• Calcium causes myosin to bind with actin
(forming cross bridges)
• Breathing stops, no energy is available to
detach cross bridges, so they stay attached
• Muscles stiffen after 3 to 4 hours, peaks at 12
hours, and decreases between 48 and 60
hours
Nerve Stimulus for Muscle Contraction
 Skeletal muscles
must be
stimulated by a
motor neuron
(nerve) to
contract
 Motor neuron
plus the fibers it
supplies is called
the “motor unit”
Nerve Stimulus for Muscle Contraction
 Neuromuscular junctions (NMJ) – site where nerve
and muscle meet
 Nerve and muscle do not make direct contact
 Each muscle fiber has only one neuromuscular junction
 Synaptic cleft – gap between nerve and muscle fiber,
filled with fluid
Nerve Stimulus for Muscle Contraction:
Neuromuscular Junction
Nerve Stimulus for Muscle Contraction
 Brain or spinal cord sends a nerve impulse down the
motor neuron in response to a stimulus
 Upon arrival of nerve impulse, a chemical called
a neurotransmitter is released by nerve
 The neurotransmitter for skeletal muscle is acetylcholine
 Neurotransmitter attaches to receptors on
the sarcolemma of the muscle fiber
Nerve Stimulus for Muscle Contraction
 Sarcolemma becomes permeable to sodium ions
(Na+)
 Sodium rushes into the cell and generates an
action potential - an explosion of electrical activity
 This electrical stimulus starts the muscle
contraction
 Once the contraction starts, it cannot be stopped ALL OR NOTHING PRINCIPLE
Neuromuscular Junction Animation
Contraction of a Skeletal Muscle
 Within a skeletal muscle, not all fibers may be
stimulated at the same time
 Different combinations of muscle fiber
contractions may give differing responses
 Graded responses – different degrees of
skeletal muscle contraction
Example: finger muscles help you pick up a
penny or a textbook
Muscle Tone
 Slightly contracted state while muscle is
at rest
 Keeps muscles ready to respond to
stimulation
 Helps maintain posture
 Stabilizes your joints
 Atrophy – decrease in muscle tone;
fibers become small and weak without
stimulation
Muscle Responses to Stimuli
a. Twitch – single, jerky contraction from a single
stimulus
 Not a normal contraction
b. Tetanus – smooth, sustained contraction from
multiple stimuli building upon each other
 Cannot go on indefinitely or results in fatigue
c. Treppe effect – increasingly forceful contractions
due to increased efficiency of muscle, even with
same stimulus
 As muscle warms up, more calcium is available,
and enzymes are more effective
Types of Muscle Contractions
1) Isotonic contractions
 Tension remains constant
 The muscle changes length so movement can
occur
a. Concentric – muscle shortens as it contracts
b. Eccentric – muscle lengthens as it contracts
2) Isometric contractions
 Tension in the muscles increases
 The muscle does not change length so
positions can be held
**Most body movements are a combination of both types.**
Force of Muscle Contraction
Depends on four things:
1)
# of muscle fibers contracting – greater the number of
motor units contracting, the greater the force
2) Size of the muscle – bigger the muscle, the greater the
force
3) Elastic elements (tendons, tissue coverings) – greater
the tension on the elastic elements, the greater the
force
4) Degree of muscle stretch – slight stretch increases the
force of contraction
Energy for Muscle Contraction
 ATP is the only energy source used for
muscle contraction
 Only 4-6 seconds worth of ATP is stored by
muscles
 After this initial time, other pathways
must be utilized to produce ATP
a. Anaerobic glycolysis/lactic acid
fermentation
b. Aerobic respiration
Energy for Muscle Contraction
 Anaerobic Glycolysis/Lactic Acid Fermentation
(NO OXYGEN AVAILABLE)
 Series of chemical reactions that makes ATP
from glucose without oxygen
 This reaction FAST, but inefficient
 Huge amounts of glucose are needed to make
ATP
 Lactic acid is a waste product of the reactions,
causes muscle fatigue and soreness
 Provides 90 seconds of energy – weightlifting,
sprints
Energy for Muscle Contraction
 Aerobic Respiration
(OXYGEN IS AVAILABLE)
 Series of chemical reactions that makes ATP
from glucose in the presence of oxygen
 This is a slow reaction, but makes huge amounts
of ATP
 Breaks down glycogen (chains of glucose) stored
in muscles and liver
 Carbon dioxide and water are waste products of
the reactions (breathing out and sweating)
 Provides 15-20 minutes of energy – long distance
running, endurance exercises
Critical Thinking Question:
After 20 minutes of exercise, your body has used up
all the stored glycogen. What do you use for
energy after that? What does this help with?
Effects of Exercise on Muscle
Results of increased muscle use:
 Increase in muscle size (size of fiber, not
number of fibers increases)
 Increase in muscle strength
 Increase in muscle efficiency (more capillaries,
more mitochondria)
 Muscle becomes more fatigue resistant (more
glycogen is stored, more mitochondria)
Muscles and Body Movements
 Movement is
attained due to a
muscle moving an
attached bone
Review:
 Origin – muscle
attachment on bone
that DOES NOT MOVE
during contraction
 Insertion – muscle
attachment to bone that
MOVES during contraction
Types of Body Movements
Review:
 Flexion
 Extension
 Rotation
 Abduction
 Circumduction
Types of Body Movements
Producing Body Movements:
Skeletal Muscle Interactions
1) Prime Mover or Agonist: muscle(s) that
provides the greatest force of a movement
2) Antagonist: muscle(s) that oppose or reverse
a particular movement, to regulate or resist
the agonist
3) Synergist: muscle that aids the agonist by:
a. Promoting the same movement
b. Reducing unnecessary movement by stabilizing or
fixating a joint and/or bone
Naming of Skeletal Muscles
1) Location of the muscle
Example: temporalis (located on the
temporal bone)
2) Number of origins
Example: triceps (three origins)
Naming of Skeletal Muscles
3) Direction of muscle fibers
 Example: rectus abdominis (straight)
4) Relative size of the muscle
 Example: gluteus maximus (largest)
Naming of Skeletal Muscles
5) Location of the muscles origin and
insertion
 Example: sternocleidomastoid (originates
on the sternum and clavicle, inserts on the
mastoid)
6) Shape of the muscle
 Example: deltoid (triangular)
7) Action of the muscle
 Example: extensor digitorum longus
(extends the fingers)
Head and Neck Muscles
Figure 6.14
Superficial Trunk and Arm Muscles
Deep Trunk and Arm Muscles
Muscles of the Anterior Pelvis, Hip, and Thigh
Muscles of the Posterior Pelvis, Hip, and Thigh
Superficial Muscles: Anterior
Superficial Muscles: Posterior
Muscular System Disorders
Chronic Fatigue Syndrome
Fibromyalgia
Duchenne’s Muscular Dystrophy
Rotator Cuff Syndrome
Anterior Compartment Syndrome
Rhabdomyolysis
Cardiomyopathy
Myasthenia Gravis
Rhabdomyosarcoma
Inguinal Hernia
Polio
Carpal Tunnel Syndrome