Download The Muscular System

The Muscular System
Chapter 6
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Muscle Tissues
• Muscles make up nearly ½ of the body’s
mass!
• Function: contraction/shortening 
movement!
• Muscle Types (Table 6.1, page 182)
– Skeletal
– Cardiac
– Smooth
Skeletal Muscle
• “fiber” – elongated cells
• Striated, voluntary
• Skeletal muscle is organ of the Muscular
System
• Surrounded by fascia separated into:
• Endomysium: surrounds each fiber
• Perimysium: surrounds groups of fibers
(fascicle)
• Epimysium: covers entire muscle
– Continuous with tendons or aponeuroses
Skeletal Muscle
• Figure 6.1, page 183
Anatomy of a Skeletal Muscle
“Bundles of Bundles”
Muscle (fascicles wrapped in epimysium)
Fascicles (muscle fibers wrapped in perimysium)
Fibers (elongated cells, enclosed with sarcolemma)
Myofibrils (organelles which fill sarcoplasm)
Myofilaments (threadlike protein filaments bundled
into myofibrils)
Muscle Function
• Producing movement
– Muscles, bones, & joints work together
• Maintaining posture & body position
• Stabilizing joints
• Generating heat (thermogenesis)
– 80% of heat comes from muscle contraction
(lost when bonds in ATP broken)
– Shivering to raise temperature when cold
Microscopic Anatomy of
Skeletal Muscle
• Cells are multinucleate
• Sarcolemma: cell membrane
• Myofibrils: ribbon-like organelles that fill the
cytoplasm
• Alternating light (I) and dark (A) bands on
myofibrils (striations)
• Sarcomere: contractile unit of muscle
(myofibrils)
– Aligned end to end
– Myofilaments within myofibrils
• Sarcoplasmic reticulum (SR): specialized
smooth ER
– Stores calcium and releases it on demand when
muscle fiber stimulated to contract
Myofilaments
• Myosin filaments: thick filaments
–
–
–
–
Made of bundles of protein myosin
Contain ATPase enzymes (split ATP)
Extend length of A band
Ends have small projections called myosin
heads (cross bridges): link thick & thin
filaments together during contraction
• Actin filaments: thin filaments
– Made of contractile protein actin
– Regulatory proteins prevent binding of myosin
heads to actin
– Anchored to the Z disc
– I band only contains actin filaments
Z Line to Z Line called Sarcomere
Properties of Skeletal
Muscle
• Excitability/responsiveness
– Receive and respond to stimuli
• Contractility
– Ability to shorten (forcibly) when adequately
stimulated
• Extensibility
– Muscle cells can be stretched
• Elasticity
– Ability to recoil and resume their resting
length after being stretched
Neuromuscular Junction
• Must be stimulated by nerve impulses
• One motor neuron (nerve cell) can stimulate
a few muscle cells or hundreds of them
• Motor unit: one motor neuron and all
skeletal muscle cells it stimulates
• Axon of neuron branches into axon terminals,
each of which forms junctions with
sarcolemma of different muscle cells
– Neuromuscular junctions: contain vesicles filled
with neurotransmitters
– NTM that stimulates skeletal muscle cells:
acetylcholine (ACh)
– Synaptic cleft: gap between axon terminal and
sarcolemma of muscle cell; filled with interstitial
fluid
Neuromuscular Junction
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Physiology of Muscle
Contraction
When nerve impulse reaches axon terminals…
1. Calcium channels open and Ca2+ enters the
terminal causing release of Ach
2. ACh diffuses across the synaptic cleft and
attaches to receptors (membrane proteins) that are
located in the sarcolemma of the muscle cell
4. Ach stimulates sarcolemma and causes
depolarization, generating an action potential (more on
that in the nervous system!)
5. Action potential spreads throughout the muscle
cell and stimulates sarcoplasmic reticulum to release
calcium ions into the cytoplasm
6. Calcium ions trigger binding of myosin to actin,
initiating filament sliding
7. Acetylcholinesterase enzyme breaks down ACh
(present on sarcolemma and in synaptic cleft).
– Single nerve impulse produces only one contraction
– Prevents continued contraction of the muscle cell in the
absence of additional nerve impulses.
Sliding Filament Theory
• Pages 191-192 (fig 6.7 & 6.8)
• Myosin heads attach to binding sites on the thin
filaments when stimulated by nervous system
(Calcium present)
• Each cross bridge attaches and detaches (bend,
break, & reform) several times during a
contraction (using energy from ATP), generating
tension that helps to pull the thin filaments
toward the center of the sarcomere (form cross
bridges further down actin filament) – power
stroke
• Myofilaments do not shorten – just slide past
each other
• Occurs simultaneously in sarcomeres throughout
the muscle cell (cell shortens)
• Attachment of myosin cross bridges to actin
requires calcium ions
• Takes a few thousandths of a second
animation
Sliding
Filament
Theory
Detailed animation
Muscle Response
• Threshold stimulus: minimal stimulus needed to
cause contraction
• All-or-none response
• Can be measured by a myogram (figure 6.9)
Contraction of Skeletal
Muscle as a Whole
• Whole muscle contraction is through
graded responses
– different degrees of shortening throughout
the whole muscle
• Produced two ways:
– Changing the frequency of muscle stimulation
– Changing the number of muscle cells being
stimulated at one time
Muscle Response to
Increasingly Rapid Stimulation
• Nerve impulses delivered to muscle at a
very rapid rate (no chance for muscle to
relax)
• Effects of successive contractions are
added together (summative)
– Causes stronger & smoother contraction
– Sustained contraction
• Fused (complete) tetanus: contractions
completely smooth and sustained
• Unfused (incomplete) tetanus: up until
the point of fused tetanus
Muscle Response to
Stronger Stimuli
• Force of muscle contraction mostly
depends on how many cells are
stimulated
• when all motor units are active and all
cells stimulated, muscle contraction is as
strong as it can get
• Muscle cells stimulated at the same time
& entire muscle contracts
• Motor units recruit other fibers
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Electromyogram
Energy for Muscle
Contraction
• Muscle store a small amount of ATP (a
few seconds’ worth) to get going
• ATP only energy source can be used
directly to power muscle activity & must
be regenerated continuously if
contraction is to continue
• ATP regeneration catalyzed by ATPase
• Three pathways for ATP regeneration:
– Direct phosphorylation of ADP by creatine
phosphate
– Aerobic respiration
– Anaerobic glycolysis & lactic acid formation
• Only 25% of energy is used – rest
released as heat!
Creatine Phosphate
• High energy molecule
found in muscle fibers
but not other cell
types
• Regenerates ATP in a
fraction of a second
by transferring a
phosphate group to
ADP
• CP supplies exhausted
in less than 15
seconds
Aerobic Respiration
• 95% of ATP used for muscle activity
• Oxidative phosphorylation: occurs in
mitochondria, metabolic pathways that
use oxygen
• Glucose broken down completely to
carbon dioxide & water, energy released
as bonds are broken  energy captured
in ATP molecules
• Generates a large amount of ATP
• Slow and requires continuous delivery of
oxygen and nutrient fuels to muscle to
keep it going
Anaerobic Glycolysis &
Lactic Acid Fermentation
• If oxygen & glucose delivery is inadequate
(intense muscle activity), glycolysis
moves into lactic acid fermentation
instead of oxidative phosphorylation
• Only 5% as much ATP produced without
oxygen
• 2.5 times faster
and provides most
of ATP needed for
30-40 seconds of
strenuous activity
• Lactic acid accumulates and produces
muscle soreness
Muscle Fatigue & Oxygen Deficit
• Muscle fatigue: unable to contract even
though being stimulated
• Oxygen deficit occurs after prolonged
muscle activity, causing fatigue
• Work that a muscle can do and how long
without fatigue depends on blood supply
• Without adequate oxygen, lactic acid
accumulates and ATP supply runs low,
which leads to fatigue
– i.e. marathon runners
Types of Muscle
Contractions
• Isotonic contractions: myofilaments slide
past each other, muscle shortens, movement
occurs
• Isometric contractions: muscles do not
shorten, tension increases in muscle but
myofilaments do not slide past each other
– Movement pitted against an immoveable object
• Muscle tone: continuous partial contractions
that occur involuntarily due to nervous
stimulation to keep a muscle firm, healthy,
and ready for action
– If no longer stimulated in this way, loses tone
and muscle is flaccid and begins to atrophy
Muscle Movement
• “Golden Rules” (Table 6.2, page 197)
– All skeletal muscles cross at least one joint
(with few exceptions)
– Bulk of skeletal muscle lies proximal to joint
crossed
– All skeletal muscles have at least two
attachments: the origin & the insertion
– Skeletal muscles can only pull; they can never
push
– During contraction, a skeletal muscle insertion
moves toward the origin
Muscle Movement
• All skeletal muscles are attached to bone
or to other connective tissue structures
at no fewer than two points
– Origin: attached to the immovable or less
movable bone
– Insertion: attached to the movable bone, and
when muscle contracts, moves toward the
origin
• Some muscles have interchangeable
origins & insertions or multiple
origins/insertions (i.e. biceps brachii)
• body movement occurs when muscles
contract across joints
Biceps Brachii
• Fig 6.12, page 197
• 2 origins at shoulder
• Insertion across elbow – attaches to ulna
Belly
Interactions of Skeletal
Muscles
• Muscles function in groups
• Groups of muscles that produce opposite
movements lie on opposite sides of a
joint
• Prime mover: muscle that has major
responsibility for causing a movement
• Antagonists: muscles that oppose or
reverse a movement
• Muscles can be both prime movers and
antagonists (i.e. biceps & triceps)
• Synergists: help prime movers by
producing same movement or reducing
undesirable movements (stabilize joints)
Interactions of Muscles
Naming Skeletal Muscles
• Direction of the muscle fibers
– External obliques
• Relative size of the muscle
– Pectoralis major
• Location of the muscle’s
origin and insertion
– Sternocleidomastoid
• Shape of the muscle
– deltoid
• Action of the muscle
– Extensor digitorum
Direction of Muscle Fibers
• External oblique
– oblique= at a slant
• Rectus abdominus
– Rectus = straight
(parallel)
• Using the midline of
the body or axis of a
long bone
Relative Size of Muscle
• Major/minor
• Vastus
– Long; covers a lot
of area; big
• Maximus/minimus
• Medius
• Longus
Location
• Frontalis
• Temporalis
• Tibialis anterior
• Biceps femoris
(not the same as the biceps
brachii in your arm!)
Action of the Muscle
• Adductor
• Extensor digitorum
Number of Attachments
• Biceps brachii
• Biceps femoris
• Triceps brachii
Also give location
(brachial; femoral)
Arrangement of Fascicles
• Determines range of motion and power
• Circular: concentric rings (sphincters)
– Ex. Orbicularis muscles (eyes & mouth)
• Convergent: fascicles converge toward a single
insertion tendon (triangular or fan shaped)
– Ex. Pectoralis major
• Parallel: length of fascilces run parallel to the
long axis of muscle (straplike)
– Fusiform: spindle-shaped with expanded “belly”
– Ex. Biceps brachii
• Pennate: short fascicles attach obliquely to
central tendon
– Unipennate (insert into one side of tendon)
– Bipennate & multipennate (most powerful)
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Circular
• Orbicularis oculi
• Orbicularis oris
Convergent
• Pectoralis Major
Parallel
• Sternocleidomastoid
• Sartorius
Fusiform
• Biceps brachii
• Tensor fasciae lata
• Semitendinosus
Unipennate
• Extensor digitorum longus
Bipennate
• Rectus femoris
Multipennate
• Deltoids
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