Muscular System
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Types of Muscle Tissue
• 3 types:
1. Skeletal
– striated & voluntary
2. Cardiac
– striated & involuntary
3. Smooth
– Smooth & involuntary
Skeletal Muscle Tissue
• most attached to bone
• striations:
– see light & dark bands under microscope
Cardiac Muscle Tissue
• found only in walls of heart chambers
• heart has a pacemaker that initiates each contraction
– called autorhymicity
– controlled by hormones & neurotransmitters
Smooth Muscle Tissue
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in walls of hollow organs
attached to hair follicles
some autorhythmic (wall of intestines)
regulated by ANS motor neuron& hormones
Functions of Muscular Tissue (#4)
• 1. producing body movements
– moving whole body or parts of body
Functions
• 2. stabilizing body position
– skeletal muscles stabilize joints & halp
maintain body positions
– postural muscles hold sustained contractions
(holding head up all day)
Functions
• 3. storing & moving substances w/in body
– storing: accompanied by sustained
contractions of ringlike bands of smooth
muscle called sphincters (hold material in
organ)
– contraction/relaxation of smooth & cardiac
muscle moves material thru bld vessels 
heart  bld vessels
Functions
• 4. generating heat
– process called thermogenesis
– most of heat generated by muscle contraction
maintains normal body temp of 37°C
– shivering: involuntary contraction of skeletal
muscle increases heat production
Properties of Muscle Tissue (#4)
• 1. electrical excitability
– ability to respond to certain stimuli by
producing electrical signals called action
potentials
– 2 main types stimuli:
1. autorhythmic electrical signals
2. chemical stimuli (neurotransmitters) released
by neurons
Properties
• 2. contractility
– ability of muscle fibers to contract forcefully
when stimulated by an action potential
– muscle fiber shortens & pulls on whatever it is
attached to
• if force > resistance of object, movement occurs
Properties
• 3. extensibility
– ability of muscle tissue to stretch w/out being
damaged
– smooth muscle fibers are stretched every time
your stomach or bladder is really full
Properties
• 4. elasticity
– ability of muscle tissue to return to original
length & shape after contraction or extension
CT Components
• fascia: sheet or broad band of fibrous CT that
supports & surrounds muscles or other organs
– 2 layers: superficial & deep
CT Components
• 2. 3 layers of CT extend from deep fascia 
deeper into muscle tissue
1. epimysium: outermost layer, encircles entire
muscle
2. perimysium: surrounds groups of 10 – 100
muscle fibers = a fascicle
3. endomysium: surrounds individual muscle
fibers
CT Components
• 3. tendon: extension of epimysium,
perimysium, & endomysium beyond muscle
that attaches the skeletal muscle to another
structure (bone or another muscle)
Muscle Histology Terms
• hypertrophy: enlargement of existing muscle
fibers
– ex: muscle growth in newborn
• hyperplasia: increase in # of muscle fibers
– ex: growth hormone causes increase in #s from
childhood  adult
• fibrosis: replacement of muscle by fibrous scar
tissue
• satellite cells: stem cells in muscle tissue;
limited capacity
Why Muscle Fibers are Multinucleated
Terms for Muscle Fiber Organelles
• sarcolemma: plasma membrane
• sarcoplasm: cytoplasm
• myofibril: contractile organelles; thread-like
structures; each extends length of muscle
fiber
Terms for Muscle Fiber Organelles
• T tubules: (transverse) invaginations of
sarcolemma into sarcoplasma; increasing
surface area
– filled with interstitial fluid
– ensures action potentials excites all parts of
muscle fiber
Terms for Muscle Fiber Organelles
• sarcoplasmic reticulum: (SR) endoplasmic
reticulum that encircles individual myofibrils
– dilated end sacs called terminal cistern
– T tubule + 2 terminal cisterns = triad
– in relaxed muscle fiber SR stores Ca++
– release of Ca++ triggers contraction
Muscle Proteins
• 3 kinds proteins in myofibrils:
1. contractile proteins
– myosin make up thick filaments, golf-club shape
(myosin head)
– actin  thin filaments
2. regulatory proteins
– tropomyosin & troponin: both in thin filaments
3. structural proteins
– ~12 different ones function in alignment,
stability, elasticity, & extensibility of myofibrils
Contraction Cycle Steps
1. ATP hydrolysis
– ATP attached to myosin head
2. attachment of myosin head to actin to form
cross bridges
3. power stroke
– cross bridges rotate  center  slides thin
filament past thick filament
4. detachment of myosin from actin
– ATP binds to myosin head & cross bridges
released
Neuromuscular Junction (NMJ)
• somatic motor neurons innervate muscle
fibers to contract
NMJ
• synapse: functional junction between 2
neurons or between a neuron & an effector
(muscle or gland); may be electrical or
chemical
• 1st side of synapse: end of axon of motor
neuron called synaptic end bulb
• then synaptic cleft (the space)
• lastly, motor end plate: part of sarcolemma
that has receptors for neurotransmitter
acetylcholine (ACh)
Steps in Nerve Action Potential 
Muscle Action Potential
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1. release of ACh
2. activation of ACh receptors
3. production of muscle action potential
4. termination of ACh activity
Release of ACh
• ACh stored in vesicles in synaptic end bulb
• action potential travels down axon  reaches
synaptic end bulb  induces exocytosis of
neurotransmitter from synaptic vesicles
• ACh diffuses across synaptic cleft toward
motor end plate
Activation of ACh receptors
• 2 molecules of ACh bind to ACh receptors
embedded in sarcolemma  opens ion
channel  allows Na+ diffuse across
membrane
Production of Muscle Action Potential
• inflow Na+ makes inside of muscle fiber more
+ charged
• this change in membrane potential triggers a
muscle action potential  propagates along
sarcolemma  T tubules
• this causes SR to release Ca++  sarcoplasm
 contraction
NMJ
• @ midpoint of muscle fiber:
– muscle action potential propagate  both
ends of fiber
– allows simultaneous activation & so
contraction of all parts of muscle fiber
Curare
• South American plant derivative
• causes paralysis by binding to & blocking ACh
receptors on motor end plates
• curare-like drugs used in general anesthesia to
relax skeletal muscles
Botulism
• disease caused by Clostridium botulinum 
toxin that blocks exocytosis of synaptic
vesicles so no ACh released so no muscle
contraction
– toxin one of most lethal chemicals known
– causes death by paralyzing skeletal muscles:
breathing stops when diaphragm & intercostal
muscles stop contracting
Isotonic Contractions
• “equal tension”
• force of contraction developed by muscle
remains almost constant while muscle
changes its length
• used for body movements & for moving
objects
• 2 types:
1. concentric isotonic contraction
2. eccentric isotonic contraction
Concentric Isotonic Contraction
• when tension generated is enough to
overcome resistance of object being moved
…muscle shortens & pulls on another
structure (tendon) ….producing movement
that reduces angle at a joint
Eccentric Isotonic Contraction
• tension exerted by the muscle resists
movement of the load (whatever was lifted
up) slowing the lengthening process
Isometric Contraction
• tension generated by the muscle is < tension
needed to overcome resistance of the object
• muscle does not change its length
Cardiac Muscle Tissue
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same arrangement actin/myosin: striations
muscle fibers branched
*intercalated discs: unique to cardiac muscle
autorhymicity alone: 75 bpm
remains contracted 10-15 x’s longer than
skeletal muscle after 1 action potential
– due to prolonged delivery of Ca++ (SR +
interstitial fluid
– larger & more #s of mitochondria
Smooth Muscle Tissue
• +thick & thin filaments but no T tubules & less
SR so no striations
Development of Muscle
• from mesoderm
• starts ~4 wks
• cardiac muscle forms
tubes  bends &
folds to form heart
Medical Terminology
• spasm: sudden involuntary contraction of a
single muscle is a large group of muscles
Medical Terminology
• tic: an involuntary twitching by muscles that
are normally under voluntary control
Medical Terminology
• tremor: rhythmic, involuntary, purposeless
contraction that produces a quivering or
shaking movement
Medical Terminology
• fasiculation: involuntary, brief twitch that is
visible under the skin; occurring irregularly &
not ass’c with movement
– seen in MS or ALS
Medical Terminology
• fibrillation: spontaneous contraction of single
muscle fiber that is not visible under skin but
can be recorded by EMG (electromyograph)
– signals destruction of motor neurons
Medical Terminology
• muscle strain: tearing of muscle due to
forceful impact + bleeding +pain
– most often affect quadraceps femoris
– tx‘d RICE (rest, ice, compression ie a wrap,
elevation)