Chapter 9
Muscular System
Three Types of Muscle Tissues
Skeletal Muscle
• usually attached
to bones
• under conscious
control
• striated
Cardiac Muscle
• wall of heart
• not under
conscious control
• striated
•Intercalated discs
Smooth Muscle
• walls of most viscera,
blood vessels, skin
• not under conscious
control
• not striated
1
Functions of Skeletal Muscle
•Produce skeletal movement
•Maintain posture and body position
•Support soft tissues
•Guard entrances and exits
•Maintain body temperature
•Store nutrient reserves
2
Structure of a Skeletal Muscle
Skeletal Muscle
• organ of the muscular system
- skeletal muscle tissue
- nervous tissue
- blood
- connective tissues
• fascia-tissue that surrounds the
entire muscle
• tendons-dense connective tissue
cord that connect muscles to bones
• aponeuroses-sheet-like tendon that
connects muscles to bones
3
Connective Tissue Coverings
• epimysium-membrane that
surrounds the whole muscle
• perimysium-membrane that
surrounds fascicles
• fascicles-groups of muscle
fibers(cells)
• endomysium-membrane that that
surrounds individual muscle cells
•muscle fibers-muscle cells
• myofibrils-protein fibers inside
muscle cells
• thick and thin filaments
4
Skeletal Muscle Fibers
• sarcolemma-cell membrane of a
muscle fiber
• sacroplasm-cytoplasm of a muscle
fiber
• sarcoplasmic reticulum-stores
calcium ions
• transverse tubule-carries muscle
impulse from sarcolemma to
searcplasmic reticulum
• triad
• pair of cisternae of
sarcoplasmic reticulum
• transverse tubule
5
Skeletal Muscle Fibers
•Myofibril-proteins
fibers
• actin filaments-thin
filaments
• myosin filamentsthick filaments
• sarcomere-functional
unit of skeletal muscle
6
Sarcomere
• I bands-thin only
• A bands-thick and
thin
• H band-thick only
• Z lines-edge of
sarcomer
• M line-mid-line of
sarcomere
• zone of overlapwhere thick and thin
filaments overlap
7
Myofilaments
Thick Filaments
• composed of myosin
• cross-bridges
Thin Filaments
• composed of actin
• associated with troponin
and tropomyosin
8
Neuromuscular Junction
• also known as
myoneural junction
• site where an axon and
muscle fiber meet
• motor neuron-neuron
that controls a muscle
fiber
• motor end plate-area of
muscle fiber that forms
synapse (junction) with
motor neuron
9
Neuromuscular Junction
•Synapse-area where the neuron
communicates with the muscle
fiber
• synaptic cleft-space between
neuron and sarcolemma
• synaptic vesicles-store and
release neurotransmitters in
neuron
• neurotransmitters-chemical
messengers that carry the signal
across the synapse
10
Motor Unit
• single motor neuron
• all muscle fibers controlled by motor neuron
11
Stimulus for Contraction
• acetylcholine (ACh)
• nerve impulse causes
release of ACh from
synaptic vesicles
• ACh binds to ACh
receptors on motor end
plate
• generates a muscle impulse
• muscle impulse eventually
reaches sarcoplasmic
reticulum
and the cisternae
12
Excitation Contraction
Coupling
• muscle impulses cause
sarcoplasmic reticulum to
release calcium ions into
cytosol
• calcium binds to troponin
to change its shape
• position of tropomyosin is
altered
• binding sites on actin are
exposed
• actin and myosin molecules
bind
13
Sliding Filament Model of Muscle
Contraction
• When sarcromeres
shorten, thick and thin
filaments slide past one
another
• H zones and I bands
narrow
• Z lines move closer
together
14
Cross-bridge Cycling
• myosin cross-bridge attaches
to actin binding site
• myosin cross-bridge pulls
thin filament
•ADP and phosphate
released from myosin
• new ATP binds to
myosin
• linkage between actin
and myosin cross-bridge
break
•ATP splits
•myosin cross-bridge goes back
to original position
15
Relaxation
• acetylcholinesterase – rapidly decomposes Ach
remaining in the synapse
• muscle impulse stops
• stimulus to sarcolemma and muscle fiber membrane
ceases
• calcium moves back into sarcoplasmic reticulum
• myosin and actin binding prevented
• muscle fiber relaxes
•Rigor mortis-muscle contract and stay contracted as calcium leaks
out and there is no ATP to break the cross bridges
16
Major Events of Muscle
Contraction and Relaxation
17
Energy Sources for
Contraction
1) Creatine phosphate
2) Cellular respiration
• creatine phosphate – stores energy that quickly converts
ADP to ATP
18
Oxygen Supply and
Cellular Respiration
•Oxygen is needed to
completely breakdown
glucose
•myoglobin
• Anaerobic Phase
• glycolysis
• occurs in cytoplasm
• produces little ATP
• Aerobic Phase
• citric acid cycle
• electron transport chain
• occurs in the mitochondria
• produces most ATP
• myoglobin stores extra
oxygen
19
Oxygen Debt
Oxygen debt – amount of oxygen needed by liver cells to use the
accumulated lactic acid to produce glucose plus the amount muscle
cells require to resynthesize ATP and creatine phosphate and restore
their original conditions
• oxygen not available
• glycolysis continues
• pyruvic acid
converted to lactic acid
• liver converts lactic
acid to glucose
•Anaerobic threshold
or lactic acid threshold
20
Muscle Fatigue
• inability to contract after persistent, prolonged use
• commonly caused from
• decreased blood flow
• ion imbalances across the sarcolemma
• accumulation of lactic acid (most common cause)
•Psychological loss of desire to continue the exercise
• cramp – sustained, painful, involuntary muscle
contraction
21
Heat Production
• by-product
of cellular respiration
• muscle cells are major source of body heat
•Helps maintain body temperature
•blood transports heat throughout body
22
Muscular Responses
Threshold Stimulus
• minimal strength required to cause contraction
Recording a Muscle
Contraction
•myogram
•twitch
• latent period
• period of contraction
• period of relaxation
• refractory period
• all-or-none response
23
Length-Tension Relationship
24
Summation
• process by which
individual twitches
combine
• produces sustained
contractions
• can lead to tetanic
contractions
25
Recruitment of Motor Units
• recruitment - increase in the number of motor units
activated
• whole muscle composed of many motor units
• more precise movements are produced with fewer
muscle fibers within a motor unit
• as intensity of stimulation increases, recruitment of
motor units continues until all motor units are
activated
26
Sustained Contractions
•smaller motor units (smaller diameter axons)
- recruited first
• larger motor units (larger diameter axons)
- recruited later
• produce smooth movements
• muscle tone – continuous state of partial contraction
27
Types of Contractions
• isotonic – muscle contracts and
changes length
• eccentric – lengthening
contraction
• concentric – shortening contraction
• isometric – muscle contracts but
does not change length
28
Fast and Slow Twitch
Muscle Fibers
Slow-twitch fibers (type I)
• always oxidative
• resistant to fatigue
• red fibers
• most myoglobin
• good blood supply
•Many mitochondria
•aerobic
Fast-twitch glycolytic fibers (type IIb)
• white fibers (less myoglobin)
• poorer blood supply
• susceptible to fatigue
•Few mitochondria
•anaerobic
Fast-twitch fatigueresistant fibers (type IIa)
• intermediate fibers
• oxidative
• intermediate
amount of
myoglobin
• pink to red in color
•resistant to fatigue
29
Smooth Muscle Fibers
Compared to skeletal muscle fibers
• shorter
• single, centrally located nucleus
• elongated with tapering ends
• myofilaments randomly organized
• lack striations
• lack transverse tubules
• sarcoplasmic reticula not well
developed
30
Types of Smooth Muscle
Visceral Smooth Muscle
• single-unit smooth muscle
• sheets of muscle fibers
• fibers held together by
gap junctions
• exhibit rhythmicity
• exhibit peristalsis
• walls of most hollow
organs
Multiunit Smooth Muscle
• less organized
• function as separate units
• fibers function separately
• irises of eye
• walls of blood vessels
31
Smooth Muscle Contraction
• Resembles skeletal muscle contraction
• interaction between actin and myosin
• both use calcium and ATP
• both are triggered by membrane impulses
• Different from skeletal muscle contraction
• smooth muscle lacks troponin
• smooth muscle uses calmodulin
• two neurotransmitters affect smooth muscle
• acetlycholine and norepinephrine
• hormones affect smooth muscle
• stretching can trigger smooth muscle contraction
• smooth muscle slower to contract and relax
• smooth muscle more resistant to fatigue
• smooth muscle can change length without changing
tautness
32
Cardiac Muscle
• located only in the heart
• striated muscle fibers joined together by intercalated discs
•Single nucleus per cells
•Cisternae are less developed and store less calcium
transverse tubules are larger
•fibers branch
• network of fibers contracts as a unit
• self-exciting and rhythmic
• longer refractory period than skeletal muscle
33
Characteristics of Muscle Tissue
34
Skeletal Muscle Actions
• origin – immovable end
• insertion – movable end
• prime mover (agonist) –
primarily responsible for
movement
• synergists – assist prime mover
• antagonist – resist prime
mover’s action and cause
movement in the opposite
direction
35
Body Movement
Four Basic Components of Lever
1. rigid bar – bones
2. fulcrum – point on which bar moves; joint
3. object - moved against resistance; weight
4. force – supplies energy for movement; muscles
36
Levers and Movement
37
Life-Span Changes
•Changes in muscular system first begin to appear in
one’s 40’s
•myoglobin, ATP, and creatine phosphate decline
• by age 80, half of muscle mass has atrophied
• adipose cells and connective tissues replace muscle
tissue
• exercise helps to maintain muscle mass and function
38
Clinical Application
Myasthenia Gravis
• autoimmune disorder
• receptors for ACh on muscle cells are attacked
• weak and easily fatigued muscles result
• difficulty swallowing and chewing
• ventilator needed if respiratory muscles are affected
• treatments include
• drugs that boost ACh
• removing thymus gland
• immunosuppressant drugs
• antibodies
39
Clinical Application
• tenanus - sustained powerful contractions of skeletal
muscles throughout the body
•Caused by Clostridium tetani
•Lockjaw
•Tetanus shot-vaccine against the toxin
40