Chapter 9 Muscular System:
Histology and Physiology
Muscular System
Functions
 Body
movement
 Maintenance of posture
 Respiration
 Production of body heat
 Communication
 Constriction of organs and vessels
 Heart beat
Criteria for Naming Muscles


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



Shape: romboideus, trapezius, biceps
Location: pectoralis (chest) intercostal (ribs)
Attachment: zygomaticus, sternocleidomastoid
Size: maximus, minimus, brevis, longis
Orientation of fibers: rectus (straight), oblique
(slanting)
Relative position (lateral, medial, internal,
external)
Function: adductor, flexor, extensor, pronator
Properties of Muscle

Contractility


Excitability


Capacity of muscle to respond to a stimulus
Extensibility


Ability of a muscle to shorten with force
Muscle can be stretched to its normal
resting length and beyond to a limited
degree
Elasticity

Ability of muscle to recoil to original resting
length after stretched
Skeletal Muscle
Smooth Muscle
Cardiac Muscle
Features
Skeletal Muscle
Smooth Muscle
Cardiac
Muscle
Location
Attached to bone
walls of hollow
organs, blood
vessels, eyes,
glands and skin
heart
Cell shape
very long,
cylindrical
Spindle shaped
Cylindrical and
branched
Nucleus
Multiple,
peripherally located
Single centrally
located
single centrally
located
Gap junctions join
visceral smooth
muscle
Intercalated disks
join cells
Involuntary
Involuntary
Spontaneous No
contraction
Yes
Yes
Function
Food movement,
urinary bladder,
blood vessels,
glands and duct
pumps blood
Special
features
Control
voluntary and
involuntary reflexes
Body movement
Skeletal Muscle Structure

Muscle fibers or
cells



Develop from
myoblasts
Numbers remain
constant
Hypertrophy –
increase in the
size of each fiber.
Connective tissue
 Nerve and blood
vessels

Connective Tissue, Nerve,
Blood Vessels

Connective tissue






Fascia


External lamina
Endomysium
Perimysium
Fasciculus
Epimysium
Binds adjacent muscles
or overlying skin.
Nerve and blood vessels

Abundant
Parts of a Muscle
Structure of Actin and Myosin
Components of Sarcomeres
Sliding Filament Model
 Actin
myofilaments sliding over
myosin to shorten sarcomeres
Actin and myosin do not change length
 Shortening sarcomeres responsible for
skeletal muscle contraction

 During
relaxation, sarcomeres
lengthen
Sarcomere Shortening
Physiology of Skeletal
Muscle

Nervous system


Controls muscle
contractions through
action potentials
Resting membrane
potentials

Membrane voltage
difference across
membranes
(polarized)
• Inside cell more
negative and more K+
• Outside cell more
positive and more Na+

Must exist for action
potential to occur
Ion Channels

Types

Ligand-gated
• Example:
neurotransmitters

Voltage-gated
• Open and close in
response to small
voltage changes
across plasma
membrane
Action Potentials

Phases

Depolarization
• Inside plasma
membrane becomes
less negative

Repolarization
• Return of resting
membrane potential

All-or-none principle


Propagate


Like camera flash
system
Spread from one
location to another
Frequency

Number of action
potential produced per
unit of time
0013.exe
Action Potential Propagation
Neuromuscular Junction

Synapse or NMJ




Presynaptic terminal
Synaptic cleft
Postsynaptic membrane or motor end-plate
Synaptic vesicles


Acetylcholine: Neurotransmitter
Acetylcholinesterase: A degrading enzyme in synaptic cleft
Function of Neuromuscular
Junction
Excitation-Contraction Coupling


Mechanism by
which an action
potential causes
muscle fiber
contraction
Involves



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

Sarcolemma
Transverse or T
tubules
Terminal cisternae
Sarcoplasmic
reticulum
Ca2+
Troponin
Action Potentials and
Muscle Contraction
Cross-Bridge Movement
Muscle Twitch


Muscle contraction in
response to a
stimulus that causes
action potential in
one or more muscle
fibers
Phases



Lag or latent
Contraction
Relaxation
Stimulus Strength and Muscle
 All-or-none law for
Contraction
muscle fibers

A motor unit contracts
with a consistent force in
response to each action
potential
• Sub-threshold stimulus
• Threshold stimulus
• Stronger than threshold

Motor units


Single motor neuron and
all muscle fibers that it
innervates
Graded for whole
muscles

Strength of contractions
range from weak to
strong depending on
stimulus strength
Multiple Motor Unit Summation


A whole muscle contracts with a small or large force
depending on number of motor units stimulated to
contract
Muscle performing delicate and precise movements
have motor units with smaller numbers of fibers
Multiple-Wave Summation

As frequency of action
potentials increase,
frequency of contraction
increases

Incomplete tetanus
• Muscle fibers partially relax
between contraction

Complete tetanus
• No relaxation between
contractions

Multiple-wave summation


Muscle tension increases
as contraction frequencies
increase
Due to increased calcium
concentration around
myofibrils and more
complete stretching of
muscle elastic elements
Treppe




Increase in the force of
contraction during the
first few contractions of a
rested muscle.
Occurs in muscle rested
for prolonged period
Each subsequent
contraction is stronger
than previous until all
equal after few stimuli
Due to Ca++ ion levels
around myofibrils and
increased temperature of
muscle

Enzymes for muscle
contraction respond
more effectively at
higher temperature.
Types of Muscle Contractions

Isometric: No change in length but
tension increases


Isotonic: Change in length but tension
constant



Postural muscles of body
Concentric: Overcomes opposing resistance
and muscle shortens
Eccentric: Tension maintained but muscle
lengthens
Muscle tone: Constant tension by
muscles for long periods of time
Muscle Length and Tension
Fatigue

Decreased capacity to work and
reduced efficiency of performance


Usually follows a period of activity
Types

Psychological (in CNS)
• Depends on emotional state of individual
• Perception that muscle is too tired (
• Home court advantage

Muscular
• Results from ATP depletion in muscle

Synaptic
• Occurs in NMJ due to lack of acetylcholine
Energy Sources

ATP provides immediate energy for muscle
contractions from 3 sources

Creatine phosphate
• During resting conditions stores energy to synthesize ATP
• Exhausted quickly (10-15 sec.)

Anaerobic respiration
• Occurs in absence of oxygen and results in breakdown of
glucose to yield ATP and lactic acid

Aerobic respiration
• Requires oxygen and breaks down glucose to produce ATP,
carbon dioxide and water
• More efficient than anaerobic

Oxygen Debt

After anaerobic respiration, aerobic respiration is higher
than normal to replace creatine phosphate and convert
lactic acid to glucose.
Slow and Fast Fibers

Slow-twitch or high-oxidative


Contract more slowly, smaller in diameter, well
developed blood supply, more mitochondria and high
myoglobin content, more fatigue-resistant than fasttwitch
Fast-twitch or low-oxidative


Respond rapidly to nervous stimulation, less blood
supply, fewer and smaller mitochondria, lower
myoglobin content than slow-twitch, fatigue easily.
Two types:
• Fast twitch fatigable fibers
• Fast twitch fatigue resistant (highly trained muscle)

Distribution of fast-twitch and slow twitch

Most muscles have both but varies for each muscle
Effects of Exercise

Training muscle increases muscular size and strength
(Hypertrophy).

Aerobic exercise can convert fast-twitch easily fatigued muscle
into fatigue-resistant fast-twitch muscle.
• Change in myosin type, increase size and number of mitochondria
and increased blood supply


Muscles that are not used Atrophy or decreases in muscle
size.
Atrophy or hypertrophy are the result of changes in the size
of individual muscle cells not the number of muscle cells.



Number of myofibrils and sacromeres changes.
Blood vessels, mitochondria and connective tissues increase.
Trained athletes:


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Have the ability to recruit large numbers of motor units
simultaneously improving coordination.
Have a greater capacity for nutrient uptake and ATP production
(increased metabolism)
Have improved circulation and more efficient respiration.
Heat Production
Heat is a biproduct of the chemical
reactions that occur in the body.
 As muscles are worked they produce
excess heat that must be disipated by
other body systems (circulatory and
integument)
 When body temperature drops muscle
shiver to generate more heat (up to 18
times that of resting muscle).

Smooth Muscle

Fig. 9.23
Characteristics


Spindle shaped
Fewer actin and
myosin
• Organized in loose
bundles.
• Not striated.



Dense bodies hold
actin filaments
together and are
attached to
noncontractile
intermediate filaments
Ca2+ required to
initiate contractions
Sarcoplamic reticulum
is not well developed.
Smooth Muscle Contraction
Types of Smooth Muscle

Visceral or Unitary Smooth Muscle



Found in digestive, urinary and reproductive
tracts.
Contains gap junctions, contracts in waves and
often has autorhythmicity.
Multiunit smooth muscle


Found in iris, blood vessels, arrector pili.
Fewer gap junctions, groups of cells act as
independent units, only contracts when
stimulated by nerves or hormones.
Electrical Properties of
Smooth Muscle
Functional Properties of
Smooth Muscle




Some visceral muscle exhibits
autorhythmic contractions
Tends to contract in response to sudden
stretch but not to slow increase in length
Exhibits relatively constant tension:
Smooth muscle tone
Amplitude of contraction remains
constant although muscle length varies
Smooth Muscle Regulation




Innervated by autonomic nervous
system
Neurotransmitter are acetylcholine and
norepinephrine
Hormones important as epinephrine
and oxytocin
Receptors present on plasma
membrane which neurotransmitters or
hormones bind determines response
Cardiac Muscle







Found only in heart
Striated
Each cell usually has one nucleus
Has intercalated disks and gap junctions
Autorhythmic cells
Action potentials of longer duration and
longer refractory period
Ca2+ regulates contraction
Types of Muscle Contraction
 Isometric

Increase in tension with no change in length during the
contraction process (postural muscles)
 Isotonic
Tension produced by muscle remains constant while
length changes.
Note - Both Isometric and Isotonic contractions are used
in most body movements




Concentric contractions
Eccentric contractions
Effects of Aging on
Skeletal Muscle
Reduced muscle mass
 Increased time for muscle to contract in
response to nervous stimuli
 Reduced stamina
 Increased recovery time
 Loss of muscle fibers
 Decreased density of capillaries in muscle
