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Chapter 8 – The Muscular System
4 properties of muscle cells that distinguish
them from all other cells
1) Contractility – ability of a cell to shorten in
length (moves structures attached to it)
2) Excitability – the ability to receive and
respond to a stimulus
3) Extensibility – ability to increase in
length/extend (stretch)
4) Elasticity – ability to return to original
form after it has contracted or stretched
-Muscular system includes only skeletal
muscle, not cardiac or smooth
-About 500 muscles accounting for 40-50% of
total body weight
3 major functions
1) Movement – integrates bones, joints,
nerves and other muscles
2) Support – connections between muscles
and bones help w/ posture + strength of
skeletal frame
3) Heat production – maintain normal body
temperature/homeostasis
Muscle Structure
-Muscle – any organ that usually reaches
from 1 bone to another, composed mainly of
skeletal muscle, but also connective and
nerve tissue
 Connective Tissue of Muscle
 Fascia – most abundant tissue of muscle;
sheet or broad band of dense connective
tissue that may occupy the space
between skin and muscle or surround
muscle/other organs
 Superficial fascia – beneath skin
 Deep fascia – associated w/ muscle;
surrounds muscle keeping it together as
one unit; route for passage of blood
vessels and nerves
- Other connective tissues associated w/
muscle:
 Epimysium (deep fascia) – outermost
covering of muscle
 Perimysium – deeper, thinner layer of
connective tissue; divides muscle into
compartments called bundles/fascicles
 Endomysium – deepest, thinnest portion
of dense connective tissue that envelopes
individual muscle cells/fibers.
-all 3 coverings transmit blood vessels +
nerves to muscle components + provide
support w/ interconnecting protein fibers
- tendon – connects muscle to bone; all 3 layers
of connective tissue merge and form a single
band of connective tissue
- tendon merges w/ periosteum to reinforce
connection
Microscopic Structure of Muscle
 Muscle fiber – a single cell of skeletal
muscle; long, cylindrical; usually length of
muscle
- Contain many nuclei (multinucleated)
 Sarcolemma – cell (plasma) membrane of
each fiber
 Sarcoplasm – cytoplasm of muscle fiber;
contains many mitochondria
 Sarcoplasmic reticulum – membranous
sac that stores calcium; necessary for
muscle contraction
 Transverse (T) tubule – found between
adjacent sacs; unite sarcolemma to s.r.;
used to transport calcium
 Myofibrils – cylindrical cords of protein
found in muscle fibers (underneath
sarcoplasmic reticulum); lie parallel to
each other lengthwise; contain 2 kinds of
protein filaments
- Thick filaments = myosin protein
- Thin filaments = actin, troponin, and
tropomyosin proteins
- Thick and thin filaments alternate along
the length of the myofibril
 A band – regions where thick and thin
filaments overlap (appear dark –
anisotropic)
 I band – regions where only thin filaments
occur (appear light – isotropic)
- Alternating dark and light bands form
visible striations that can be seen w/ a
microscope
 Z line – section on I-band where
perpendicular protein fibers intersect w/
thin filaments
 Sarcomere – segment of a myofibril
between Z lines (contain half of 2 I-bands
and an A band in between)
 H zone – less dense central region w/ in A
band; no overlap of thin filaments = thick
filaments only
Nerve Supply
 Resting Membrane Potential – difference in
charge that exists between the outside and
inside of the cell membrane, which causes a
small difference in voltage
o Caused by fewer positive ions inside
nerve cells (neurons)
 Action Potential – reversal of charges across
the plasma membrane that occurs when
the cell is stimulated (conducting an
impulse)
o Positive ions move into the cell (very
brief)
 Motor neurons – carry nervous impulses
(action potentials) from brain to skeletal
muscles
 Motor unit – a single motor neuron and the
muscle fibers it stimulates
 Neuromuscular junction – where the
terminal end of a motor neuron meets the
motor end plate (very folded, many
mitochondria) of a muscle cell (including
the space between called the synaptic cleft)
 Synaptic vesicles – tiny sacs inside the
terminal end of a motor neuron that
contain neurotransmitters (chemicals that
carry a signal from 1 nerve terminal to
another neuron or muscle cell)
 Ach (acetylcholine) – neurotransmitter
found in motor neurons that causes
muscles to contract
Physiology of Muscle Contraction
-1 motor neuron stimulates 25-3000 muscle
fibers which contract simultaneously to provide
a smooth contraction (average = 150 fibers)
-sliding filament mechanism – once stimulus
(action potential from nerve cell) reaches
muscle fiber, thin filaments in A band slide
inward toward H-zone, causing each sarcomere
along myofibril to shorten
The Fiber at Rest (before impulse reaches
muscle)
-Ca 2+ ions are stored in the sarcoplasmic
reticulum
-ATP is bound to myosin proteins of thick
filaments
-thin filaments are intact w/ all proteins (actin,
troponin, tropomyosin)
Role of the Stimulus
-ACh is released into synaptic cleft and reaches
the motor end plate of the muscle fiber; an
action potential is generated that travels down
the sarcolemma, down T-tubules, and through
sarcoplasmic reticulum
-membrane of sarcoplasmic reticulum releases
calcium ions into sarcoplasm
-Calcium ions diffuse into myofibrils
Muscle Contraction
-Calcium ions bind to troponin molecules in thin
filaments, causing actin and troponin molecules
to change shape, exposing binding sites on thin
filaments
-cross bridges of thick filaments bind to these
attachment sites on thin filaments
-Ca 2+ ions activate the breakdown of ATP
(attached to thick filaments); myosin breaks
phosphate off of ATP
-energy released is used to move cross bridges
+ released as heat
-movement of cross bridges cause thin
filaments to be drawn toward the center of the
sarcomere (H-zone)
-cross bridges break (detach) when another
ATP binds to myosin (cross bridges)
-cross bridges reattach to binding sites on thin
filaments (after troponin moves exposing
binding site), move, and move thin filaments
closer to the centers of sarcomeres
-process continues until thin filaments have
moved as far inward as they will go
Return to Rest
-after an action potential passes down the
motor neuron, ACh release stops, but stimulus
does not end until all ACh molecules on motor
end plate are inactivated
-done by an enzyme in the sarcolemma called
AChE (acetylcholinesterase)
-calcium ions are then returned to sarcoplasmic
reticulum by active transport (requires ATP)
-original shape of thin filaments is restored
Energy for Contraction
3 major activities requiring energy (ATP)
1) Move cross bridges
2) Break cross bridge attachments from thin
filaments
3) Return calcium to sarcoplasmic reticulum
-ATP is made during cellular respiration in
mitochondria
-sugar molecules are broken down, then
converted to ATP (usable energy source) +
stored
-once muscle contraction begins, stored ATP is
used up in seconds; rate of use exceeds rate of
production  other energy sources must be
available
-creatine phosphate – high energy molecule
that can be stored longer than ATP, more
abundant; energy released from it regenerates
ATP
-ATP + creatine phosphate = 15 seconds of
muscle contraction
-glycogen – storage form of glucose; when
broken down, it produces enough ATP to
sustain contraction for several minutes
-after this, fat molecules are used
Oxygen Debt
-oxygen is required for muscle contraction b/c
it is used in cellular respiration to make ATP
-during strenuous exercise, the production of
ATP reaches maximum rate, however, after
several minutes, respiratory and cardiovascular
systems cannot bring in enough oxygen to meet
demands = oxygen levels are depleted (called
oxygen debt)
-causes buildup of lactic acid (by-product of
cellular respiration) in muscle fibers = soreness
-muscle fatigue – inability of a muscle to
contract normally; caused by changes in the
muscle fiber that occur b/c of lactic acid
accumulation (decreases pH)
-cramp – muscle contracts spasmodically w/o
relaxing; caused by a lack of ATP to return
calcium ions to sarcoplasmic reticulum;
prevents muscle relaxation
Smooth and Cardiac Muscle
-structure of smooth and cardiac muscle cells
and internal arrangement of proteins makes
them contract differently than skeletal muscles
*Cardiac Muscle
-single nucleus
-rectangular shape
-branches connect adjacent cells
-intercalated discs – thickenings of cell
membranes that connect cells and help
conduct impulses between cardiac cells,
allowing cells to function as a unit
-thin and thick filaments arranged into
sarcomeres
-striations
-contraction is not as forceful (intermediate) as
skeletal muscle, but lasts longer
-does not develop oxygen debt + does not
fatigue
-contraction is autorhythmic (does not require
a stimulus to begin contracting)
*Smooth Muscle
-single nucleus
-small, spindle-shaped cells
-no striations
-no troponin, fewer actin fibers in thin
filaments
-no T-tubules or sarcoplasmic reticula
-slowest and weakest contraction
-does not develop oxygen debt
-usually require an external stimulus to
contract
-greatest ability to remain contracted
*Skeletal Muscle
-multinucleated
-long, cylindrical shape
-striations
-sarcomeres
-fastest, strongest contraction
-shortest duration of contraction
Muscular Responses
*All-or-None Response- a muscle fiber either
contracts or it doesn’t; it does not partially
contract
-threshold stimulus – weakest stimulus that can
initiate a contraction
-subthreshold stimulus – any stimulus too weak
to cause a contraction
How do we adjust the strength of a handshake?
-A motor unit stimulates an average of 150
muscle fibers that contract simultaneously
-However, individual motor units in an entire
muscle have different thresholds
-When only motor units w/ low thresholds are
stimulated, the muscle does not contract w/ as
much force as it would if motor units w/ higher
thresholds are stimulated
*recruitment – adding of motor units as
stimulus strength increases
Types of Muscle Contraction
-frequency of stimuli received by the muscle
varies; a change in stimulus frequency has an
immediate effect on the nature of muscle
contraction producing different types
*Twitch – rapid response to a single stimulus
that is slightly over threshold
-1/10 second
-myogram – a recording of muscle contraction
taken by a myograph
-latent period – delay of contraction after
stimulus is applied (time it takes for calcium
ions to be released, myosin to be activated, +
cross bridges to attach; flat on recording
-period of contraction – the muscle is pulling at
attachments, shortening; upward slope on
recording
-period of relaxation – muscle returns to
original shape; downward slope on recording
*Treppe
-muscle is allowed to rest in between
contractions (stimuli) increasing slightly in
strength
-enables muscles to warm up prior to full
contraction
*Wave Summation
-muscle is not allowed to relax completely in
between contractions (stimuli); receives a 2nd
stimulus before the 1st contraction cycle is
complete; 2nd stimulus will be stronger than 1st
*Tetanus
-Incomplete tetanus – wave summation
reaches maximum value and contraction is
sustained w/ partial relaxation until stimuli
stop; occurs when stimuli arrive at muscle
between 20-30 per second
-Complete tetanus – contraction sustained w/o
any relaxation between stimuli; 35-50 stimuli
per second
-muscle contraction by tetanus provides usual
means of body movement
*Isometric and Isotonic
-tension – force exerted by a muscle
contraction, requires use of energy
-isotonic contraction – provides movement as
the muscle pulls an attached structure, usually
a bone, toward a more stationary structure
-isometric contraction – produces muscle
tension, but no body movement results
(pushing against a wall)
Production of Movement
-each movement is determined by many
factors; how muscle forms its attachments,
structure of joint, interactions of nearby
muscles
*Origin + Insertion
-most muscles extend from one bone to
another, crossing the joint in between
-origin – point of attachment of a muscle to the
more stationary bone
-insertion – point of attachment of a muscle to
the more movable bone
*Group Actions - the coordinated response of a
group of muscles to bring about a body
movement
-prime mover - causes desired action
-antagonist – must relax during desired action
-synergist – steady the movement
-fixator – stabilize the origin of the prime mover