INTRODUCTION TO MUSCLE
Movement is a fundamental characteristic of
all living things
 Cells capable of shortening and converting the
chemical energy of ATP into mechanical energy
 Types of muscle

 skeletal,

cardiac and smooth
Physiology of skeletal muscle
 basis
of warm-up, strength, endurance and fatigue
11-1
THE FUNCTIONS OF MUSCLES
Movement of body parts and organ contents
 Maintain posture and prevent movement
 Communication - speech, expression and
writing
 Control of openings and passageways
 Heat production

11-2
CHARACTERISTICS OF MUSCLE

Responsiveness (excitability)
 to
chemical signals, stretch and electrical changes
across the plasma membrane

Conductivity
 local
electrical change triggers a wave of excitation
that travels along the muscle fiber
Contractility -- shortens when stimulated
 Extensibility -- capable of being stretched
 Elasticity -- returns to its original resting length
after being stretched

11-3
SKELETAL MUSCLE


Each muscle is composed of muscle tissue, blood
vessels, nerve fibers, and connective tissue
The three connective tissue sheaths are:
 Endomysium – fine sheath of connective
Epimysium
tissue composed of reticular fibers
surrounding each muscle fiber


Deep fascia
Endomysium surrounds muscle fiber
Perimysium – fibrous connective tissue that
surrounds groups of muscle fibers called
fascicles


Tendon
Perimysium surrounds fascicles of muscle fibers
Epimysium – an overcoat of dense regular
connective tissue that surrounds the entire
muscle

Epimysium surrounds entire muscle
Perimysium
Endomysium
11-4
MUSCLE ATTACHMENTS




Direct (fleshy) attachment to bone
 epimysium is continuous with periosteum
 intercostal muscles
Indirect attachment to bone
 epimysium continues as tendon or aponeurosis
that merges into periosteum as perforating fibers
 biceps brachii or abdominal muscle
Attachment to dermis
Stress will tear the tendon before pulling the tendon
loose from either muscle or bone
11-5
SKELETAL MUSCLE
Voluntary striated muscle with multiple nuclei
 Muscle fibers (myofibers) as long as 30 cm
 Exhibits alternating light and dark transverse
bands or striations

 reflects
overlapping arrangement of
internal contractile proteins

Under conscious control (voluntary)
11-6
MUSCLE FIBERS


Multiple flattened nuclei inside cell membrane
 fusion of multiple myoblasts during development
 unfused satellite cells nearby can multiply to produce a small number of
new myofibers
Sarcolemma has tunnel-like infoldings or transverse (T) tubules that
penetrate the cell
 carry electric current to cell interior form triad with sacoplasmic reticulum
(SR)
11-7
MUSCLE FIBERS 2


Sarcoplasm is filled with
 myofibrils (bundles of myofilaments)
 glycogen for stored energy and myoglobin for binding oxygen
Sarcoplasmic reticulum = smooth ER
 network around each myofibril
 dilated end-sacs (terminal cisternea) store calcium
 triad = T tubule and 2 terminal cisternae
11-8
THICK FILAMENTS

Made of 200 to 500 myosin molecules
2

entwined polypeptides (golf clubs)
Arranged in a bundle with heads directed
outward in a spiral array around the bundled
tails
 central
area is a bare zone with no heads
11-9
THIN FILAMENTS

Two intertwined strands fibrous (F) actin
 globular

Groove holds tropomyosin molecules
 each

(G) actin with an active site
blocking 6 or 7 active sites of G actins
One small, calcium-binding troponin molecule on each
tropomyosin molecule
11-10
ELASTIC FILAMENTS



Springy proteins called titin
Anchor each thick filament to Z disc
Prevents overstretching of sarcomere
11-11
STRIATIONS = ORGANIZATION OF FILAMENTS


Dark A bands (regions) alternating with lighter I bands (regions)
A band is thick filament region


lighter, central H band area contains no thin filaments
I band is thin filament region


bisected by Z disc protein called connectin, anchoring elastic and thin
filaments
from one Z disc (Z line) to the next is a sarcomere
11-12
STRIATIONS AND SARCOMERES
11-13
OVERLAP OF THICK AND THIN FILAMENTS
11-14
CONTRACTILE AND REGULATORY PROTEINS


Myosin and actin are contractile proteins
Tropomyosin and troponin = regulatory proteins



switch that starts and stops shortening of muscle cell
contraction activated by release of calcium into sarcoplasm and its
binding to troponin,
troponin moves tropomyosin off the actin active sites
11-15
RELAXED AND CONTRACTED SARCOMERES

Muscle cells shorten because
their individual sarcomeres
shorten
 pulling
Z discs closer
together
 pulls on sarcolemma


Notice neither thick nor thin
filaments change length
during shortening
Their overlap changes as
sarcomeres shorten
Animation
11-16
NEUROMUSCULAR JUNCTIONS (SYNAPSE)



Functional connection between
nerve fiber and muscle cell
Neurotransmitter
(acetylcholine/ACh) released from
nerve fiber stimulates muscle cell
Components of synapse (NMJ)
 synaptic knob is swollen end of
nerve fiber (contains ACh)
 junctional folds region of
sarcolemma with ACh receptors
 synaptic cleft = tiny gap
between nerve and muscle cells
 basal lamina = thin layer of
collagen and glycoprotein over
all of muscle fiber
11-17
ELECTRICALLY EXCITABLE CELLS

Plasma membrane is polarized or charged
 resting
membrane potential due to Na+ outside of
cell and K+ and other anions inside of cell
 difference in charge across the membrane = resting
membrane potential (-90 mV)

Stimulation opens ion gates in membrane
 ion
gates open (Na+ rushes into cell and K+ rushes
out of cell)
 quick
up-and-down voltage shift = action potential
 spreads
over cell surface (both in muscle and nerve
cells)
11-18
MUSCLE CONTRACTION AND RELAXATION

Four actions involved in this process
 excitation
= nerve action potentials lead to action
potentials in muscle fiber
 excitation-contraction coupling = action potentials
on the sarcolemma activate myofilaments
 contraction = shortening of muscle fiber
 relaxation = return to resting length

Images will be used to demonstrate the steps
of each of these actions
11-19
EXCITATION (STEPS 1 AND 2)

Nerve signal opens voltage-gated calcium channels. Calcium
stimulates exocytosis of synaptic vesicles containing ACh = ACh
release into synaptic cleft.
11-20
EXCITATION (STEPS 3 AND 4)
Binding of ACh to receptor proteins opens Na+ and K+
channels resulting in jump in RMP from -90mV to +75mV
forming an end-plate potential (EPP).
11-21
EXCITATION (STEP 5)
Voltage change in end-plate region (EPP) opens nearby
voltage-gated channels producing an action potential
11-22
EXCITATION-CONTRACTION COUPLING
(STEPS 6 AND 7)
Action potential spreading over sarcolemma enters T
tubules -- voltage-gated channels open in T tubules causing
calcium gates to open in SR
11-23
EXCITATION-CONTRACTION COUPLING
(STEPS 8 AND 9)


Calcium released by SR binds to troponin
Troponin-tropomyosin complex changes shape and
exposes active sites on actin
11-24
CONTRACTION (STEPS 10 AND 11)


Myosin ATPase in myosin head hydrolyzes an ATP molecule,
activating the head and “cocking” it in an extended position
It binds to actin active site forming a cross-bridge
11-25
CONTRACTION (STEPS 12 AND 13)


Power stroke = myosin head
releases ADP and phosphate as it
flexes pulling the thin filament past
the thick
With the binding of more ATP, the
myosin head extends to attach to a
new active site
 half of the heads are bound to a
thin filament at one time
preventing slippage
 thin and thick filaments do not
become shorter, just slide past
each other (sliding filament
theory)
11-26
RELAXATION (STEPS 14 AND 15)
Nerve stimulation ceases and acetylcholinesterase removes
ACh from receptors. Stimulation of the muscle cell ceases.
11-27
RELAXATION (STEP 16)


Active transport needed to
pump calcium back into SR
to bind to calsequestrin
ATP is needed for muscle
relaxation as well as muscle
contraction
11-28
RELAXATION (STEPS 17 AND 18)

Loss of calcium from
sarcoplasm moves troponintropomyosin complex over
active sites


stops the production or
maintenance of tension
Muscle fiber returns to its
resting length due to recoil of
series-elastic components
and contraction of
antagonistic muscles
11-29
RIGOR MORTIS





Stiffening of the body beginning 3 to 4 hours after death
Deteriorating sarcoplasmic reticulum releases calcium
Calcium activates myosin-actin cross-bridging and muscle
contracts, but can not relax.
Muscle relaxation requires ATP and ATP production is no
longer produced after death
Fibers remain contracted until myofilaments decay
11-30
NEUROMUSCULAR TOXINS

Pesticides (cholinesterase inhibitors)
 bind
to acetylcholinesterase and prevent it from
degrading ACh
 spastic paralysis and possible suffocation

Flaccid paralysis (limp muscles) due to curare
that competes with ACh
 respiratory
arrest
11-31
NERVE-MUSCLE RELATIONSHIPS
Skeletal muscle must be stimulated by a
nerve or it will not contract
 Axons of somatic motor neurons = somatic
motor fibers

 terminal

branches supply one muscle fiber
Each motor neuron and all the muscle fibers
it innervates = motor unit
11-32
MOTOR UNITS

A motor neuron and the muscle fibers it
innervates




Fine control



dispersed throughout the muscle
when contract together causes weak contraction over
wide area
provides ability to sustain long-term contraction as
motor units take turns resting (postural control)
small motor units contain as few as
20 muscle fibers per nerve fiber
eye muscles
Strength control

gastrocnemius muscle has 1000
fibers per nerve fiber
11-33
LENGTH-TENSION RELATIONSHIP




Amount of tension generated depends on
length of muscle before it was stimulated
 length-tension relationship
Overly contracted (weak contraction results)
 thick filaments too close to Z discs and
can’t slide
Too stretched (weak contraction results)
 little overlap of thin and thick does not
allow for very many cross bridges too
form
Optimum resting length produces greatest
force when muscle contracts
 central nervous system maintains
optimal length producing muscle tone
or partial contraction
11-34
MUSCLE TWITCH IN FROG

Threshold = voltage producing an action potential


a single brief stimulus at that voltage produces a quick cycle of contraction
and relaxation called a twitch (lasting less than 1/10 second)
A single twitch contraction is not strong enough to do
any useful work
11-35
MUSCLE TWITCH IN FROG 2

Phases of a twitch contraction
 latent period (2 msec delay)
only internal tension is generated
 no visible contraction occurs – elastic components are being stretched


contraction phase


external tension develops as muscle shortens
relaxation phase

loss of tension and return to resting length as calcium returns to SR
11-36
CONTRACTION STRENGTH OF TWITCHES



Threshold stimuli produces twitches
Twitches unchanged despite increased voltage
“Muscle fiber obeys an all-or-none law” contracting to
its maximum or not at all
 not a true statement since twitches vary in strength


depending upon, Ca2+ concentration, previous stretch of the
muscle, temperature, pH and hydration
Closer stimuli produce stronger twitches
11-37
RECRUITMENT AND STIMULUS INTENSITY


Stimulating the whole nerve with higher and higher voltage
produces stronger contractions
More motor units are being recruited


called multiple motor unit summation
lift a glass of milk versus a whole gallon of milk
11-38
TWITCH AND TREPPE CONTRACTIONS

Muscle stimulation at variable frequencies

low frequency (up to 10 stimuli/sec)


each stimulus produces an identical twitch response
moderate frequency (between 10-20 stimuli/sec)

each twitch has time to recover but develops more tension than
the one before (treppe phenomenon)


calcium was not completely put back into SR
heat of tissue increases myosin ATPase efficiency
11-39
INCOMPLETE AND COMPLETE TETANUS

Higher frequency stimulation (20-40 stimuli/second) generates
gradually more strength of contraction

each stimuli arrives before last one recovers



temporal summation or wave summation
incomplete tetanus = sustained fluttering contractions
Maximum frequency stimulation (40-50 stimuli/second)



muscle has no time to relax at all
twitches fuse into smooth, prolonged contraction called complete tetanus
rarely occurs in the body
11-40
ISOMETRIC AND ISOTONIC CONTRACTIONS


Isometric muscle contraction
 develops tension without changing length
 important in postural muscle function and
antagonistic muscle joint stabilization
Isotonic muscle contraction
 tension while shortening = concentric
 tension while lengthening = eccentric
11-41
ATP SOURCES


All muscle contraction depends on ATP
Pathways of ATP synthesis
 anaerobic fermentation (ATP production limited)

without oxygen, produces toxic lactic acid
 aerobic respiration (more ATP produced)
 requires continuous oxygen supply, produces H2O and CO2
11-42
IMMEDIATE ENERGY NEEDS

Short, intense exercise (100 m dash)


Phosphagen system



oxygen need is supplied by myoglobin
myokinase transfers Pi groups
from one ADP to another forming ATP
creatine kinase transfers Pi groups from
creatine phosphate to make ATP
Result is power enough for 1 minute
brisk walk or 6 seconds of sprinting
11-43
SHORT-TERM ENERGY NEEDS

Glycogen-lactic acid system takes over

produces ATP for 30-40 seconds of maximum activity
 playing

basketball or running around baseball diamonds
muscles obtain glucose from blood and stored glycogen
11-44
LONG-TERM ENERGY NEEDS



Aerobic respiration needed for prolonged exercise
 Produces 36 ATPs/glucose molecule
After 40 seconds of exercise, respiratory and
cardiovascular systems must deliver enough oxygen
for aerobic respiration
 oxygen consumption rate increases for first 3-4
minutes and then levels off to a steady state
Limits are set by depletion of glycogen and blood
glucose, loss of fluid and electrolytes
11-45
FATIGUE

Progressive weakness from use
ATP synthesis declines as glycogen is consumed
 sodium-potassium pumps fail to maintain membrane
potential and excitability
 lactic acid inhibits enzyme function
 accumulation of extracellular K+ hyperpolarizes the cell
 motor nerve fibers use up their acetylcholine

11-46
ENDURANCE

Ability to maintain high-intensity exercise for >5
minutes
 determined
by maximum oxygen uptake
 VO2
max is proportional to body size, peaks at age 20, is
larger in trained athlete and males
 nutrient
availability
 carbohydrate
loading used by some athletes
packs glycogen into muscle cells
 adds water at same time (2.7 g water with each gram/glycogen)


side effects include “heaviness” feeling
11-47
OXYGEN DEBT


Heavy breathing after strenuous exercise
 known as excess postexercise oxygen consumption
(EPOC)
 typically about 11 liters extra is consumed
Purposes for extra oxygen
 replace oxygen reserves (myoglobin, blood hemoglobin,
in air in the lungs and dissolved in plasma)
 replenishing the phosphagen system
 reconverting lactic acid to glucose in kidneys and liver
 serving the elevated metabolic rate that occurs as long
as the body temperature remains elevated by exercise
11-48
SLOW- AND FAST-TWITCH FIBERS

Slow oxidative, slow-twitch fibers
 more
mitochondria, myoglobin and capillaries
 adapted for aerobic respiration and resistant to fatigue
 soleus and postural muscles of the back
(100msec/twitch)
11-49
SLOW AND FAST-TWITCH FIBERS

Fast glycolytic, fast-twitch fibers
 rich
in enzymes for phosphagen and glycogen-lactic
acid systems
 sarcoplasmic reticulum releases calcium quickly so
contractions are quicker (7.5 msec/twitch)
 extraocular eye muscles, gastrocnemius and biceps
brachii

Proportions genetically determined
11-50
MUSCULAR DYSTROPHY
Hereditary diseases - skeletal muscles degenerate
and are replaced with adipose
 Disease of males

appears as child begins to walk
 rarely live past 20 years of age


Dystrophin links actin filaments to cell membrane


leads to torn cell membranes and necrosis
Fascioscapulohumeral MD -- facial and shoulder
muscle only
11-51
MYASTHENIA GRAVIS

Autoimmune disease - antibodies attack NMJ
and bind ACh receptors in clusters
 receptors
removed
 less and less sensitive to ACh
 drooping
eyelids and double vision, difficulty swallowing,
weakness of the limbs, respiratory failure
Disease of women between 20 and 40
 Treated with cholinesterase inhibitors, thymus
removal or immunosuppressive agents

11-52
MYASTHENIA GRAVIS
Drooping eyelids and weakness of muscles of eye movement
11-53