MUSCLE II
Skeletal muscle
Muscle force, muscle work, muscle fatigue
Smooth muscle
Heart muscle
Skeletal muscle – types of contraction
Isometric contraction
• Muscle contracts against a force transducer
• Muscle does not shorten during contraction
Isotonic contraction
• Muscle shortens against a fixed load
• Muscle shortens, tension remains constant
– Myographic curve on kymograph
Types of myographic curves - tetanus
Muscle contraction – twitch
•Summation
•Superposition
Tetanus
•Smooth - multiple summation
•Undulating – multiple superposition
Relation of muscle length to force of contraction
Resulting force of contraction is a sum of:
• Active tension during contraction
• Passive tension (resting tension) (sarcolemma, vessels, nerves)
Relation of velocity of contraction to load
maximal – when muscle contracts against no load
zero - when muscle contracts against maximal load
Muscle force
• Muscle force depends on the number of motor
unit
• Motor unit – all muscle fibers innervated by a
single motoneuron (from 2-3 up to several
hundred muscle fibers)
• The total strength of the muscles in the human
body is 250,000 N (Newtons).
– 1 Newton (N) = 1 kg.m/s2
• Muscle force is influenced
– genetically
– hormonally – testosterone, anabolics
Muscle hypertrophy
Enlargement of a total mass of muscle
• Increased number of myofibrils
• Increased amount of mitochondrial enzymes - up
to 120%
• Increased amount of ATP and kreatinphosphate up to 160 - 180%
• Increased amount of glycogen - up to 150%
• Increased amount fat deposits - up to 175-200%
Types of skeletal muscle fibers
Every muscle is composed
of a mixture of fast and
slow muscle fibers
• Fast fibers – white
Longer fibers for great strength of contraction
Extensive sarcoplasmic reticulum for rapid release of Ca2+
Large amounts of glycolytic enzymes for rapid release of
energy
Less excessive blood supply (oxidative metabolism is of
secondary importance)
Fewer mitochondria
Adapted for rapid, powerful contraction (jumping, sprints)
Types of skeletal muscle fibers
Every muscle is composed
of a mixture of fast and
slow muscle fibers
• Slow fibers - red
Smaller fibers
Innervated by smaller nerve fibers
More capillaries – to supply extra amount of oxygenu
Increased numbers of mitochondrias (high level of oxidative
metabolism)
Large amount of myoglobin (Fe containing protein, similar to
hemoglobin - red color)
Adapted for prolonged, continuous muscle activity (antigravity
muscle, long-distance races)
Muscle work
• Muscle work = the effect of the muscle force on a
certain path (A = F . s) (J)
• Dynamic (during isotonic contraction, movement)
More extensive blood supply
Intensive circulation during relaxation
• Static (changes of muscle tension without the
shortening of the muscle)
Lesser blood supply
Limited circulation - accumulation of metabolites –
development of muscle fatigue
Muscle fatigue
• Acute (recovery - within 24 hours) and chronic (may
be followed by a complete exhaustion)
• Decrease force of muscle contraction
• Fatigue is localized in the neuromuscular junction
– Accumulation of extracellular K+ may lead to a disturbance
in depolarization, reduction of the amplitude of the action
potential and conduction velocity
• Muscle fatigue is increasing parallel to decreasing
amounts of muscle glycogen
• Accumulation of lactate – lower pH, increase of K+,
stimulation of the free nervous endings – pain,
edemas
• Contracture – long-lasting muscle contraction –
without action potentials, exhaustion of ATP
Orbeli phenomenon
• Effect of epinephrine – a transitory increase of
muscle force contraction
• appearance of the fatigue is postponed by the
sympathetic stimulation
• Fatigue appears before the energetic reserves of
the organism are exhausted
• Fatigue is a physiological protective phenomenon
preventing the damage to the organism
Sources of energy for muscle contraction
• ATP – maintains contraction for 1 to 2 seconds
• phosphocreatine – 5 times as great as ATP, sufficient for 78 s contraction
• Glycogen
– Enzymatic breakdown of the glycogen to pyruvate and
lactate liberates energy that is used to convert ADP to
ATP, glycolysis can sustain contraction for about 1 min
• Twofold importance of glycolysis
– Reactions occurs in the absence of oxygen (muscle
contraction can be sustained for a short time when
oxygen is not available)
– The rate of formation of ATP is 2.5 times as rapid as
ATP formation with oxygene
– Accumulation of many end-products
• Oxidative metabolism – the final source of energy
– 95% of all energy used by the muscle
Function of ATP
ATP is necessary for
• Muscle contraction – detachment of the head of
myosin from the actin
• Function of Na+/K+ pump
• Function of Ca++ pump
Physiological depletion of sources of ATP
(reversible) – contracture
Irreversible loss of all ATP – rigor mortis
– Lack of energy for the separation of cross-bridges
– Rigor is faster after muscle fatigue and exhaustion
– Muscles remain in rigor until muscle proteins are
destroyed by autolysis (15-25 hours)
Smooth muscle - structure
Contains actin and myosin,
it does not contain troponin
Dense bodies – analog of
Z-lines – attachment of
actin filaments
Actin – long filaments
15 times as many actin as
myosin
•Contraction of smooth muscle is about 30 times slower than
contraction of skeletal muscle
•Great ability to shorten with full force of contraction
•Some contractile units of smooth muscle have optimal
overlapping of their actin and myosin filaments at one length
of the muscle and others at other length
Types of smooth muscles
• Single-unit (visceral)
– Hundreds to millions of muscle fibers contract together
as a single unit – syncithial smooth muscle
– The cell membranes are joined by gap junction –
through which ions can flow freely from one cell to the
next cell
– Visceral organs of GIT, gut, bile ducts, ureters, uterus,
vessels
• Multiunite
– Is composed of discrete smooth muscle fibers
– Each fiber operates independently of others
– Is innervated by a single nerve ending
– The ciliary muscle of the eye (parasympathetic control)
– The piloerector muscles (sympathetic control)
Contraction of smooth muscle
• Initiating event in smooth muscle contraction is an
increase in intracelullar Ca2+ ions cause by:
–
–
–
–
Nerve stimulation
Stretch of the fiber
Hormonal stimulation
Changes in the chemical environment of the fiber
• Strength of contraction depends on extracellular
Ca2+
• Removal of Ca2+ ions is achieved by calcium pump,
calcium pump is much slower in comparison with a
pump of skeletal muscle – longer contraction
Mechanism of contraction
• Beginning of contraction
4 Ca2+ bind with regulatory protein calmodulin
Complex Ca-calmodulin activates enzyme miosin
kinase (a phosphorylating enzyme)
Light chain of of each myosin head (regulatory
chain) become phosphorylated, the head has the
capability of binding with the actin filaments
• Cessation of contraction:
When the concentration of Ca2+ falls bellow a critical
level, all processes automatically reverse except
for the fphosphorilation of myosin head
Enzyme myosin phosphatase splits the phosphate
from the regulatory light chain
Smooth muscle - contraction
Smooth muscle – membrane potential
Slow wave
•Resting potential –50
to –60 mV
•Spontaneous slow
wave (some smooth
muscle is selfexcitatory)
•Slow wave can initiate
action potentials (-35
mV)
•The more AP, the
stronger contraction
Smooth muscle has more voltage-gated calcium channels
and very few voltage-gated sodium channels than skeletal m.
Importance of Ca2+ ions in generating smooth muscle action
potential – phase plateau of AP, contraction
Contraction without action potentials
• In multiunite smooth muscle, Ca2+ ions can
flow into the cell through the ligand-gated
Ca2+ channel
– ligand – acetylcholine, norepinephrine
• Action potentials most often do not develop
• Membrane potential do not reach a critical
level for generating action potential because
the Na+ pump pumps sodium ions out of the
cell
Regulation of smooth muscle
Smooth muscle are regulated by autonomic nerves
Nerve fibers do not make direct contact with smooth muscle
fibers – they formed so-called diffuse junction
Terminal axons have multiple varicosities, containing vesicules
In the multiunite type of smooth cells, the contact junctions are
similar to the end plate of skeletal muscle
Heart muscle - structure
•The cardiac muscle is striated
as skeletal muscle
•Has typical myofibrils that
contain actin and myosin
filaments
•Heart muscle is a syncitium -gap junctions connection
•Intercalated discs –
membrane with low resistance
•AP travel from one cardiac
muscle cell to to another
Heart muscle - action potential
•Resting potential –80 to –95 mV
•Amplitude 105 mV
•Phase of plateau
•Duration 200 – 300 ms
Regulation of cardiac muscle
Sympathetic nervous
system
Parasympathetic
nervous system
• Norepinephrine binds to
adrenergic receptors
and activates Ca2+
channel (Ca2+ flows into
the cell – increases
positivity inside)
• Increases frequency
• Increases strength of
contraction
• Increases conductance
• Increases excitability
• Acetylcholine binds to
muscarine cholinergic
receptors and activates
K+ channel (K+ flow from
the cell – increases
negativity inside)
• Hyperpolarization makes
excitable tissue much
less excitable
• Slowing transmission
• Decrease of frequency
Sarcomere
Nucleus
Sarkopl. reticulum
T-tubules
Skeletal m.
yes
a lot of
Smooth muscle
no
one
well developed moderately
yes
Content of actomyosin higher
Ratio A:M
2:1
Actin filaments
short
Plasticity
low
Conductance
faster
Velocity of contraction rapid
Resting potential
-80 to –90
mV
no (caveoli)
lower
15:1
long
high (10x)
slower
slower
-50 to –60 mV
unstable
Skeletal m.
ExcitationCa- TN-C
contraction coupling TN-I
Cessation of
Ca decrease
contraction
Energy from ATP
Autonom. contraction
Control
Fatigue
Smooth muscle
Ca-calmodulin
myozin kinase
Ca decrease
myozin
spontaneously
phosphatase
high
low
no
yes pacemaker
motoneurons autonomic NS
humoral
mechanical
yes
practically not