Download Myofilaments

Skeletal Muscle Tissue
Keri Muma
Bio 6
Functions of Skeletal Muscle

Movement – muscles attach directly or indirectly to
bone, pull on bone or tissue when they contract

Maintain posture / body position – muscles are
continuously contracting to make adjustments to
maintain posture

Stabilize joints – tendons crossing joints and
muscle tone

Thermogenesis - generate heat when contracting
Functional Properties of Muscle Tissue

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Excitable – respond to nerve stimulus
Contractible – shorten when stimulated
Extensible – can stretch beyond resting
length when relaxed
Elastic – can recoil/rebound to original resting
length after contraction or stretching
Gross Anatomy of Skeletal Muscle

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Endomysium – surrounds
individual muscle fibers
Perimysium – surrounds
bundles of muscle fibers
called fascicles
Epimysium – dense irregular
CT, surrounds the entire
muscle
Microscopic Anatomy of Skeletal Muscle

Sarcolemma – plasma membrane surrounding
muscle fiber

Sarcoplasm

Contains a lot of mitochondria, glycogen, myoglobin, and
contractile organelles called myofibrils
Microscopic Anatomy of Skeletal Muscle

Myofibril
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Long fiber-like organelle that fills the sarcoplasm of
the cell
Runs parallel to muscle fiber
Myofibrils

Sarcomere
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Aligned end to end along the length of the myofibril
Z-line – boundary at the end of each sarcomere
Microscopic Anatomy of Skeletal Muscle

Myofilaments
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Contractile proteins in the sarcomere
Arrangement gives muscle its striations
Summary:
Myofilaments
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Myosin (thick) filaments
Tail with split head
 Arranged in bundles
 Heads contain:
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ATP binding site
ATPase enzyme which
splits ATP to provide energy
during contraction
Actin binding site
Myofilaments

Actin (thin) filament
Attached to Z – line and
extends towards the
center of the sarcomere
 Contains active binding
site for myosin heads

Arrangement of Actin and Myosin
Myofilaments

Proteins that regulate the binding of myosin
 Tropomyosin – spirals around actin, blocks active
site when muscle is relaxed
 Troponin – contains three binding sites
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Binds to actin
Binds to tropomyosin and controls its position on active
binding site
Contains calcium binding site
Myofilaments
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Titin (elastic) filaments
Large protein attached to z-line and runs through the
center of thick filaments
 Gives muscle elastic property; uncoils when muscle
stretches yet stiffens to avoid over extension, and
recoil when muscle relaxes

Titin
Microscopic Anatomy of Skeletal Muscle

Sarcoplasmic reticulum (SR) – specialized smooth
endoplasmic reticulum
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Surrounds each myofibril
Terminal cisternae – expanded ends of SR
Stores and releases calcium when muscle fiber is
stimulated to contract
Microscopic Anatomy of Skeletal Muscle

Transverse Tubules (T-tubule) – deep
indentations of the sarcolemma into the muscle fiber
Lies between two terminal cisternae
 Conducts electrical impulse from the sarcolemma into the
muscle fiber
 Coordinates muscle contraction by triggering calcium
release from the terminal cisternae

Skeletal Muscle Contraction

In order to contract, a skeletal muscle must:
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Be stimulated by a somatic motor neuron
Propagate an electrical current, or action
potential, along its sarcolemma
Have a rise in intracellular Ca2+ levels, the final
trigger for contraction
Linking the electrical signal to the contraction
is excitation-contraction coupling
Neuromuscular Junction

The neuromuscular junction is formed from:
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Axon terminals, which have synaptic vesicles
that contain acetylcholine (ACh)
Synaptic cleft
The motor end plate, which is a specific part of
the sarcolemma that contains ACh receptors
Neuromuscular Junction

When a nerve impulse reaches the end of an
axon at the neuromuscular junction:
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Voltage-regulated calcium channels open and allow
Ca2+ to enter the axon
Ca2+ inside the axon terminal causes vesicles to fuse
with the axon membrane and release ACh into the
synaptic cleft by exocytosis
Neuromuscular Junction

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ACh diffuses across the synaptic cleft to ACh
receptors on the motor end plate
Binding of ACh to its receptors initiates an action
potential in the muscle
ACh bound to ACh receptors is quickly destroyed by
acetylcholinesterase
Excitation-Contraction Coupling

Once generated, the
action potential:
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Is propagated along
the sarcolemma
Travels down the T
tubules
Triggers Ca2+ release
from terminal cisternae
Excitation-Contraction Coupling

Ca2+ binds to troponin and causes:
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A conformational change in troponin
Tropomyosin rolls off the actin active binding sites
allowing them to be exposed for myosin to attach
Sliding Filament Theory
Sliding Filament Theory
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Cross bridge formation –
myosin cross bridge attaches
to actin filament
Working (power) stroke –
myosin head pivots and pulls
actin filament toward M line
Sliding Filament Theory
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Cross bridge detachment –
ATP attaches to myosin head
and the cross bridge detaches
“Cocking” of the myosin head
– energy from hydrolysis of
ATP cocks the myosin head
into the high-energy state
Sliding Filament Theory
Muscle Relaxation

When the nerve stimulation ceases:
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Ca2+ is removed and actively transported back
into the SR (requires ATP)
Tropomyosin roles back over the binding sites,
and the muscle fiber relaxes
Muscle Contraction

Tension – force muscle exerts on an object
when contracted

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Load or resistance is the opposing force exerted
on the muscle
Muscle tension must over come the force of the
load in order to shorten
Phases of Muscle Contraction
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Latent period – first few
milliseconds, excitationcontraction coupling
Period of contraction –
cross bridge cycling,
tension increases
Period of relaxation –
calcium transported back
into SR, cross bridge
cycling ends, tension
decreases
Graded Muscle Response

Individual muscle fiber contraction is an “all or
none” response, but whole muscles can vary in
tension produced and length of contraction
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Graded muscle responses are:
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Variations in the degree of muscle contraction
Required for proper control of skeletal movement
Responses are graded by:
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Changing the frequency of stimulation
Changing the strength of the stimulus
Factors Effecting Tension
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Intensity of stimulus – number of motor units
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Motor unit – a single motor neuron and all the
muscle fibers it supplies
Recruitment – calling
additional motor units,
therefore stimulating more
fibers and increasing
muscle tension
Factors Effecting Tension
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Intensity of stimulus
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Threshold stimulus – minimal
stimulus needed to invoke
visible muscle contraction
Maximal stimulus – all motor
units are recruited, strongest
contraction produced
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Asychronous recruitment of motor
units – alternates motor units
Factors Effecting Tension
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Frequency of stimulation
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Twitch – single impulse, contraction followed by
relaxation
Wave summation – when impulses are delivered in
succession the second twitch will be stronger then the
first
Complete tetanus – rapid stimulation results in sustained
smooth contraction without periods of relaxation
Factors Effecting Tension
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Frequency of stimulation
Refractory Period in Skeletal Muscle
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Contractile response
lasts longer, far beyond
the refractory period of
the action potential
This is important in
skeletal muscle’s ability
to produce tetanus
Treppe: The Staircase Effect

Treppe – increased contraction in response
to multiple stimuli of the same strength

Different than summation because relaxation
occurs
Treppe: The Staircase Effect
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Contractions increase because:
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There is increasing availability of Ca2+ in the
sarcoplasm
Reduced slack of the elastic series component
Muscle enzyme systems become more efficient
because heat is increased as muscle contracts
Factors Effecting Tension
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Size of Muscle
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Number of muscle fibers per muscle
Size of individual muscle fibers – fibers produce
more myofilaments in response to demands
placed on them. Fibers hypertrophy.
Factors Effecting Tension
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Optimal operating length – the resting length in
which maximum contraction can be generated
(70 -130%)
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Occurs when the muscle is slightly stretched and filaments
barely overlap
Types of Contractions
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Isotonic – same tension
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Muscle tension remains constant during
contraction
Muscle length changes during contraction,
shortens or lengthens
Isotonic Contractions
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Concentric - muscle shortens and does work
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Examples: pick up pencil, kick soccer ball
Isotonic Contractions
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Eccentric – muscle contracts as it lengthens
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Helps counter act gravity or prevent joint injury
“muscle braking”
Example: squats – quadriceps stretch but are
contracted to counter act gravity and control
movement
Types of Contractions
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Isometric – same length
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Tension increases but muscle length remains the same
Muscle is unable to produce enough force to overcome the
load
Example: pushing against a stationary wall
Muscle Tone
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Constant low level of tension in relaxed
muscles
Maintained by spinal reflexes that activate
alternating motor units
Keeps muscles firm and ready to respond to
stimuli
Muscle Metabolism
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Role of ATP - muscles need a constant supply
of ATP to carry out contractions
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For cross bridge formation and power stroke
For disconnecting of cross bridges
For active transport of calcium back into the terminal
cisternae
Sodium-potassium pumps
Muscles only have enough ATP stored for 4-6
seconds worth of contraction
Therefore ATP must be constantly regenerated
Sources of ATP
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Three pathways that supply additional ATP:
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Creatine phosphate (direct phosphorylation)
Oxidative phosphorylation
Glycolysis
Sources of ATP
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Direct phosphorylation
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Creatine phosphate,
transfers energy and a
phosphate to ADP forming
ATP
Creatine phosphate + ADP
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Creatine + ATP
Creatine phosphate is
stored in the muscle fibers
Provides a rapid source of
energy for 10 – 15 seconds
of contraction
Glycolysis
Glycogen
Stores
Glucose from
blood
Glycolysis
2 ATP
Pyruvic Acid
O2
O2 - stored by myoglobin or
delivered by the blood
Lactic Acid
Aerobic
Respiration
Sources of ATP
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Oxidative Phosphorylation Aerobic Respiration
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Occurs in the mitochondria
Main source when O2 is
present
Fueled by glycogen stores and
glucose and fatty acids
delivered by the blood
Can provide hours of muscle
contraction for prolonged
moderate activity
Slower because it requires the
delivery of oxygen and glucose
Sources of ATP
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There are cardiovascular limits to the amount of
nutrients that can be delivered to muscle
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CV system cannot keep up with O2 demands
Blood vessels in the muscle are compressed during
maximal contraction
Oxidative phosporylation may not be able to produce
enough ATP quick enough to keep up with the demands
Sources of ATP
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Glycolysis - anaerobic
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Glucose is broken down to
pyruvic acid and produces
2ATP
In the absence of O2 pyruvic
acid is converted to lactic
acid
Produces minimal amounts of
ATP but occurs quickly
Provides 30-60 seconds of
high level activity
Muscle Fatigue
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Fatigue - decline in muscle tension as a result of
previous activity
Muscles are unable to contract despite being
stimulated
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Results from a deficit of ATP (not a total absence)
Anaerobic respiration becomes less efficient as lactic
acid accumulates and pH drops in the muscle fiber
Muscle fibers lose K+ as the Na-K pump is unable to
restore ion balance since it requires ATP
Neuromuscular fatigue – caused by a shortage of
neurotransmitters at the NMJ
Muscle
Fatigue
Oxygen Debt
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Oxygen debt - the amount of extra oxygen the
body must take in to restore muscle chemistry
back to resting state
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The liver converts lactic acid in the blood to pyruvic
acid which can be converted to glucose or enter
aerobic respiration now that O2 is available
Glycogen stores are replenished in muscles and liver
Creatine is re-phosphorylated into creatine phosphate
and stored in muscles
O2 rebinds to myoglobin
Summary of Muscle Metabolism
Muscle Fiber Types
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Muscle fibers differ in their methods of
metabolism based on:
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Pathways they use to produce ATP
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Duration of muscle contraction
How quickly their ATPases work
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Speed (velocity) of contraction
Muscle Fiber Types
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Slow – oxidative (red)
Slow to contraction but most resistant to
fatigue
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Good for endurance and continuous contraction
Better equipped for oxidative phosphorylation
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Numerous mitochondria and rich supply of capillaries
Small in diameter
High myoglobin content
Slow myosin ATPase activity
Muscle Fiber Types
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Fast – oxidative (pink)
Fast to contraction but resistant to fatigue
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Also equipped for oxidative phosphorylation
Fast myosin ATPase activity
Muscle Fiber Types
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Fast glycolytic fibers (white)
Fast to contract but fatigue quickly
Good for power and speed for short durations
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High glycogen reserves and relies mainly on
glycolysis
Fatigue quickly due to lactic acid build up
Large fibers generate more force but poor nutrient
diffusion
Light in color due to reduced myoglobin
Fewer capillaries and mitochondria
Effects of Exercise on Muscle Fibers
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Aerobic Exercise
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Results in more efficient muscle metabolism and
resistance to fatigue
Increases capillaries, mitochondria and myoglobin
Also increases efficiency of the heart, lungs, body
metabolism, and neuromuscular coordination
Effects of Exercise on Muscle
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Resistance – weight lifting and isometric
contractions
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Fibers produce more myofilaments and myofibrils
causing muscle fibers to hypertrophy
Increases glycogen stores
Results in increased muscle size and strength
Smooth Muscle Tissue
Chapter 12
Smooth Muscle
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Composed of spindle-shaped fibers
Organized into two layers (longitudinal and
circular)
Found in walls of hollow organs (except the
heart)
Innervation of Smooth Muscle
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Smooth muscle lacks neuromuscular junctions
Innervating nerves have bulbous swellings
called varicosities that release neurotransmitters
Some neurotransmitters are excitatory and some
are inhibitory, depending on the receptor
Smooth Muscle
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Structural Characteristics
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SR is less developed than in skeletal muscle and
lacks a specific pattern
T tubules are absent
Thin filaments only contain tropomyosin (NO
troponin)
Thick and thin filaments are arranged diagonally,
causing smooth muscle to contract in a corkscrew
manner
Contraction Mechanism
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Actin and myosin
interact according to
the sliding filament
mechanism
Calcium influx from
the extracellular space
triggers Ca2+ release
from the SR
The trigger for
contractions is a rise
in intracellular Ca2+
Role of Calcium Ion
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Ca2+ binds to calmodulin
Activated calmodulin activates the myosin
light chain kinase enzyme which transfers
phosphate from ATP to myosin cross bridges
Phosphorylated cross bridges interact with
actin to produce shortening
Smooth muscle relaxes when intracellular
Ca2+ levels drop
Smooth Muscle Contraction
Ca2+
Calmodulin
Ca2+ -calmodulin
Inactive myosin kinase
Active myosin kinase
Pi
Inactive myosin
Phosphorylated myosin
(can bind with actin)
Types of Smooth Muscle: Single Unit
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Visceral smooth muscle is autonomous

Smooth muscle pacemaker cells display rhythmic,
spontaneous variations in membrane potentials.
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Known as slow wave potentials
Self-excitable (myogenic) – can produce spontaneous
action potentials without external stimulation
Cells are electrically coupled to one another via gap
junctions and contract rhythmically as a unit
Smooth Muscle Activity

Pacemaker smooth muscle cells (Interstitial cells of
Cajal) membrane potential oscillates closer and
further away from threshold
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If threshold is reached a burst of action potentials is
triggered causing
rhythmic smooth
muscle contractions
Drive several digestive
processes
(e.g., peristalsis and
segmentation)
Gap Junctions
Pacemaker smooth muscle cell
Spontaneous action potential
induced by pacemaker potential
Action potential spread
to nonpacemaker cell
Gap junction
Nonpacemaker
smooth muscle cell
Smooth Muscle Activity

Hormones, paracrines, mechanical stress,
and nerve stimuli determines the starting
point of the slow wave potentials

Example:
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Food in the GI tract – closer to threshold
Empty GI tract – further away from threshold
Response to Stretch

Smooth muscle exhibits a phenomenon
called stress-relaxation response in which:
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Smooth muscle responds to stretch only briefly,
and then adapts to its new length
The new length, however, retains its ability to
contract
This enables organs such as the stomach and
bladder to temporarily store contents
Types of Smooth Muscle: Multiunit

Multiunit smooth muscles are found:
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In large airways to the lungs, large arteries,
arrector pili muscles attached to hair follicles, and
in the internal eye muscles
Their characteristics include:
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Rare gap junctions
Structurally independent muscle fibers
A rich nerve supply, forms motor units
Graded contractions in response to
neural stimuli