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MUSCULAR SYSTEM
FUNCTION OF MUSCLE tissue;
(through sustained contraction of
alternating contraction and
relaxation, muscle performs 5 key
functions)

1
...
g
...


Stabilize body positions (e
...

muscles for holding the axial
skeleton – back muscles)

2
...
g
...


Moving substances within the body
(propulsion through hollow organs)
(e
...
digestive tract)

5
...
g
...
Electrical excitability - able to respond to certain
stimuli (from inside and outside the body) by
producing electrical signals such as action potential
...
Contractility – ability to shorten and thicken
(contract) to generate force to do work
3
...

4
...
g
...
g
...
g
...
Connective tissue components
2
...
Function: provides pathway
for nerves, blood vessels, stores fat, insulates and
protects muscles from trauma
...
Holds
muscles with similar functions together
...
e
...

-All fibrous sheaths are continuous with one another and
contains blood vessels that supply muscle fibers
...
This inner fibrous sheath is called
perimysium
•ENDOMYSIUM – The perimysium‟s fibrous tissue continues
inside each fascile to wrap around each individual muscle
fiber
...

Just deep to the endomysium is the sarcolemma

STRUCTURE OF A MUSCLE ORGAN

Skeletal muscle – Other Connective
tissue components

• TENDON – is an
extension of dense
fibrous connective tissue
that is in the shape of a
strap or band and it is
attached to the
periosteum
...


Nerve and Blood supply
Nerves – (containing motor neurons) convey impulses for
muscular contraction
...

• A few myoblasts persist in mature skeletal muscle as
satellite cells
...

- When the muscle fiber is stimulated, an action potential
arises and reverses the membrane resting potential
...

-They quickly spread the muscle action potential to all parts
of the muscle fiber
SARCOPLASM – the muscle cell cytoplasm, contains large
amount of glycogen for energy production and myoglobin for
oxygen storage

SKELETAL MUSCLE CELL

SKELETAL MUSCLE CELL

Nucleus – a muscle fiber cell is multinucleated and are against
the inside of the sarcolemma
...

- The SR encircles each myofibril
- It lies within the sarcolemma covering
...

- Ca 2+ exits SR through Ca 2+ release channel
- The Ca 2+ released diffuses through the sarcoplasm
among parallel bundles of protein myofilaments
(Relaxing muscle fiber)
- Stores Ca 2+

Clinical Application
• Muscular atrophy - wasting away of muscles
• Muscular hypertrophy – an increase in the diameter
of muscle fibers

Filaments and sarcomere

• Myofibrils are composed of thick and thin filaments
arranged in units called sarcomeres
...

- Extends from z disc to z disc
- There are numerous sarcomeres in each myofibril
- Each sarcomeres shortens as the muscle fiber contracts
• Striations (I and A bands) occur due to size and
density differences between thick filaments and thin
filaments
• A band is the dark middle portion consisting primarily
of thick filaments with some thin filaments overlapping
the thick ones
...
Lie on either side of dark bands
...

• Made of thin transverse protein structure in the
center of the I band that serves as an attachment
site for thin filament ends
...

Relaxed muscle – H zone is lighter since its only
thick fibers and there is no overlapping thin
filaments
...


• M line – made of thin transverse protein meshwork
structure in the center of the H zone of a relaxed
fiber
...

• Clinical Application: Exercise can result in torn
sarcolemma, damaged myofibrils, and disrupted Z
discs

Muscle proteins
• Contractile proteins generate force during
contraction
Myosin - the main component of thick filaments
...
Motor proteins push
or pull their cargo to achieve movement by converting
energy from ATP into mechanical energy of motion or
force

• Actin – the main component of thin filaments
...

• Individual actin molecules (G (Globular) actin) join to
form an actin filament (F (filamentous) actin) that is
twisted into a helix
...


Regulatory proteins
• They assist in switching contractions on and off
• Tropomyosin and Troponin – are part of the thin
filament
Relaxed muscle – Tropomyosim, which is held in place by
troponin blocks the myosin-binding sites on actin,
preventing myosin from binding to actin
...

Function: When Ca 2+ ions
bind to one of its subunits,
troponin changes shape,
causing the tropomyosin to
move off the actin active
site, and thus allows myosin
to bind to actin
...

2) Attaches to tropomyosin
to hold it in place over the
surface of the actin
3) Functionally, troponin
provides a binding site for
calcium ions
...
Because it
can stretch and then spring back unharmed, it accounts for
much of the elasticity and extensibility of myofibrils
...

• Nebulin – wraps around entire length of each thin filament
...


Other structural proteins in skeletal
muscle fibers
• Dystrophin – links thin filaments to integral
membrane proteins in sarcolemma which are attached
to proteins in the connective tissue matrix that
surrounds muscle fibers
...
SKELETAL MUSCLE CELL
2
...
CARDIAC MUSCLE CELL

MUSCLE CELL TYPES
1
...
SMOOTH MUSCLE CELL
STRUCTURE – Spindle shaped, one centrally located nucleus,
not striated
NERVOUS SYSTEM CONTROL – involuntary („unconscious
control” and by autonomic nervous system)
LOCATION – found in walls of hollow viscera, airways, blood
vessels, e
...
c
...
CARDIAC MUSCLE CELL
STRUCTURE – Striated, branched with usually one centrally
located nuclei
- have intercalated discs that join neighbouring
cells
NERVOUS SYSTEM CONTROL – Involuntary (“unconscious” )
control by the autonomic nervous system)
LOCATION – Heart

Skeletal
muscle

Requirements for skeletal muscle
contraction
1
...

2
...

3
...


Contraction and relaxation of skeletal
muscle fibers – sliding filament mechanism
• During muscle contraction, myosin cross-bridges pull
on thin filaments, causing them to slide inward toward
the H zone
• The z discs come toward each other and the
sarcomeres shortens, but the thick and thin filaments
do not change in length
...
cross bridge formation: Activated myosin
head binds to actin forming a cross bridge;
Inorganic phosphate released; Bond between
myosin and actin becomes stronger

2
...
cross bridge detachment: Link between mysoin head
and actin weakens when another ATP attaches to myosin
head; Myosin head detaches

4
...

• When binding sites are free the contraction cycle
begins
...

It includes: i) ATP hydrolysis,
ii) attachment of myosin to actin to form
cross-bridges,
iii) the power stroke,
and iv) detachment of myosin from actin

• Sequence of events that converts action potentials in
a muscle fiber to a contraction;
• Action potential travels across entire sarcolemma;
• Occurs during hidden (latent) period, between AP
initiation and the beginning of mechanical activity
(contraction);
• Electrical signal does not act directly on
myofilaments (it causes a rise in intracellular Ca2+ ion
concentration that allows filaments to slide)

Excitation-contraction
coupling
• An increase in Ca2+ ion concentration in the cytosol
starts muscle contraction; a decrease stops it
...


• The use of Ca2+ ions to remove the contraction
inhibitor and the joining of actin and myosin
constitute the excitation-contraction coupling, the
steps that connect excitation (a muscle action
potential propagation through the T tubules) to
contraction of the muscle fiber
...

• Rigor mortis, a state of muscular rigidity following
death, results from the lack of ATP to split myosin –
actin cross-bridges
...

Action potential
travels across entire
sarcolemma and are
rapidly conducted to
interior of muscle
fibers by transverse
(T) tubules

Step 2
...


• Ca2+ binds to troponin and removes the blocking action
of tropomyosin; when Ca2+ binds, troponin changes
shape, exposing binding sites for myosin on the thin
filaments

Step 3
...
At the end of the action potential, Ca2+ is actively taken
up by Ca2+ active transport pumps in SR that use ATP to move Ca2+
constantly from cytosol into the SR
...
Ca2+
channels flow into the cytosol more rapidly than they are
transported back by the pumps
...
As the pump move
Ca2+ back into the SR, the concentration of Ca2+ in the
cytosol quickly decreases
...

- The levels of Ca2+ in the SR is 10,000 times higher in the SR
than the cytosol in a relaxed muscle fiber
...


Role of Calcium (Ca2+) in Contraction
• At low intracellular Ca2+ concentration:
Tropomyosin blocks the active sites on actin
...

• At higher intracellular Ca2+ concentrations:
Ca2+ binds to troponin
...
Events of the cross
bridge cycle occurs
...


NEUROMUSCULAR JUNCTION
DEFINITION –This is the junction between the terminal
of a motor neuron and a muscle fiber
...

- It is a kind of chemical synapse
...

- The ACh are released when nerve impulses (action
potentials) traveling down the motor neurons of the
sensory-somatic branch of the nervous system
...


The neuromuscular Junction
• Muscle action potentials arise at the neuromuscular
junction (NMJ), the synapse between a somatic motor
neuron and a skeletal muscle fiber
...

• A nerve action potential elicits a muscle action potential
through the release of acetylcholine, activation of ACh
receptors, production of a muscle action potential, and
termination of ACh activity

NEUROMUSCULAR JUNCTION

NEUROMUSCULAR JUNCTION

NEUROMUSCULAR JUNCTION

MYSTHENIA GRAVIS
DESCRIBE
- It is an autoimmune disorder, in which muscle weakness is
caused by circulating antibodies
...

- Myasthenia is treated medically with cholinesterase
inhibitors or immunosuppressant
...

- The symptoms usually starts in the face muscles and spread
to the other parts of the body as the disease progresses
...

- All of these symptoms are worst with repeated use and
improves with rest
...


MOTOR UNIT
DEFINITION – This is a motor neuron plus all skeletal
muscle fiber it stimulates
...


- We generally see 2 patterns;
1
...
MUSCLES CONTROLLING POWERFUL GROSS
MOVEMENTS

MOTOR UNIT

MOTOR UNIT
- 2 patterns of the numbers of motor units (and type
of motor units) in a skeletal muscle;

MUSCLE

PHYSIOLOGY

- RECRUITMENT OF MOTOR UNITS
...


Length - tension Relationship
• The forcefulness of muscle contraction depends on
the length of the sarcomeres within a muscle before
contraction begins
...
e
...

- GREATEST TENSION (= FORCE) is produced when there is
optimal overlap between thick and thin filaments
...
e
...

• Within limits, the more the elastic components are
stretched, the greater the passive tension
...


- FACTORS affecting force of contraction;
1
...
g
...
Recent contraction of muscle fibers

MUSCLE

PHYSIOLOGY

• CONTRACTIONS can be graded;
i
...
Contractions can be graded (varying sizes) to match
different tasks
...
g
...

• The grade of muscle contraction will be different
...

• Amount of tension that a skeletal muscle develops depends
on;
1
...
e
...
Length of muscle fibers just before they contract
3
...
e
...
Structural components of muscle itself
...

MYOGRAM
- i
...
is a graph record of a twitch muscle contraction
...

WHAT HAPPENS HERE?
- Ca 2+ released from SR
- Filaments start to exert force
- Shortening finally begins

ii) CONTRACTION PERIOD (10 – 100 msec)
- Is the upward segment of the tracing (shortening of
muscle fibre)

MUSCLE

PHYSIOLOGY

• MYOGRAM
iii)
RELAXATION PERIOD (10 – 100msec)
- Is shown by the downward segment of tracing
WHAT HAPPENS HERE?
- Active transport of Ca2+ back into SR
- Duration depends on type of muscle fiber;
a) FAST- TWITCH FIBERS
E
...
Fibers that move the eyeballs
Briefly contracts and relaxes ~ 10 msec
b) SLOW –TWITCH FIBERS
E
...
Fibers that moves the leg
Longer contracting and relaxation time ~ 10msec

MUSCLE

PHYSIOLOGY

• MYOGRAM
REFRACTORY PERIOD of a Twitch
- i
...
if 2 stimuli applied one immediately after the
other, the muscle fibers responds only to the 1rst
stimulus but not to the 2nd stimulus
...

- Varies with different muscle type
E
...
Skeletal muscle ~ 5 msec
Cardiac muscle ~ 300 msec

MUSCLE

PHYSIOLOGY

• Effect of frequency of stimulation
- TETANUS
- COMPLETE TETANUS
- STAIRCASE EFFECT

MUSCLE

PHYSIOLOGY

• FREQUENCY OF STIMULATION
(i
...
the effect of frequency of stimulation on muscle
contraction)
- 1rst stimuli applied
muscle fiber contracts (=
twitch)
- 2nd stimuli applied when;
i) Refractory period of 1rst stimuli has completed
...


MUSCLE
•

PHYSIOLOGY

TETANUS

- Can be described in to 2 parts;
a) INCOMPLETE (unfused) TETANUS
b) COMPLETE (fused) TETANUS

MUSCLE

PHYSIOLOGY

• TETANUS
a) INCOMPLETE (unfused) TETANUS
DESCRIBE – Skeletal muscle is stimulated at a rate 20 -30
times per second
...

- Sustained contraction
- Lacks partial relaxation between stimuli
RESULT: COMPLETE
(fused) TETANUS

MUSCLE

PHYSIOLOGY

• EXPLAINATION OF THE 2 TYPES
(fused and unfused tetanus) CONTRACTION PATTERN
...

After first few contractions, muscle reaches peak
performance
undergoes its strongest contractions
...
(i
...
at a faster rate than the active transport
pumps can take back in)
Ca2+ progressively builds-up in sarcoplasm
At a certain point, Ca2+ builds up and binds to troponin
RESULT: More powerful stroke occurs
(How? Filament sliding intensifies)
ii) (OTHER EFFECTS)
The environment within and surrounding muscle fiber has
changed
and it‟s favorable for muscle contraction
...
g
...
Acetyl cholinesterase (AChE) activity
2
...


RELAXATION

Acetyl cholinesterase (AChE) activity

ACTION: AChE breaks down the ACh released into
synaptic cleft as consequence of a transmitted
action potential from motor neuron
...

DISTRIBUTION: Attached to collagen fibers in
extracellular matrix in synaptic cleft
RESULT OF ACTION (i
...
breaking down of ACh)
i)
Muscle action potential stops
ii) Closing of Ca2+ release channels in SR

MUSCLE

RELAXATION

2
...


MUSCLE

PHYSIOLOGY

• ISOTONIC AND ISOMETRIC CONTRACTION
1
...

- Tension is constant
- There are 2 types of isotonic contractions

MUSCLE

PHYSIOLOGY

• ISOTONIC AND ISOMETRIC CONTRACTION
2) ISOMETRIC CONTRACTION
DESCRIBE: Muscle contractions are characterized by;
- muscle tension
- NO change in muscle length
IMPORTANCE: Stabilizes some joints as others are moved
...


MUSCLE ATROPHY

MUSCLE
•

PHYSIOLOGY

MUSCLE HYPERTROPHY

MUSCLE

HYPERTROPHY

MUSCLE

PHYSIOLOGY

• MUSCLE TONE
DEFINITION – Is firmness in relaxed skeletal muscle

Muscle of back posture

MUSCLE

PHYSIOLOGY

•
MUSCLE METABOLISM
- Contraction of muscle fiber needs ATP
- ATP in muscle fiber is only enough for contractions
that last a few seconds
...

(i
...
they switch from inactivity to great activity rapidly)
- For STRENUOUS EXERCISE
the contractions are
more than a few seconds
special muscle metabolic
biochemical systems are switched on
so ATP
available
...
PHOSPHAGEN system
2
...
AEROBIC system

MUSCLE METABOLISM
1) PHOSPHAGEN SYSTEM
- Molecule unique to muscle fibers
PHOSPHATE
...


MUSCLE METABOLISM

2) GLYCOGEN – LACTIC ACID SYSTEM

MUSCLE METABOLISM
2) GLYCOGEN – LACTIC ACID SYSTEM

Oxidation

reduction

Oxidation is gain of oxygen
...


Oxidation is loss of hydrogen
...


Oxidation is loss of electrons
...


- Oxidising agents give oxygen to
another substance
...

An oxidising agent oxidises something else
...

That means that an oxidising agent takes
electrons from that other substance
...

Or you could think it out like this:
An oxidising agent oxidises something else
...

Reduction is gain of electrons (OIL RIG)
...


- Reducing agents remove oxygen
from another substance
...


Fates of Glucose 6-Phosphate
Glucose 6-phosphate derived from glycogen can (1) be used as a fuel for
anaerobic or aerobic metabolism as in, for instance, muscle; (2) be converted into
free glucose in the liver and subsequently released into the blood; (3) be
processed by the pentose phosphate pathway to generate NADPH or ribose in a
variety of tissues
...


Intermediates of krebs cycle

MUSCLE METABOLISM
3) AEROBIC SYSTEM
- WHEN IS THIS SYSTEM SWITCHED - ON? – for
muscular activity > 30mins
DESCRIBE MECHANISM ?

MUSCLE METABOLISM
3
...
g
...
e
...

•1 Acetyl-CoA = 3 NADH + 1 FADH2 + 1 GTP (=ATP)
during Krebs cycle
...

-However 2 ATP molecules were used for the initial
activation of every fatty acid that is going to be oxidized
in the mitochondria
...

FACTORS THAT INFLUENCE IT;
i) Gender (> for males)
ii) Age (Highest at ~ 20 years)
iii)Size ( body size)
E
...
Highly trained athletes
- Maximal Oxygen uptake is 2 times greater
- Because of i) Training
- ii) Heredity

MUSCEL FATIGUE

DEFINE: Is when muscle is unable to maintain its
strength of contraction or tension
WHAT HAPPENS AT CELL LEVEL?
Muscle can not produce enough ATP to meet its
needs
FACTORS CONTRIBUTING TO MUSCLE FATIGUE
i)
Insufficient oxygen
ii) Depleted glycogen
iii) Build-up lactic acid
iv) Failure for action potential in motor neuron to
release enough ATP

TYPES OF SKELETAL MUSCLE FIBERS
1
...

EXAMPLE:
- “Red muscle fibers”- has a high myoglobin content
...

*Important associations with amount of myoglobin present
are;
i) MITOCHONDRIA – the more the myoglobin, the
more the mitochondria present
...


TYPES OF SKELETAL MUSCLE FIBERS
2) CONTRACTION VELOCITY (
There are 2 types
(i
...
depends on type of myosin ATPase activity found on
myosin head);
i) FAST – TWITCH
- Caused by rapid splitting of ATP
ii) SLOW – TWITCH
- Caused by slow splitting of ATP
3) METABOLIC REACTIONS
SIGNIFICANCE: The type of metabolic reactions
influences how quickly the muscle fatigues

TYPES OF SKELETAL MUSCLE FIBERS
There are 3 different types of skeletal muscle fibers;
1
...
FAST OXIDATIVE (type IIA) fibers
3
...
White muscle fiber
2
...
Red muscle fiber

TYPES OF SKELETAL MUSCLE FIBERS

1) SLOW OXIDATIVE (type I) fibers
FEATURES:
- Another name is “slow twitch, fatigue-resistant
fibers
- Smallest in diameter
- Large amount of myoglobin (which gives its)
- Has many mitochondria
- Has many blood capillaries
METABOLISM – Aerobic system for producing ATP
- It is fatigue resistant
CONTRACTION VELOCITY
- Slow (= slow twitch)
...
e
...
g
...
(It is fatigue resistant but not as much as slow
oxidative fibers)
CONTRACTION VELOCITY – Fast (= fast- twitch)
i
...
splits ATP rapidly
DISTRIBUTION
E
...
Sprinters – a large proportion
is in the leg muscles

TYPES OF SKELETAL MUSCLE FIBERS

3) FAST GLYCOLYTIC (type IIB) fiber
FEATURES:
- Another name is “fast-twitch B and fatigue resistant
fibers”
- Has low amount of myoglobin (Gives its white colour)
- Has few mitochondria
- Has few blood capillaries
METABOLISM – Anaerobic process (glycolysis) to produce
ATP
...
e
...

DISTRIBUTION
E
...
Muscles of arms

MUSCLE FIBER TYPES

EXTRA NOTES
• Skeletal muscle contains mixtures of the 3 types of muscle fibers
- the proportion depends on the muscle function
E
...
Postural muscle of neck – Have large amounts of slow oxidative
fibers
...
g
...

• Different types of exercise changes the types of muscle fibers
...
g
...
Contribution by SMOOTH MUSCLE
2
...
Contribution by SMOOTH MUSCLE
DISTRIBUTION; - Walls of skin arterioles and also small
arteries
...


MUSCLE

PHYSIOLOGY

• HOMEOSTASIS OF BODY TEMPERATURE
2

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