Muscular System
Anatomy & Physiology
Muscles
• From the Latin mus meaning little mouse (Flexing
muscles looked like mice scurrying under the skin)
• Have ability to transform ATP into mechanical energy
• Muscles can only pull, never push, which allows them to
exert a force
ATP --> ADP + P + Energy
ADP + P + Energy ---> ATP
Muscle Types
Myosin
Actin
• Skeletal - Slow to Fast twitch
• Cardiac - Fast twitch
• Smooth - Slow twitch
• Twitch = contraction
• Skeletal & smooth muscle cells are elongated &
called fibers
• All have contractile myofilaments actin & myosin
Muscle Fxn’s
-Produce movement
• Skeletal – locomotion & manipulation in response to
the environment
• Cardiac – moves blood
• Smooth – propels (squeezes) stuff through the
digestive, urinary, circulatory, and reproductive
systems
-Maintaining posture
-Stabilizing joints
-Generating heat (40% of your body heat)
Functional Characteristics of Muscle
• Excitability (Irritability) – the ability to respond to a
stimulus
• Contractility – the ability to shorten forcibly when
adequately stimulated
• Extensibility – the ability to be stretched or extended
• Elasticity – the ability of a muscle fiber to recoil &
resume its resting length after being stretched
Skeletal
Muscle
•
•
•
•
•
Striated
Multinucleate
Voluntary muscles
Can generate great power but fatigue quickly
Non-rhythmic contraction
Gross Anatomy of Skeletal Muscle
• Epimysium – outermost layer of dense irregular
connective tissue (Surrounds the whole muscle)
• Fascicle – bundle of muscle fibers
• Perimysium – fibrous C.T. which surrounds the
fascicle
• Endomysium – each muscle fiber is surrounded
by reticular C.T.
Sarcoplasmic Reticulum
Sarcoplasm = (Cytoplasm with
lots of glycogen stored)
Sarcolemma
(Plasma membrane)
Sarcomere
Protein Titan
Functional unit of muscle
Nerve & Blood Supply
Each muscle is served by:
• 1 nerve
• 1 artery
• 1 or more veins
Skeletal Muscle Attachment
Skeletal muscles attach to bones in at least 2 places
• When the muscle contracts, the moveable bone (Insertion),
moves toward the immovable or less-moveable bone (Origin)
• Direct muscle attachment – epimysium fused to periosteum
• Indirect muscle attachment – tendon or aponeurosis (flat,
sheet-like tendon)
Skeletal Muscle Contraction
• Sarcomere - contractile unit (z-line to z-line)
• Myosin - thick filaments (contain ATPase which is used to
split ATP to power muscle contraction) found in the dark Aband. Myosin heads form cross-bridges when attached to
actin
• Actin - thin filaments found in the light I-band which are
anchored to the z-line
Sliding Filament Theory
•
Hugh Huxley 1954 proposed that during contraction
actin will slide past myosin which result in overlapping
filaments
1.
Cross bridge attachment
2.
Power stroke (Myosin head pivots pulling actin)
3.
Cross bridge detachment (ATP binds to myosin head
loosening the bond to actin)
4.
“Cocking” the myosin head – ATPase hydrolyzes ATP to
ADP & Pi returning the myosin head to it’s cocked
position
Tropomyosin & Troponin
• Tropomyosin – stiffen the actin protein & block
myosin binding sites in relaxed muscle fibers,
preventing myosin & actin from forming a crossbridge
• Troponin – regulates cross-bridge formation. In
the presence of Ca2+ troponin moves tropomyosin,
thereby exposing the myosin binding sites
Z- Line - Defines each end of the sacromere. Thin filaments
of adjacent sarcomeres are linked together here.
A band - Consists of overlapping thin and thick filaments.
I band - Only thin filaments.
H zone - Only thick filaments. This also shortens during
contraction.
Thin Fibers - Each thin filament is made of three different proteins.
Actin - Actin filaments are made of subunits called G-actin; these are
globular proteins which are linked together to form a filament.
Myosin binding site - Each G-actin contains a binding site for myosin head
groups (part of the thick filament)
Troponin - This protein is associated with actin and it binds calcium.
Tropomyosin -The third protein of the group; when the muscle fiber is
not contracting, tropomyosin covers the myosin binding site, preventing
the myosin head groups from binding to actin.
Thick Fibers Consists of a bundle of proteins called myosin
Myosin Tails Each tail has two head groups at the same end and
each has two binding sites.
heads form cross bridges
Actin binding site - This binds to the myosin binding site on actin, when it
is exposed as tropomyosin moves.
ATP-binding site - Binds to ATP; splits the molecule and the released
energy is used to drive the movement of the myosin head groups.
Power Stroke
Calcium
• Sarcoplasmic reticulum – regulates intracellular
Ca2+ by storing & releasing Ca2+ when a stimulus
causes the muscle to contract
• T (Transverse) Tubules – extensions of the
sarcolemma which allow for rapid impulse
transmission through the muscle which ensures a
single muscle contraction
Step 6. The trans port of calcium ions
back into the s arcoplas mic reticulum.
Step 5. The hydrolysis of ATP,
whichleads to the re-energizing
and repos itioning of the
cros s bridge.
Step 1. The infux of calcium,
triggering the exposure of
binding sites on actin.
Step 2. The bindin g of
myos in to actin.
Step 4. The binding of
ATP to the cros s bridge,
which results in the cross
bridge dis connecting from
actin.
Step 3 . The po wer s troke of
the cro ss brid ge that causes the
s liding o f the thin filaments .
Regulation of Contraction
• Muscle contraction is stimulated
by an action potential from a nerve
• The neuromuscular junction (motor
end plate) in skeletal muscle is
regulated by acetylcholine (ACh)
• Ach needs to be broken down as
soon as it is used; the enzyme
acetlycholinesterase serves this
function.
1. ACH released
2. AP propagated along
membrane and at Ttubules
3. Ca released from SR
voltage gated Ca
channel opens
4. Ca binds to Troponin-C
conformation changes
favor tropomyosin opens
actin sites
5. myosin cross-bridges
attach-detach from
actin...pulls filament
toward M-line
6. Ca removed (uptake by
SR)
7. tropomyosin blocks actin
sites relaxation
a.
b.
c.
Generating an Action Potential
• Depolarization (Na channels open)
• Repolarization (Na channels close K channels open)
• Refractory Period (K channels close)
• Na/K pump
Homeostatic Imbalance
• Myasthenia gravis – autoimmune disease where ACh receptors
are broken down by ACh antibodies resulting in drooping
eyelids and general muscle fatigue
• Curare – arrowhead poison used in South America which
blocks ACh receptors resulting in respiratory arrest & death
• Cobra venom – same as curare
• Botulinum toxin prevents ACh release
• Black widow spider venom releases all Ach
• Nerve gasses inhibit AChase which keeps cleft flooded with
ACh
ACh destruction
• After ACh initiates the action potential the ACh
is broken down by acetylcholinesterase
• This prevents continued muscle contraction in
the absence of additional nerve stimuli
Mysostatins control muscle growth
Absence of results in enlarged
muscle development
Energy for muscle contraction
ATP is needed for…
1) contraction - Cocking and detachment of the
myosin head.
2) calsequestrin - Pumping calcium into the SR of the
sarcoplasm.
3) Na+/K+-ATPase - Needed for impulse conduction..
ATP Production
• ATP lasts - Only a few seconds during active muscle contraction as
ATP stores are used up.
• ATP is quickly reconstituted -There are several mechanisms
that replenish the ATP stores.
• Sources of energy for ATP production
1) phosphocreatine (creatine phosphate, CP)
ATP is
produced from another high energy molecule called phosphocreatine.
• Creatine kinase
• Breaks down phosphocreatine, releasing a phosphate and energy. The
energy is used to make new ATP.
• This store of energy lasts about 8-15 seconds.
ATP Production
2) Anaerobic Respiration & Glycolysis
Glycogen - A polymer of glucose.
For the muscle to recover the lactic acid needs to be removed quickly; a
well vascularized muscle serves this purpose.
lasts about 2 minutes.
Lactic acid/pyruvic acid
The product of anaerobic respiration is pyruvate; as no oxygen is available
the pyruvate is converted to lactic acid.
As lactic acid builds up in the muscles it changes the pH of the tissues in
the muscle, which causes a decrease in the efficiency of proteins and
enzymes.
This leads to soreness and fatigue.
ATP Production
• Oxidative metabolism a.k.a. Aerobic Respiration - Oxygen
used by mitochondria to produce 36 net ATPs (2 from
glycolysis and 34 from Krebs and ETC)
• Can produce ATP indefinitely as long as you have oxygen
and energy stores (fat, proteins or glucose.)