Download MUSCLES

MUSCLE Mania!!
Sarah Bartley
Lauren Thames
Annie Lee
Contracting Muscles (49.6 in text)
• All animal movement is based on one of two basic
contractile systems, both of which consume energy in
moving protein strands against one another
– Microtubules: beating of cilia and undulations of flagella
– Microfilaments: amoeboid movement, contractile elements of
muscle cells
• Muscles always contract (extend only passively)
– The ability to move body parts in opposite directions requires the
muscles to be attached to the skeleton in antagonistic pairs
– Each member of the pair is working against the other
– Example: Flexing and extending your arm
• To understand how a muscle contracts, we need to
understand its structure….
Vertebrate Skeletal Muscle
Muscle
•
Skeletal muscle is attached to the bones and
responsible for their movement
– Consists of a bundle of long fibers running
parallel to the length of the muscle
– Each fiber is a single cell with multiple nuclei
(formed by the fusion of many embryonic
cells)
– A fiber itself is a bundle of myofibrils
arranged longitudinally
• These are composed of myofilaments:
– Thin filaments- two stands of actin,
one strand of regulatory protein
coiled around one another
– Thick filaments- staggered arrays of
myosin molecules
Bundle of
muscle fibers
Nuclei
Single muscle fiber
(cell)
Plasma membrane
Myofibril
Z line
Light
band
Dark band
Sarcomere
TEM
I band
0.5 m
A band
I band
M line
Thick
filaments
(myosin)
Thin
filaments
(actin)
Z line
H zone
Sarcomere
Z line
Contd…
• Skeletal muscle is also called striated muscle because
the regular arrangement of the myofilaments creates a
pattern of light and dark bands
– Each repeating unit is a sarcomere, the basic contractile unit of
the muscle
– Z lines border the sarcomere
– Area near the edge of the sarcomere where there are only thin
filaments is called the I band
– A Band- the broad region that corresponds to the length of the
thick filaments
• H Zone is the center
– This arrangement of thick and thin filaments is the key to how the
sarcomere, and hence the whole muscle, contracts
Sliding-Filament Model
of Muscle Contraction
• According to the model, neither the thin filaments nor the thick
filaments change in length when the sarcomere shortens
• The filaments slide past each other longitudinally, producing more
overlap between filaments
• As a result, both the region occupied by thin filaments (the I Band)
and the region occupied by thick filaments (the H zone) will shrink.
• The sliding is based on the interaction between the actin and myosin
molecules that make up the filaments
– Myosin molecules have a tail region that adheres to the tails of other
myosin molecules that form the thick filaments
– The head it the center of bioenergetic reactions that power muscle
contractions
• It can bind ATP into ADP and inorganic phosphate
• See model on next slide
Myosin-actin interaction
Thick filament
Thin filaments
1 Starting here, the myosin head is
bound to ATP and is in its lowenergy confinguration.
5 Binding of a new molecule of ATP releases the
myosin head from actin,
and a new cycle begins.
Thin filament
Myosin head (lowenergy configuration)
ATP
ATP
Thick
filament
Thin filament moves
toward center of sarcomere.
+
4 Releasing ADP and ( P i), myosin
relaxes to its low-energy configuration,
sliding the thin filament.
Actin
Pi
ADP
Pi
Cross-bridge
binding site
ADP
Myosin head (lowenergy configuration)
ADP
2 The myosin head hydrolyzes
ATP to ADP and inorganic
phosphate ( P I ) and is in its
high-energy configuration.
Pi
Cross-bridge
Myosin head (highenergy configuration)
13 The myosin head binds to
actin, forming a crossbridge.
Role of Calcium and Regulatory
Proteins
• A skeletal muscle fiber contracts only when stimulated by
a motor neuron
• When a muscle is at rest, the mysosin binding sites on
the thin filament are blocked by the regulatory protein
tropomyosin
Tropomyosin
Actin
Ca2+-binding sites
(a) Myosin-binding sites blocked
Troponin complex
• For a muscle fiber to contract, those binding sites must
be uncovered
• This can happen when calcium ions bind to another set
of regulatory proteins (troponin complex) which
controls the position of the tropomyosin on the thin
Ca
filamen
2+
Myosinbinding site
(b) Myosin-binding sites exposed
The stimulus leading to the contraction of a skeletal muscle fiber is an action
potential in a motor neuron that makes a synapse with the muscle fiber. The
Synaptic terminal of a motor neuron releases acetylcholine, which depolarizes
the plasma membrane of the muscle fiber. The depolarization causes action
potentials to sweep across the fiber and trigger the release of calcium from the
plasmic reticulum into the cytosol. Calcium is what initiates the sliding of filaments
through the binding of myosin to actin.
Neural Control of Muscle Tension
• When action potential in a motor neuron releases acetylcholine on a skeletal muscle fiber, the muscle fiber
respons by producing a brief contraction called a twitch
• Contraction of a whole muscle, however, is graded
• 2 basic mechanisms:
– Varying the number of muscle fibers that contract
– Varying the rate at which muscle fibers are stimulated
Motor units- Each muscle fiber has a single synapse with one motor neuron, but
each motor neuron typically synapses with several or many muscle fibers. A
motor neuron and all the muscles fibers it make up a motor unit.
Types of Muscle Fibers
• All skeletal muscle fibers contract when stimulated by an
action potential in a motor neuron, but the speed at
which they contract differs among muscle fibers
• Mainly due to the rate at which the myosin heads
hydrolyze ATP
• Based on speed of contraction, we can classify muscles
as fast or slow
– Fast- brief, rapid, powerful contractions
– Slow- long contractions (less sarcoplasmic reticulum and slower
calcium pumps..more calcium in cytosol longer)
• Can also be classified by the major metabolic pathway they use for
producing ATP
• Oxidative fibers rely on aerobic respiration. They are specialized to
make use of a steady supply of energy.
– Myoglobin is an oxygen- storing protein that is the brownish pigment in
the dark meat of poultry and fish that binds oxygen more tightly than
does hemoglobin, so it an effectively extract oxygen from the blood
– Can either be fast or slow
• Glycolitic fibers rely on glycolysis, all fast
• Therefore, considering both contraction speed and ATP synthesis,
we can identify three main types of skeletal muscle fibers:
– Slow oxidative
– Fast oxidative
– Fast glycolitic
Most human skeletal muscles contain all three fiber types, but the muscles of the
eye and hand lack slow oxidative fibers. If a muscle is used repeatedly for
activities requiring high endurance, some fast glycolitic fibers can develop into
fast oxidative fibers. Since fast oxidative fibers fatigue more slowly than fast glycolitic fibers, the muscle as a whole will become more resistant to fatigue.
Other Types of Muscle
•
The vertebrate cardiac muscle is found in the heart. While skeletal muscle
fibers will not produce action potentials unless stimulated by a motor
neuron, cardiac muscle cells have ion channels in their plasma membrane
that cause rhythmic depolarizations, triggering action potentials without
input from the nervous system.
– Action potential is up to 20 times longer than skeletal muscle fibers
– Play a key role in controlling the duration of contraction
– Plasma membranes of adjacent cardiac muscle cells interlock at
specialized regions called intercalated disks, where gap junctions
provide a direct electrical coupling, generating an action potential in one
part of the heart that will spread to all other cardiac muscle cells, and
the whole heart will contract.
Contd.
• Smooth muscle is found in the walls of hollow organs
like blood vessels. Instead of regularly arrayed
filaments, the thick filaments are scattered throughout
the cytoplasm, and thin filaments are attached to
structures called dense bodies, some tethered to the p.
membrane.
• Contract relatively slowly but over a much greater range
of length than striated muscle. Some only contract when
stimulated by neurons, but others can generate action
potentials without neural input and are electrically
coupled to one another.