Download Cellular movement and muscles

Cellular movement and Muscles
Muscles – general information
Vertebrates and many invertebrates have three main classes of muscle
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Skeletal muscle connect bones are are used for complex coordianted
activities.
Smooth muscles surround internal organs such as the large and small
intestines, the uterus, and large blood vessels
The contraction and relaxation of smooth muscles controls the
diameter of blood vessels and also propels food along the
gastrointestinal tract.
Compared with skeletal muscles, smooth muscle cells contract and
relax slowly, and they can create and maintain tension for long
periods of time.
Cardiac muscle: Striated muscle of the heart.
Muscles - introduction
B. Skeletal muscle from the neck of a hamster
C. Heart muscle from a rat
D. Smooth muscle from the urinary bladder of a guinea pig
E. Myoepithelial cells in a secretory alveolus from a lactating rat mammary
gland
Microtubule Function
• Move subcellular components
• Use motor proteins kinesin and dynein
• e.g., Rapid change in skin color
Microtubules Show Dynamic Instability
Balance between growth and shrinkage
Factors
• Local concentrations of tubulin
• Dynamic instability
• Microtubule-associated proteins (MAPs)
• Temperature
Chemicals can disrupt the dynamics (e.g., plant poisons)
Movement Along Microtubules
Direction is determined by polarity and the type of motor protein
• Kinesin move in + direction
• Dynein moves in – direction
Fueled by ATP
Rate of movement is determined by the ATPase domain of the protein and
regulatory proteins
Dynein is larger than kinesin and moves 5-times faster
Cilia and Flagella
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Cilia – numerous, wavelike motion
Flagella – single or in pairs, whiplike movement
Composed of microtubules
Arranged into axoneme
Movement results from asymmetric activation of dynein
Microtubules and Physiology
Microfilaments
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Other type of cytoskeletal fiber
Polymers composed of the protein actin
Often use the motor protein myosin
Found in all eukaryotic cells
Movement arises from
• Actin polymerization
• Sliding filament model using myosin (more common)
Microfilament Structure and Growth
• Polymers of G-actin
called F-actin
• Spontaneous growth
(6-10X faster at +
end)
• Treadmilling when
length is constant
• Capping proteins
increase length by
stabilizing minus end
Microfilament Arrangement
Actin Polymerization
Amoeboid movement
Two types
• Filapodia are rodlike
extensions
• Neural connections
• Microvilli of digestive
epithelia
• Lamellapodia
resemble
pseudopodia
• Leukocytes
• Macrophages
Skeletal muscle (striated muscle)
• Skeletal muscle cells are one of the largest cells in the body
• Are multinucleate formed by the fusion of myoblasts
• Diameters range from 50 to 150 microns with lengths ranging from
mm to cm
• Muscle fibers contract in response to an electrical signal ie
depolarization
• The signal is generated at the synapse (the neuromuscular
junction) and propagated through an action potential via the
muscle fiber membrane
• The membrane of the cell has specialized invaginations called
Transverse-tubules (T-tubules) that enter into the cell (at every 1-2
microns)
• The action potential can be rapidly transmitted deep into the
interior of the cell resulting a delay of only 3-5 msec between the
depolarization at the synapse and the first muscle fiber tension.
• The T-tubule network is so extensive that 50-80% of the plasma
membrane is in the T-tubules.
Neuromuscular junction – things to remember
• Each muscle fiber is innervated by a motorneuron
• One motorneuron can innervate one or multiple fibers
• Each motorneuron plus its complement of muscle fibers is called a
motor unit as well all contract in concert.
• The synpase between the motorneuron and the muscle fiber is
called a neuromuscular junction(NMJ)
• Nerve terminal contains many mitochondria and vesicles which can
be seen lined up in double rows along side the voltage-gated Ca2+
channels attached to presynaptic membrane => active zone.
• The vesicles of the NMJ have very high concentrations of
neurotransmitter (2,000 to 10,000 molecules of ACH per vesicle)
• Excess of neurotransmitter is released to ensure that the resulting
post-synaptic depolarization is strong enough to generate an action
potential - "safety margin"
Neuromuscular junction – things to know
The nACHR - again
• The receptor is made up of 4 different transmembrane proteins one of
which (the alpha subunit) is repeated to give 5 subunits to create the ion
channel
• ACH binds to the alpha subunit and thus it takes two molecules of
acetylcholine to open the channel
• One nACHR opens and allow 1.5 x 104 Na+ ions/msec of open time
• The channel opens on average 1 msec.
• 1 vesicle contains enough neurotransmitter to open ~3000 receptors
(wow!) and because two molecules of Ach is needed to open one receptor
there must be a minimum of ~6,000 molecules Ach per vesicle.
• Studies have shown that the amount of neurotransmitter contained in one
vesicle causes an post-synaptic potential of ~ 1 mV.
• If the average depolarization generated at a NMJ
of a muscle fiber is 40 mV then there must be at
least 40 vesicles released and in the order of
120,000 receptors activated at the NMJ. WOW!
Striated muscle channels and action potentials
Striated muscle channels and action potentials
Pumps and transporters
1) Na+/K+ ATPase pump - to establish the electrochemical gradients of
Na+ and K+
2) Ca2+ ATPase pump - uses energy from ATP to remove 2 Ca2+ from the
inside to the outside of the cell to ensure that internal Ca2+
concentrations remain low (10-7 mM internal)
3) Na+/Ca2+ cotransporter - to also remove Ca2+ from the inside of the cell
and uses the energy from the cotransport of 3 Na+ molecules to
export 1 Ca2+
4) Muscle Ca2+ ATPase pump - a different pump from number 2 above.
Found highly concentrated on the sarcoplasmic reticulum (SR)
(constitutes 80% of the protein found in the SR membranes).
The muscle Ca2+ ATPase pumps 2 Ca2+ into the SR to lower cytosolic
Ca 2+ and to concentrate Ca 2+ into internal stores.
Striated muscle channels and action potentials
Channels - muscle cells share many of the same ion channels as
neurons
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Leak channels - besides the leak K+ channel, skeletal muscle cells have
a high concentration of Cl- leak channels. The high permeability to Clhelps repolarize the membrane after an action potential
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Voltage-gated Na+ channels .
3) Voltage-gated K+ channel - the delayed rectifier K+
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Voltage gated Ca2+channels - high threshold Ca2+ channels
Skeletal muscle (striated muscle)
Terminology
• Muscle cell: Muscle fiber
• Myofibrils: Main intracellular structures in striated muscles. Are bundles of
contractile and elastic proteins
• Sarcolemma: Cell membrane of a muscle cell
• Cytoplasm: Sarcoplasm
• Sarcoplasmic reticulum: wraps around each myofibril like a piece of lace.
Release and sequester Ca2+ ions
Skeletal muscle (striated muscle)
•The sarcoplasmic reticulum (SR) regulates the cytosolic Ca2+ levels in
skeletal muscle
•Myofibril: A long bundle of actin, myosin and associated proteins in
muscle cell.
•Transverse (T) tubules: invaginations of the plasma membrane, enter
myofibers at the Z disks, where they come in close contact with the
terminal cisternae of the SR
•Terminal cisternae: store Ca2+ ions and connect with the lacelike
network of SR tubules that overlie the A band.
T-Tubules and SRs
Transverse tubules
• Sarcolemmal invaginations
• Enhance action potential
penetration
• More developed in larger,
faster twitching muscles
• Sarcoplasmic reticulum (SR)
• Stores Ca2+
• Terminal cisternae -  storage
Triads and Sarcoplasmic reticulum
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The link between depolarization and Ca2+ release or excitationcontraction coupling occurs at the junctions between the T-tubule and
the sarcoplasmic reticulum
80% of the T-tubules membrane is associated with the sarcoplasmic
reticulum at triads
The voltage-gated Ca2+ channels are concentrated in the T-tubules in
the triads
The Ca2+ release channel found in the sarcoplasmic reticulum
membrane is associated with the voltage-gated Ca2+ channel at this
point
This close association allows for the rapid signaling from action
potential to Ca2+ release.
General sequence of events
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Resting [Ca2+]i = 0.1 μM
AP propagation along Sarcolemma and into T- tubules
Depolarization opens the voltage-gated Ca2+ channels at triad junctions
This results in a release of Ca2+ through the Ca2+ release channels from
the SR
Cytosolic [Ca2+]i reaches 1-10 μM
Diffusion and binding of Ca2+ to TnC
Contraction events
[Ca2+] to resting levels:
1. After the action potential is passed and the voltage-gated Ca2+
channels close, the Ca2+ release channels close
2. Ca2+ is recycled back into the SR through the Ca2+ ATPases
3. Ca2+ binds to calsequesterin
Ca2+ channels and its release
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Release of Ca2+ stores mediated by ryanodine receptors (RYRs) in
skeletal muscle
Voltage sensing dihydropyridine (DHP) receptors in the plasma
membrane contact ryanodine receptors located in the membrane of the
SR
In response to a change in voltage, the dihydropyridine receptors
undergo a conformational change
This produces a conformational change in the associated RYRs, opening
them so that Ca2+ ions can exit into the cytosol.
The voltage-gated Ca2+ channel is either closely localized to or makes a
physical connection to the Ca2+ release channels in the SR
Cont……..
Ca2+ channels and its release
• Not all Ca2+ release channels are associated with voltage-gated Ca2+
channels
• These non-associated channels are thought to be opened solely by Ca2
influx into the cytosol from the voltage-gated Ca2+ channels.
• The Ca2+ release channel in the SR of most muscle cells (smooth, cardiac,
skeletal) is a Ca2+ activated Ca2+ channel
• The Ca2+ release channel is stimulated to open at low concentrations of
Ca2+ ( up to 0.1 mM) in the cytosol but inhibited by high concentrations of
Ca2+ in the cytosol (0.5 mM and higher for cardiac cells)
• So as Ca2+ is released from the SR it starts to inhibit the Ca2+ release
channel.
Depolarization induced Ca2+ release
Ca2+ induced Ca2+ release
Experiment
Sarcomeres
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Skeletal muscle is made up of bundles of multinucleate muscle cells
(myofibers)
Each cell contains myofibrils that are composed of repeated units of
actin and myosin called sacromeres
Thick and thin filaments arranged into sarcomeres
Repeated in parallel and
in series
Features
• Z-disk
• A-band
• I-band
• M-lines
Sarcomeres
• Electron micrograph of a longitudinal section through a skeletal muscle
cell of a rabbit
• Schematic diagram of a single sarcomere
• Z discs: At each end of the sarcomere
• Attachment sites for the plus ends of actin filaments (thin filaments)
• M line: Midline.
• Location of proteins that link adjacent myosin II filaments (thick filaments)
to one another
• Dark bands: mark the location of the thick filaments = A bands
• Light bands: which contain only thin filaments and therefore have a lower
density of protein = I bands.
Sarcomeres
Myofibril
• A single continuous stretch of interconnected sarcomeres
• Runs the length of the muscle cell
• More myofibrils in parallel  more force
Striated Muscle Cell Structure
Composed of thick and thin
filaments
• Thick: myosin
• 300 myosin II hexamers
• Thin: actin
• Capped by tropomodulin
(-) and CapZ (+) to
stabilize
• Decorated by troponin
and tropomyosin
• Globular protein (G-actin)
• Form long chains called
F-actin
• In skeletal muscle 2 Factin polymers twist
together
Actin and Myosin Function
Myosin
• Motor protein used by actin
• Sliding filament model
• Most common type of movement
• Myosin is an ATPase
• Converts E released
• from ATP to mechanical E
• 17 classes of myosin with
• multiple isoforms
• Similar structure
• Head, tail, and neck
Sliding Filament Model
Analogous to pulling yourself along a rope
• Actin: the rope
• Myosin: your arm
Sliding Filament Model
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Two processes
• Chemical
• Myosin binds to actin
(Cross-bridge)
• Structural
• Myosin bends
(Power stroke)
Cross-bridge cycle
• Formation of crossbridge, power stroke,
and release
Need ATP to attach and
release
• No ATP  rigor mortis
Sliding filament model
Sliding Filament Model, Cont.
• Myosin is bound to actin in the
absence of ATP and this is the
"rigor" state i.e. gives rigidity to
the muscle
• ATP binds to the myosin causing
the head domain to dissociate
from actin
• ATP is then hydrolyzed causing
a conformational change in the
mysoin head to move it to a new
position and bind to actin
• Pi is released causing the
myosin head to change
conformation again and it is this
movement that moves the actin
• ADP is released
Ca2+ Allows Myosin to Bind to Actin
• Ca2+ binds to TnC
• Reorganization of troponin-tropomyosin
• Expose myosin-binding site on actin
Ca2+ Allows Myosin to Bind to Actin
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Ca2+ levels increase in cytosol
Ca2+ binds to troponin C
Troponin-Ca2+ complex pulls tropomyosin away form
G-actin binging site
Myosin binds to actin and completes power stroke
Actin filament moves
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Sliding Filament Model
Contractile Force
• Depends on sarcomere length: distance
between the Z-disks
• Number of myofibrils
• Number of cells (recruitment)
Isotonic and isometric contraction
Regulation of Contraction
• Excitation-contraction coupling
• Depolarization of the muscle plasma
membrane (sarcolemma)
• Elevation of intracellular Ca2+
• Contraction
• Relaxation when the sarcolemma
repolarizes and Ca2+ returns to resting
levels
Excitation – contraction coupling
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ACH released at the NMJ
Net entry of Na+ initiates a
muscle action potential
AP in T- tubule alters
conformation of DHP receptor
DHP receptor opens Ca2+ release
channels in SR and Ca2+ enters
the cytoplasm
Ca2+ binds to troponin C
Troponin-Ca2+ complex pulls
tropomyosin away form G-actin
binging site
Myosin binds to actin and
completes power stroke
Actin filament moves
Time Course of Depolarization
Time Course of Depolarization
Cause of Depolarization
Myogenic
• Spontaneous
• e.g., Vertebrate heart
• Pacemaker
• Cells that depolarize fastest
• Unstable resting membrane
potential
Neurogenic
• Excited by
neurotransmitters
• e.g., Vertebrate skeletal
muscle
• Can have multiple (tonic)
or single (twitch)
innervation sites
Relaxation
• Repolarization
• Reestablish Ca2+ gradients
• Extracellular
• Ca2+ ATPase
• Na+/Ca2+ exchanger (NaCaX) in reverse
• Intracellular
• Ca2+ ATPase (SERCA)
• Parvalbumin – cytosolic Ca2+ buffer
The Z disk
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The Z disk is complex of proteins
Responsible for anchoring the actin filaments to ensure that the
sacromere will shorten during contraction
Actin filaments are capped at both ends to ensure that the actin will not
depolymerize
Titin-nebulin filament system stabilizes the alignment of thick and thin
filaments in skeletal muscle
Thick filaments are connected at both ends to Z disks through titin
Nebulin is associated with a thin filament from its (+) end at the Z disk to
its other end
Accessory proteins in a sarcomere
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Each giant titin molecule extends from the Z disc to the M line
Part of each titin molecule is closely associated with a myosin thick
filament
The rest of the titin molecule is elastic and changes length as the
sarcomere contracts and relaxes
Each nebulin molecule is exactly the length of a thin filament
The actin filaments are also coated with tropomyosin and troponin and are
capped at both ends. Tropomodulin caps the minus end of the actin
filaments, and CapZ anchors the plus end at the Z disc, which also
contains a-actinin.
Skeletal muscle metabolism
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Muscles require a large source of ATP to allow for contraction and for
transport of Ca2+
ATP requirements are normally met by glycolysis or respiration
Skeletal muscles contain large glycogen stores
Skeletal muscles contain creatine phosphate that generates ATP:
creatine phosphate + ADP = creatine + ATP
Muscles have lots of mitochondria which extend through out the
myofibrils and are red coloured due to a large blood flow and myoglobin
(which stores oxygen)
The breakdown of glycogen stores in muscles can be stimulated by both
Ca2+ and epinephrine
Asynchronous Insect Flight Muscles
• Wing beats: 250 to 1000 Hz
• Fastest vertebrate
contraction: 100 Hz
(toadfish sonic muscle)
• Asynchronous because nervous
stimulation is not synchronized
to contraction
• Due to stretch-activation
• Contracted: Ca2+
insensitive
• Stretched: Ca2+ sensitive
Trans-Differentiation
• Cells with novel properties
• Heater organs
• Electric organs
Muscles – general information again
Vertebrates and many invertebrates have three main classes of muscle
•Skeletal muscle
•Smooth muscles surround internal organs such as the large and small
intestines, the uterus, and large blood vessels
•Cardiac muscle: Striated muscle of the heart.
Smooth Muscle - introduction
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Slow, regular contractions
Prolonged contractions
Contribute to many systems
Key differences from skeletal muscle
• Lack sarcomeres (no striations)
• No t-tubules
• Minimal SR
• Gap junctions
• Contract in all dimensions
• More complex regulation
Smooth muscles - introduction
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Smooth muscle cells have multiple receptors and activation
mechanisms
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Smooth muscle cells can be activated by neurotransmitters,
hormones, neighbouring cells
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Important: The overall goal is always the same.... change levels of
cytosolic Ca2+ to change the degree of contraction.
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Composed of elongated spindle-shaped cells, each with a single
nucleus
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Packed with thick and thin filaments but these filaments are not
organized into well-ordered sarcomeres and thus smooth muscle
is not striated
Smooth muscles – more introduction
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The sarcoplasmic reticulum network is sparse
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Majority of the increase in cytosolic Ca2+ needed for muscle
contraction enters the cell via the plasma-membrane Ca2+ channel
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Changes in the cytosolic Ca2+ level occur much more slowly in
smooth muscle (seconds to minutes).
Nerve innervation of smooth muscle cells is from the autonomic
nervous system
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Many smooth muscle cells have the ability to spontaneously
activate
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Contractions can occur over minutes rather than milliseconds as
was seen with skeletal and hundreds of milliseconds as was seen
with cardiac cells.
Smooth muscles – more introduction
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Filaments in smooth muscle are
gathered into loose bundles, which
are attached to dense bodies in the
cytosol
Dense bodies serve the same
function as Z disks in skeletal
muscle
The other end of the thin filaments
in many smooth muscle cells is
connected to attachment plaques.
Like a Z disk, an attachment
plaque is rich in the actin-binding
protein alpha-actinin; it also
contains a second protein,
vinculin, which binds to an integral
membrane protein in the plaque
and to alpha-actinin
Smooth muscles
Smooth muscle contraction
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Smooth muscle contraction is not controlled by the binding of Ca2+ to
the troponin complex as it is in cardiac and skeletal muscles
Ca2+ control of myosin attachment to the actin is through an
intermediate step of Ca2+/calmodulin and it is this that controls
contraction in smooth muscle cells
Calmodulin = intracellular second messenger that binds Ca2+
Troponin is not found in smooth muscle cells (tropomyosin is)
Caldesmon = regulatory protein on smooth muscle actin. Binds to actin
and prevents myosin from binding actin
Caldesmon and Ca2+/calmodulin
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The activation of smooth muscle myosin can be regulated by caldesmon
(CD) which in low Ca2+ levels (10-6 M), binds to tropomyosin and actin and
blocks myosin binding to actin
As Ca2+ levels increase, Ca2+ activated calmodulin binds to caldesmon
which releases caldesmon from the tropomyosin/actin complex
Now myosin is free to bind and move along the thin filaments to contract
the cell
Phosphorylation by several kinases, including MAP kinase, and
dephosphorylation by phosphatases also regulate caldesmon’s actinbinding activity
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Myosin light chain kinase (MLCK) and Ca2+/calmodulin
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In vertebrate smooth muscle, phosphorylation of the myosin regulatory
light chains on site X by Ca2+-dependent myosin LC kinase activates
contraction
At Ca2+ concentrations < 10-6 M, the myosin LC kinase is inactive
A myosin LC phosphatase, which is not dependent on Ca2+ for activity,
dephosphorylates the myosin LC, causing muscle relaxation
Contraction – simple
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Intracellular Ca2+ increase and Ca2+ is
released from the SR
Ca2+ binds to calmodulin (CaM)
Ca2+ - calmodulin complex activates
MLCK
MLCK phosphorylates light chains in
myosin heads and increases myosin
ATPase activity
Active myosin crossbridges slide along
actin and create muscle
Relaxation - simple
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Free Ca2+ in cytosol decreases when
Ca2+ is pumped out of the cell or back
into the SR released from the SR
Ca2+ unbinds from calmodulin
Myosin phosphatase removes
phosphate from myosin, which
decreases myosin ATPase
Less myosin ATPase activity results in
decreased muscle tension
Regulation of smooth muscle contraction
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The major means that control smooth muscle contraction is controlled
is through changes in resting membrane potential
Depolarization causes a greater increase in cytosolic Ca2+ and thus
greater contraction
Hyperpolarization causes a reduced amount of cytosolic Ca2+ and thus
relaxes the muscle cell
However it is important to note that release of Ca2+ from internal stores
may also lead to greater contraction through G protein mediated
cascades that have nothing to do with changes in membrane
depolarization.
Norepinephrine and epinephrine
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Depending on the type of receptor norepinephrine and
epinephrine can have different results on the smooth muscle cell
Epinephrine bound to beta-adrenergic receptors on smooth
muscle cells of the intestine causes them to relax
Epinephrine also binds to the alpha2-adrenergic receptor found
on smooth muscle cells lining the blood vessels in the intestinal
tract, skin, and kidneys
Epinephrine bound to alpha2 receptors causes the arteries to
contract (constrict), reducing circulation to these organs
Acetylcholine and Nitric Oxide
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ACH is released by autonomic nerves in the walls of a blood vessel, and
it causes smooth muscle cells in the vessel wall to relax
ACH acts indirectly by inducing the nearby endothelial cells to make and
release NO, which then signals the underlying smooth muscle cells to
relax.
Regulation of contractility of arterial smooth muscle by NO and cGMP:
NO synthesized in endothelial cells diffuses locally through tissue
and activates guanylate cyclase in nearby smooth muscle cells
The resulting rise in cGMP leads to the relaxation of the muscle and
vasodilation.
Cont…..
Acetylcholine and Nitric Oxide
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Schematic diagram of the structure of soluble guanylate cyclase
Binding of NO to the heme group stimulates the enzyme’s catalytic
activity, leading to formation of cGMP from GTP.
More about Nitric Oxide
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Catalyzed by the enzyme NO synthase from arginine
Rapidly diffuses out of the cell into neighboring cells
Very short half life (5-10 seconds) - so acts only locally
In many target cells, NO binds to iron in the active site of the enzyme
guanylyl cyclase, stimulating this enzyme to produce cyclic GMP.
The effects of NO can occur within seconds, because the normal rate of
turnover of cyclic GMP is high
Increased cGMP activates a kinase that subsequently leads to the
inhibition of calcium influx into the smooth muscle cell, and decreased
calcium-calmodulin stimulation of myosin light chain kinase (MLCK).
Decreases the phosphorylation of myosin light chains, thereby
decreasing smooth muscle tension development and causing
vasodilation.
Other regulators
Nitroglycerine
• Has been used for about 100 years to treat patients with angina (pain
resulting from inadequate blood flow to the heart muscle)
• Converted to NO, which relaxes blood vessels
• This reduces the workload on the heart and reduces the oxygen levels
needed by the heart muscle.
Viagra
• The drug sildenafil [Viagra] inhibits this cyclic GMP phosphodiesterase
and increases the amount of time that cyclic GMP levels remain elevated.
• The cyclic GMP keeps blood vessels relaxed and in certain parts of the
male anatomy blood pools and the resulting effect has sales of Viagra
soaring. It is interesting to note however that Viagra is not specific to the
penis it will affect cGMP levels throughout the body and can have some
interesting side effects.
More about ACH
Tissue
Vasculature (endothelial cells)
Eye iris (pupillae sphincter muscle)
Ciliary muscle
Salivary glands and lacrimal glands
Bronchi
Heart
Gastrointestinal tract
Urinary bladder
Sweat glands
Reproductive tract, male
Uterus
Effects of ACh
Release of endothelium-derived
relaxing factor (nitric oxide) and
vasodilation
Contraction and miosis
Contraction and accommodation of lens
to near vision
Secretion—thin and watery
Constriction, increased secretions
Bradycardia, decreased conduction
(atrioventricular block at high doses),
small negative inotropic action
Increased tone, increased
gastrointestinal secretions, relaxation at
sphincters
Contraction of detrusor muscle,
relaxation of the sphincter
Diaphoresis
Erection
Variable, dependent on hormone
influence
Cardiac muscle
Cardiac muscle – general info
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Many similar properties to skeletal muscles but there are some
important differences
Hearts of course vary greatly in size, shape and complexity from
animal to animal - ranging from insects with a simple tube that
pumps blood or hemolymph around an open circulatory system to
our closed circulatory system and a four chambered heart
Cardiac muscle – general info
• The heart contains pace-maker cells that produce the depolarization and
action potentials to drive cardiac cell contraction
• Heart contraction is not neuronally driven but self-driven or myogenically.
• Each muscle cell is a single cell not multinucleate like skeletal muscle
• Like skeletal muscle cells each cell contains multiple myofibrils and in the
cases of higher vertebrates an extensive sarcoplasmic reticulum
• Cardiac muscle cells are linked to each other with gap junctions
Cardiac muscle – general info cont.
• There are different types of cardiac muscle cells ranging from the
pacemaker cells in the sinoatrial node to the atrial and ventricular cells
that produce the contraction of the heart chambers
• The action potential in cardiac cells is quite different from skeletal
muscle and neuronal action potentials in that voltage-gated Ca2+
channels play a much larger role
• The mechanism of triggering the Ca2+ release channel in the
sarcoplasmic reticulum is not the same as in vertebrate skeletal
muscle cells
Pacemaker Cells
• Derived from cardiac muscle cells
• Differences from most cardiac muscle
• Small with few myofibrils, mitochondria or other organelles
• Do not contract
• Have unstable resting membrane potential (pacemaker potential)
that slowly drifts upwards until it reaches a threshold and
activates and action potential
Cardiac muscle channels and action potentials
Pumps and transporters
• Na+/K+ ATPase pump - to establish the electrochemical gradients of
Na+ and K+
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Ca2+ ATPase pump - uses energy from ATP to remove 2 Ca2+ from the
inside to the outside of the cell or into the sarcoplasmic reticulum to
ensure that internal Ca2+ concentrations remain low (10-7 mM internal)
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Some cardiac cells (i.e. lower vertebrates, invertebrates) do not have
an extensive sarcoplasmic reticulum and thus most of the Ca2+ that is
used to trigger contraction is from extracellular sources
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Na+/Ca2+ cotransporter - to also remove Ca2+ from the inside of the cell
and uses the energy from the cotransport of 3 Na+ molecules to export
1 Ca2+.
Excitation-contraction coupling
Cardiac muscle channels and action potentials
Channels
• leak channels - leak K+channel
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voltage-gated Na+ channels
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voltage-gated K+ channel - the delayed rectifier K+ channel
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voltage gated Ca2+ channels
- Note: In cardiac cells the Ca2+ channel plays a much greater role
during the action potential
- High threshold Ca2+ channels, called L channel or DHP
(dihydropyridine channel)
Action potential in ventricular (and atrial) cardiac cells
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Resting potentials in these cells is set by a large K+ permeability due to a
combination of the leak K+ channel and a voltage-gated K+ channel
(called the inward rectifier K+ channel) that is open at rest
This means that rest is very close to EK+
The rising phase of the action potential is set by the cardiac voltagegated Na+ channel
Action potential in ventricular (and atrial) cardiac cells
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As the voltage-gated Na+ channels produce the rising phase and then
start to inactivate two channels will now be opening, the delayed rectifier
K+ channel and the voltage-gated Ca2+ channel (L or DHP channel)
There are many Ca2+ channels in these cells and thus this channel
dominates the membrane potential producing a long plateau of
depolarization
This plateau is a balance between the open Ca2+ channels and the open
K+ channels
Ca2+ channel only slowly inactivates and thus this plateau can persist for
100-200 msec.
Action potential in ventricular (and atrial) cardiac cells
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Voltage-gated Ca2+ channel inactivates and the voltage-gated K+
channels will now dominate and the membrane potential will repolarize
to rest (EK+ in these cells).
Then the voltage-gated K+ channels will close, the voltage-gated Na+
channels will switch from the inactive to the closed state and the
membrane is set back at 4) ready to fire again.
The long Ca2+ plateau allows Ca2+ inside the cell to elevate enough to
generate contraction in the case of those cardiac cells that rely on
external Ca2+ sources.
Action potential in sinoatrial cardiac cells
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Sinoatrical cells have the ability to spontaneously fire action potentials
in a repeated fashion without any external influence.
These cells are the pacemaker cells of the heart and once an action
potential fires in these cells it is propagated via gap junctions to other
regions of the heart first to the atrial cells and then eventually making it
to the ventricular cells.
Action potential in sinoatrial cardiac cells
Very similar to the ventricular cardiac cells with a few major exceptions
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Do not have a stable rest.
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The action potential is driven by the voltage-gated Ca2+ channel in most
SA cells
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The rising phase is due the opening of the voltage-gated Ca2+ channel
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As the Ca2+ channel inactivates the membrane is repolarized by the
delayed rectifier K+channel as in other excitable cells
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Spontaneously depolarize once the delayed K+ channel has closed
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Due to presence of an ion channel that is activated by hyperpolarization
– the funny channel.
The funny channel
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Activated by hyperpolarization
As the membrane repolarizes after the action potential the threshold for
opening of the funny channel is reached at about -50 mV
The channel opens and allows Na+ to preferentially flow into the cell
Also called the HCN channel or hyperpolarization, cyclic nucleotide
gated ion channel
cAMP can have dramatic influences on this channel and shift its
threshold of activation from -50 mV to -40 mV.
The funny channel actually looks very much like a voltage-gated K+
channel but has differences in its pore to allow Na+ influx and in the
voltage sensing/opening mechanism.
Putting it all together……
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Located in the right atrium at the superior vena cava is the sinus node
(sinoatrial or SA node) which consists of specialized muscle cells
The SA nodal cells are self-excitatory, pacemaker cells
They generate an action potential at the rate of about 70 per minute in
humans (your heart beat)
From the sinus node, activation propagates throughout the atria, but
cannot propagate directly across the boundary between atria and
ventricles
This boundary serves to ensure a delay between the activation of the
atria and the ventricles
The atrioventricular node (AV node) is located at the boundary between
the atria and ventricles
In a normal heart, the AV node provides the only
conducting path from the atria to the ventricles
Putting it all together……
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Propagation from the AV node to the ventricles is provided by a
specialized muscle cells called the bundle of His conduct the signal
system
Further down the bundle separates into two bundle branches which
travel along each side of the septum, constituting the right and left
bundle branches.
Even more distally the bundles split into Purkinje fibers that branch and
contact the inner sides of the ventricular walls.
From the inner side of the ventricular wall, these activation sites cause
the formation of a wave of depolarization which propagates through gap
junctions between the ventricular cells toward the outer wall
After each ventricular muscle region has
depolarized, repolarization occurs.
Electrocardiogram - ECG
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P wave - an impulse is generated at the sinoatrial node and
spreads across both atria, causing them to contract
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Delay: The Fibro-fatty atrioventricular groove insulates the
ventricles from the atrial impulse
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The AV node is the only normal gateway of conduction to the
ventricles
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QRS wave - The impulse travels down the AV bundle and it's
branches and reaches the Purkinje fibers
The ventricles are stimulated to contract
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T wave - correlates with repolarization of the
ventricles.
Electrocardiogram - ECG
Electrocardiogram - ECG
Increasing the heart rate
Epinephrine and norepinephrine
• Released from the sympathetic nervous system
• Epinephrine and norepinephrine are synthesized and released
into the blood by the adrenal medulla, an endocrine organ
• Epinephrine and the related norepinephrine are all synthesized
from tyrosine and contain the catechol moiety; hence they are
referred to as catecholamines
• Nerves that synthesize and use epinephrine or norepinephrine
are termed adrenergic
• Adrenergic receptors: bind epinephrine and norepinephrine.
Because different receptors are linked to different G proteins, the
activation leads to different signal transduction cascades
• More Na+ and Ca2+ channels open
• Rate of depolarization and action potentials increase
Increasing the heart rate cont.
Epinephrine and norepinephrine cont….
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In sinoatrial cells: norepinephrine binds to the b-adrenergic
receptor which is a G protein associated membrane receptor
This triggers a signal transduction cascade outlined below that
activates the G protein (Gs - stimulates) that activates adenylate
cyclase to produce cAMP.
Beta-blockers: Drugs which are used to slow heart contractions
in the treatment of cardiac arrhythmia and angina, are beta1adrenergic receptor antagonists
They bind the beta1-adrenergic receptor to block the receptor and
thus slow heart contraction
Cardiac muscle cells possess beta1 adenergic receptors
Decreasing the heart rate
Acetylcholine: released from parasympathetic nervous system
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Muscarinic acetylcholine receptor: a G protein associated receptor.
The G protein activated in this case is a Gi subunit that inhibits
adenylate cyclase
More K+ channels open
Pacemaker cells hyperpolarize
Time for depolarization takes longer
Modulating the funny channel
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Through G protein coupled receptors various
hormones/neurotransmitters or drugs can increase or decrease the heart
rate by simply increasing or decreasing the ability of the funny channel
to open.
Modulating the funny channel
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Binding of hormone (e.g., epinephrine,
glucagon) to a Gs protein coupled
receptor
Gs protein relays the hormone signal to
the effector protein, ie adenylyl cyclase
Gs cycles between an inactive form with
bound GDP and an active form with
bound GTP
Dissociation of the active form yields the
Gsalpha · GTP complex, which directly
activates adenylyl cyclase
The increase in cAMP physically binds to
the funny channel and makes the channel
open more easily
Funny channel will open sooner during
the repolarization stage of the sinoatrial
action potential and a second action
potential will be triggered sooner
Increase heart rate.
Modulation of Ca2+ channel
Epinephrine:
• Causes an increase in cAMP that stimulates PKA (protein kinase A)
which in turn phosphorylates the voltage-gated Ca2+ channel (L channel)
• Results in a protein conformational change that enhances the channels
activity
• This new conformation of Ca2+ channel opens more readily (i.e. less time
between action potentials) and opens for longer (i.e. more Ca2+ flow into
the cell = greater [Ca2+] intracellular = greater contraction).
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Stimulates glycogen breakdown in skeletal muscles
Caffeine (mmmmhhh):
• Blocks the activty of phosphodiesterases. Phosphodiesterases break
down cyclic nucleotides. cAMP levels remain elevated and thus the
funny channel continues to open more readily.
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Affects the Ca2+ release channel or ryanodine receptor such that more
Ca2+ is released through the channel. Therefore heart contractions are
stronger in the presence of caffeine as well
Modulating the funny channel
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Acetylcholine works to block any rise in cAMP and reduces cAMP
levels in the cell
Therefore the funny channel will now not open so readily and the slow
depolarziation of the membrane will occur later thus resulting in a
longer time to generate a second action potential.
Modulating the voltage-gated K+ channels
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Acetylcholine-induced opening of K+ channels in the heart muscle
plasma membrane
Binding of ACH by muscarinic ACH receptors triggers activation of a
transducing G protein by catalyzing exchange of GTP for GDP on the
alpha subunit
The released beta/gamma subunit then binds to and opens a K+ channel
The increase in K+ permeability hyperpolarizes the membrane, which
reduces the frequency of heart muscle contraction
Activation is terminated when the GTP bound alpha subunit is
hydrolyzed to GDP and Galpha · GDP recombines with Gbeta/gamma.
Modulating the voltage-gated K+ channels
Application of acetylcholine (or muscarine) to frog heart muscle produces,
after a lag period of about 40 ms (not visible in graph), a hyperpolarization
of 2 3 mV, which lasts several seconds
From receptor to control of muscle cell contraction
From receptor to control of muscle cell contraction
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The cardiovascular system is highly regulated so that there is
always an adequate supply of oxygenated blood to the body
tissues under a wide range of circumstances
There are receptors that respond to the degree of blood pressure
and provide mechanical (barosensory) information about
pressure in the arteries system
There are receptors that provide information about the level of
oxygen and carbon dioxide in the blood
These sensory systems provide input to the respiratory control
centers of the brain which in turn control the parasympathetic and
sympathetic nerves that will control the heart, blood vessels and
diaphragm muscles for breathing.
From receptor to control of muscle cell contraction
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We will concentrate only on the chemoreceptors which are
located primarily in the aortic and carotid bodies
These are small, specialized organs located at the bifurcation of
the common carotid arteries (some chemosensory tissue is also
found in the aorta)
The chemoreceptors in the carotid bodies and aorta provide
information about the partial pressure of oxygen (PO2) and carbon
dioxide (PCO2) in the blood
This information is relayed by second order neurons to the
hypothalamus and other regions in the brainstem
This information about blood gas levels works in a reflex to
modulate the autonomic nervous system to control smooth and
cardiac muscles
From receptor to control of muscle cell contraction
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The carotid chemosensory cells detect levels of PO2 in the blood by
simply depolarizing in response to decreased levels of oxygen
The mechanism appears to be an O2 sensitive K+ channel, that in the
presence of normal levels of PO2 is open
Therefore the Vm is close to EK+
As oxygen levels drop the K+ channel closes and Vm depolarizes
allowing the voltage-gated Ca2+ channel to open and to trigger vesicle
fusion and neurotransmitter release
From receptor to control of muscle cell contraction
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PO2 levels can have a direct effect on smooth muscles around blood
vessels
Many of these cells have K+ channel that is inhibited by ATP
As PO2 drops so does respiration and ATP production
This reduction in ATP results in the opening of K + channels and the
inhibition of smooth muscle contraction
This results in the relaxation of the smooth muscles the relaxation of the
blood vessels and the increase blood flow into the tissue that is
experiencing reduced PO2
Conversely an increase in PO2 results in greater inhibition of the ATP
sensitive K + channels and thus a greater degree of depolarization
More Ca 2+channels are open and thus there is greater cytosolic Ca2+
levels, greater degree of smooth muscle contraction
This causes the blood vessel to constrict (vasoconstriction) and less PO2
transfer to the surrounding tissues.
From receptor to control of muscle cell contraction