Muscle I and II Flashcards
Contractility
presence of myosin and actin with ATP
Conductivity
allows muscle cells to transmit electrical impulses to other cells and receive impulses from other cells
Skeletal Muscle Fibers
- attached to skeleton, some visceral organs
- voluntary
- striated
- used for locomotion, respiration, etc
- tongue, pharynx, upper esophagus (visceral examples that help with swallowing and speech)
Smooth Muscle Fibers
- walls of internal organs
- NO striations
- INvoluntary
Cardiac Muscle Fibers
- found in heart
- striated
- INvoluntary
Sarcolemma
specialized plasmalemma of muscle cells
forms long processes called T-tubules that extend into the cytoplasm and are important for carrying the wave of depolarization deep into the sarcoplasm.
Sarcoplasm
specialized cytoplasm in muscle cells
contain myofibrils and filamentous mitochondria
Sarcoplasmic Reticulum
Specialized SER in muscle
used as a depot of Ca++. When excited, the sarcoplasmic reticulum releases Ca++ into cytoplasm initiating the muscular contraction
Skeletal Muscle
- striated, voluntary, quick contraction but tire fast
- originate from myoblasts that fuse to form LONG MULTINUCLEATED postmitotic myotubes
Each of the “cells” of the skeletal muscle is actually a ____ that forms a ___
multinucleated syncytium
skeletal muscle fiber
Nuclei of skeletal muscle cells
peripheral in location and are found immediately beneath the sarcolemma
filamentous mitochondria
lie between myofibrils and close to the sarcolemma. They represent the source of ATP for the myofibrils.
Myoglobin
an oxygen-binding protein, is present in skeletal muscle cells
Named skeletal muscles (e.g. the biceps)
consist of fascicles or bundles of skeletal muscle fibers surrounded by a connective tissue sheath.
Fascicles
are formed by groups of skeletal muscle fibers.
myofibril
formed of thick and thin filaments
extend the length of a muscle fiber
Microstructure of a muscular fiber
composed of hundreds of myofibrils that span the entire length of the muscle cell
Within the myofibril the thin (or actin) filaments are arranged in a hexagonal array with 6 thin filaments surrounding 1 thick (myosin) filament.
Thin filaments
composed of several proteins:
- F Actin
- Tropomyosin
- Troponin Complex (T,I,C)
F-actin
forms double stranded helical filament
Tropomyosin
forms filaments that lie in the grooves between actin monomers.
In a RESTING muscle it masks the binding sites for on actin filament (block contraction)
Troponin Complex
attached to tropomyosin and includes three globular subunits.
T, I, C
Troponin T
binds to tropomyosin, anchoring the troponin complex.
Troponin I
binds to actin inhibiting the interaction with myosin
Troponin C
is the smallest subunit; it binds to Ca++, which is an essential step in the initiation of the muscular contraction.
Thick Filaments
formed by hundreds of myosin molecules. Myosin II molecules consist of: 2 pairs of heavy chains and 2 pairs of light chains
Myosin II:
Heavy Chains
coiled A-helices and two globular heads that exhibit ATPase motor activity. Heads contain binding sites for actin/ATP
Myosin II:
Light Chains
attach to the globular heads of heavy chains
Microstructure of a myofibril
INSERT PICTURE
exhibits striations formed by alternating dark (A-bands) and light (I-bands) regions
H-zone, M Line, Z disk, Sarcomere
A-Band
thick and thin filaments
central part is H-zone
H-zone
Pale area in the middle of A-band and contains THICK filaments
M-line
formed by accessory proteins like myomesin that hold the thick filaments in palce
I-Band
pale area formed by mostly thin filaments. contains proteins like titin and nebulin. Is bisected by thre Z-disk
Z-disk
accessory proteins; functions to provide anchoring proteins for thin filaments and support the architecture of the myofibril
Sarcomere
portion of a myofibril between two adjacent Z-disks. It is the basic contractile unit of skeletal muscle. It measures 2-3 μm in relaxed muscle. The sarcomeres of individual myofibrils are in register in one muscle fiber, so the entire muscle cell exhibits cross-striations.
Skeletal muscle contraction is due to:
the sliding of filaments which results in the shortening of the sarcomere, while the length of individual filaments does not change. The length of the A-band does not change during contraction, but the I-band and H-zone shrink during contraction due to the increase of overlap between thick and thin filaments.
Where are accessory proteins primarily found in a motor unit?
Z disk and M line
Myomesin
holds thick filaments in place at M-line
a-Actinin
anchors thin filaments to Z-disk
Nebulin
protein that is attached to the Z-disk and runs parallel to thin filaments. It helps to anchor the thin filaments and to regulate the length of thin filaments during muscular fiber development
Desmin
an intermediate filament that forms a lattice that surrounds Z-disks and attaches them to one another and also attaches Z-disks to the plasma membrane
Dystrophin
a membrane-associated protein complex that links actin cytoskeleton to the extracellular matrix stabilizing the thin filaments
Electrophysiology of the muscle
- Sarcolemma is an electric capacitor. The voltage inside the cell is negative, while it is zero outside the cell, so there is a negative membrane potential in the resting cell, which is achieved by actively pumping Na+ ions out of the cell.
- The depolarized membrane develops a positive membrane potential when it becomes more permeable for Na+ ions. The depolarization of the membrane of the muscle cell starts a cascade of reactions that cause muscular contraction.
- Action potentials are brief positive going changes in the membrane potential that are propagated along the length of the membrane at speed up to 120 m/sec.
Membrane triads
formed by both the sarcolemma and the sarcoplasmic reticulum. Each triad consists of one T-tubule and two cisternae of sarcoplasmic reticulum running parallel to it.
T-tubular system (transverse tubule system)
formed by deep invaginations of the sarcolemma. T-tubules allow for the impulse to travel down into the cell and excite the terminal cisternae of the sarcoplasmic reticulum.
Terminal cisternae
long chambers formed by sarcoplasmic reticulum
parallel to T-tubules on both sides of it, forms triads
contain high levels of CA++
running near boundary of A/I-bands
action potential
travels along the membrane, it descends down into the cell along the T-tubules, which causes excitation of the SR and release of Ca++ into sarcoplasm.
Skeletal muscle: contraction
- In the resting muscle cell the myosin-
binding site on the actin filament is
concealed by the tropomyosin filament. - In the presence of high concentration of
Ca++ ions, Ca++ binds to troponin C. - This changes the spatial configuration of the
troponin molecule and causes the tropomyosin
filament to shift, which opens up myosin-binding
site on the actin filament. - Myosin starts “walking” along the actin
fibers. - Myosin uses ATP energy to slide the actin
along, so myosin is an ATPase.
Skeletal muscle: relaxation
- Following the depolarization, Ca++-activated ATPase membrane pumps transport Ca++ back into the sarcoplasmic reticulum. Ca++ disassociates from the troponin C.
- As a result the troponin complex returns to its original configuration and pulls the tropomyosin filament over the myosin binding site blocking the actin-myosin interaction. This stops the contraction unless there are new waves of depolarization coming with the nerve impulses.
- This is a rapid action that only takes 30 msec.
Regeneration of skeletal muscle
due to the presence of satellite cells, scattered between the skeletal muscle fibers. After injury satellite cells become activated, proliferate and give rise to new myoblasts, which fuse to form a new fiber. Extensive damage results in the formation of a connective tissue scar. Muscles respond to aging by increasing the diameter, to exercise by hypertrophy, and to disuse by atrophy
Endomysium
the most internal layer of reticular fibers that surrounds individual muscle fibers.
Perimysium
thicker layer of collagenous connective tissue that surrounds groups of muscle fibers, known as fascicles.
Epimysium
most external thick layer of connective tissue that surrounds a group of fascicles that constitutes a named muscle
Smooth muscle
- simplest type of muscle
- no striation (contractile elements are less organized)
- specialized for slow/rhythmic prolonged contractions of visceral organs
- except: precise contraction in eye
- involuntary
Smooth muscle cells
elongated spindle-shaped or fusiform cells that are 20-200 μm in length (20 μm in the walls of smallest blood vessels and up to 500 μm in the pregnant uterus). The nuclei of smooth muscle cells are centrally placed and long (cigar-shaped) with tapered ends.
Most organelles near tapered ends of cells. Myofilaments randomly distributed throughout.
Sarcolemma possess invaginations
SmM Thin Filaments
attached to anchoring points called dense bodies, formed primarily by the actin-binding protein α-actinin. Dense bodies are anchored into the network of intermediate filaments, formed by desmin. Dense bodies of the smooth muscle are somewhat analogous to the Z-disks of skeletal muscle fibers.
Sarcolemma of smooth muscle cells possesses large number of caveolae-like invaginations.
Caveolae-like invaginations act as sarcolemmal vesicles and deliver the depolarization to the chambers of sarcoplasmic reticulum, located beneath the caveolae. So it is thought that caveolae act similar to the T-tubules of skeletal muscle.
communicating junctions
found in the smooth muscle interconnecting individual muscle cells. Small molecules or ions can pass through these junctions from cell to cell and regulate contraction of the entire bundle of muscle cells. Smooth muscle cell can initiate contraction of neighboring muscle cells by allowing ions to pass through gap junctions and depolarize plasma membranes of other cells.
Smooth muscle cells are enclosed by the ___ and a network of ___. These fibers play an important role in the ____ in the smooth muscle tissue.
basal lamina
reticular fibers
force transduction
How do smooth muscle contractile filaments differ from skeletal muscle?
- Thin filaments of smooth muscle cells resemble the thin filaments of skeletal muscle fibers but without the troponin complex.
a. F-Actin is the principal component of thin filaments in the smooth muscle.
b. Tropomyosin wraps around actin in a fashion similar to skeletal muscle.
c. No troponin is present in the thin filaments of smooth muscle.
d. Caldesmon is a smooth muscle-specific actin-binding protein that masks the myosin-binding site on the actin fibers.
What part of SmM contraction is similar to SkM?
- Thick filaments are formed by myosin II.
a. Myosin II is composed of two heavy polypeptide chains and four light chains.
i. Smooth muscle myosin binds to actin only when phosphorylated.
ii. Myosin molecule is folded (inactive!) when dephosphorylated.
Excitation of smooth muscle fibers (3 ways)
- The neural stimulation of smooth muscle occurs through the postganglionic fibers of the autonomic nervous system. The neuromuscular junctions are not as specialized as in the skeletal muscle. The neurotransmitter is released in the close proximity of a muscle cell and has to diffuse to the muscle cells through the connective tissue that surrounds the muscle cells. Not all smooth muscle cells receive terminal nerves and impulse transmission from cell to cell occurs through gap junctions.
- Chemical stimulation of a smooth muscle contraction can be elicited by various hormones, angiotensin II, vasopressin, and other agents.
- Mechanical stimulation, such as passive stretching of the organ, can lead to the initiation of a muscular contraction.
Contraction of smooth muscle
driven by ^Ca++ in sarcoplasm
- in relaxed muscle the highest concentration is in the Sarcoplasmic reticulum
- Excitation: Ca released to sarcoplasm
- Ca binds to calmodulin
- Ca-calmodulin complex binds to caldesmon, releasing it from actin to open myosin binding site on actin
- complex activates myosin light chain kinase
- MLCK phosphorylates light chain of myosin
- PO4: myosin unfolds, actin binding site opens, myosin binds to actin
- filaments slide, cell shortens, nucleus folds to corkscrew, takes over 1 sec
Relaxation of smooth muscle
- Ca++ is pumped back into sarcoplasmic reticulum.
- Ca++ levels in the sarcoplasm drop.
- Calmodulin disassociates from the light-chain kinase, which deactivates the latter.
- Myosin is dephosphorylated and becomes inactive.
- Caldesmon binds to the myosin-binding site on the actin filament.
- This is slow action, but has a prolonged effect on the tissue and requires very little energy spent compared to the skeletal muscle (only 10% of ATP that would have been used by the skeletal muscle).
- Smooth muscles possess a secondary mechanism that allows them to maintain long-term contractions with minimum energy spent. This is called a latch state of the smooth muscle and is caused by the decrease of ATP activity while the myosin head is attached to actin. This causes prolonged contraction used to sustain the tone of blood vessels. This condition resembles the rigor mortis of the skeletal muscle
Location of Smooth Muscle
Walls of large hollow organs
- veins, arteries, larger lymphatics
- extramural part of large ducts of glands
- GI tract (distal esophagus to anus)
- reproductive and urinary tracts
Iris and ciliary body (pupil)
Dermis of skin (thermoregulation via arrectores pilorum)
Smooth muscle regeneration and blood supply
- Smooth muscles have moderate blood supply through capillaries in the connective tissue immediately surrounding the muscle cells.
- Smooth muscle is capable of active regenerative response. The smooth muscle cells can go through mitosis and replace the damaged or lost cells.
Cardiac Muscle
- have cross striations like skeletal
- involuntary rhythmic contractions for pumping blood
- receives extensive blood supply
- NO ABILITY FOR REGENERATION. local injury results in scarring and loss of function (MI)
- cells are individual, unlike skeletal. rectangular, form chains
- intercalated disks join the chains. Joining of two cells often reveals branching (unique)
- central round nuclei, large flat mitos, lots of myofibrils
Intercalated disks
junctions between individual CM cells. have transverse and lateral portions. several junction types.
Transverse Portion of Intercalated Disk
runs across fibers at a right angle, contains anchoring junctions (mechanical stability and prevents cells from separating), both adherens and desmosomes
Adherens Junctions
connect the microfilaments of the two neighboring cardiac muscle cells. They are similar to the zonulae adherentes found in the epithelial cells.
Desmosomes
connect intermediate filaments (formed by desmin) of two adjacent cells.
Lateral portion of the intercalated disk
runs parallel to the myofilaments. It contains communicating, or gap junctions that provide ionic continuity between adjacent cardiac muscle cells and allow the signal to contract to pass from cell to cell and generate a wave of contraction.
Cardiac muscle: T-tubules
- large T-tubules that are found at the Z-disks and not at the junction of A and I bands, as was in the skeletal muscle fibers
- The sarcoplasmic reticulum is not as well-developed as in the skeletal muscle. There are no large terminal cisternae; instead there are small chambers that run parallel to the T-tubules only on one side of the tubule forming diads.
- The contraction is Ca++ dependent and similar to contraction of the skeletal muscle.
Cardiac conducting system
The heartbeat is initiated, coordinated, and regulated by modified cardiac muscle cells that form the cardiac conducting system.
Cardiac conducting cells called the Purkinje fibers are modified for the conduction of electric impulses similar to nerve cells. Purkinje fibers form nodes and bundles including the sinoatrial and atrioventricular nodes and the bundle of His.
Describe the levels of Skeletal Muscle, Starting with the muscle and ending with the smallest aspect
Muscle
Fascicle
Fiber (aka cell)
Myofibiril
Myofilaments (thick or thin)
